Ihdr crc error

For the purposes of this specification the following
definitions apply.

alpha

a value representing a pixel degree of opacity. The more
opaque a pixel, the more it hides the background against which
the image is presented. Zero alpha represents a completely
transparent pixel, maximum alpha represents a completely opaque
pixel.

alpha compaction

an implicit representation of transparent
pixels. If every
pixel with a specific colour or greyscale value is fully
transparent and all other pixels are fully opaque, the
alpha
channel may be
represented implicitly.

alpha separation

separating an alpha channel in which every
pixel is fully
opaque; all alpha values are the maximum value.
The fact that all pixels are fully opaque is represented implicitly.

alpha table

indexed table of alpha sample values, which in an indexed-colour image defines the alpha
sample values of the reference image. The alpha table has the
same number of entries as the palette.

ancillary chunk

class of chunk that provides additional
information. A PNG decoder, without processing an
ancillary chunk, can still produce a meaningful image, though not
necessarily the best possible image.

animated image

Optional animation, consisting of a series of frames.
The first frame may be,
but need not be,
the static image.

bit depth

for indexed-colour images, the number of bits
per palette index. For other images, the
number of bits per sample in the image. This is the value
that appears in the
11.2.1 IHDR Image
header chunk.

byte

8 bits; also called an octet. The highest bit (value 128) of
a byte is numbered bit 7; the lowest bit (value 1) is numbered
bit 0.
It represents an unsigned integer limited to the range 0 to
2⁸-1.

byte order

ordering of bytes for multi-byte data values within a
PNG file
or PNG datastream. PNG uses
network byte order.

canvas

the area on the output device on which the frames are to be displayed.
The contents of the canvas are not necessarily available to the decoder.
If a bKGD chunk exists,
it may be used to fill the canvas if there is no preferable background

channel

array of all per-pixel information of a particular kind
within a reference image. There are five kinds of
information: red, green, blue, greyscale, and alpha. For example the alpha
channel is the array of alpha values within a reference
image.

chromaticity

pair of CIE x,y values [COLORIMETRY] that precisely specify a colour,
except for the brightness information.

chunk

section of a PNG datastream. Each chunk has a chunk
type. Most chunks also include data. The format and meaning of
the data within the chunk are determined by the chunk type.
Each chunk is either a
critical chunk or an
ancillary chunk.

composite (verb)

to form an image by merging a foreground image and a
background image, using transparency information to determine
where and to what extent the background should be visible. The
foreground image is said to be «composited against» the
background.

critical chunk

chunk
that
shall be understood and processed by the decoder in order to
produce a meaningful image from a PNG datastream.

datastream

sequence of bytes. This term is used rather than
«file» to describe a byte sequence that may be only a portion of
a file. It is also used to emphasize that the sequence of bytes
might be generated and consumed «on the fly», never appearing in
a stored file at all.

deflate

name of a particular compression algorithm. This algorithm is
used, in compression mode 0, in conforming
PNG
datastreams. Deflate is a member of the LZ77 family of
compression methods. It is defined in [RFC1951].

delivered image

image constructed from a decoded
PNG datastream.

frame buffer

the final digital storage area for the image shown by most
types of computer display. Software causes an image to appear on
screen by loading the image into the frame buffer.

fully transparent black

the red, green, blue and alpha components are all set to zero.

gamma value

value of the exponent of a gamma transfer function

gamma

power-law transfer function

full-range image

Image where reference black and white correspond to sample values
0 and 2bit depth - 1, respectively.

greyscale

image representation in which each pixel is defined by a single
sample of
colour information, representing overall
luminance (on a
scale from black to white), and optionally an
alpha sample (in
which case it is called greyscale with alpha).

image data

1-dimensional array of scanlines within an image.

indexed-colour

image representation in which each pixel of the original image is
represented by a single index into a palette. The selected palette entry
defines the actual colour of the pixel.

interlaced PNG image

sequence of reduced images generated from the
PNG image
by pass extraction.

lossless

method of data compression that permits reconstruction of the
original data exactly, bit-for-bit.

luminance

formal definition of luminance is in [COLORIMETRY].
Informally it is the perceived brightness, or
greyscale
level, of a colour. Luminance and chromaticity together fully define
a perceived colour.

LZ77

data compression algorithm described in
[Ziv-Lempel].

narrow-range image

Image where reference black and white do not correspond to sample values
0 and 2bit depth - 1, respectively.

network byte order

byte order in which the most significant byte comes first,
then the less significant bytes in descending order of
significance (MSB LSB for two-byte integers,
MSB B2 B1
LSB for four-byte
integers).

output buffer

The output buffer is a pixel array
with dimensions specified by the width and height parameters of the PNG IHDR chunk. Conceptually, each frame is constructed in the output buffer before being composited onto the canvas. The contents of the output buffer are available to the decoder. The corners of the output buffer are mapped to the corners of the canvas.

palette

indexed table of three 8-bit sample values, red, green, and blue,
which with an indexed-colour image defines the red,
green, and blue sample values of the
reference image. In other cases, the palette may be a suggested
palette that viewers may use to present the image on
indexed-colour display hardware. Alpha samples may be defined for palette
entries via the alpha table and may be used to
reconstruct the alpha sample values of the reference image.

organizing a PNG image as a sequence of reduced images
to change the order of transmission and enable
progressive display.

pixel

information stored for a single grid point in an image. A
pixel consists of (or points to) a sequence of samples from all
channels. The
complete image is a rectangular array of pixels.

PNG datastream

result of encoding a PNG image. A PNG
datastream
consists of a PNG signature followed by a sequence of
chunks.

PNG decoder

process or device which reconstructs the
reference image from a PNG datastream and generates a
corresponding delivered image.

PNG editor

process or device which creates a modification of an existing
PNG datastream, preserving unmodified ancillary
information wherever possible, and obeying the
chunk ordering
rules, even for unknown chunk types.

PNG encoder

process or device which constructs a
reference image from a source image, and generates a
PNG datastream representing the reference image.

PNG file

PNG datastream stored as a file.

PNG four-byte unsigned integer

a four-byte unsigned integer limited to the range 0 to 2³¹-1.

Note

The restriction is imposed in order to accommodate languages
that have difficulty with unsigned four-byte values.

PNG image

result of transformations applied by a
PNG encoder to
a reference image, in preparation for encoding as a
PNG datastream, and the result of decoding a PNG
datastream.

PNG signature

sequence of bytes appearing at the start of every
PNG datastream. It differentiates a PNG datastream from
other types of datastream and allows early detection of
some transmission errors.

reduced image

pass of the interlaced PNG image extracted from the
PNG image by pass extraction.

reference image

rectangular array of rectangular pixels, each having the same number
of samples, either three (red, green, blue)
or four (red, green, blue, alpha). Every reference image can be
represented exactly by a PNG datastream and every PNG datastream
can be converted into a reference image. Each
channel has a
sample depth in the range 1 to 16. All samples in the same
channel have the same sample depth. Different channels may have
different sample depths.

sample

intersection of a channel and a pixel in an image.

sample depth

number of bits used to represent a sample value. In an
indexed-colour PNG image, samples are stored in
the palette and thus the sample depth is
always 8 by definition of the palette. In other types of PNG
image it is the same as the bit depth.

scanline

row of pixels within an image or
interlaced PNG image.

source image

image which is presented to a PNG encoder.

static image

non-animated image
corresponding to the reference image
encoded in the IDAT chunk.
All PNG and APNG images contain a static image,
and non animation-capable displays (such as printers)
will display this rather than the animation.

transfer function

function relating image luminance with image samples

white point

chromaticity of a computer display’s
nominal white value.

zlib

deflate-style compression method.

Note

The format is defined in [rfc1950].

Note

Also refers to the name of a library containing a sample
implementation of this method.

APNG

Animated PNG, a type of PNG
which — in addition to a
static image —
also contains an animated image.

Cyclic Redundancy Code

CRC

type of check value designed to detect most transmission errors.

Note

A decoder calculates the CRC for the received data and checks
by comparing it to the CRC calculated by the encoder and appended to the data.
A mismatch indicates that the data or the CRC were corrupted in transit.

Cathode Ray Tube

CRT

vacuum tube containing one or more electron guns, which emit electron beams that are manipulated to display images on a phosphorescent screen.

LSB

Least Significant Byte of a multi-byte value.

MSB

Most Significant Byte of a multi-byte value.

All PNG images contain a single
static image.
Some PNG images —
called APNG
for Animated PNG —
also contain a frame-based animation sequence,
the animated image.
The first frame of this may be —
but need not be —
the static image.

The static image,
and each individual frame of an
animated image,
corresponds to a reference image
and is stored as a PNG image.

This specification specifies the PNG datastream, and
places some requirements on PNG encoders, which generate PNG
datastreams, PNG decoders, which interpret PNG datastreams, and
PNG editors, which transform one PNG datastream into another. It
does not specify the interface between an application and either
a PNG encoder, decoder, or editor. The precise form in which an
image is presented to an encoder or delivered by a decoder is not
specified. Four kinds of image are distinguished.

1. The source image is the image presented to a PNG
   encoder.
2. The reference image, which only exists conceptually,
   is a rectangular array of rectangular pixels, all having the same
   width and height, and all containing the same number of unsigned
   integer samples, either three (red, green, blue) or four (red,
   green, blue, alpha). The array of all samples of a particular
   kind (red, green, blue, or alpha) is called a channel. Each
   channel has a sample depth in the range 1 to 16, which is the
   number of bits used by every sample in the channel. Different
   channels may have different sample depths. The red, green, and
   blue samples determine the intensities of the red, green, and
   blue components of the pixel’s colour; if they are all zero, the
   pixel is black, and if they all have their maximum values
   (2^sampledepth-1), the pixel is white. The alpha sample
   determines a pixel’s degree of opacity, where zero means fully
   transparent and the maximum value means fully opaque. In a
   three-channel reference image all pixels are fully opaque. (It is
   also possible for a four-channel reference image to have all
   pixels fully opaque; the difference is that the latter has a
   specific alpha sample depth, whereas the former does not.) Each
   horizontal row of pixels is called a scanline. Pixels are ordered
   from left to right within each scanline, and scanlines are
   ordered from top to bottom. A PNG encoder may transform the source
   image directly into a PNG image, but conceptually it first
   transforms the source image into a reference image, then
   transforms the reference image into a PNG image. Depending on the
   type of source image, the transformation from the source image to
   a reference image may require the loss of information. That
   transformation is beyond the scope of this International Standard.
   The reference image, however, can always be recovered
   exactly from a PNG datastream.
3. The PNG image is obtained from the reference image by
   a series of transformations: alpha separation, indexing, RGB
   merging, alpha compaction, and sample depth scaling. Five types
   of PNG image are defined (see 6.1 Colour types and values). (If the PNG encoder actually transforms the
   source image directly into the PNG image, and the source image
   format is already similar to the PNG image format, the encoder
   may be able to avoid doing some of these transformations.)
   Although not all sample depths in the range 1 to 16 bits are
   explicitly supported in the PNG image, the number of significant
   bits in each channel of the reference image may be recorded. All
   channels in the PNG image have the same sample depth. A PNG
   encoder generates a PNG datastream from the PNG image. A PNG
   decoder takes the PNG datastream and recreates the PNG
   image.
4. The delivered image is constructed from the PNG image
   obtained by decoding a PNG datastream. No specific format is
   specified for the delivered image. A viewer presents an image to
   the user as close to the appearance of the original source image
   as it can achieve.

The relationships between the four kinds of image are
illustrated in Figure 1.

The relationships between samples, channels, pixels, and
sample depth are illustrated in Figure 2.

The RGB colour space in which colour samples are situated may
be specified in one of four ways:

1. by CICP image format signaling metadata;
2. by an ICC profile;
3. by specifying explicitly that the colour space is sRGB when
   the samples conform to this colour space;
4. by specifying a gamma value and the 1931 CIE x,y
   chromaticities of the red, green, and blue primaries used in the
   image and the reference white point.

For high-end applications the first two methods provides the most
flexibility and control. The third method enables one particular,
but extremely common,
colour space to be indicated. The fourth method,
which was standardized before ICC profiles were widely adopted,
enables the exact
chromaticities of the RGB data to be specified, along with the
gamma correction to be applied (see C. Gamma and chromaticity).
However, colour-aware applications will prefer one of the first three methods,
while colour-unaware applications will typically ignore all four methods.

Gamma correction is not applied to the alpha channel, if
present. Alpha samples always represent a linear fraction of full
opacity.

The transformation of the reference image results in one of
five types of PNG image (see Figure 6) :

1. Truecolour with alpha: each pixel is defined by
   samples, representing red, green, and blue intensities, and an
   alpha sample.
2. Greyscale with alpha: each pixel consists of two samples:
   grey and alpha.
3. Truecolour: each pixel is defined by
   samples, representing red, green, and blue intensities. The alpha channel
   may be represented by a single RGB pixel value. Matching pixels are fully
   transparent, and all others are fully opaque. If the alpha channel is not
   represented in this way, all pixels are fully opaque.
4. Greyscale: each pixel consists of a single sample: grey. The
   alpha channel may be represented by a single greyscale pixel value,
   similar to
   the previous case. If the alpha channel is not represented in
   this way, all pixels are fully opaque.
5. Indexed-colour: each pixel consists of an index into a palette (and into an associated table of alpha values, if present).

