rfc9649.original.xml		rfc9649.xml
	<?xml version="1.0" encoding="US-ASCII"?>		<?xml version="1.0" encoding="UTF-8"?>
	<!DOCTYPE rfc [		<!DOCTYPE rfc [
	<!ENTITY nbsp "&#160;">		<!ENTITY nbsp "&#160;">
	<!ENTITY zwsp "&#8203;">		<!ENTITY zwsp "&#8203;">
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	<rfc xmlns:xi="http://www.w3.org/2001/XInclude"		<rfc xmlns:xi="http://www.w3.org/2001/XInclude"
	submissionType="IETF"		submissionType="IETF"
	category="info"		category="info"
	consensus="true"		consensus="true"
	docName="draft-zern-webp-15"		docName="draft-zern-webp-15"
			number="9649"
	ipr="trust200902"		ipr="trust200902"
	obsoletes=""		obsoletes=""
	updates=""		updates=""
	xml:lang="en"		xml:lang="en"
	tocInclude="true"		tocInclude="true"
	tocDepth="4"		tocDepth="4"
	symRefs="true"		symRefs="true"
	sortRefs="true"		sortRefs="true"
	version="3">		version="3">
	<front>		<front>
	<title>WebP Image Format</title>		<title>WebP Image Format</title>
	<seriesInfo name="Internet-Draft" value="draft-zern-webp-15" />		<seriesInfo name="RFC" value="9649"/>
	<seriesInfo status="informational" name="" value="draft-zern-webp-15" />
	<author fullname="James Zern" initials="J." surname="Zern">		<author fullname="James Zern" initials="J." surname="Zern">
	<organization>Google LLC</organization>		<organization>Google LLC</organization>
	<address>		<address>
	<postal>		<postal>
	<street>1600 Amphitheatre Parkway</street>		<street>1600 Amphitheatre Parkway</street>
	<city>Mountain View</city>		<city>Mountain View</city>
	<region>CA</region>		<region>CA</region>
	<code>94043</code>		<code>94043</code>
	<country>United States of America</country>		<country>United States of America</country>
	</postal>		</postal>
	skipping to change at line 54 ¶		skipping to change at line 54 ¶
	<address>		<address>
	<email>pascal.massimino@gmail.com</email>		<email>pascal.massimino@gmail.com</email>
	</address>		</address>
	</author>		</author>
	<author fullname="Jyrki Alakuijala" initials="J." surname="Alakuijala">		<author fullname="Jyrki Alakuijala" initials="J." surname="Alakuijala">
	<organization>Google LLC</organization>		<organization>Google LLC</organization>
	<address>		<address>
	<email>jyrki.alakuijala@gmail.com</email>		<email>jyrki.alakuijala@gmail.com</email>
	</address>		</address>
	</author>		</author>
	<date year="2024" month="April"/>		<date year="2024" month="October"/>
	<area>art</area>		<area>art</area>
	<keyword>VP8</keyword>		<keyword>VP8</keyword>
	<keyword>WebP</keyword>		<keyword>WebP</keyword>
	<abstract>		<abstract>
	<t>This document defines the WebP image format and registers a media type		<t>This document defines the WebP image format and registers a media type
	supporting its use.</t>		supporting its use.</t>
	</abstract>		</abstract>
	</front>		</front>
	skipping to change at line 87 ¶		skipping to change at line 87 ¶
	image pixel data, including alpha (transparency) information. Lossy		image pixel data, including alpha (transparency) information. Lossy
	compression is achieved using VP8 intra-frame encoding <xref		compression is achieved using VP8 intra-frame encoding <xref
	target="RFC6386"/>. The <xref target="webp-lossless">lossless		target="RFC6386"/>. The <xref target="webp-lossless">lossless
	algorithm</xref> stores and restores the pixel values exactly,		algorithm</xref> stores and restores the pixel values exactly,
	including the color values for fully transparent pixels. A universal algo rithm for sequential data compression		including the color values for fully transparent pixels. A universal algo rithm for sequential data compression
	<xref target="LZ77"/>, <xref target="Huffman">prefix coding</xref>,		<xref target="LZ77"/>, <xref target="Huffman">prefix coding</xref>,
	and a color cache are used for compression of the bulk data.</t>		and a color cache are used for compression of the bulk data.</t>
	</section>		</section>
	<section anchor="webp-container" numbered="true" toc="default">		<section anchor="webp-container" numbered="true" toc="default">
	<name>WebP Container Specification</name>		<name>WebP Container Specification</name>
	<t>Note that this section is based on the documentation in the <xref		<aside><t>Note that this section is based on the documentation in the <xref
	target="webp-riff-src">libwebp source repository</xref>.</t>		target="webp-riff-src">libwebp source repository</xref>.</t></aside>
	<section numbered="true" toc="default">		<section numbered="true" toc="default">
