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<!DOCTYPE html>
<html lang="en">
<head>
<title>Portable Network Graphics (PNG) Specification (Third Edition)</title>
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publisher: "O'Reilly & Associates Inc",
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</head>
<body>
<section id="abstract">
<p>This document describes PNG (Portable Network Graphics), an extensible file format for the <a>lossless</a>, portable,
well-compressed storage of static and animated raster images. PNG provides a patent-free replacement for GIF and can also
replace many common uses of TIFF. <a>Indexed-color</a>, <a>greyscale</a>, and <a>truecolor</a> images are supported, plus an optional alpha
channel. Sample depths range from 1 to 16 bits.</p>
<p>PNG is designed to work well in online viewing applications, such as the World Wide Web, so it is fully streamable with a
progressive display option. PNG is robust, providing both full file integrity checking and simple detection of common
transmission errors. Also, PNG can store color space data for improved color matching on heterogeneous platforms.</p>
<p>This specification defines two Internet Media Types, image/png and image/apng.</p>
<!--
needs update, comment out for now
<p>The PNG specification enjoys a good level of <a href="http://www.libpng.org/pub/png/pngstatus.html">implementation</a> with good interoperability. At the time of this publication more than 180 <a href="http://www.libpng.org/pub/png/pngapvw.html">image viewers</a> could display PNG images and over 100 <a href="http://www.libpng.org/pub/png/pngaped.html">image editors</a> could read and write valid PNG files. Full support of PNG is required for conforming <a href="/Graphics/SVG">SVG</a> viewers; at the time of publication all eighteen <a href="/Graphics/SVG/SVG-Implementations.htm8#viewer">SVG viewers</a> had PNG support. HTML has no required image formats, but over 60 <a href="http://www.libpng.org/pub/png/pngapbr.html">HTML browsers</a> had at least basic support of PNG images.</p> -->
</section>
<section id="sotd">
<p>This specification is intended to become an International Standard, but is not yet one. It is inappropriate to refer to this
specification as an International Standard.</p>
</section>
<!-- *********************************************************************
FROM HERE ON THIS FILE IS IDENTICAL TO THE ISO VERSION
with these exceptions:
- id added to any headings that did not have one, to comply with pubrules and allow indexing into the document
- URL for this document updated in Annex E and the words " [to be completed when published]" removed
************************************************************************** -->
<h2 id="Introduction">Introduction</h2>
<p>The design goals for this specification were:</p>
<ol>
<li>Portability: encoding, decoding, and transmission should be software and hardware platform independent.</li>
<li>Completeness: it should be possible to represent <a>truecolor</a>, <a>indexed-color</a>, and <a>greyscale</a> images, in each case with
the option of transparency, color space information, and ancillary information such as textual comments.
</li>
<li>Serial encode and decode: it should be possible for datastreams to be generated serially and read serially, allowing the
datastream format to be used for on-the-fly generation and display of images across a serial communication channel.</li>
<li>Progressive presentation: it should be possible to transmit datastreams so that an approximation of the whole image can be
presented initially, and progressively enhanced as the datastream is received.</li>
<li>Robustness to transmission errors: it should be possible to detect datastream transmission errors reliably.</li>
<li>Losslessness: filtering and compression should preserve all information.</li>
<li>Performance: any filtering, compression, and progressive image presentation should be aimed at efficient decoding and
presentation. Fast encoding is a less important goal than fast decoding. Decoding speed may be achieved at the expense of
encoding speed.</li>
<li>Compression: images should be compressed effectively, consistent with the other design goals.</li>
<li>Simplicity: developers should be able to implement the standard easily.</li>
<li>Interchangeability: any standard-conforming PNG decoder shall be capable of reading all conforming PNG datastreams.</li>
<li>Flexibility: future extensions and private additions should be allowed for without compromising the interchangeability of
standard PNG datastreams.</li>
<li>Freedom from legal restrictions: no algorithms should be used that are not freely available.</li>
</ol>
<section>
<!-- Maintain a fragment named "1Scope" to preserve incoming links to it -->
<h2 id="1Scope">Scope</h2>
<p>This specification specifies a datastream and an associated file format, Portable Network Graphics (PNG, pronounced "ping"),
for a <a>lossless</a>, portable, compressed individual computer graphics image or frame-based animation, transmitted across the
Internet.