The format of each pixel depends on the PNG image type and the
bit depth. For PNG image types other than indexed-colour,
the bit depth specifies the number of bits per sample, not the
total number of bits per pixel.
For indexed-colour images, the bit depth specifies the
number of bits in each palette index, not the sample depth of the
colours in the palette or alpha table. Within the pixel the
samples appear in the following order, depending on the PNG image
type.

1. Truecolour with alpha: red, green, blue, alpha.
2. Greyscale with alpha: grey, alpha.
3. Truecolour: red, green, blue.
4. Greyscale: grey.
5. Indexed-colour: palette index.

Ancillary information may be associated with an image.
Decoders may ignore all or some of the ancillary information. The
types of ancillary information provided are described in Table 1.

Errors in a PNG datastream fall into two general classes:

1. transmission errors or damage to a computer file system,
   which tend to corrupt much or all of the datastream;
2. syntax errors, which appear as invalid values in chunks, or
   as missing or misplaced chunks. Syntax errors can be caused not
   only by encoding mistakes, but also by the use of registered or
   private values, if those values are unknown to the decoder.

PNG decoders should detect errors as early as possible,
recover from errors whenever possible, and fail gracefully
otherwise. The error handling philosophy is described in detail
in 13.1 Error handling.

This section is non-normative.

The PNG format exposes several extension points:

• chunk type;
• text keyword; and
• private field values.

Some of these extension points are reserved by W3C, while others are
available for private use.

The PNG datastream consists of a PNG signature followed by a sequence of chunks.

The first eight bytes of a PNG datastream always contain the
following (decimal) values:

137 80 78 71 13 10 26 10

which are (in hexadecimal):

89 50 4E 47 0D 0A 1A 0A

This signature indicates that the remainder of the datastream
contains a single PNG image, consisting of a series of chunks
beginning with an
IHDR chunk and ending with an IEND chunk.

Each chunk consists of three or four fields (see Figure 10).
The meaning of the fields is described in Table 4.
The chunk data field may be empty.

The chunk data length may be any number of bytes up to the
maximum; therefore, implementors cannot assume that chunks are
aligned on any boundaries larger than bytes.

Chunk types are chosen to be meaningful names when the bytes
of the chunk type are interpreted as ISO 646 letters [ISO646]. Chunk types
are assigned so that a decoder can determine some properties of a
chunk even when the type is not recognized. These rules allow
safe, flexible extension of the PNG format, by allowing a PNG
decoder to decide what to do when it encounters an unknown chunk.

The naming rules are normally of interest only when the decoder does not
recognize the chunk’s type, as specified at 13. PNG decoders and viewers.

Four bits of the chunk type, the property bits, namely bit 5
(value 32) of each byte, are used to convey chunk properties.
This choice means that a human can read off the assigned
properties according to whether the letter corresponding to each
byte of the chunk type is uppercase (bit 5 is 0) or lowercase
(bit 5 is 1).

The property bits are an inherent part of the chunk type, and
hence are fixed for any chunk type. Thus, CHNK and cHNk would be
unrelated chunk types, not the same chunk with different
properties.

The semantics of the property bits are
defined in
Table 5.

EXAMPLE The hypothetical chunk type «cHNk» has the property bits:

cHNk  <-- 32 bit chunk type represented in text form
||||
|||+- Safe-to-copy bit is 1 (lower case letter; bit 5 is 1)
||+-- Reserved bit is 0     (upper case letter; bit 5 is 0)
|+--- Private bit is 0      (upper case letter; bit 5 is 0)
+---- Ancillary bit is 1    (lower case letter; bit 5 is 1)

Therefore, this name represents an ancillary, public,
safe-to-copy chunk.

CRC fields are calculated using standardized CRC methods with
pre and post conditioning, as defined by [ISO_3309]
and [ITU-T_V.42].
The CRC polynomial employed—
which is identical to that used in the
GZIP file format specification [RFC1952]—
is

x³² + x²⁶ + x²³ +
x²² + x¹⁶ + x¹² + x¹¹
+ x¹⁰ + x⁸ + x⁷ + x⁵
+ x⁴ + x² + x + 1

In PNG, the 32-bit CRC is initialized to all 1’s, and then the
data from each byte is processed from the least significant bit
(1) to the most significant bit (128). After all the data bytes
are processed, the CRC is inverted (its ones complement is
taken). This value is transmitted (stored in the datastream) MSB
first. For the purpose of separating into bytes and ordering, the
least significant bit of the 32-bit CRC is defined to be the
coefficient of the x31 term.

Practical calculation of the CRC often employs a precalculated
table to accelerate the computation. See D. Sample CRC
implementation.

The constraints on the positioning of the individual chunks
are listed in
Table 6 and illustrated diagrammatically
for static images in Figure 11 and Figure 12,
for animated images where the static image forms the first frame in
Figure 13 and
Figure 14, and
for animated images where the static image is not part of the animation in
Figure 15 and
Figure 16.
These lattice diagrams represent the constraints on positioning
imposed by this specification. The lines in the diagrams
define partial ordering relationships. Chunks higher up shall
appear before chunks lower down. Chunks which are horizontally
aligned and appear between two other chunk types (higher and
lower than the horizontally aligned chunks) may appear in any
order between the two higher and lower chunk types to which they
are connected. The superscript associated with the chunk type is
defined in
Table 7. It indicates whether the chunk is mandatory,
optional, or may appear more than once. A vertical bar between
two chunk types indicates alternatives.

Values greater than or equal to 128 in the following fields are
private field values:

• compression method
• interlace method
• filter method

These private field values are neither defined nor reserved by this
specification.

Private field values MAY be used for experimental or private
semantics.

Private field values SHOULD NOT appear in publicly available
software or datastreams since they can result in datastreams that are
unreadable by PNG decoders as detailed at 13. PNG decoders and viewers.

As explained in 4.5 PNG image there are five types of PNG

image. Corresponding to each type is a colour type, which is the
sum of the following values: 1 (palette used), 2 (truecolour
used) and 4 (alpha used). Greyscale and truecolour images may
have an explicit alpha channel. The PNG image types and
corresponding colour types are listed in Table 8.

The allowed bit depths and sample depths for each PNG image
type are listed in Image
header.

Greyscale samples represent luminance if the transfer curve is
indicated (by
gAMA,
sRGB, or
iCCP) or device-dependent greyscale if not.
RGB samples represent calibrated colour information if the colour
space is indicated (by
gAMA and
cHRM, or
sRGB, or
iCCP,
or uncalibrated device-dependent colour
if not.

Sample values are not necessarily proportional to light
intensity; the
gAMA chunk specifies the relationship between
sample values and display output intensity. Viewers are strongly
encouraged to compensate properly. See 4.3 Colour spaces, 13.13 Decoder gamma
handling and C. Gamma and chromaticity.

In a PNG datastream transparency may be represented in one of
four ways, depending on the PNG image type (see alpha separation
and alpha compaction).

1. Truecolour with alpha, greyscale with alpha: an alpha channel
   is part of the image array.
2. truecolour, greyscale: A
   tRNS chunk contains a single pixel value
   distinguishing the fully transparent pixels from the fully opaque
   pixels.
3. Indexed-colour: A
   tRNS chunk contains the alpha table that
   associates an alpha sample with each palette entry.
4. truecolour, greyscale, indexed-colour: there is no tRNS chunk present and
   all pixels are fully opaque.

An alpha channel included in the image array has 8-bit or
16-bit samples, the same size as the other samples. The alpha
sample for each pixel is stored immediately following the
greyscale or RGB samples of the pixel. An alpha value of zero
represents full transparency, and a value of
2^sampledepth — 1 represents full opacity. Intermediate
values indicate partially transparent pixels that can be
composited against a background image to yield the delivered
image.

The colour values in a pixel are not premultiplied by the
alpha value assigned to the pixel. This rule is sometimes called
«unassociated» or «non-premultiplied» alpha. (Another common
technique is to store sample values premultiplied by the alpha
value; in effect, such an image is already composited against a
black background. PNG does not use premultiplied alpha.
In consequence an image editor can take a PNG image and easily
change its transparency.) See 12.3 Alpha channel
creation
and 13.16 Alpha channel
processing.

All integers that require more than one byte shall be in
network byte order (as illustrated in Figure 17
): the most significant byte
comes first, then the less significant bytes in descending order
of significance (MSB LSB for two-byte integers, MSB B2 B1 LSB for
four-byte integers). The highest bit (value 128) of a byte is
numbered bit 7; the lowest bit (value 1) is numbered bit 0.
Values are unsigned unless otherwise noted. Values explicitly
noted as signed are represented in two’s complement notation.

PNG four-byte unsigned integers are limited to the range 0 to
2³¹-1 to accommodate languages that have difficulty
with unsigned four-byte values.

A PNG image (or pass, see 8. Interlacing and pass
extraction) is a rectangular pixel array, with pixels
appearing left-to-right within each scanline, and scanlines
appearing top-to-bottom. The size of each pixel is determined by
the number of bits per pixel.

Pixels within a scanline are always packed into a sequence of
bytes with no wasted bits between pixels. Scanlines always begin
on byte boundaries. Permitted bit depths and colour types are
restricted so that in all cases the packing is simple and
efficient.

In PNG images of colour type 0 (greyscale) each pixel is a single sample, which may have precision less than a byte (1, 2, or 4 bits). These samples are packed into bytes with the leftmost sample in the high-order bits of a byte followed by the other samples for the scanline.

In PNG images of colour type 3 (indexed-colour) each pixel is a single palette index. These indices are packed into bytes in the same way as the samples for colour type 0.

When there are multiple pixels per byte, some low-order bits
of the last byte of a scanline may go unused. The contents of
these unused bits are not specified.

PNG images that are not indexed-colour images may have sample
values with a bit depth of 16. Such sample values are in network
byte order (MSB first, LSB second). PNG permits multi-sample
pixels only with 8 and 16-bit samples, so multiple samples of a
single pixel are never packed into one byte.

A filter method is a transformation applied to an
array of scanlines with the aim of improving their compressibility.

PNG standardizes one filter method and several filter types
that may be used to prepare image data for compression. It
transforms the byte sequence into an equal length
sequence of bytes preceded by a filter type byte (see Figure 18 for an
example).

The encoder shall use only a single filter method for
an interlaced PNG image, but may use different filter types for
each scanline in a reduced image. An intelligent encoder can
switch filters from one scanline to the next. The method for
choosing which filter to employ is left to the encoder.

The filter type byte is not considered part of
the image data, but it is included in the datastream sent to the
compression step. See 9. Filtering.

Pass extraction (see
figure 4.8) splits a PNG image into a
sequence of reduced images (the interlaced PNG image) where the
first image defines a coarse view and subsequent images enhance
this coarse view until the last image completes the PNG image.
This allows progressive display of the interlaced PNG image by
the decoder and allows images to «fade in» when they are being
displayed on-the-fly. On average, interlacing slightly expands
the datastream size, but it can give the user a meaningful
display much more rapidly.

Two interlace methods are defined in this International
Standard, methods 0 and 1. Other values of interlace method are
reserved for future standardization.

With interlace method 0, the null method, pixels are extracted
sequentially from left to right, and scanlines sequentially from
top to bottom. The interlaced PNG image is a single reduced
image.

Interlace method 1, known as Adam7, defines seven distinct
passes over the image. Each pass transmits a subset of the pixels
in the reference image. The pass in which each pixel is
transmitted (numbered from 1 to 7) is defined by replicating the
following 8-by-8 pattern over the entire image, starting at the
upper left corner:

1 6 4 6 2 6 4 6
7 7 7 7 7 7 7 7
5 6 5 6 5 6 5 6
7 7 7 7 7 7 7 7
3 6 4 6 3 6 4 6
7 7 7 7 7 7 7 7
5 6 5 6 5 6 5 6
7 7 7 7 7 7 7 7

Figure 4.8
shows the seven passes of interlace method 1. Within each pass,
the selected pixels are transmitted left to right within a
scanline, and selected scanlines sequentially from top to bottom.
For example, pass 2 contains pixels 4, 12, 20, etc. of scanlines
0, 8, 16, etc. (where scanline 0, pixel 0 is the upper left
corner). The last pass contains all of scanlines 1, 3, 5, etc.
The transmission order is defined so that all the scanlines
transmitted in a pass will have the same number of pixels; this
is necessary for proper application of some of the filters. The
interlaced PNG image consists of a sequence of seven reduced
images. For example, if the PNG image is 16 by 16 pixels, then
the third pass will be a reduced image of two scanlines, each
containing four pixels (see
figure 4.8).

Scanlines that do not completely fill an integral number of
bytes are padded as defined in 7.2 Scanlines.

NOTE If the reference image contains fewer than
five columns or fewer than five rows, some passes will be
empty.

Filtering transforms the PNG image with the goal of
improving compression.
The overall process is depicted in
Figure 7
while the specifics of serializing and filtering a scanline
are shown in Figure 18.