	<name>Introduction (from "WebP Container Specification")</name>		<name>Introduction (from "WebP Container Specification")</name>
	<t>WebP is an image format that uses either (i) the VP8 intra-frame		<t>WebP is an image format that uses either (i) the VP8 intra-frame
	encoding <xref target="RFC6386"/> to compress image data in a lossy		encoding <xref target="RFC6386"/> to compress image data in a lossy
	way or (ii) the <xref target="webp-lossless">WebP lossless		way or (ii) the <xref target="webp-lossless">WebP lossless
	encoding</xref>. These encoding schemes should make it more		encoding</xref>. These encoding schemes should make it more
	efficient than older formats, such as JPEG, GIF, and PNG. It is o ptimized for fast		efficient than older formats, such as JPEG, GIF, and PNG. It is o ptimized for fast
	image transfer over the network (for example, for websites). The WebP		image transfer over the network (for example, for websites). The WebP
	format has feature parity (color profile, metadata, animation,		format has feature parity (color profile, metadata, animation,
	etc.) with other formats as well. This section describes the		etc.) with other formats as well. This section describes the
	skipping to change at line 248 ¶		skipping to change at line 248 ¶
	</section>		</section>
	<section anchor="simple-file-format-lossy" numbered="true" toc="default">		<section anchor="simple-file-format-lossy" numbered="true" toc="default">
	<name>Simple File Format (Lossy)</name>		<name>Simple File Format (Lossy)</name>
	<t>This layout <bcp14>SHOULD</bcp14> be used if the image requires lossy encoding and		<t>This layout <bcp14>SHOULD</bcp14> be used if the image requires lossy encoding and
	does not require transparency or other advanced features provided by		does not require transparency or other advanced features provided by
	the extended format. Files with this layout are smaller and supported		the extended format. Files with this layout are smaller and supported
	by older software.</t>		by older software.</t>
	<t>
	Simple WebP (lossy) file format:
	</t>
	<figure>		<figure>
	<name>Simple WebP (Lossy) File Format</name>		<name>Simple WebP (Lossy) File Format</name>
	<artwork name="" type="" align="left" alt=""><![CDATA[		<artwork name="" type="" align="left" alt=""><![CDATA[
	0 1 2 3		0 1 2 3
	0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1		0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
	+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+		+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
	| |		| |
	| WebP file header (12 bytes) |		| WebP file header (12 bytes) |
	| |		| |
	+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+		+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
	: 'VP8 ' Chunk :		: 'VP8 ' Chunk :
	+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+		+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
	]]></artwork>		]]></artwork>
	</figure>		</figure>
	<t>
	'VP8 ' Chunk:
	</t>
	<figure>		<figure>
	<name>'VP8 ' Chunk</name>		<name>'VP8 ' Chunk</name>
	<artwork name="" type="" align="left" alt=""><![CDATA[		<artwork name="" type="" align="left" alt=""><![CDATA[
	0 1 2 3		0 1 2 3
	0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1		0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
	+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+		+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
	| ChunkHeader('VP8 ') |		| ChunkHeader('VP8 ') |
	| |		| |
	+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+		+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
	: VP8 data :		: VP8 data :
	+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+		+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
	]]></artwork>		]]></artwork>
	</figure>		</figure>
	<dl newline="true" spacing="normal" indent="4">		<dl newline="true" spacing="normal" indent="4">
	<dt>VP8 data: <em>Chunk Size</em> bytes</dt>		<dt>VP8 data: <em>Chunk Size</em> bytes</dt>
	<dd>VP8 bitstream data.</dd>		<dd>VP8 bitstream data.</dd>
	</dl>		</dl>
	<t>Note that the fourth character in the 'VP8 ' FourCC is an ASCII space		<aside><t>Note that the fourth character in the 'VP8 ' FourCC is an ASCII
	(0x20).</t>		space
			(0x20).</t></aside>
	<t>The VP8 bitstream format specification is described in <xref		<t>The VP8 bitstream format specification is described in <xref
	target="RFC6386"/>. Note that the VP8 frame header contains the VP8		target="RFC6386"/>.</t><aside><t>Note that the VP8 frame header contains the VP8