</section>
<section>
<!-- Maintain a fragment named "3Defsandabbrevs" to preserve incoming links to it -->
<h2 id="3Defsandabbrevs">Terms, definitions, and abbreviated terms</h2>
<p>For the purposes of this specification the following definitions apply.</p>
<dl>
<!-- Maintain a fragment named "3byte" to preserve incoming links to it -->
<dt id="3byte"><dfn>byte</dfn>
</dt>
<dt>octet</dt>
<dd>8-bit binary integer in the range [0, 255] where the most significant bit is bit 7 and the least significant
bit is bit 0.</dd>
<!-- Maintain a fragment named "3byteOrder" to preserve incoming links to it -->
<dt id="3byteOrder"><dfn>byte order</dfn>
</dt>
<dd>
ordering of <a>bytes</a> for multi-byte data values.
</dd>
<!-- Maintain a fragment named "3chromaticity" to preserve incoming links to it -->
<dt id="3chromaticity"><dfn>chromaticity</dfn>
</dt>
<dd>pair of <i>x</i> and <i>y</i> values in the <i>xyY</i> space specified at [[COLORIMETRY]].
<p class="note">Chromaticity is a measure of the quality of a color regardless of its luminance.</p></dd>
<!-- Maintain a fragment named "3composite" to preserve incoming links to it -->
<dt id="3composite"><dfn data-lt="composited|composite">composite (verb)</dfn>
</dt>
<dd>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.
<p class="note">The foreground image is said to be <a>composited</a> against the background.</p>
</dd>
<!-- Maintain a fragment named "3datastream" to preserve incoming links to it -->
<dt id="3datastream"><dfn>datastream</dfn>
</dt>
<dd>
sequence of <a>bytes</a>.
</dd>
<!-- Maintain a fragment named "3deflate" to preserve incoming links to it -->
<dt id="3deflate"><dfn>deflate</dfn>
</dt>
<dd>
member of the <a>LZ77</a> family of compression methods.
<p>SOURCE: [[RFC1951]]</p>
</dd>
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<!-- ************Page Break******************* -->
<dt><dfn>frame</dfn></dt>
<dd>For static PNG, the <a>static image</a>
is considered to be the first (and only) frame.
For animated PNG, each image that forms part
of the <a href="#apng-frame-based-animation">frame-based animation</a> sequence
is a frame.
Thus, for animated PNG, when the static image is not the first frame,
the static image is not considered to be a frame.
</dd>
<!-- Maintain a fragment named "3frameBuffer" to preserve incoming links to it -->
<dt id="3frameBuffer"><dfn>frame buffer</dfn>
</dt>
<dd>
the final digital storage area for the image shown by most types of computer display.
<p class="note">Software causes an image to appear on screen by loading the image into the <a>frame buffer</a>.</p>
</dd>
<dt><dfn>fully transparent black</dfn>
</dt>
<dd>pixel where the red, green, blue and alpha components are all equal to zero.</dd>
<dt><dfn>gamma value</dfn>
</dt>
<dd>
value of the exponent of a <a>gamma</a> <a>transfer function</a>.
</dd>
<!-- Maintain a fragment named "3gamma" to preserve incoming links to it -->
<dt id="3gamma"><dfn>gamma</dfn>
</dt>
<dd>
power-law <a>transfer function</a>.
</dd>
<dt>high dynamic range (<dfn>HDR</dfn>)
</dt>
<dd>
an image format capable of storing images with a relatively high dynamic range similar to or in excess of the human visual system's instantaneous dynamic range (~12-14 <a>stops</a>). PNG allows the use of two <a>HDR</a> formats, <a>HLG</a> and <a>PQ</a> [[ITU-R-BT.2100]].