PNG allows for a number of filter methods.
All the reduced
images in an interlaced image shall use a single filter method.
Only filter method 0
is defined by this specification. Other filter methods
are reserved for future standardization.
Filter method 0 provides a set of five filter types,
and individual scanlines in each reduced image may use
different filter types.

PNG imposes no additional restriction on which filter types
can be applied to an interlaced PNG image. However, the filter
types are not equally effective on all types of data. See
12.7 Filter selection.

Filtering transforms the byte sequence in a scanline to an
equal length sequence of bytes preceded by the filter type.
Filter type bytes are associated only with non-empty scanlines.
No filter type bytes are present in an empty pass. See 13.10 Interlacing and
progressive display.

Filters are applied to bytes, not to pixels,
regardless of the bit depth or colour type of the image. The
filters operate on the byte sequence formed by a scanline that
has been represented as described in 7.2 Scanlines. If the image
includes an alpha channel, the alpha data is filtered in the same
way as the image data.

Filters may use the original values of the following bytes to
generate the new byte value:

Figure 19 shows the relative positions of the bytes x,
a, b,
and c.

Filter method 0 defines five basic filter types as listed
in
Table 10. Orig(y) denotes the original (unfiltered)
value of byte y. Filt(y) denotes the value
after a filter type has been applied. Recon(y) denotes the
value after the corresponding reconstruction function has been
applied. The Paeth filter type
PaethPredictor [Paeth] is defined below.

Filter method 0 specifies exactly this set of five filter
types and this shall not be extended.
This ensures that decoders need not decompress the data
to determine whether it contains unsupported filter types:
it is sufficient to check the filter method in 11.2.1 IHDR Image
header.

For all filters, the bytes «to the left of» the first pixel in
a scanline shall be treated as being zero. For filters that refer
to the prior scanline, the entire prior scanline and bytes «to
the left of» the first pixel in the prior scanline shall be
treated as being zeroes for the first scanline of a reduced
image.

To reverse the effect of a filter requires the decoded values
of the prior pixel on the same scanline, the pixel immediately
above the current pixel on the prior scanline, and the pixel just
to the left of the pixel above.

Unsigned arithmetic modulo 256 is used, so that both the
inputs and outputs fit into bytes. Filters are applied to each
byte regardless of bit depth. The sequence of Filt
values is transmitted as the filtered scanline.

The sum Orig(a) + Orig(b) shall be performed without
overflow (using at least nine-bit arithmetic). floor()
indicates that the result of the division is rounded to the next
lower integer if fractional; in other words, it is an integer
division or right shift operation.

The Paeth filter type computes a simple linear function of
the three neighbouring pixels (left, above, upper left), then
chooses as predictor the neighbouring pixel closest to the
computed value. The algorithm used in this specification
is an adaptation of the technique due to Alan W. Paeth
[Paeth].

The PaethPredictor function is defined in the code below. The
logic of the function and the locations of the bytes a,
b, c, and x are shown in Figure 20.
Pr is the predictor for byte x.

p = a + b - c
pa = abs(p - a)
pb = abs(p - b)
pc = abs(p - c)
if pa <= pb and pa <= pc then Pr = a
else if pb <= pc then Pr = b
else Pr = c
return Pr

The calculations within the PaethPredictor function shall be
performed exactly, without overflow.

The order in which the comparisons are performed is
critical and shall not be altered. The function tries to
establish in which of the three directions (vertical, horizontal,
or diagonal) the gradient of the image is smallest.

Exactly the same PaethPredictor function is used by both
encoder and decoder.

Only PNG compression method 0 is defined by this International
Standard. Other values of compression method are reserved for
future standardization. PNG compression method 0 is
deflate compression with a sliding window
(which is an upper bound on the distances appearing in the
deflate stream) of at most
32768 bytes. Deflate compression is derived from LZ77.

Deflate-compressed datastreams within PNG are stored in the
zlib format, which has the structure:

zlib is specified at [rfc1950].

For PNG compression method 0, the zlib compression
method/flags code shall specify method code 8 (deflate
compression) and an LZ77 window size of not more than 32768
bytes. The zlib compression method number is not the same as the
PNG compression method number in the IHDR chunk. The additional
flags shall not specify a preset dictionary.

If the data to be compressed contain 16384 bytes or fewer, the
PNG encoder may set the window size by rounding up to a power of
2 (256 minimum). This decreases the memory required for both
encoding and decoding, without adversely affecting the
compression ratio.

The compressed data within the zlib datastream are stored as a
series of blocks, each of which can represent raw (uncompressed)
data, LZ77-compressed data encoded with fixed Huffman codes, or
LZ77-compressed data encoded with custom Huffman codes. A marker
bit in the final block identifies it as the last block, allowing
the decoder to recognize the end of the compressed datastream.
Further details on the compression algorithm and the encoding are
given in the deflate specification [rfc1951].

The check value stored at the end of the zlib datastream is
calculated on the uncompressed data represented by the
datastream. The algorithm used to calculate this is not the same
as the CRC calculation used for PNG chunk CRC field values. The
zlib check value is useful mainly as a cross-check that the
deflate algorithms are implemented correctly.
Verifying the individual PNG chunk CRCs provides confidence that
the PNG datastream has been transmitted undamaged.

The sequence of filtered scanlines is compressed and the
resulting data stream is split into
IDAT chunks. The concatenation of the
contents of all the
IDAT chunks makes up a zlib datastream. This
datastream decompresses to filtered image data.

It is important to emphasize that the boundaries between IDAT chunks are
arbitrary and can fall anywhere in the zlib datastream. There is
not necessarily any correlation between
IDAT chunk boundaries and deflate block
boundaries or any other feature of the zlib data. For example, it
is entirely possible for the terminating zlib check value to be
split across
IDAT chunks.

Similarly, there is no required correlation between the
structure of the image data (i.e., scanline boundaries) and
deflate block boundaries or
IDAT chunk boundaries. The complete filtered
PNG image is represented by a single zlib datastream that is
stored in a number of
IDAT chunks.

PNG also uses compression method 0 in
iTXt,
iCCP,
and
zTXt chunks. Unlike the image data, such
datastreams are not split across chunks; each such chunk contains
an independent zlib datastream (see 10.1 Compression method 0).

This clause defines chunk used in this specification.

This clause gives requirements and recommendations for encoder
behaviour. A PNG encoder shall produce a PNG datastream from a
PNG image that conforms to the format specified in the preceding
clauses. Best results will usually be achieved by following the
additional recommendations given here.

See C. Gamma and chromaticity for a brief introduction
to gamma issues.

PNG encoders capable of full colour management will perform more
sophisticated calculations than those described here and may
choose to use the
iCCP chunk. If it is known that the image
samples conform to the sRGB specification [SRGB], encoders are strongly encouraged to write
the sRGB chunk
without performing additional gamma handling. In both cases it is
recommended that an appropriate
gAMA chunk be generated for use by PNG
decoders that do not recognize the
iCCP or
sRGB chunks.

A PNG encoder has to determine:

1. what value to write in the
   gAMA chunk;
2. how to transform the provided image samples into the values
   to be written in the PNG datastream.

The value to write in the
gAMA chunk is that value which causes a PNG
decoder to behave in the desired way. See 13.13 Decoder gamma
handling.

The transform to be applied depends on the nature of the image
samples and their precision. If the samples represent light
intensity in floating-point or high precision integer form
(perhaps from a computer graphics renderer), the encoder may
perform gamma encoding (applying a power function with exponent
less than 1) before quantizing the data to integer values for
inclusion in the PNG datastream. This results in fewer banding
artifacts at a given sample depth, or allows smaller samples
while retaining the same visual quality. An intensity level
expressed as a floating-point value in the range 0 to 1 can be
converted to a datastream image sample by:

integer_sample =
floor((2sampledepth-1) * intensityencoding_exponent
+ 0.5)

If the intensity in the equation is the desired output
intensity, the encoding exponent is the gamma value to be used in
the gAMA
chunk.

If the intensity available to the PNG encoder is the original
scene intensity, another transformation may be needed. There is
sometimes a requirement for the displayed image to have higher
contrast than the original source image. This corresponds to an
end-to-end transfer function from original scene to display
output with an exponent greater than 1. In this case:

gamma = encoding_exponent/end_to_end_exponent

If it is not known whether the conditions under which the
original image was captured or calculated warrant such a contrast
change, it may be assumed that the display intensities are
proportional to original scene intensities, i.e. the end-to-end
exponent is 1 and hence:

gamma = encoding_exponent

If the image is being written to a datastream only, the
encoder is free to choose the encoding exponent. Choosing a value
that causes the gamma value in the
gAMA chunk to be 1/2.2 is often a reasonable
choice because it minimizes the work for a PNG decoder displaying
on a typical video monitor.

Some image renderers may simultaneously write the image to a
PNG datastream and display it on-screen. The displayed pixels
should be gamma corrected for the display system and viewing
conditions in use, so that the user sees a proper representation
of the intended scene.

If the renderer wants to write the displayed sample values to
the PNG datastream, avoiding a separate gamma encoding step for
the datastream, the renderer should approximate the transfer
function of the display system by a power function, and write the
reciprocal of the exponent into the
gAMA chunk. This will allow a PNG
decoder to reproduce what was displayed on screen for the
originator during rendering.

However, it is equally reasonable for a renderer to compute
displayed pixels appropriate for the display device, and to
perform separate gamma encoding for data storage and
transmission, arranging to have a value in the gAMA chunk more
appropriate to the future use of the image.

Computer graphics renderers often do not perform gamma
encoding, instead making sample values directly proportional to
scene light intensity. If the PNG encoder receives sample values
that have already been quantized into integer values, there is no
point in doing gamma encoding on them; that would just result in
further loss of information. The encoder should just write the
sample values to the PNG datastream. This does not imply that the
gAMA chunk
should contain a gamma value of 1.0 because the desired
end-to-end transfer function from scene intensity to display
output intensity is not necessarily linear. However, the desired
gamma value is probably not far from 1.0. It may depend on
whether the scene being rendered is a daylight scene or an indoor
scene, etc.

When the sample values come directly from a piece of hardware,
the correct gAMA
value can, in principle, be inferred from the transfer function
of the hardware and lighting conditions of the scene. In the case
of video digitizers («frame grabbers»), the samples are probably
in the sRGB colour space, because the sRGB specification was
designed to be compatible with modern video standards. Image
scanners are less predictable. Their output samples may be
proportional to the input light intensity since CCD sensors
themselves are linear, or the scanner hardware may have already
applied a power function designed to compensate for dot gain in
subsequent printing (an exponent of about 0.57), or the scanner
may have corrected the samples for display on a monitor. It may
be necessary to refer to the scanner’s manual or to scan a
calibrated target in order to determine the characteristics of a
particular scanner. It should be remembered that gamma relates
samples to desired display output, not to scanner input.

Datastream format converters generally should not attempt to
convert supplied images to a different gamma. The data should be
stored in the PNG datastream without conversion, and the gamma
value should be deduced from information in the source datastream
if possible. Gamma alteration at datastream conversion time
causes re-quantization of the set of intensity levels that are
represented, introducing further roundoff error with little
benefit. It is almost always better to just copy the sample
values intact from the input to the output file.

If the source datastream describes the gamma characteristics
of the image, a datastream converter is strongly encouraged to
write a gAMA
chunk. Some datastream formats specify the display exponent (the
exponent of the function which maps image samples to display
output rather than the other direction). If the source file’s
gamma value is greater than 1.0, it is probably a display
exponent, and the reciprocal of this value should be used for the
PNG gamma value. If the source file format records the
relationship between image samples and a quantity other than
display output, it will be more complex than this to deduce the
PNG gamma value.

If a PNG encoder or datastream converter knows that the image
has been displayed satisfactorily using a display system whose
transfer function can be approximated by a power function with
exponent display_exponent, the image can be marked as
having the gamma value:

gamma = 1/display_exponent

It is better to write a
gAMA chunk with a value that is approximately
correct than to omit the chunk and force PNG decoders to guess an
approximate gamma value. If a PNG encoder is unable to infer the gamma
value, it is preferable to omit the
gAMA chunk. If a guess has to be made
this should be left to the PNG decoder.

gamma does not apply to alpha samples; alpha is always
represented linearly.

See also 13.13 Decoder gamma
handling.

See C. Gamma and chromaticity for references to colour
issues.

PNG encoders capable of full colour management will perform more
sophisticated calculations than those described here and may
choose to use the
iCCP chunk. If it is known that the image
samples conform to the sRGB specification [SRGB], PNG encoders are strongly encouraged to
use the sRGB
chunk.

If it is possible for the encoder to determine the
chromaticities of the source display primaries, or to make a
strong guess based on the origin of the image, or the hardware
running it, the encoder is strongly encouraged to output the cHRM chunk. If this
is done, the
gAMA chunk should also be written; decoders
can do little with a
cHRM chunk if the
gAMA chunk is missing.

There are a number of recommendations and standards for
primaries and white points, some of which are linked to
particular technologies, for example the CCIR 709 standard [ITU-R_BT.709]
and the SMPTE-C standard [SMPTE_170M].

There are three cases that need to be considered:

1. the encoder is part of the generation system;
2. the source image is captured by a camera or scanner;
3. the PNG datastream was generated by translation from some
   other format.