	frame width and height. That is assumed to be the width and height of		frame width and height. That is assumed to be the width and height of
	the canvas.</t>		the canvas.</t></aside>
	<t>The VP8 specification describes how to decode the image into Y'CbCr		<t>The VP8 specification describes how to decode the image into Y'CbCr
	format. To convert to RGB, <xref target="rec601">Recommendation 601</xre f> <bcp14>SHOULD</bcp14>		format. To convert to RGB, <xref target="rec601">Recommendation 601</xre f> <bcp14>SHOULD</bcp14>
	be used. Applications <bcp14>MAY</bcp14> use another conversion method , but visual		be used. Applications <bcp14>MAY</bcp14> use another conversion method , but visual
	results may differ among decoders.</t>		results may differ among decoders.</t>
	</section>		</section>
	<section anchor="simple-file-format-lossless" numbered="true"		<section anchor="simple-file-format-lossless" numbered="true"
	toc="default">		toc="default">
	<name>Simple File Format (Lossless)</name>		<name>Simple File Format (Lossless)</name>
	<t>Note: Older readers may not support files using the lossless		<aside><t>Note: Older readers may not support files using the lossless
	format.</t>		format.</t></aside>
	<t>This layout <bcp14>SHOULD</bcp14> be used if the image requires lossle ss encoding		<t>This layout <bcp14>SHOULD</bcp14> be used if the image requires lossle ss encoding
	(with an optional transparency channel) and does not require advanced		(with an optional transparency channel) and does not require advanced
	features provided by the extended format.</t>		features provided by the extended format.</t>
	<t>
	Simple WebP (lossless) file format:
	</t>
	<figure>		<figure>
	<name>Simple WebP (Lossless) File Format</name>		<name>Simple WebP (Lossless) File Format</name>
	<artwork name="" type="" align="left" alt=""><![CDATA[		<artwork name="" type="" align="left" alt=""><![CDATA[
	0 1 2 3		0 1 2 3
	0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1		0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
	+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+		+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
	| |		| |
	| WebP file header (12 bytes) |		| WebP file header (12 bytes) |
	| |		| |
	+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+		+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
	: 'VP8L' Chunk :		: 'VP8L' Chunk :
	+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+		+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
	]]></artwork>		]]></artwork>
	</figure>		</figure>
	<t>
	'VP8L' Chunk:
	</t>
	<figure>		<figure>
	<name>'VP8L' Chunk</name>		<name>'VP8L' Chunk</name>
	<artwork name="" type="" align="left" alt=""><![CDATA[		<artwork name="" type="" align="left" alt=""><![CDATA[
	0 1 2 3		0 1 2 3
	0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1		0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
	+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+		+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
	| ChunkHeader('VP8L') |		| ChunkHeader('VP8L') |
	| |		| |
	+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+		+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
	: VP8L data :		: VP8L data :
	+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+		+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
	]]></artwork>		]]></artwork>
	</figure>		</figure>
	<dl newline="true" spacing="normal" indent="4">		<dl newline="true" spacing="normal" indent="4">
	<dt>VP8L data: <em>Chunk Size</em> bytes</dt>		<dt>VP8L data: <em>Chunk Size</em> bytes</dt>
	<dd>VP8L bitstream data.</dd>		<dd>VP8L bitstream data.</dd>
	</dl>		</dl>
	<t>The specification of the VP8L bitstream can be found in <xref		<t>The specification of the VP8L bitstream can be found in <xref
	target="webp-lossless"/>. Note that the VP8L header contains the		target="webp-lossless"/>.</t><aside><t>Note that the VP8L header contai ns the
	VP8L image width and height. That is assumed to be the width and		VP8L image width and height. That is assumed to be the width and
	height of the canvas.</t>		height of the canvas.</t></aside>
	</section>		</section>
	<section anchor="ext-file-form" numbered="true" toc="default">		<section anchor="ext-file-form" numbered="true" toc="default">
	<name>Extended File Format</name>		<name>Extended File Format</name>
	<t>Note: Older readers may not support files using the extended		<aside><t>Note: Older readers may not support files using the extended
	format.</t>		format.</t></aside>
	<t>An extended format file consists of:</t>		<t>An extended format file consists of:</t>