</dd>
<dt>hybrid log-gamma (<dfn>HLG</dfn>)
</dt>
<dd>
<a>transfer function</a> defined in [[ITU-R-BT.2100]] Table 5. (A relative scene-referred system.)
</dd>
<dt><dfn>full-range image</dfn>
</dt>
<dd>image where reference black and white correspond, respectively, to sample values <code>0</code> and <code>2<sup>bit depth</sup> -
1</code>.</dd>
<!-- Maintain a fragment named "3imageData" to preserve incoming links to it -->
<dt id="3imageData"><dfn>image data</dfn>
</dt>
<dd>
1-dimensional array of <a>scanlines</a> within an image.
</dd>
<!-- Maintain a fragment named "3interlacedPNGimage" to preserve incoming links to it -->
<dt id="3interlacedPNGimage"><dfn>interlaced PNG image</dfn>
</dt>
<dd>
sequence of <a>reduced images</a> generated from the <a>PNG image</a> by <a>pass extraction</a>.
</dd>
<!-- Maintain a fragment named "3losslessCompression" to preserve incoming links to it -->
<dt id="3losslessCompression"><dfn>lossless</dfn>
</dt>
<dd>method of data compression that permits reconstruction of the original data exactly, bit-for-bit.</dd>
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<dt id="3luminance"><dfn>luminance</dfn>
</dt>
<dd>
an objective measurement of the visible light intensity, taking into account the sensitivity of the human eye to different wavelengths.
<p class="note">Luminance and <a>chromaticity</a> together fully define a measured colour. See <a href="https://colorusage.arc.nasa.gov/lum_and_chrom.php">Luminance and Chromaticity</a>
or, for a formal definition [[COLORIMETRY]].</p>
</dd>
<!-- Maintain a fragment named "3LZ77" to preserve incoming links to it -->
<dt><dfn id="3LZ77">LZ77</dfn>
</dt>
<dd>data compression algorithm described in [[Ziv-Lempel]].</dd>
<dt><dfn>narrow-range image</dfn>
</dt>
<dd>Image where reference black and white do not correspond, respectively, to sample values <code>0</code> and
<code>2<sup>bit depth</sup> - 1</code>.</dd>
<!-- Maintain a fragment named "3networkByteOrder" to preserve incoming links to it -->
<dt id="3networkByteOrder"><dfn>network byte order</dfn>
</dt>
<dd>
<a>byte order</a> 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).
</dd>
<dt>perceptual quantizer (<dfn>PQ</dfn>)
</dt>
<dd>
<a>transfer function</a> defined in [[ITU-R-BT.2100]] Table 4. (An absolute display-referred system.)
</dd>
<p class="note">
Only RGB may be used in PNG, ICtCp is NOT supported.
</p>
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<!-- Maintain a fragment named "3PNGdecoder" to preserve incoming links to it -->
<dt id="3PNGdecoder">PNG decoder</dt>
<dd>
process or device that reconstructs the <a>reference image</a> from a <a>PNG datastream</a> and generates a
corresponding <a>delivered image</a>.
</dd>
<!-- Maintain a fragment named "3PNGeditor" to preserve incoming links to it -->
<dt id="3PNGeditor"><dfn>PNG editor</dfn>
</dt>
<dd>
process or device that creates a modification of an existing <a>PNG datastream</a>, preserving unmodified ancillary
information wherever possible, and obeying the <a>chunk</a> ordering rules, even for unknown chunk types.
</dd>
<!-- Maintain a fragment named "3PNGencoder" to preserve incoming links to it -->
<dt id="3PNGencoder">PNG encoder
</dt>
<dd>
process or device which constructs a <a>reference image</a> from a <a>source image</a>, and generates a <a>PNG
datastream</a> representing the reference image.
</dd>
<!-- Maintain a fragment named "3PNGfile" to preserve incoming links to it -->
<dt id="3PNGfile">PNG file
</dt>
<dd>
<a>PNG datastream</a> stored as a file.