In the case of hand-drawn or digitally edited images, it is
necessary to determine what monitor they were viewed on when
being produced. Many image editing programs allow the type of
monitor being used to be specified. This is often because they
are working in some device-independent space internally. Such
programs have enough information to write valid cHRM and gAMA chunks, and are
strongly encouraged to do so automatically.

If the encoder is compiled as a portion of a computer image
renderer that performs full-spectral rendering, the monitor
values that were used to convert from the internal
device-independent colour space to RGB should be written into the
cHRM chunk. Any
colours that are outside the gamut of the chosen RGB device
should be mapped to be within the gamut; PNG does not store
out-of-gamut colours.

If the computer image renderer performs calculations directly
in device-dependent RGB space, a
cHRM chunk should not be written unless the
scene description and rendering parameters have been adjusted for
a particular monitor. In that case, the data for that monitor
should be used to construct a
cHRM chunk.

A few image formats store calibration information, which can
be used to fill in the
cHRM chunk. For example, TIFF_6.0 files [TIFF_6.0] can
optionally store calibration information, which if present should
be used to construct the
cHRM chunk.

Video created with recent video equipment probably uses the
CCIR 709 primaries and D65 white point [ITU-R_BT.709],
which are given in
Table 38.

An older but still very popular video standard is SMPTE-C [SMPTE_170M]
given in
Table 39.

It is not recommended that datastream format
converters attempt to convert supplied images to a different RGB
colour space. The data should be stored in the PNG datastream
without conversion, and the source primary chromaticities should
be recorded if they are known. Colour space transformation at
datastream conversion time is a bad idea because of gamut
mismatches and rounding errors. As with gamma conversions, it is
better to store the data losslessly and incur at most one
conversion when the image is finally displayed.

See 13.14 Decoder colour
handling.

The alpha channel can be regarded either as a mask that
temporarily hides transparent parts of the image, or as a means
for constructing a non-rectangular image. In the first case, the
colour values of fully transparent pixels should be preserved for
future use. In the second case, the transparent pixels carry no
useful data and are simply there to fill out the rectangular
image area required by PNG. In this case, fully transparent
pixels should all be assigned the same colour value for best
compression.

Image authors should keep in mind the possibility that a
decoder will not support transparency control in full (see
13.16 Alpha channel
processing). Hence, the colours assigned to
transparent pixels should be reasonable background colours
whenever feasible.

For applications that do not require a full alpha channel, or
cannot afford the price in compression efficiency, the tRNS transparency chunk
is also available.

If the image has a known background colour, this colour should
be written in the
bKGD chunk. Even decoders that ignore
transparency may use the
bKGD colour to fill unused screen area.

If the original image has premultiplied (also called
«associated») alpha data, it can be converted to PNG’s
non-premultiplied format by dividing each sample value by the
corresponding alpha value, then multiplying by the maximum value
for the image bit depth, and rounding to the nearest integer. In
valid premultiplied data, the sample values never exceed their
corresponding alpha values, so the result of the division should
always be in the range 0 to 1. If the alpha value is zero, output
black (zeroes).

When encoding input samples that have a sample depth that
cannot be directly represented in PNG, the encoder shall scale
the samples up to a sample depth that is allowed by PNG. The most
accurate scaling method is the linear equation:

output = floor((input * MAXOUTSAMPLE / MAXINSAMPLE) + 0.5)

where the input samples range from 0 to MAXINSAMPLE
and the outputs range from 0 to MAXOUTSAMPLE (which is
2^sampledepth-1).

A close approximation to the linear scaling method is achieved
by «left bit replication», which is shifting the valid bits to
begin in the most significant bit and repeating the most
significant bits into the open bits. This method is often faster
to compute than linear scaling.

EXAMPLE Assume that 5-bit samples are being scaled up to 8
bits. If the source sample value is 27 (in the range from 0-31),
then the original bits are:

4 3 2 1 0
---------
1 1 0 1 1

Left bit replication gives a value of 222:

7 6 5 4 3  2 1 0
----------------
1 1 0 1 1  1 1 0
|=======|  |===|
    |      Leftmost Bits Repeated to Fill Open Bits
    |
Original Bits

which matches the value computed by the linear equation. Left
bit replication usually gives the same value as linear scaling,
and is never off by more than one.

A distinctly less accurate approximation is obtained by simply
left-shifting the input value and filling the low order bits with
zeroes. This scheme cannot reproduce white exactly, since it does
not generate an all-ones maximum value; the net effect is to
darken the image slightly. This method is not recommended in
general, but it does have the effect of improving compression,
particularly when dealing with greater-than-8-bit sample depths.
Since the relative error introduced by zero-fill scaling is small
at high sample depths, some encoders may choose to use it.
Zero-fill shall not be used for alpha channel
data, however, since many decoders will treat alpha values of all
zeroes and all ones as special cases. It is important to
represent both those values exactly in the scaled data.

When the encoder writes an
sBIT chunk, it is required to do the scaling
in such a way that the high-order bits of the stored samples
match the original data. That is, if the
sBIT chunk specifies a sample depth of
S, the high-order S bits of the stored data shall agree with the
original S-bit data values. This allows decoders to recover the
original data by shifting right. The added low-order bits are not
constrained. All the above scaling methods meet this
restriction.

When scaling up source image data, it is recommended that the
low-order bits be filled consistently for all samples; that is,
the same source value should generate the same sample value at
any pixel position. This improves compression by reducing the
number of distinct sample values. This is not a mandatory
requirement, and some encoders may choose not to follow it. For
example, an encoder might instead dither the low-order bits,
improving displayed image quality at the price of increasing file
size.

In some applications the original source data may have a range
that is not a power of 2. The linear scaling equation still works
for this case, although the shifting methods do not. It is
recommended that an
sBIT chunk not be written for such images,
since sBIT
suggests that the original data range was exactly
0..2^S-1.

Suggested palettes may appear as
sPLT chunks in any PNG datastream, or as
a PLTE chunk in
truecolour PNG datastreams. In either case, the suggested palette
is not an essential part of the image data, but it may be used to
present the image on indexed-colour display hardware. Suggested
palettes are of no interest to viewers running on truecolour
hardware.

When an sPLT
chunk is used to provide a suggested palette, it is recommended
that the encoder use the frequency fields to indicate the
relative importance of the palette entries, rather than leave
them all zero (meaning undefined). The frequency values are most
easily computed as «nearest neighbour» counts, that is, the
approximate usage of each RGBA palette entry if no dithering is
applied. (These counts will often be available «for free» as a
consequence of developing the suggested palette.) Because the
suggested palette includes transparency information, it should be
computed for the un-composited image.

Even for indexed-colour images,
sPLT can be used to define alternative reduced
palettes for viewers that are unable to display all the colours
present in the
PLTE chunk.
If the PLTE
chunk appears without the
bKGD chunk in an image of colour type 6, the
circumstances under which the palette was computed are
unspecified.

An older method for including a suggested palette in a
truecolour PNG datastream uses the
PLTE chunk. If this method is used, the
histogram (frequencies) should appear in a separate hIST chunk. The PLTE chunk does not
include transparency information. Hence for images of colour type
6 (truecolour with alpha), it is recommended that a bKGD chunk appear and
that the palette and histogram be computed with reference to the
image as it would appear after compositing against the specified
background colour. This definition is necessary to ensure that
useful palette entries are generated for pixels having fractional
alpha values. The resulting palette will probably be useful only
to viewers that present the image against the same background
colour. It is recommended that PNG editors delete or recompute
the palette if they alter or remove the
bKGD chunk in an image of colour type
6.

For images of colour type 2 (truecolour), it is recommended
that the PLTE
and hIST chunks
be computed with reference to the RGB data only, ignoring any
transparent-colour specification. If the datastream uses
transparency (has a
tRNS chunk), viewers can easily adapt the
resulting palette for use with their intended background colour
(see 13.17 Histogram and suggested palette usage).

For providing suggested palettes,
the sPLT
chunk is more flexible than the
PLTE chunk in
the following ways:

1. With sPLT
   multiple suggested palettes may be provided. A PNG decoder may
   choose an appropriate palette based on name or number of
   entries.
2. In a PNG datastream of colour type 6 (truecolour with alpha
   channel), the
   PLTE chunk represents a palette already
   composited against the
   bKGD colour, so it is useful only for display
   against that background colour. The
   sPLT chunk provides an un-composited
   palette, which is useful for display against backgrounds chosen
   by the PNG decoder.
3. Since the
   sPLT chunk is an ancillary chunk, a PNG editor
   may add or modify suggested palettes without being forced to
   discard unknown unsafe-to-copy chunks.
4. Whereas the
   sPLT chunk is allowed in PNG datastreams for
   colour types 0, 3, and 4 (greyscale and indexed), the PLTE chunk cannot be
   used to provide reduced palettes in these cases.
5. More than 256 entries may appear in the sPLT chunk.

A PNG encoder that uses the
sPLT chunk may choose to write a suggested
palette represented by
PLTE and
hIST chunks as well, for compatibility with
decoders that do not recognize the
sPLT chunk.

This specification defines two interlace methods,
one of which is no interlacing. Interlacing provides a convenient
basis from which decoders can progressively display an image, as
described in 13.10 Interlacing and
progressive display.

For images of colour type 3 (indexed-colour), filter type 0
(None) is usually the most effective. Colour images with 256 or
fewer colours should almost always be stored in indexed-colour
format; truecolour format is likely to be much larger.

Filter type 0 is also recommended for images of bit depths
less than 8. For low-bit-depth greyscale images, in rare cases,
better compression may be obtained by first expanding the image
to 8-bit representation and then applying filtering.

For truecolour and greyscale images, any of the five filters
may prove the most effective. If an encoder uses a fixed filter,
the Paeth filter type is most likely to be the best.

For best compression of truecolour and greyscale images,
the recommended approach is
adaptive filtering in which a filter type is
chosen for each scanline. The following simple heuristic has
performed well in early tests: compute the output scanline using
all five filters, and select the filter that gives the smallest
sum of absolute values of outputs. (Consider the output bytes as
signed differences for this test.) This method usually
outperforms any single fixed filter type choice. However, it is likely
that better heuristics will be found as more experience is
gained with PNG.

Filtering according to these recommendations is effective in
conjunction with either of the two interlace methods defined in
this specification.

The encoder may divide the compressed datastream into IDAT chunks however it
wishes. (Multiple
IDAT chunks are allowed so that encoders may
work in a fixed amount of memory; typically the chunk size will
correspond to the encoder’s buffer size.) A PNG datastream in
which each IDAT
chunk contains only one data byte is valid, though remarkably
wasteful of space. (Zero-length
IDAT chunks are also valid, though even more
wasteful.)

A nonempty keyword shall be provided for each text chunk. The
generic keyword «Comment» can be used if no better description of
the text is available. If a user-supplied keyword is used,
encoders should check that it meets the restrictions on
keywords.

For the tEXt
and zTXt chunks,
PNG text strings are expected to use the Latin-1 character set.
Encoders should avoid storing characters that are not defined in
Latin-1, and should provide character code remapping if the local
system’s character set is not Latin-1. The iTXt chunk provides
support for international text, represented using the UTF-8
encoding of UCS. Encoders should discourage the creation of
single lines of text longer than 79 characters, in order to
facilitate easy reading. It is recommended that text items less
than 1024 bytes in size should be output using uncompressed
text chunks. It is
recommended that the basic title and author keywords be output
using uncompressed text chunks.
Placing large text chunks after the
image data (after the
IDAT chunks) can speed up image display in
some situations, as the decoder will decode the image data first.
It is recommended that small text chunks, such as the image
title, appear before the
IDAT chunks.

This clause gives some requirements and recommendations for PNG
decoder behaviour and viewer behaviour. A viewer presents the
decoded PNG image to the user. Since viewer and decoder behaviour
are closely connected, decoders and viewers are treated together
here. The only absolute requirement on a PNG decoder is that it
successfully reads any datastream conforming to the format
specified in the preceding chapters. However, best results will
usually be achieved by following these additional
recommendations.

PNG decoders shall support all valid combinations of bit
depth, colour type, compression method, filter method, and
interlace method that are explicitly defined in this
International Standard.

Errors in a PNG datastream will fall into two general classes,
transmission errors and syntax errors (see 4.10 Error handling).

Examples of transmission errors are transmission in «text» or
«ascii» mode, in which byte codes 13 and/or 10 may be added,
removed, or converted throughout the datastream; unexpected
termination, in which the datastream is truncated; or a physical
error on a storage device, in which one or more blocks (typically
512 bytes each) will have garbled or random values. Some examples
of syntax errors are an invalid value for a row filter, an
invalid compression method, an invalid chunk length, the absence
of a PLTE chunk
before the first
IDAT chunk in an indexed image, or the
presence of multiple
gAMA chunks. A PNG decoder should handle
errors as follows:

1. Detect errors as early as possible using the PNG signature
   bytes and CRCs on each chunk. Decoders should verify that all
   eight bytes of the PNG signature are correct. A decoder can
   have additional confidence in the datastream’s integrity if the
   next eight bytes begin an
   IHDR chunk with the correct chunk length. A
   CRC should be checked before processing the chunk data. Sometimes
   this is impractical, for example when a streaming PNG decoder is
   processing a large
   IDAT chunk. In this case the CRC should be
   checked when the end of the chunk is reached.
2. Recover from an error, if possible; otherwise fail
   gracefully. Errors that have little or no effect on the
   processing of the image may be ignored, while those that affect
   critical data shall be dealt with in a manner appropriate to the
   application.
3. Provide helpful messages describing errors, including
   recoverable errors.