	<ul spacing="normal">		<ul spacing="normal">
	<li>A 'VP8X' Chunk with information about features used in the		<li>A 'VP8X' Chunk with information about features used in the
	file.</li>		file.</li>
	<li>An optional 'ICCP' Chunk with a color profile.</li>		<li>An optional 'ICCP' Chunk with a color profile.</li>
	<li>An optional 'ANIM' Chunk with animation control data.</li>		<li>An optional 'ANIM' Chunk with animation control data.</li>
	<li>Image data.</li>		<li>Image data.</li>
	<li>An optional 'EXIF' Chunk with Exif metadata.</li>		<li>An optional 'EXIF' Chunk with Exif metadata.</li>
	<li>An optional 'XMP ' Chunk with XMP metadata.</li>		<li>An optional 'XMP ' Chunk with XMP metadata.</li>
	skipping to change at line 387 ¶		skipping to change at line 373 ¶
	<ul spacing="normal">		<ul spacing="normal">
	<li>An optional <xref target="alpha">alpha subchunk</xref>.</li>		<li>An optional <xref target="alpha">alpha subchunk</xref>.</li>
	<li>A <xref target="bitstream-vp8vp8l">bitstream subchunk</xref>.</li>		<li>A <xref target="bitstream-vp8vp8l">bitstream subchunk</xref>.</li>
	</ul>		</ul>
	<t>For an <em>animated image</em>, the <em>image data</em> consists of		<t>For an <em>animated image</em>, the <em>image data</em> consists of
	multiple frames. More details about frames can be found in <xref		multiple frames. More details about frames can be found in <xref
	target="animation"/>.</t>		target="animation"/>.</t>
	<t>All chunks necessary for reconstruction and color correction, that is		<t>All chunks necessary for reconstruction and color correction, that is,
	'VP8X', 'ICCP', 'ANIM', 'ANMF', 'ALPH', 'VP8 ' and 'VP8L',		'VP8X', 'ICCP', 'ANIM', 'ANMF', 'ALPH', 'VP8 ', and 'VP8L',
	<bcp14>MUST</bcp14> appear in the order described earlier. Readers		<bcp14>MUST</bcp14> appear in the order described earlier. Readers
	<bcp14>SHOULD</bcp14> fail when chunks necessary for reconstruction and		<bcp14>SHOULD</bcp14> fail when chunks necessary for reconstruction and
	color correction are out of order.</t>		color correction are out of order.</t>
	<t><xref target="metadata">Metadata</xref> and <xref		<t><xref target="metadata">Metadata</xref> and <xref
	target="unknown-chunks">unknown</xref> chunks MAY appear out of		target="unknown-chunks">unknown chunks</xref> MAY appear out of
	order.</t>		order.</t>
	<aside><t>Rationale: The chunks necessary for reconstruction should		<aside><t>Rationale: The chunks necessary for reconstruction should
	appear first in the file to allow a reader to begin decoding an image		appear first in the file to allow a reader to begin decoding an image
	before receiving all of the data. An application may benefit from		before receiving all of the data. An application may benefit from
	varying the order of metadata and custom chunks to suit the		varying the order of metadata and custom chunks to suit the
	implementation.</t></aside>		implementation.</t></aside>
	<t>
	Extended WebP file header:
	</t>
	<figure anchor="extended_header">		<figure anchor="extended_header">
	<name>Extended WebP File Header</name>		<name>Extended WebP File Header</name>
	<artwork name="" type="" align="left" alt=""><![CDATA[		<artwork name="" type="" align="left" alt=""><![CDATA[
	0 1 2 3		0 1 2 3
	0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1		0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
	+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+		+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
	| |		| |
	| WebP file header (12 bytes) |		| WebP file header (12 bytes) |
	| |		| |
	+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+		+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
	skipping to change at line 467 ¶		skipping to change at line 450 ¶
	<t>Future specifications may add more fields. Unknown fields <bcp14>MUST< /bcp14> be		<t>Future specifications may add more fields. Unknown fields <bcp14>MUST< /bcp14> be
	ignored.</t>		ignored.</t>
	<section numbered="true" toc="default">		<section numbered="true" toc="default">
	<name>Chunks</name>		<name>Chunks</name>
	<section anchor="animation" numbered="true" toc="default">		<section anchor="animation" numbered="true" toc="default">
	<name>Animation</name>		<name>Animation</name>
	<t>An animation is controlled by 'ANIM' and 'ANMF' Chunks.</t>		<t>An animation is controlled by 'ANIM' and 'ANMF' Chunks.</t>
	<t anchor="anim_chunk">'ANIM' Chunk:</t>		<figure anchor="anim_chunk">
	<t>For an animated image, this chunk contains the <em>global