</dd>
<!-- Maintain a fragment named "3PNGfourByteUnSignedInteger" to preserve incoming links to it -->
<dt id="3PNGfourByteUnSignedInteger"><dfn>PNG four-byte unsigned integer</dfn>
</dt>
<dd>
<p>a four-byte unsigned integer limited to the range 0 to 2<sup>31</sup>-1.</p>
<p class="note">The restriction is imposed in order to accommodate languages that have difficulty with unsigned four-byte
values.</p>
</dd>
<dt id="3PNGtwoByteUnSignedInteger"><dfn>PNG two-byte unsigned integer</dfn>
</dt>
<dd>
<p>a two-byte unsigned integer in network byte order.</p>
</dd>
<!-- Maintain a fragment named "3sample" to preserve incoming links to it -->
<dt id="3sample"><dfn>sample</dfn>
</dt>
<dd>
intersection of a <a>channel</a> and a <a>pixel</a> in an image.
</dd>
<!-- Maintain a fragment named "3sampleDepth" to preserve incoming links to it -->
<dt id="3sampleDepth">sample depth</dt>
<dd>
number of bits used to represent a <a>sample</a> value.
</dd>
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<dt id="3scanline"><dfn>scanline</dfn>
</dt>
<dd>
row of <a>pixels</a> within an image or <a>interlaced PNG image</a>.
</dd>
<dt>standard dynamic range (<dfn>SDR</dfn>)
</dt>
<dd>
an image format capable of storing images with a relatively low dynamic range of 5-8 <a>stops</a>. Examples include [[SRGB]], [[Display-P3]], [[ITU-R-BT.709]].
<p class="note">Standard dynamic range is independent of the primaries and hence, gamut. Wide color gamut <a>SDR</a> formats are supported by PNG.</p></dd>
<dt><dfn>stop</dfn>
</dt>
<dd>
a change in scene light luminance of a factor of 2.
</dd>
<dt><dfn>transfer function</dfn>
</dt>
<dd>function relating image luminance with image samples.</dd>
<!-- Maintain a fragment named "3whitePoint" to preserve incoming links to it -->
<dt id="3whitePoint"><dfn>white point</dfn>
</dt>
<dd>
<a>chromaticity</a> of a computer display's nominal white value.
</dd>
<!-- Maintain a fragment named "3zlib" to preserve incoming links to it -->
<dt id="3zlib"><dfn>zlib</dfn>
</dt>
<dd>
<p><a>deflate</a>-style compression method.</p>
<p>SOURCE: [[rfc1950]]</p>
<p class="note">Also refers to the name of a library containing a sample implementation of this method.</p>
</dd>
<!-- Maintain a fragment named "3CRC" to preserve incoming links to it -->
<dt id="3CRC">Cyclic Redundancy Code</dt>
<dt><abbr title="Cyclic Redundancy Code">CRC</abbr></dt>
<dd>
<p>type of check value designed to detect most transmission errors.</p>
<p class="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.</p>
</dd>
<!-- Maintain a fragment named "3CRT" to preserve incoming links to it -->
<dt id="3CRT">Cathode Ray Tube</dt>
<dt><abbr title="Cathode Ray Tube">CRT</abbr></dt>
<dd>vacuum tube containing one or more electron guns, which emit electron beams that are manipulated to display images on a
phosphorescent screen.</dd>
<!-- Maintain a fragment named "3LSB" to preserve incoming links to it -->
<dt id="3LSB">Least Significant Byte</dt>
<dt><abbr title="Least Significant Byte">LSB</abbr></dt>
<dd>
Least significant byte of a multi-<a>byte</a> value.
</dd>
<!-- Maintain a fragment named "3MSB" to preserve incoming links to it -->
<dt id="3MSB">Most Significant Byte</dt>
<dt><abbr title="Most Significant Byte">MSB</abbr></dt>
<dd>
Most significant byte of a multi-<a>byte</a> value.