Three classes of PNG chunks are relevant to this philosophy.
For the purposes of this classification, an «unknown chunk» is
either one whose type was genuinely unknown to the decoder’s
author, or one that the author chose to treat as unknown, because
default handling of that chunk type would be sufficient for the
program’s purposes. Other chunks are called «known chunks». Given
this definition, the three classes are as follows:

1. known chunks, which necessarily includes all of the critical
   chunks defined in this specification (IHDR, PLTE, IDAT, IEND)
2. unknown critical chunks (bit 5 of the first byte of the chunk
   type is 0)
3. unknown ancillary chunks (bit 5 of the first byte of the
   chunk type is 1)

See 5.4 Chunk naming conventions for a description
of chunk naming conventions.

PNG chunk types are marked «critical» or «ancillary» according
to whether the chunks are critical for the purpose of extracting
a viewable image (as with
IHDR,
PLTE, and
IDAT) or critical to understanding the
datastream structure (as with
IEND). This is a specific kind of criticality
and one that is not necessarily relevant to every conceivable
decoder. For example, a program whose sole purpose is to extract
text annotations (for example, copyright information) does not
require a viewable image. Another decoder might consider the tRNS and gAMA chunks essential to
its proper execution.

Syntax errors always involve known chunks because syntax
errors in unknown chunks cannot be detected. The PNG decoder has
to determine whether a syntax error is fatal (unrecoverable) or
not, depending on its requirements and the situation. For
example, most decoders can ignore an invalid IEND chunk; a
text-extraction program can ignore the absence of IDAT; an image viewer
cannot recover from an empty
PLTE chunk in an indexed image but it can
ignore an invalid
PLTE chunk in a truecolour image; and a
program that extracts the alpha channel can ignore an invalid gAMA chunk, but may
consider the presence of two
tRNS chunks to be a fatal error. Anomalous
situations other than syntax errors shall be treated as
follows:

1. Encountering an unknown ancillary chunk is never an error.
   The chunk can simply be ignored.
2. Encountering an unknown critical chunk is a fatal condition
   for any decoder trying to extract the image from the datastream.
   A decoder that ignored a critical chunk could not know whether
   the image it extracted was the one intended by the encoder.
3. A PNG signature mismatch, a CRC mismatch, or an unexpected
   end-of-stream indicates a corrupted datastream, and may be
   regarded as a fatal error. A decoder could try to salvage
   something from the datastream, but the extent of the damage will
   not be known.

When a fatal condition occurs, the decoder should fail
immediately, signal an error to the user if appropriate, and
optionally continue displaying any image data already visible to
the user (i.e. «fail gracefully»). The application as a whole
need not terminate.

When a non-fatal error occurs, the decoder should signal a
warning to the user if appropriate, recover from the error, and
continue processing normally.

When decoding an indexed-color PNG, if out-of-range indexes are encountered,
decoders have historically varied in their handling of this error.
Displaying the pixel as opaque black is one common error recovery tactic,
although such behaviour is not required by this specification
and must not be relied on by encoders.

Decoders that do not compute CRCs should interpret apparent
syntax errors as indications of corruption (see also 13.2 Error checking).

Errors in compressed chunks (
IDAT,
zTXt,
iTXt,
iCCP) could lead to buffer overruns.
Implementors of deflate decompressors should guard against this
possibility.

APNG is designed to allow incremental display of frames
before the entire datastream has been read.
This implies that some errors may not be detected
until partway through the animation.
It is strongly recommended that when any error is encountered
decoders should discard all subsequent frames,
stop the animation,
and revert to displaying the static image.
A decoder which detects an error
before the animation has started
should display the static image.
An error message may be displayed to the user if appropriate.

Decoders shall treat
out-of-order APNG chunks
as an error.
APNG-aware PNG editors
should restore them to correct order,
using the sequence numbers.

The PNG error handling philosophy is described in 13.1 Error handling.

An unknown chunk type is not to be treated as an error
unless it is a critical chunk.

The chunk type can be checked for plausibility by seeing
whether all four bytes are in the range codes 65-90 and 97-122
(decimal); note that this need be done only for unrecognized
chunk types. If the total datastream size is known (from file
system information, HTTP protocol, etc), the chunk length can be
checked for plausibility as well. If CRCs are not checked,
dropped/added data bytes or an erroneous chunk length can cause
the decoder to get out of step and misinterpret subsequent data
as a chunk header.

For known-length chunks, such as
IHDR, decoders should treat an
unexpected chunk length as an error. Future extensions to this
specification will not add new fields to existing chunks;
instead, new chunk types will be added to carry new
information.

Unexpected values in fields of known chunks (for example, an
unexpected compression method in the
IHDR chunk) shall be checked for and
treated as errors. However, it is recommended that unexpected
field values be treated as fatal errors only in critical
chunks. An unexpected value in an ancillary chunk can be handled
by ignoring the whole chunk as though it were an unknown chunk
type. (This recommendation assumes that the chunk’s CRC has been
verified. In decoders that do not check CRCs, it is safer to
treat any unexpected value as indicating a corrupted
datastream.)

Standard PNG images shall be compressed with compression
method 0. The compression method field of the
IHDR chunk is
provided for possible future standardization or proprietary
variants. Decoders shall check this byte and report an error if
it holds an unrecognized code. See 10. Compression for
details.

A PNG datastream is composed of a collection of explicitly
typed chunks. Chunks whose contents are defined by the
specification could actually contain anything, including
malicious code. But there is no known risk that such malicious
code could be executed on the recipient’s computer as a result of
decoding the PNG image.

The possible security risks associated with future chunk types cannot be
specified at this time. Security issues will be considered when defining future
public chunks. There is no additional security risk associated with unknown or
unimplemented chunk types, because such chunks will be ignored, or at most be
copied into another PNG datastream.

The iTXt, tEXt, and zTXt chunks contain keywords
and data
that are meant to be displayed as plain text. The iCCP
and
sPLT chunks contain keywords that are meant to be displayed as plain text. It is
possible that if the decoder displays such text without filtering
out control characters, especially the ESC (escape) character,
certain systems or terminals could behave in undesirable and
insecure ways. It is recommended that decoders filter out control
characters to avoid this risk; see 13.7 Text chunk
processing.

Every chunk begins with a length field, which makes it easier
to write decoders that are invulnerable to fraudulent chunks that
attempt to overflow buffers. The CRC at the end of every chunk
provides a robust defence against accidentally corrupted data.
The PNG signature bytes provide early detection of common file
transmission errors.

A decoder that fails to check CRCs could be subject to data
corruption. The only likely consequence of such corruption is
incorrectly displayed pixels within the image. Worse things might
happen if the CRC of the
IHDR chunk is not checked and the width or
height fields are corrupted. See 13.2 Error checking.

A poorly written decoder might be subject to buffer overflow,
because chunks can be extremely large, up to 2³¹-1
bytes long. But properly written decoders will handle large
chunks without difficulty.

No privacy considerations
have been raised on this specification.

Decoders shall recognize chunk types by a simple four-byte
literal comparison; it is incorrect to perform case conversion on
chunk types. A decoder encountering an unknown chunk in which the
ancillary bit is 1 may safely ignore the chunk and proceed to
display the image. A decoder trying to extract the image, upon
encountering an unknown chunk in which the ancillary bit is 0,
indicating a critical chunk, shall indicate to the user that the
image contains information it cannot safely interpret.

Decoders should test the properties of an unknown chunk type by numerically
testing the specified bits. Testing whether a character is uppercase or
lowercase is inefficient, and even incorrect if a locale-specific case
definition is used.

Decoders should not flag an error if the reserved bit is set
to 1, however, as some future version of the PNG specification
could define a meaning for this bit. It is sufficient to treat a
chunk with this bit set in the same way as any other unknown
chunk type.

Decoders do not need to test the chunk type private bit, since it has no
functional significance and is used to avoid conflicts between chunks defined by
W3C and those defined privately.

All ancillary chunks are optional; decoders may ignore them.
However, decoders are encouraged to interpret these chunks when
appropriate and feasible.

Non-square pixels can be represented (see 11.3.4.3 pHYs
Physical pixel dimensions), but viewers are not
required to account for them; a viewer can present any PNG
datastream as though its pixels are square.

Where the pixel aspect ratio of the display differs from the
aspect ratio of the physical pixel dimensions defined in the PNG
datastream, viewers are strongly encouraged to rescale images for
proper display.

When the
pHYs chunk has a unit specifier of 0
(unit is unknown), the behaviour of a decoder may depend on the
ratio of the two pixels-per-unit values, but should not depend on
their magnitudes. For example, a
pHYs chunk
containing (ppuX, ppuY, unit) = (2, 1, 0) is equivalent
to one containing (1000, 500, 0); both are equally valid
indications that the image pixels are twice as tall as they are
wide.

One reasonable way for viewers to handle a difference between
the pixel aspect ratios of the image and the display is to expand
the image either horizontally or vertically, but not both. The
scale factors could be obtained using the following
floating-point calculations:

image_ratio = pHYs_ppuY / pHYs_ppuX
display_ratio = display_ppuY / display_ppuX
scale_factor_X = max(1.0, image_ratio/display_ratio)
scale_factor_Y = max(1.0, display_ratio/image_ratio)

Because other methods such as maintaining the image area are
also reasonable, and because ignoring the
pHYs chunk is
permissible, authors should not assume that all viewing
applications will use this scaling method.

As well as making corrections for pixel aspect ratio, a viewer
may have reasons to perform additional scaling both horizontally
and vertically. For example, a viewer might want to shrink an
image that is too large to fit on the display, or to expand
images sent to a high-resolution printer so that they appear the
same size as they did on the display.

If practical, PNG decoders should have a way to display to the
user all the
iTXt,
tEXt, and
zTXt chunks found in the datastream. Even if
the decoder does not recognize a particular text keyword, the
user might be able to understand it.

When processing
tEXt and
zTXt chunks, decoders could encounter
characters other than those permitted. Some can be safely
displayed (e.g., TAB, FF, and CR, decimal 9, 12, and 13,
respectively), but others, especially the ESC character (decimal
27), could pose a security hazard (because unexpected actions may
be taken by display hardware or software). Decoders should not
attempt to directly display any non-Latin-1 characters (except
for newline and perhaps TAB, FF, CR) encountered in a tEXt or zTXt chunk. Instead,
they should be ignored or displayed in a visible notation such as
«nnn«. See 13.3 Security
considerations.

Even though encoders are recommended to represent newlines as
linefeed (decimal 10), it is recommended that decoders not rely
on this; it is best to recognize all the common newline
combinations (CR, LF, and CR-LF) and display each as a single
newline. TAB can be expanded to the proper number of spaces
needed to arrive at a column multiple of 8.

Decoders running on systems with non-Latin-1 character set
encoding should provide character code remapping so that Latin-1
characters are displayed correctly. Some systems may not provide
all the characters defined in Latin-1. Mapping unavailable
characters to a visible notation such as «nnn» is a
good fallback. Character codes 127-255 should be displayed only
if they are printable characters on the decoding system. Some
systems may interpret such codes as control characters; for
security, decoders running on such systems should not display
such characters literally.

Decoders should be prepared to display text chunks that
contain any number of printing characters between newline
characters, even though it is recommended that encoders avoid
creating lines in excess of 79 characters.

The compression technique used in this specification
does not require the entire compressed datastream to be available
before decompression can start. Display can therefore commence
before the entire decompressed datastream is available. It is
extremely unlikely that any general purpose compression methods
in future versions of this specification will not have
this property.

It is important to emphasize that
IDAT chunk boundaries have no semantic
significance and can occur at any point in the compressed
datastream. There is no required correlation between the
structure of the image data (for example, scanline boundaries) and
deflate block boundaries or
IDAT chunk boundaries. The complete image data
is represented by a single zlib datastream that is stored in some
number of IDAT
chunks; a decoder that assumes any more than this is incorrect.
Some encoder implementations may emit datastreams in which some
of these structures are indeed related, but decoders cannot rely
on this.

To reverse the effect of a filter, the decoder may need
to use the decoded values of the prior pixel on the same line,
the pixel immediately above the current pixel on the prior line,
and the pixel just to the left of the pixel above. This implies
that at least one scanline’s worth of image data needs to be
stored by the decoder at all times. Even though some filter types
do not refer to the prior scanline, the decoder will always need
to store each scanline as it is decoded, since the next scanline
might use a filter type that refers to it. See
7.3 Filtering.

Decoders are required to be able to read interlaced images. If
the reference image contains fewer than five columns or fewer
than five rows, some passes will be empty. Encoders and decoders
shall handle this case correctly. In particular, filter type
bytes are associated only with nonempty scanlines; no filter type
bytes are present in an empty reduced image.