	parameters</em> of the animation.</t>
	<figure>
	<name>'ANIM' Chunk</name>		<name>'ANIM' Chunk</name>
	<artwork name="" type="" align="left" alt=""><![CDATA[		<artwork name="" type="" align="left" alt=""><![CDATA[
	0 1 2 3		0 1 2 3
	0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1		0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
	+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+		+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
	| ChunkHeader('ANIM') |		| ChunkHeader('ANIM') |
	| |		| |
	+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+		+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
	| Background Color |		| Background Color |
	+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+		+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
	| Loop Count |		| Loop Count |
	+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+		+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
	]]></artwork>		]]></artwork>
	</figure>		</figure>
			<t>For an animated image, this chunk contains the <em>global
			parameters</em> of the animation.</t>
	<dl newline="true" spacing="normal" indent="4">		<dl newline="true" spacing="normal" indent="4">
	<dt>Background Color: 32 bits (<em>uint32</em>)</dt>		<dt>Background Color: 32 bits (<em>uint32</em>)</dt>
	<dd>		<dd>
	<t>The default background color of the canvas in [Blue, Green,		<t>The default background color of the canvas in [Blue, Green,
	Red, Alpha] byte order. This color <bcp14>MAY</bcp14> be used t o fill the		Red, Alpha] byte order. This color <bcp14>MAY</bcp14> be used t o fill the
	unused space on the canvas around the frames, as well as the		unused space on the canvas around the frames, as well as the
	transparent pixels of the first frame. The background color is		transparent pixels of the first frame. The background color is
	also used when the Disposal method is <tt>1</tt>.</t>		also used when the Disposal method is <tt>1</tt>.</t>
	<t>Note:</t>		<t>Notes:</t>
	<ul spacing="normal">		<ul spacing="normal">
	<li>The background color <bcp14>MAY</bcp14> contain a nonopaque alpha value,		<li>The background color <bcp14>MAY</bcp14> contain a nonopaque alpha value,
	even if the <em>Alpha</em> flag in the <xref		even if the <em>Alpha</em> flag in the <xref
	target="extended_header">'VP8X' Chunk</xref> is unset.</li>		target="extended_header">'VP8X' Chunk</xref> is unset.</li>
	<li>Viewer applications <bcp14>SHOULD</bcp14> treat the backgro und color value		<li>Viewer applications <bcp14>SHOULD</bcp14> treat the backgro und color value
	as a hint and are not required to use it.</li>		as a hint and are not required to use it.</li>
	<li>The canvas is cleared at the start of each loop. The		<li>The canvas is cleared at the start of each loop. The
	background color <bcp14>MAY</bcp14> be used to achieve this.< /li>		background color <bcp14>MAY</bcp14> be used to achieve this.< /li>
	</ul>		</ul>
	</dd>		</dd>
	<dt>Loop Count: 16 bits (<em>uint16</em>)</dt>		<dt>Loop Count: 16 bits (<em>uint16</em>)</dt>
	<dd>The number of times to loop the animation. If it is <tt>0</tt>, this means		<dd>The number of times to loop the animation. If it is <tt>0</tt>, this means
	infinitely.</dd>		infinitely.</dd>
	</dl>		</dl>
	<t>This chunk <bcp14>MUST</bcp14> appear if the <em>Animation</em> fl ag in the 'VP8X'		<t>This chunk <bcp14>MUST</bcp14> appear if the <em>Animation</em> fl ag in the 'VP8X'
	Chunk is set. If the <em>Animation</em> flag is not set and this		Chunk is set. If the <em>Animation</em> flag is not set and this
	chunk is present, it <bcp14>MUST</bcp14> be ignored.</t>		chunk is present, it <bcp14>MUST</bcp14> be ignored.</t>
	<t>'ANMF' Chunk:</t>
	<t>For animated images, this chunk contains information about a
	<em>single</em> frame. If the <em>Animation flag</em> is not set,
	then this chunk <bcp14>SHOULD NOT</bcp14> be present.</t>
	<figure>		<figure>
	<name>'ANMF' Chunk</name>		<name>'ANMF' Chunk</name>
	<artwork name="" type="" align="left" alt=""><![CDATA[		<artwork name="" type="" align="left" alt=""><![CDATA[
	0 1 2 3		0 1 2 3
	0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1		0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
	+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+		+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
	| ChunkHeader('ANMF') |		| ChunkHeader('ANMF') |
	| |		| |
	+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+		+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
	| Frame X | ...		| Frame X | ...