</dd>
</dl>
</section>
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<!-- ************Page Break******************* -->
<section>
<!-- Maintain a fragment named "4Concepts" to preserve incoming links to it -->
<h2 id="4Concepts">Concepts</h2>
<section>
<h2>Static and Animated images</h2>
<p>All PNG images contain a single <dfn>static image</dfn>.</p>
<p>Some PNG images — called <dfn class="lint-ignore">Animated PNG</dfn> (<abbr title="Animated PNG">APNG</abbr>) — also
contain a frame-based animation sequence, the <dfn>animated image</dfn>. The first frame of this may be — but need not be —
the <a>static image</a>. Non-animation-capable displays (such as printers) will display the <a>static image</a> rather than
the animation sequence.</p>
<p>The <a>static image</a>, and each individual frame of an <a>animated image</a>, corresponds to a <em>reference image</em>
and is stored as a <em>PNG image</em>.</p>
</section>
<section>
<!-- Maintain a fragment named "4Concepts.Sourceimage" to preserve incoming links to it -->
<h2 id="4Concepts.Sourceimage">Images</h2>
<p>This specification specifies the PNG datastream, and places some requirements on PNG encoders, which generate PNG
datastreams, PNG decoders, which interpret PNG datastreams, and <a>PNG editors</a>, 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.</p>
<dl>
<!-- Maintain a fragment named "3sourceImage" to preserve incoming links to it -->
<dt><dfn id="3sourceImage">Source image</dfn></dt>
<dd>The <i>source image</i> is the image presented to a PNG encoder.</dd>
<!-- Maintain a fragment named "3referenceImage" to preserve incoming links to it -->
<!-- Maintain a fragment named "3pixel" to preserve incoming links to it -->
<dt><dfn id="3referenceImage">Reference image</dfn></dt>
<dd>The <a>reference image</a>, which only exists conceptually, is a rectangular array of rectangular <dfn id="3pixel">pixels</dfn>, 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
<!-- Maintain a fragment named "3channel" to preserve incoming links to it -->
<dfn id="3channel">channel</dfn>. 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 color; if they are all zero, the pixel is black, and if they all have
<!-- Maintain a fragment named "3alpha" to preserve incoming links to it -->
their maximum values (2<sup>sampledepth</sup>-1), the pixel is white. The <dfn id="3alpha">alpha</dfn> 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. Every reference image can be represented exactly by a <a>PNG datastream</a> and every <a>PNG datastream</a> can be converted into a reference image. 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.</dd>
<!-- Maintain a fragment named "3PNGimage" to preserve incoming links to it -->
<dt><dfn id="3PNGimage">PNG image</dfn></dt>
<dd>The <i>PNG image</i> is obtained from the reference image by a series of transformations: <a>alpha separation</a>,
<a>indexing</a>, <a>RGB merging</a>, <a>alpha compaction</a>, and <a>sample depth scaling</a>. Five types of PNG image are defined
(see <a href="#6Colour-values"></a>). (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.
</dd>
<!-- Maintain a fragment named "3deliveredImage" to preserve incoming links to it -->
<dt><dfn id="3deliveredImage">Delivered image</dfn></dt>
<dd>The <a>delivered image</a> is constructed from the PNG image obtained by decoding a PNG datastream. No specific format
is specified for the <a>delivered image</a>. A viewer presents an image to the user as close to the appearance of the
original source image as it can achieve.