When receiving images over slow transmission links, viewers
can improve perceived performance by displaying interlaced images
progressively. This means that as each reduced image is received,
an approximation to the complete image is displayed based on the
data received so far. One simple yet pleasing effect can be
obtained by expanding each received pixel to fill a rectangle
covering the yet-to-be-transmitted pixel positions below and to
the right of the received pixel. This process can be described by
the following ISO C code [ISO_9899]:

/*
    variables declared and initialized elsewhere in the code:
        height, width
    functions or macros defined elsewhere in the code:
        visit(), min()
 */

int starting_row[7]  = { 0, 0, 4, 0, 2, 0, 1 };
int starting_col[7]  = { 0, 4, 0, 2, 0, 1, 0 };
int row_increment[7] = { 8, 8, 8, 4, 4, 2, 2 };
int col_increment[7] = { 8, 8, 4, 4, 2, 2, 1 };
int block_height[7]  = { 8, 8, 4, 4, 2, 2, 1 };
int block_width[7]   = { 8, 4, 4, 2, 2, 1, 1 };

int pass;
long row, col;

pass = 0;
while (pass < 7)
{
    row = starting_row[pass];
    while (row < height)
    {
        col = starting_col[pass];
        while (col < width)
        {
            visit(row, col,
                  min(block_height[pass], height - row),
                  min(block_width[pass], width - col));
            col = col + col_increment[pass];
        }
        row = row + row_increment[pass];
    }
    pass = pass + 1;
}

The function visit(row,column,height,width) obtains
the next transmitted pixel and paints a rectangle of the
specified height and width, whose upper-left corner is at the
specified row and column, using the colour indicated by the
pixel. Note that row and column are measured from 0,0 at the
upper left corner.

If the viewer is merging the received image with a background
image, it may be more convenient just to paint the received pixel
positions (the visit() function sets only the pixel at the
specified row and column, not the whole rectangle). This produces
a «fade-in» effect as the new image gradually replaces the old.
An advantage of this approach is that proper alpha or
transparency processing can be done as each pixel is replaced.
Painting a rectangle as described above will overwrite
background-image pixels that may be needed later, if the pixels
eventually received for those positions turn out to be wholly or
partially transparent. This is a problem only if the background
image is not stored anywhere offscreen.

To achieve PNG’s goal of universal interchangeability,
decoders shall accept all types of PNG image: indexed-colour,
truecolour, and greyscale. Viewers running on indexed-colour
display hardware need to be able to reduce truecolour images to
indexed-colour for viewing. This process is called «colour
quantization».

A simple, fast method for colour quantization is to reduce the
image to a fixed palette. Palettes with uniform colour spacing
(«colour cubes») are usually used to minimize the per-pixel
computation. For photograph-like images, dithering is recommended
to avoid ugly contours in what should be smooth gradients;
however, dithering introduces graininess that can be
objectionable.

The quality of rendering can be improved substantially by
using a palette chosen specifically for the image, since a colour
cube usually has numerous entries that are unused in any
particular image. This approach requires more work, first in
choosing the palette, and second in mapping individual pixels to
the closest available colour. PNG allows the encoder to supply
suggested palettes, but not all encoders will do so, and the
suggested palettes may be unsuitable in any case (they may have
too many or too few colours). Therefore, high-quality viewers
will need to have a palette selection routine at hand. A large
lookup table is usually the most feasible way of mapping
individual pixels to palette entries with adequate speed.

Numerous implementations of colour quantization are available.
The PNG sample implementation, libpng (http://www.libpng.org/pub/png/libpng.html),
includes code for the purpose.

Decoders may wish to scale PNG data to a lesser sample depth
(data precision) for display. For example, 16-bit data will need
to be reduced to 8-bit depth for use on most present-day display
hardware. Reduction of 8-bit data to 5-bit depth is also
common.

The most accurate scaling is achieved by the linear
equation

output = floor((input * MAXOUTSAMPLE / MAXINSAMPLE) +
0.5)

where

MAXINSAMPLE = (2sampledepth)-1
MAXOUTSAMPLE = (2desired_sampledepth)-1

A slightly less accurate conversion is achieved by simply
shifting right by (sampledepth - desired_sampledepth)
places. For example, to reduce 16-bit samples to 8-bit, the
low-order byte can be discarded. In many situations the shift
method is sufficiently accurate for display purposes, and it is
certainly much faster. (But if gamma correction is being done,
sample rescaling can be merged into the gamma correction lookup
table, as is illustrated in 13.13 Decoder gamma
handling.)

If the decoder needs to scale samples up (for example, if the
frame buffer has a greater sample depth than the PNG image), it
should use linear scaling or left-bit-replication as described in
12.4 Sample depth scaling.

When an sBIT
chunk is present, the reference image data can be recovered by
shifting right to the sample depth specified by sBIT. Note that linear
scaling will not necessarily reproduce the original data, because
the encoder is not required to have used linear scaling to scale
the data up. However, the encoder is required to have used a
method that preserves the high-order bits, so shifting always
works. This is the only case in which shifting might be said to
be more accurate than linear scaling. A decoder need not pay
attention to the
sBIT chunk; the stored image is a valid PNG
datastream of the sample depth indicated by the IHDR chunk; however,
using sBIT to
recover the original samples before scaling them to suit the
display often yields a more accurate display than ignoring sBIT.

When comparing pixel values to
tRNS chunk values to detect transparent
pixels, the comparison shall be done exactly. Therefore,
transparent pixel detection shall be done before reducing sample
precision.

See C. Gamma and chromaticity for a brief introduction
to gamma issues.

Viewers capable of full colour management will perform more
sophisticated calculations than those described here.

For an image display program to produce correct tone
reproduction, it is necessary to take into account the
relationship between samples and display output, and the transfer
function of the display system. This can be done by
calculating:

sample = integer_sample / (2sampledepth -
1.0)
display_output = sample1.0/gamma
display_input = inverse_display_transfer(display_output)

framebuf_sample = floor((display_input *
MAX_FRAMEBUF_SAMPLE)+0.5)

where integer_sample is the sample value from the
datastream, framebuf_sample is the value to write into
the frame buffer, and MAX_FRAMEBUF_SAMPLE is the maximum
value of a frame buffer sample (255 for 8-bit, 31 for 5-bit,
etc). The first line converts an integer sample into a normalized
floating point value (in the range 0.0 to 1.0), the second
converts to a value proportional to the desired display output
intensity, the third accounts for the display system’s transfer
function, and the fourth converts to an integer frame buffer
sample. Zero raised to any positive power is zero.

A step could be inserted between the second and third to
adjust display_output to account for the difference
between the actual viewing conditions and the reference viewing
conditions. However, this adjustment requires accounting for
veiling glare, black mapping, and colour appearance models, none
of which can be well approximated by power functions. Such
calculations are not described here. If viewing conditions are
ignored, the error will usually be small.

The display transfer function can typically be approximated by
a power function with exponent display_exponent, in
which case the second and third lines can be merged into:

display_input = sample1.0/(gamma *
display_exponent) =
sampledecoding_exponent

so as to perform only one power calculation. For colour
images, the entire calculation is performed separately for R, G,
and B values.

The gamma value can be taken directly from the gAMA chunk.
Alternatively, an application may wish to allow the user to
adjust the appearance of the displayed image by influencing the
gamma value. For example, the user could manually set a
parameter user_exponent which defaults to 1.0, and the
application could set:

gamma = gamma_from_file / user_exponent
decoding_exponent = 1.0 / (gamma * display_exponent)
   = user_exponent / (gamma_from_file * display_exponent)

The user would set user_exponent greater than 1 to
darken the mid-level tones, or less than 1 to lighten them.

A gAMA chunk containing zero is
meaningless but could appear by mistake.
Decoders should ignore it,
and editors may discard it and issue a warning to the user.

It is not necessary to perform a transcendental
mathematical computation for every pixel. Instead, a lookup table
can be computed that gives the correct output value for every
possible sample value. This requires only 256 calculations per
image (for 8-bit accuracy), not one or three calculations per
pixel. For an indexed-colour image, a one-time correction of the
palette is sufficient, unless the image uses transparency and is
being displayed against a nonuniform background.

If floating-point calculations are not possible, gamma
correction tables can be computed using integer arithmetic and a
precomputed table of logarithms. Example code appears in [PNG-EXTENSIONS].

When the incoming image has unknown gamma value (gAMA, sRGB, and iCCP
all absent), standalone image viewers should choose
a likely default gamma value, but allow the user to select a new
one if the result proves too dark or too light. The default gamma value
may depend on other knowledge about the image, for example
whether it came from the Internet or from the local system.
For consistency, viewers for document formats such as HTML,
or vector graphics such as SVG, should treat embedded or
linked PNG images with unknown gamma value in the same way
that they treat other untagged images.

In practice, it is often difficult to determine what value of
display exponent should be used. In systems with no built-in
gamma correction, the display exponent is determined entirely by
the CRT. A display exponent of 2.2 should be used unless detailed
calibration measurements are available for the particular CRT
used.

Many modern frame buffers have lookup tables that are used to
perform gamma correction, and on these systems the display
exponent value should be the exponent of the lookup table and CRT
combined. It may not be possible to find out what the lookup
table contains from within the viewer application, in which case
it may be necessary to ask the user to supply the display
system’s exponent value. Unfortunately, different manufacturers
use different ways of specifying what should go into the lookup
table, so interpretation of the system gamma value is
system-dependent.

The response of real displays is actually more complex than
can be described by a single number (the display exponent). If
actual measurements of the monitor’s light output as a function
of voltage input are available, the third and fourth lines of the
computation above can be replaced by a lookup in these
measurements, to find the actual frame buffer value that most
nearly gives the desired brightness.

See C. Gamma and chromaticity for references to colour
issues.

In many cases, the image data in PNG datastreams will be
treated as device-dependent RGB values and displayed without
modification (except for appropriate gamma correction). This
provides the fastest display of PNG images. But unless the viewer
uses exactly the same display hardware as that used by the author
of the original image, the colours will not be exactly the same
as those seen by the original author, particularly for darker or
near-neutral colours. The
cHRM chunk provides information that allows
closer colour matching than that provided by gamma correction
alone.

The cHRM data
can be used to transform the image data from RGB to XYZ and
thence into a perceptually linear colour space such as CIE LAB.
The colours can be partitioned to generate an optimal palette,
because the geometric distance between two colours in CIE LAB is
strongly related to how different those colours appear (unlike,
for example, RGB or XYZ spaces). The resulting palette of
colours, once transformed back into RGB colour space, could be
used for display or written into a
PLTE chunk.

Decoders that are part of image processing applications might
also transform image data into CIE LAB space for analysis.

In applications where colour fidelity is critical, such as
product design, scientific visualization, medicine, architecture,
or advertising, PNG decoders can transform the image data from
source RGB to the display RGB space of the monitor used to view
the image. This involves calculating the matrix to go from source
RGB to XYZ and the matrix to go from XYZ to display RGB, then
combining them to produce the overall transformation. The PNG
decoder is responsible for implementing gamut mapping.

Decoders running on platforms that have a Colour Management
System (CMS) can pass the image data,
gAMA, and
cHRM values to the CMS for display or further
processing.

PNG decoders that provide colour printing facilities can use
the facilities in Level 2 PostScript to specify image data in
calibrated RGB space or in a device-independent colour space such
as XYZ. This will provide better colour fidelity than a simple
RGB to CMYK conversion. The PostScript Language Reference manual
[PostScript] gives examples. Such decoders
are responsible for implementing gamut mapping between source RGB
(specified in the
cHRM chunk) and the target printer. The
PostScript interpreter is then responsible for producing the
required colours.

PNG decoders can use the
cHRM data to calculate an accurate greyscale
representation of a colour image. Conversion from RGB to grey is
simply a case of calculating the Y (luminance) component of XYZ,
which is a weighted sum of R, G, and B values. The weights depend
upon the monitor type, i.e. the values in the cHRM chunk. PNG decoders
may wish to do this for PNG datastreams with no cHRM chunk. In this
case, a reasonable default would be the CCIR 709 primaries [ITU-R_BT.709]. The original NTSC primaries
should not be used unless the PNG image really
was colour-balanced for such a monitor.

The background colour given by the
bKGD chunk will typically be used to
fill unused screen space around the image, as well as any
transparent pixels within the image. (Thus, bKGD is valid and useful
even when the image does not use transparency.) If no bKGD chunk is present,
the viewer will need to decide upon a suitable background colour.
When no other information is available, a medium grey such as 153
in the 8-bit sRGB colour space would be a reasonable choice.
Transparent black or white text and dark drop shadows, which are
common, would all be legible against this background.

Viewers that have a specific background against which to
present the image (such as web browsers) should ignore the bKGD chunk, in
effect overriding
bKGD with their preferred background colour or
background image.

The background colour given by the
bKGD chunk is not to be considered
transparent, even if it happens to match the colour given by the
tRNS chunk (or,
in the case of an indexed-colour image, refers to a palette index
that is marked as transparent by the
tRNS chunk). Otherwise one would have to
imagine something «behind the background» to composite against.
The background colour is either used as background or ignored; it
is not an intermediate layer between the PNG image and some other
background.