	skipping to change at line 545 ¶		skipping to change at line 520 ¶
	+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+		+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
	... | Frame Height Minus One |		... | Frame Height Minus One |
	+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+		+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
	| Frame Duration | Reserved |B|D|		| Frame Duration | Reserved |B|D|
	+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+		+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
	: Frame Data :		: Frame Data :
	+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+		+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
	]]></artwork>		]]></artwork>
	</figure>		</figure>
			<t>For animated images, this chunk contains information about a
			<em>single</em> frame. If the <em>Animation flag</em> is not set,
			then this chunk <bcp14>SHOULD NOT</bcp14> be present.</t>
	<dl newline="true" spacing="normal" indent="4">		<dl newline="true" spacing="normal" indent="4">
	<dt>Frame X: 24 bits (<em>uint24</em>)</dt>		<dt>Frame X: 24 bits (<em>uint24</em>)</dt>
	<dd>The X coordinate of the upper left corner of the frame is		<dd>The X coordinate of the upper left corner of the frame is
	<tt>Frame X * 2</tt>.</dd>		<tt>Frame X * 2</tt>.</dd>
	<dt>Frame Y: 24 bits (<em>uint24</em>)</dt>		<dt>Frame Y: 24 bits (<em>uint24</em>)</dt>
	<dd>The Y coordinate of the upper left corner of the frame is		<dd>The Y coordinate of the upper left corner of the frame is
	<tt>Frame Y * 2</tt>.</dd>		<tt>Frame Y * 2</tt>.</dd>
	<dt>Frame Width Minus One: 24 bits (<em>uint24</em>)</dt>		<dt>Frame Width Minus One: 24 bits (<em>uint24</em>)</dt>
	<dd>The <em>1-based</em> width of the frame. The frame width is		<dd>The <em>1-based</em> width of the frame. The frame width is
	<tt>1 + Frame Width Minus One</tt>.</dd>		<tt>1 + Frame Width Minus One</tt>.</dd>
	skipping to change at line 623 ¶		skipping to change at line 602 ¶
	dst.RGB * dst.A * (1 - src.A / 255)) / blend.A		dst.RGB * dst.A * (1 - src.A / 255)) / blend.A
	]]></sourcecode>		]]></sourcecode>
	</li>		</li>
	<li>Alpha-blending <bcp14>SHOULD</bcp14> be done in linear co lor space by		<li>Alpha-blending <bcp14>SHOULD</bcp14> be done in linear co lor space by
	taking into account the <xref target="color-profile">color		taking into account the <xref target="color-profile">color
	profile</xref> of the image. If the color profile is not		profile</xref> of the image. If the color profile is not
	present, standard RGB (sRGB) is to be assumed. (Note th at sRGB also		present, standard RGB (sRGB) is to be assumed. (Note th at sRGB also
	needs to be linearized due to a gamma of ~2.2.)</li>		needs to be linearized due to a gamma of ~2.2.)</li>
	</ul>		</ul>
	</dd>		</dd>
	<dt>Frame Data: <em>Chunk Size</em> - <tt>16</tt> bytes</dt>		<dt>Frame Data: <em>Chunk Size</em> bytes - <tt>16</tt></dt>
	<dd><t>Consists of:</t>		<dd><t>Consists of:</t>
	<ul spacing="normal">		<ul spacing="normal">
	<li>An optional <xref target="alpha">alpha subchunk</xref>		<li>An optional <xref target="alpha">alpha subchunk</xref>
	for the frame.</li>		for the frame.</li>
	<li>A <xref target="bitstream-vp8vp8l">bitstream		<li>A <xref target="bitstream-vp8vp8l">bitstream
	subchunk</xref> for the frame.</li>		subchunk</xref> for the frame.</li>
	<li>An optional list of <xref		<li>An optional list of <xref
	target="unknown-chunks">unknown chunks</xref>.</li>		target="unknown-chunks">unknown chunks</xref>.</li>
	</ul>		</ul>
	</dd>		</dd>
	</dl>		</dl>
	<t>Note: The 'ANMF' payload, <em>Frame Data</em>, consists		<aside><t>Note: The 'ANMF' payload, <em>Frame Data</em>, consists
	of individual <em>padded</em> chunks, as described by the <xref		of individual <em>padded</em> chunks, as described by the <xref
	target="riff-file-format">RIFF file format</xref>.</t>		target="riff-file-format">RIFF file format</xref>.</t></aside>
	</section>		</section>
	<section anchor="alpha" numbered="true" toc="default">		<section anchor="alpha" numbered="true" toc="default">
	<name>Alpha</name>		<name>Alpha</name>
	<figure>		<figure>
	<name>'ALPH' Chunk</name>		<name>'ALPH' Chunk</name>
	<artwork name="" type="" align="left" alt=""><![CDATA[		<artwork name="" type="" align="left" alt=""><![CDATA[