</dd>
</dl>
<p>The relationships between the four kinds of image are illustrated in <a href="#image-relationship"></a>.</p>
<figure id="image-relationship">
<!-- Maintain a fragment named "figure41" to preserve incoming links to it -->
<object id="figure41" data="figures/image-relationships.svg" type="image/svg+xml">
</object>
<figcaption>
Relationships between source, reference, PNG, and display images
</figcaption>
</figure>
<!-- ************Page Break******************* -->
<!-- ************Page Break******************* -->
<p>The relationships between samples, channels, pixels, and sample depth are illustrated in <a href=
"#sample-pixel-channel-relationship"></a>.</p>
<figure id="sample-pixel-channel-relationship">
<!-- Maintain a fragment named "figure42" to preserve incoming links to it -->
<object id="figure42" data="figures/sample-pixel-channel-relationship.svg" type="image/svg+xml" width="640" height="290">
</object>
<figcaption>
Relationships between sample, sample depth, pixel, and channel
</figcaption>
</figure>
</section>
<!-- Maintain a fragment named "4Concepts.ColourSpaces" to preserve incoming links to it -->
<section id="4Concepts.ColourSpaces">
<h2>Color spaces</h2>
<p>The RGB color space in which color samples are situated may be specified in one of four ways:</p>
<!-- <ol start="1"> -->
<ol>
<li>by CICP image format signaling metadata;</li>
<li>by an ICC profile;</li>
<li>by specifying explicitly that the color space is sRGB when the samples conform to this color space;</li>
<li>by specifying a <a>gamma value</a> and the 1931 CIE <i>x,y</i> chromaticities of the red, green, and blue primaries
used in the image and the reference <a>white point</a>.
</li>
</ol>
<p>For high-end applications the first two methods provides the most flexibility and control. The third method enables one
particular, but extremely common, color 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 <a>gamma</a> correction
to be applied (see <a href="#C-GammaAppendix"></a>). However, color-aware applications will prefer one of the first three
methods, while color-unaware applications will typically ignore all four methods.</p>
<p><a>Gamma</a> correction is not applied to the alpha channel, if present. Alpha samples always represent a linear fraction
of full opacity.</p>
<p>Mastering metadata may also be provided</p>
</section>
<!-- Maintain a fragment named "4Concepts.PNGImageTransformation" to preserve incoming links to it -->
<section id="4Concepts.PNGImageTransformation">
<h2>Reference image to PNG image transformation</h2>
<!-- Maintain a fragment named "4Concepts.Introduction" to preserve incoming links to it -->
<section class="introductory" id="4Concepts.Introduction">
<h3>Introduction</h3>
<p>A number of transformations are applied to the reference image to create the PNG image to be encoded (see <a href=
"#reference-to-png-transformation"></a>). The transformations are applied in the following sequence, where square brackets
mean the transformation is optional:</p>
<pre>
[alpha separation]
indexing or ( [RGB merging] [alpha compaction] )
sample depth scaling
</pre>
<p>When every pixel is either fully transparent or fully opaque, the <a>alpha separation</a>, <a>alpha compaction</a>, and
<a>indexing</a> transformations can cause the recovered reference image to have an alpha sample depth different from the
original reference image, or to have no alpha channel. This has no effect on the degree of opacity of any pixel. The two
reference images are considered equivalent, and the transformations are considered lossless. Encoders that nevertheless
wish to preserve the alpha sample depth may elect not to perform transformations that would alter the alpha sample
depth.</p>
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<!-- ************Page Break******************* -->
<figure id="reference-to-png-transformation">
<!-- Maintain a fragment named "figure43" to preserve incoming links to it -->
<object id="figure43" data="figures/reference-to-png-transformation.svg" type="image/svg+xml" height="525" width="370">
</object>
<figcaption>
Reference image to PNG image transformation
</figcaption>
</figure>
</section>
<!-- Maintain a fragment named "4Concepts.Implied-alpha" to preserve incoming links to it -->
<section id="4Concepts.Implied-alpha">
<!-- Maintain a fragment named "3alphaSeparation" to preserve incoming links to it -->
<h2><dfn id="3alphaSeparation">Alpha separation</dfn></h2>
<p>If all alpha samples in a reference image have the maximum value, then the alpha channel may be omitted, resulting in an
equivalent image that can be encoded more compactly.</p>
</section>
<!-- Maintain a fragment named "4Concepts.Indexing" to preserve incoming links to it -->
<section id="4Concepts.Indexing">
<h2>Indexing</h2>
<p>If the number of distinct pixel values is 256 or less, and the RGB sample depths are not greater than 8, and the alpha
channel is absent or exactly 8 bits deep or every pixel is either fully transparent or fully opaque, then the alternative
<a>indexed-color</a> representation, achieved through an <dfn id="3indexing">indexing</dfn> transformation, may be more efficient for encoding.