Indeed, it will be common that the
bKGD and
tRNS chunks specify the same colour, since
then a decoder that does not implement transparency processing
will give the intended display, at least when no
partially-transparent pixels are present.

The alpha channel can be used to composite a foreground image
against a background image. The PNG datastream defines the
foreground image and the transparency mask, but not the
background image. PNG decoders are not required to
support this most general case. It is expected that most will be
able to support compositing against a single background
colour.

The equation for computing a composited sample value is:

output = alpha * foreground + (1-alpha) * background

where alpha and the input and output sample values are
expressed as fractions in the range 0 to 1. This computation
should be performed with intensity samples (not gamma-encoded
samples). For colour images, the computation is done separately
for R, G, and B samples.

The following code illustrates the general case of compositing
a foreground image against a background image. It assumes that
the original pixel data are available for the background image,
and that output is to a frame buffer for display. Other variants
are possible; see the comments below the code. The code allows
the sample depths and gamma values of foreground image and
background image all to be different and not necessarily suited
to the display system. In practice no assumptions about equality
should be made without first checking.

This code is ISO C [ISO_9899], with line numbers added for
reference in the comments below.

01  int foreground[4];  /* image pixel: R, G, B, A */
02  int background[3];  /* background pixel: R, G, B */
03  int fbpix[3];       /* frame buffer pixel */
04  int fg_maxsample;   /* foreground max sample */
05  int bg_maxsample;   /* background max sample */
06  int fb_maxsample;   /* frame buffer max sample */
07  int ialpha;
08  float alpha, compalpha;
09  float gamfg, linfg, gambg, linbg, comppix, gcvideo;

    /* Get max sample values in data and frame buffer */
10  fg_maxsample = (1 << fg_sample_depth) - 1;
11  bg_maxsample = (1 << bg_sample_depth) - 1;
12  fb_maxsample = (1 << frame_buffer_sample_depth) - 1;
    /*
     * Get integer version of alpha.
     * Check for opaque and transparent special cases;
     * no compositing needed if so.
     *
     * We show the whole gamma decode/correct process in
     * floating point, but it would more likely be done
     * with lookup tables.
     */
13  ialpha = foreground[3];

14  if (ialpha == 0) {
        /*
         * Foreground image is transparent here.
         * If the background image is already in the frame
         * buffer, there is nothing to do.
         */
15      ;
16  } else if (ialpha == fg_maxsample) {
        /*
         * Copy foreground pixel to frame buffer.
         */
17      for (i = 0; i < 3; i++) {
18          gamfg = (float) foreground[i] / fg_maxsample;
19          linfg = pow(gamfg, 1.0 / fg_gamma);
20          comppix = linfg;
21          gcvideo = pow(comppix, 1.0 / display_exponent);
22          fbpix[i] = (int) (gcvideo * fb_maxsample + 0.5);
23      }
24  } else {
        /*
         * Compositing is necessary.
         * Get floating-point alpha and its complement.
         * Note: alpha is always linear; gamma does not
         * affect it.
         */
25      alpha = (float) ialpha / fg_maxsample;
26      compalpha = 1.0 - alpha;

27      for (i = 0; i < 3; i++) {
            /*
             * Convert foreground and background to floating
             * point, then undo gamma encoding.
             */
28          gamfg = (float) foreground[i] / fg_maxsample;
29          linfg = pow(gamfg, 1.0 / fg_gamma);
30          gambg = (float) background[i] / bg_maxsample;

31          linbg = pow(gambg, 1.0 / bg_gamma);
            /*
             * Composite.
             */
32          comppix = linfg * alpha + linbg * compalpha;
            /*
             * Gamma correct for display.
             * Convert to integer frame buffer pixel.
             */
33          gcvideo = pow(comppix, 1.0 / display_exponent);
34          fbpix[i] = (int) (gcvideo * fb_maxsample + 0.5);
35      }
36  }

Variations:

1. If output is to another PNG datastream instead of a frame
   buffer, lines 21, 22, 33, and 34 should be changed along the
   following lines

   /*
    * Gamma encode for storage in output datastream.
    * Convert to integer sample value.
    */
   gamout = pow(comppix, outfile_gamma);
   outpix[i] = (int) (gamout * out_maxsample + 0.5);

   Also, it becomes necessary to process background pixels when
   alpha is zero, rather than just skipping pixels. Thus, line 15
   will need to be replaced by copies of lines 17-23, but processing
   background instead of foreground pixel values.

2. If the sample depths of the output file, foreground file, and
   background file are all the same, and the three gamma values also
   match, then the no-compositing code in lines 14-23 reduces to
   copying pixel values from the input file to the output file if
   alpha is one, or copying pixel values from background to output
   file if alpha is zero. Since alpha is typically either zero or
   one for the vast majority of pixels in an image, this is a
   significant saving. No gamma computations are needed for most
   pixels.
3. When the sample depths and gamma values all match, it may
   appear attractive to skip the gamma decoding and encoding (lines
   28-31, 33-34) and just perform line 32 using gamma-encoded sample
   values. Although this does not have too bad an effect on image
   quality, the time savings are small if alpha values of zero and
   one are treated as special cases as recommended here.
4. If the original pixel values of the background image are no
   longer available, only processed frame buffer pixels left by
   display of the background image, then lines 30 and 31 need to
   extract intensity from the frame buffer pixel values using code
   such as

   /*
    * Convert frame buffer value into intensity sample.
    */
   gcvideo = (float) fbpix[i] / fb_maxsample;
   linbg = pow(gcvideo, display_exponent);

   However, some roundoff error can result, so it is better to have
   the original background pixels available if at all possible.

5. Note that lines 18-22 are performing exactly the same gamma
   computation that is done when no alpha channel is present. If the
   no-alpha case is handled with a lookup table, the same lookup
   table can be used here. Lines 28-31 and 33-34 can also be done
   with (different) lookup tables.
6. Integer arithmetic can be used instead of floating point,
   providing care is taken to maintain sufficient precision
   throughout.

NOTE In floating point, no overflow or underflow
checks are needed, because the input sample values are guaranteed
to be between 0 and 1, and compositing always yields a result
that is in between the input values (inclusive). With integer
arithmetic, some roundoff-error analysis might be needed to
guarantee no overflow or underflow.

When displaying a PNG image with full alpha channel, it is
important to be able to composite the image against some
background, even if it is only black. Ignoring the alpha channel
will cause PNG images that have been converted from an
associated-alpha representation to look wrong. (Of course, if the
alpha channel is a separate transparency mask, then ignoring
alpha is a useful option: it allows the hidden parts of the image
to be recovered.)

Even if the decoder does not implement true compositing logic,
it is simple to deal with images that contain only zero and one
alpha values. (This is implicitly true for greyscale and
truecolour PNG datastreams that use a
tRNS chunk; for indexed-colour PNG
datastreams it is easy to check whether the tRNS chunk contains any
values other than 0 and 255.) In this simple case, transparent
pixels are replaced by the background colour, while others are
unchanged.

If a decoder contains only this much transparency capability,
it should deal with a full alpha channel by treating all nonzero
alpha values as fully opaque or by dithering. Neither approach
will yield very good results for images converted from
associated-alpha formats, but this is preferable to doing
nothing. Dithering full alpha to binary alpha is very much like
dithering greyscale to black-and-white, except that all fully
transparent and fully opaque pixels should be left unchanged by
the dither.

For viewers running on indexed-colour hardware attempting to
display a truecolour image, or an indexed-colour image whose
palette is too large for the frame buffer, the encoder may have
provided one or more suggested palettes in sPLT chunks. If one of
these is found to be suitable, based on size and perhaps name,
the PNG decoder can use that palette. Suggested palettes with a
sample depth different from what the decoder needs can be
converted using sample depth rescaling (see 13.12 Sample depth
rescaling).

When the background is a solid colour, the viewer should
composite the image and the suggested palette against that
colour, then quantize the resulting image to the resulting RGB
palette. When the image uses transparency and the background is
not a solid colour, no suggested palette is likely to be
useful.

For truecolour images, a suggested palette might also be
provided in a
PLTE chunk. If the image has a tRNS chunk and the
background is a solid colour, the viewer will need to adapt the
suggested palette for use with its desired background colour. To
do this, the palette entry closest to the
tRNS colour should be replaced with the
desired background colour; or alternatively a palette entry for
the background colour can be added, if the viewer can handle more
colours than there are
PLTE entries.

For images of colour type 6 (truecolour with alpha), any PLTE chunk should
have been designed for display of the image against a uniform
background of the colour specified by the
bKGD chunk. Viewers should probably
ignore the palette if they intend to use a different background,
or if the bKGD
chunk is missing. Viewers can use a suggested palette for display
against a different background than it was intended for, but the
results may not be very good.

If the viewer presents a transparent truecolour image against
a background that is more complex than a uniform colour, it is
unlikely that the suggested palette will be optimal for the
composite image. In this case it is best to perform a truecolour
compositing step on the truecolour PNG image and background
image, then colour-quantize the resulting image.

In truecolour PNG datastreams, if both
PLTE and
sPLT chunks appear, the PNG decoder may choose
from among the palettes suggested by both, bearing in mind the
different transparency semantics described above.

The frequencies in the
sPLT and
hIST chunks are useful when the viewer cannot
provide as many colours as are used in the palette in the PNG
datastream. If the viewer has a shortfall of only a few colours,
it is usually adequate to drop the least-used colours from the
palette. To reduce the number of colours substantially, it is
best to choose entirely new representative colours, rather than
trying to use a subset of the existing palette. This amounts to
performing a new colour quantization step; however, the existing
palette and histogram can be used as the input data, thus
avoiding a scan of the image data in the
IDAT chunks.

If no suggested palette is provided, a decoder can develop its
own, at the cost of an extra pass over the image data in the IDAT chunks.
Alternatively, a default palette (probably a colour cube) can be
used.

See also 12.5 Suggested
palettes.

Authors are encouraged to look existing chunk types in both this
specification and [PNG-EXTENSIONS] before considering introducing a new chunk
types. The chunk types at [PNG-EXTENSIONS] are expected to be less widely
supported than those defined in this specification.

Two
examples of PNG editors are a program that adds or modifies text
chunks, and a program that adds a suggested palette to a
truecolour PNG datastream. Ordinary image editors are not PNG
editors because they usually discard all unrecognized information
while reading in an image.

To allow new chunk types to be added to PNG, it is necessary
to establish rules about the ordering requirements for all chunk
types. Otherwise a PNG editor does not know what to do when it
encounters an unknown chunk.

EXAMPLE Consider a hypothetical new ancillary chunk type that
is safe-to-copy and is required to appear after PLTE if PLTE is present. If a
program attempts to add a
PLTE chunk and does not recognize the new
chunk, it may insert the
PLTE chunk in the wrong place, namely after
the new chunk. Such problems could be prevented by requiring PNG
editors to discard all unknown chunks, but that is a very
unattractive solution. Instead, PNG requires ancillary chunks not
to have ordering restrictions like this.

To prevent this type of problem while allowing for future
extension, constraints are placed on both the behaviour of PNG
editors and the allowed ordering requirements for chunks. The
safe-to-copy bit defines the proper handling of unrecognized
chunks in a datastream that is being modified.

1. If a chunk’s safe-to-copy bit is 1, the chunk may be copied
   to a modified PNG datastream whether or not the PNG editor
   recognizes the chunk type, and regardless of the extent of the
   datastream modifications.
2. If a chunk’s safe-to-copy bit is 0, it indicates that the
   chunk depends on the image data. If the program has made
   any changes to critical chunks, including
   addition, modification, deletion, or reordering of critical
   chunks, then unrecognized unsafe chunks shall
   not be copied to the output PNG datastream. (Of
   course, if the program does recognize the chunk,
   it can choose to output an appropriately modified version.)
3. A PNG editor is always allowed to copy all unrecognized
   ancillary chunks if it has only added, deleted, modified, or
   reordered ancillary chunks. This implies that it is not
   permissible for ancillary chunks to depend on other ancillary
   chunks.
4. PNG editors shall terminate on encountering an unrecognized
   critical chunk type, because there is no way to be certain that a
   valid datastream will result from modifying a datastream
   containing such a chunk. (Simply discarding the chunk is not good
   enough, because it might have unknown implications for the
   interpretation of other chunks.) The safe/unsafe mechanism is
   intended for use with ancillary chunks. The safe-to-copy bit will
   always be 0 for critical chunks.

The rules governing ordering of chunks are as follows.

1. When copying an unknown unsafe-to-copy ancillary
   chunk, a PNG editor shall not move the chunk relative to any
   critical chunk. It may relocate the chunk freely relative to
   other ancillary chunks that occur between the same pair of
   critical chunks. (This is well defined since the editor shall not
   add, delete, modify, or reorder critical chunks if it is
   preserving unknown unsafe-to-copy chunks.)
2. When copying an unknown safe-to-copy ancillary
   chunk, a PNG editor shall not move the chunk from before IDAT to after IDAT or vice versa.
   (This is well defined because
   IDAT is always present.) Any other reordering
   is permitted.
3. When copying a known ancillary chunk type, an editor
   need only honour the specific chunk ordering rules that exist for
   that chunk type. However, it may always choose to apply the above
   general rules instead.