	0 1 2 3		0 1 2 3
	0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1		0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
	+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+		+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
	| ChunkHeader('ALPH') |		| ChunkHeader('ALPH') |
	skipping to change at line 683 ¶		skipping to change at line 662 ¶
	<li><tt>0</tt>: None.</li>		<li><tt>0</tt>: None.</li>
	<li><tt>1</tt>: Horizontal filter.</li>		<li><tt>1</tt>: Horizontal filter.</li>
	<li><tt>2</tt>: Vertical filter.</li>		<li><tt>2</tt>: Vertical filter.</li>
	<li><tt>3</tt>: Gradient filter.</li>		<li><tt>3</tt>: Gradient filter.</li>
	</ul>		</ul>
	<t>For each pixel, filtering is performed using the following		<t>For each pixel, filtering is performed using the following
	calculations. Assume the alpha values surrounding the current		calculations. Assume the alpha values surrounding the current
	<tt>X</tt> position are labeled as:</t>		<tt>X</tt> position are labeled as:</t>
	<figure>		<figure>
			<name>Pixels Used in Alpha Filtering</name>
	<artwork name="" type="" align="left" alt=""><![CDATA[		<artwork name="" type="" align="left" alt=""><![CDATA[
	C | B |		C | B |
	---+---+		---+---+
	A | X |		A | X |
	]]></artwork>		]]></artwork>
	</figure>		</figure>
	<t>We seek to compute the alpha value at position X. First, a		<t>We seek to compute the alpha value at position X. First, a
	prediction is made depending on the filtering method:</t>		prediction is made depending on the filtering method:</t>
	skipping to change at line 735 ¶		skipping to change at line 715 ¶
	(x-1, 0) on the left.</li>		(x-1, 0) on the left.</li>
	</ul>		</ul>
	</dd>		</dd>
	<dt>Compression method (C): 2 bits</dt>		<dt>Compression method (C): 2 bits</dt>
	<dd><t>The compression method used:</t>		<dd><t>The compression method used:</t>
	<ul spacing="normal">		<ul spacing="normal">
	<li><tt>0</tt>: No compression.</li>		<li><tt>0</tt>: No compression.</li>
	<li><tt>1</tt>: Compressed using the WebP lossless format.</li>		<li><tt>1</tt>: Compressed using the WebP lossless format.</li>
	</ul>		</ul>
	</dd>		</dd>
	<dt>Alpha bitstream: <em>Chunk Size</em> - <tt>1</tt> bytes</dt>		<dt>Alpha bitstream: <em>Chunk Size</em> bytes - <tt>1</tt></dt>
	<dd>Encoded alpha bitstream.</dd>		<dd>Encoded alpha bitstream.</dd>
	</dl>		</dl>
	<t>This optional chunk contains encoded alpha data for this frame. A		<t>This optional chunk contains encoded alpha data for this frame. A
	frame containing a 'VP8L' Chunk <bcp14>SHOULD NOT</bcp14> contain t his chunk.</t>		frame containing a 'VP8L' Chunk <bcp14>SHOULD NOT</bcp14> contain t his chunk.</t>
	<aside><t>Rationale: The transparency information is already part of the		<aside><t>Rationale: The transparency information is already part of the
	'VP8L' Chunk.</t></aside>		'VP8L' Chunk.</t></aside>
	<t>The alpha channel data is stored as uncompressed raw data		<t>The alpha channel data is stored as uncompressed raw data
	(when the compression method is '0') or compressed using the		(when the compression method is '0') or compressed using the
	lossless format (when the compression method is '1').</t>		lossless format (when the compression method is '1').</t>
	<ul spacing="normal">		<ul spacing="normal">
	<li>Raw data: This consists of a byte sequence of length = width *		<li>Raw data: This consists of a byte sequence of length = width *
	height, containing all the 8-bit transparency values in scan		height, containing all the 8-bit transparency values in scan
	order.</li>		order.</li>
	<li><t>Lossless format compression: The byte sequence is a		<li><t>Lossless format compression: The byte sequence is a
	compressed image-stream (as described in <xref		compressed image-stream (as described in <xref
	target="webp-lossless"/>) of implicit dimensions width x		target="webp-lossless"/>) of implicit dimensions width x
	height. That is, this image-stream does NOT contain any		height. That is, this image-stream does NOT contain any
	headers describing the image dimensions.</t>		headers describing the image dimensions.</t></li></ul>
	<t>Rationale: The dimensions are already known from other sources,		<aside><t>Rationale: The dimensions are already known from other so