<!-- Maintain a fragment named "3indexing" to preserve incoming links to it -->
<!-- Maintain a fragment named "3palette" to preserve incoming links to it -->
<!-- Maintain a fragment named "3alphaTable" to preserve incoming links to it -->
<!-- Maintain a fragment named "3indexedColour" to preserve incoming links to it -->
In the <dfn id="3indexedColour">indexed-color</dfn> representation, each pixel is replaced by an index into a palette. The <dfn id="3palette">palette</dfn>
is a list of entries each containing three 8-bit samples (red, green, blue). If an alpha channel is present, there is also
a parallel table of 8-bit alpha samples, called the <dfn id="3alphaTable">alpha table</dfn>.</p>
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<!-- ************Page Break******************* -->
<figure id="indexed-colour-image">
<!-- Maintain a fragment named "figure44" to preserve incoming links to it -->
<object id="figure44" height="450" width="660" data="figures/indexed-colour-image.svg" type="image/svg+xml">
</object>
<figcaption>
Indexed-color image
</figcaption>
</figure>
<p>A suggested palette or palettes may be constructed even when the PNG image is not <a>indexed-color</a> in order to assist
viewers that are capable of displaying only a limited number of colors.</p>
<p>For <a>indexed-color</a> images, encoders can rearrange the palette so that the table entries with the maximum alpha value are
grouped at the end. In this case the table can be encoded in a shortened form that does not include these entries.</p>
<p>Encoders creating indexed-color PNG must not insert index values greater than the actual length of the palette table; to
do so is an error, and decoders will vary in their handling of this error.</p>
</section>
<!-- Maintain a fragment named "4Concepts.RGBMerging" to preserve incoming links to it -->
<section id="4Concepts.RGBMerging">
<!-- Maintain a fragment named "3RGBmerging" to preserve incoming links to it -->
<h2><dfn id="3RGBmerging">RGB merging</dfn>
</h2>
<p>If the red, green, and blue channels have the same sample depth, and, for each pixel, the values of the red, green, and
blue samples are equal, then these three channels may be merged into a single greyscale channel.</p>
</section>
<!-- Maintain a fragment named "4Concepts.Alpha-indexing" to preserve incoming links to it -->
<section id="4Concepts.Alpha-indexing">
<!-- Maintain a fragment named "3alphaCompaction" to preserve incoming links to it -->
<h2><dfn id="3alphaCompaction">Alpha compaction</dfn></h2>
<p>For non-indexed images, if there exists an RGB (or greyscale) value such that all pixels with that value are fully
transparent while all other pixels are fully opaque, then the alpha channel can be represented more compactly by merely
identifying the RGB (or greyscale) value that is transparent.</p>
</section>
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<section id="4Concepts.Scaling">
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<h2><dfn id="3sampleDepthScaling">Sample depth scaling</dfn>
</h2>
<p>In the PNG image, not all sample depths are supported (see <a href="#6Colour-values"></a>), and all channels shall have
the same sample depth. All channels of the PNG image use the smallest allowable sample depth that is not less than any
sample depth in the reference image, and the possible sample values in the reference image are linearly mapped into the
next allowable range for the PNG image. <a href="#scaling-sample-values"></a> shows how samples of depth 3 might be mapped
into samples of depth 4.</p>
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<figure id="scaling-sample-values">
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<figcaption>
Scaling sample values
</figcaption>
</figure>
<p>Allowing only a few sample depths reduces the number of cases that decoders have to cope with. <a>Sample depth
scaling</a> is reversible with no loss of data, because the reference image sample depths can be recorded in the PNG
datastream. In the absence of recorded sample depths, the reference image sample depth equals the PNG image sample depth.
See <a href="#12Sample-depth-scaling"></a> and <a href="#13Sample-depth-rescaling"></a>.</p>
<figure id="possible-pixel-types">
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</object>