These rules are expressed in terms of copying chunks from an
input datastream to an output datastream, but they apply in the
obvious way if a PNG datastream is modified in place.

See also 5.4 Chunk naming conventions.

PNG editors that do not change the image data should not
change the tIME
chunk. The Creation Time keyword in the
tEXt,
zTXt, and
iTXt chunks may be used for a user-supplied
time.

This updates the existing
image/png Internet Media type, under the
image top level type.
This appendix is in conformance with
BCP 13 and
W3CRegMedia.

Media type name:

image

Media subtype name:

png

Required parameters:

None

Optional parameters:

None

Encoding considerations:

binary

Security considerations:

A PNG document is composed of a collection of explicitly typed «chunks».
For each of the chunk types defined in the PNG specification (except
for gIFx), the only effect associated with those chunks is to cause
an image to be rendered on the recipient’s display or printer.

The gIFx chunk type is used
to encapsulate Application Extension
data, and some use of that data might present security risks, though
no risks are known. Likewise, the security risks associated with
future chunk types cannot be evaluated, particularly unregistered
chunks. However, it is the intention of the PNG Working Group to disallow
chunks containing «executable» data to become registered chunks.

The text chunks,
tEXt,
iTXT and
zTXt,
contain data that can be displayed in
the form of comments, etc. Some operating systems or terminals might
allow the display of textual data with embedded control characters to
perform operations such as re-mapping of keys, creation of files, etc.
For this reason, the specification recommends that the text chunks be
filtered for control characters before direct display.

The PNG format is specifically designed to facilitate early detection
of file transmission errors, and makes use of cyclical redundancy
checks to ensure the integrity of the data contained in its chunks.

Interoperability considerations:

Network byte order used throughout.

Published specification:

Portable Network Graphics (PNG) Specification,
https://www.w3.org/TR/PNG/

Applications which use this media:

PNG is widely implemented in all Web browsers, image viewers, and image creation tools

Fragment identifier considerations:

N/A

Restrictions on usage:

N/A

Provisional registration? (standards tree only):

No

Additional information:

Deprecated alias names for this type:

N/A

Magic number(s):

89 50 4E 47 0D 0A 1A 0A

File extension(s):

.png

Macintosh file type code:

PNGf

Object Identifiers:

N/A

General Comments:

This registration updates the earlier one:

1. The old one points to an expired Internet Draft. This updated registration points to a W3C Recommendation.
2. The old contact person is sadly deceased. The new contact email is a publicly archived W3C mailing list for the PNG Working Group.
3. Change controller is W3C

Person to contact for further information:

Name:

PNG Working Group

Email:

public-png@w3.org

Intended usage:

Common

Author/Change controller:

W3C

This appendix is in conformance with
BCP 13 and
W3CRegMedia.

Media type name:

image

Media subtype name:

apng

Required parameters:

None

Optional parameters:

None

Encoding considerations:

binary

Security considerations:

An APNG document is composed of a collection of explicitly typed «chunks».
For each of the chunk types defined in the PNG specification (except
for gIFx), the only effect associated with those chunks is to cause
an animated image to be rendered on the recipient’s display.

The gIFx chunk type is used
to encapsulate Application Extension
data, and some use of that data might present security risks, though
no risks are known. Likewise, the security risks associated with
future chunk types cannot be evaluated, particularly unregistered
chunks. However, it is the intention of the PNG Working Group to disallow
chunks containing «executable» data to become registered chunks.

The text chunks,
tEXt,
iTXt and
zTXt,
contain data that can be displayed in
the form of comments, etc. Some operating systems or terminals might
allow the display of textual data with embedded control characters to
perform operations such as re-mapping of keys, creation of files, etc.
For this reason, the specification recommends that the text chunks be
filtered for control characters before direct display.

The PNG format is specifically designed to facilitate early detection
of file transmission errors, and makes use of cyclical redundancy
checks to ensure the integrity of the data contained in its chunks.

Interoperability considerations:

Network byte order used throughout.

Published specification:

Portable Network Graphics (PNG) Specification,
https://www.w3.org/TR/PNG/

Applications which use this media:

Animated PNG (APNG) is widely implemented
in all Web browsers, image viewers, and animation and image creation tools

Fragment identifier considerations:

N/A

Restrictions on usage:

N/A

Provisional registration? (standards tree only):

No

Additional information:

Deprecated alias names for this type:

N/A

Magic number(s):

89 50 4E 47 0D 0A 1A 0A

File extension(s):

.apng

Object Identifiers:

N/A

General Comments:

This registration brings the PNG specification
into alignment with already widely-deployed reality.

Person to contact for further information:

Name:

PNG Working Group

Email:

public-png@w3.org

Intended usage:

Common

Author/Change controller:

W3C

This annex gives the locations of some Internet resources for
PNG software developers. By the nature of the Internet, the list
is incomplete and subject to change.

This specification can be found at
http://www.w3.org/TR/2022/WD-PNG-20221025"/.

ICC profile specifications are available at: http://www.color.org/

There is a World Wide Web site for PNG at http://www.libpng.org/pub/png/.
This page is a central location for current information about PNG
and PNG-related tools.

Additional documentation and portable C code for deflate,
and an optimized implementation of the CRC algorithm are
available from the zlib web site,
http://www.zlib.org/.

A sample implementation in portable C, libpng, is
available at http://www.libpng.org/pub/png/libpng.html.
Sample viewer and encoder applications of libpng are available at
http://www.libpng.org/pub/png/book/sources.html
and are described in detail in PNG: The Definitive Guide
[ROELOFS]. Test images can also be
accessed from the PNG web site.

• The three previously defined, but unofficial, chunks
  for APNG have been added.
  This brings the PNG specification into alignment
  with widely deployed industry practice.
• Added the cICP chunk,
  Coding-independent code points for video signal type identification,
  to contain image format metadata defined in [ITU-T_H.273]
  which enables PNG to contain High Dynamic Range (HDR) and Wide Colour Gamut (WCG)
  images.
• The previously defined eXIf chunk
  has been moved from the PNG-Extensions document [PNG-EXTENSIONS]
  into the main body of this specification,
  to reflect it’s widespread use.
• Incorporation of all
  PNG Second Edition Errata
• Various editorial clarifications in response to community feedback
• References updated to latest versions
• Markup corrections and link fixes
• Document source reformatted to use ReSpec

For the list of changes between W3C
Recommendation PNG Specification Version 1.0 and
PNG Second Edition,
see
PNG
Second Edition changelist

[BCP47]

Tags for Identifying Languages. A. Phillips, Ed.; M. Davis, Ed.. IETF. September 2009. Best Current Practice. URL: https://www.rfc-editor.org/rfc/rfc5646

[CIPA_DC-008]

Exchangeable image file format for digital still cameras: Exif Version 2.32. Camera & Imaging Products Association. 2019-05-17. URL: https://www.cipa.jp/std/documents/download_e.html?DC-008-Translation-2019-E

[COLORIMETRY]

Colorimetry, Fourth Edition. CIE 015:2018. CIE. 2018. URL: http://www.cie.co.at/publications/colorimetry-4th-edition

[ICC]

ICC.1:2010 (Profile version 4.3.0.0). International Color Consortium. December 2010. URL: http://www.color.org/specification/ICC1v43_2010-12.pdf

[ICC-2]

Specification ICC.2:2019 (Profile version 5.0.0 — iccMAX). International Color Consortium. 2019. URL: https://www.color.org/specification/ICC.2-2019.pdf

[ISO_15076-1]

ISO_15076-1:2010 Image technology colour management — Architecture, profile format and data structure — Part 1: Based on ICC.1:2010. ISO. 2010-12. URL: https://www.iso.org/standard/54754.html

[ISO_3309]

ISO/IEC 3309:1993, Information Technology — Telecommunications and information exchange between systems — High-level data link control (HDLC) procedures — Frame structure.. ISO. 1993.

[ISO_8859-1]

ISO/IEC 8859-1:1998, Information technology — 8-bit single-byte coded graphic character sets — Part 1: Latin alphabet No. 1.. ISO. 1998.

[ISO_9899]

ISO/IEC 9899:2018 Information technology — Programming languages — C. ISO. 2018-6. URL: https://www.iso.org/standard/74528.html

[ISO646]

Information technology — ISO 7-bit coded character set for information interchange. International Organization for Standardization (ISO). December 1991. Published. URL: https://www.iso.org/standard/4777.html

[ITU-R_BT.709]

ITU-R_BT.709, SERIES H: AUDIOVISUAL AND MULTIMEDIA SYSTEMS Infrastructure of audiovisual services – Coding of moving video Coding-independent code points for video signal type identification. ITU. URL: https://www.itu.int/rec/R-REC-BT.709

[ITU-T_H.273]

ITU-T_H.273, SERIES H: AUDIOVISUAL AND MULTIMEDIA SYSTEMS Infrastructure of audiovisual services – Coding of moving video Coding-independent code points for video signal type identification. ITU. URL: https://www.itu.int/rec/T-REC-H.273

[ITU-T_V.42]

ITU Recommendation V.42 : Error-correcting procedures for DCEs using asynchronous-to-synchronous conversion. ITU. 2002-03-29. URL: https://www.itu.int/rec/T-REC-V.42-200203-I/en

[JPEG]

JPEG File Interchange Format. Eric Hamilton. C-Cube Microsystems. Milpitas, CA, USA. September 1992. URL: https://www.w3.org/Graphics/JPEG/jfif3.pdf

[Paeth]

Image File Compression Made Easy, in Graphics Gems II, pp. 93-100. Paeth, A.W. Academic Press. 1991. URL: https://www.sciencedirect.com/science/article/pii/B9780080507545500293

[PNG-EXTENSIONS]

Extensions to the PNG Third Edition Specification, Version 1.6.0. W3C. 2021. URL: https://w3c.github.io/PNG-spec/extensions/Overview.html

[rfc1123]

Requirements for Internet Hosts — Application and Support. R. Braden, Ed.. IETF. October 1989. Internet Standard. URL: https://www.rfc-editor.org/rfc/rfc1123

[rfc1950]

ZLIB Compressed Data Format Specification version 3.3. P. Deutsch; J-L. Gailly. IETF. May 1996. Informational. URL: https://www.rfc-editor.org/rfc/rfc1950

[RFC1951]

DEFLATE Compressed Data Format Specification version 1.3. P. Deutsch. IETF. May 1996. Informational. URL: https://www.rfc-editor.org/rfc/rfc1951

[RFC1952]

GZIP file format specification version 4.3. P. Deutsch. IETF. May 1996. Informational. URL: https://www.rfc-editor.org/rfc/rfc1952

[RFC2119]

Key words for use in RFCs to Indicate Requirement Levels. S. Bradner. IETF. March 1997. Best Current Practice. URL: https://www.rfc-editor.org/rfc/rfc2119

[rfc3629]

UTF-8, a transformation format of ISO 10646. F. Yergeau. IETF. November 2003. Internet Standard. URL: https://www.rfc-editor.org/rfc/rfc3629

[RFC8174]

Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words. B. Leiba. IETF. May 2017. Best Current Practice. URL: https://www.rfc-editor.org/rfc/rfc8174

[SMPTE_170M]

Television — Composite Analog Video Signal — NTSC for Studio Applications. Society of Motion Picture and Television Engineers. 2004-11-30. URL: https://standards.globalspec.com/std/892300/SMPTE%20ST%20170M

[SRGB]

Multimedia systems and equipment — Colour measurement and management — Part 2-1: Colour management — Default RGB colour space — sRGB. IEC. URL: https://webstore.iec.ch/publication/6169

[XMP]

Extensible metadata platform (XMP) specification — Part 1. Adobe Systems Incorporated. ISO/IEC. 15 February 2012. URL: http://www.iso.org/iso/home/store/catalogue_tc/catalogue_detail.htm?csnumber=57421

[Ziv-Lempel]

A Universal Algorithm for Sequential Data Compression, IEEE Transactions on Information Theory, vol. IT-23, no. 3, pp. 337 — 343. J. Ziv; A. Lempel. IEEE. 1977-05. URL: https://ieeexplore.ieee.org/document/1055714

[COLOR_FAQ]

Color FAQ. Poynton, C.. 2009-10-19. URL: http://poynton.ca/ColorFAQ.html

[EBU_R_103]

Video Signal Tolerance in Digital Television Systems. EBU. 2020-05. URL: https://tech.ebu.ch/docs/r/r103.pdf

[GAMMA_FAQ]

Gamma FAQ. Poynton, C.. 1998-08-04. URL: http://poynton.ca/GammaFAQ.html

[GIF]

Graphics Interchange Format. CompuServe Incorporated. 31 July 1990. URL: https://www.w3.org/Graphics/GIF/spec-gif89a.txt

[Hill]

Comparative analysis of the quantization of color spaces on the basis of the CIELAB color-difference formula. Hill, B.; Roger, Th.; Vorhagen, F.W.. 1997-04. URL: https://dl.acm.org/doi/10.1145/248210.248212

[ITU-R_BT.2100]

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[ROELOFS]

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