	so storing them again would be redundant and prone to errors.</t>		urces,
	<t>Once the image-stream is decoded into Alpha, Red, Green, Blue (A		so storing them again would be redundant and prone to errors.</t></
	RGB) color values,		aside>
			<ul empty="true">
			<li><t>Once the image-stream is decoded into Alpha, Red, Green, Bl
			ue (ARGB) color values,
	following the process described in the lossless format		following the process described in the lossless format
	specification, the transparency information must be extracted		specification, the transparency information must be extracted
	from the green channel of the ARGB quadruplet.</t>		from the green channel of the ARGB quadruplet.</t></li></ul>
	<t>Rationale: The green channel is allowed extra transformation		<aside><t>Rationale: The green channel is allowed extra transform
			ation
	steps in the specification -- unlike the other channels -- that		steps in the specification -- unlike the other channels -- that
	can improve compression.</t></li>		can improve compression.</t></aside>
	</ul>
	</section>		</section>
	<section anchor="bitstream-vp8vp8l" numbered="true" toc="default">		<section anchor="bitstream-vp8vp8l" numbered="true" toc="default">
	<name>Bitstream (VP8/VP8L)</name>		<name>Bitstream (VP8/VP8L)</name>
	<t>This chunk contains compressed bitstream data for a single		<t>This chunk contains compressed bitstream data for a single
	frame.</t>		frame.</t>
	<t>A bitstream chunk may be either (i) a 'VP8 ' Chunk, using 'VP8 '		<t>A bitstream chunk may be either (i) a 'VP8 ' Chunk, using 'VP8 '
	(note the significant fourth-character space) as its FourCC,		(note the significant fourth-character space) as its FourCC,
	<em>or</em> (ii) a 'VP8L' Chunk, using 'VP8L' as its FourCC.</t>		<em>or</em> (ii) a 'VP8L' Chunk, using 'VP8L' as its FourCC.</t>
	<t>The formats of' VP8 ' and 'VP8L' Chunks are as described in Sectio ns <xref		<t>The formats of' VP8 ' and 'VP8L' Chunks are as described in Sectio ns <xref
	target="simple-file-format-lossy" format="counter"/> and <xref		target="simple-file-format-lossy" format="counter"/> and <xref
	skipping to change at line 810 ¶		skipping to change at line 790 ¶
	target="ICC">ICC specification</xref> for details.</t>		target="ICC">ICC specification</xref> for details.</t>
	<t>If this chunk is not present, sRGB <bcp14>SHOULD</bcp14> be assume d.</t>		<t>If this chunk is not present, sRGB <bcp14>SHOULD</bcp14> be assume d.</t>
	</section>		</section>
	<section anchor="metadata" numbered="true" toc="default">		<section anchor="metadata" numbered="true" toc="default">
	<name>Metadata</name>		<name>Metadata</name>
	<t>Metadata can be stored in 'EXIF' or 'XMP ' Chunks.</t>		<t>Metadata can be stored in 'EXIF' or 'XMP ' Chunks.</t>
	<t>There <bcp14>SHOULD</bcp14> be at most one chunk of each type ('EX IF' and 'XMP '). If there are more such chunks, readers <bcp14>MAY</bcp14> ignor e all except		<t>There <bcp14>SHOULD</bcp14> be at most one chunk of each type ('EX IF' and 'XMP '). If there are more such chunks, readers <bcp14>MAY</bcp14> ignor e all except
	the first one.</t>		the first one.</t>
	<t>The chunks are defined as follows:</t>		<t>The chunks are defined as follows:</t>
	<t>
	'EXIF' Chunk:
	</t>
	<figure>		<figure>
	<name>'EXIF' Chunk</name>		<name>'EXIF' Chunk</name>
	<artwork name="" type="" align="left" alt=""><![CDATA[		<artwork name="" type="" align="left" alt=""><![CDATA[
	0 1 2 3		0 1 2 3
	0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1		0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
	+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+		+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
	| ChunkHeader('EXIF') |		| ChunkHeader('EXIF') |
	| |		| |
	+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+		+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
	: Exif Metadata :		: Exif Metadata :
	+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+		+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
	]]></artwork>		]]></artwork>
	</figure>		</figure>
	<dl newline="true" spacing="normal" indent="4">		<dl newline="true" spacing="normal" indent="4">
	<dt>Exif Metadata: <em>Chunk Size</em> bytes</dt>		<dt>Exif Metadata: <em>Chunk Size</em> bytes</dt>
	<dd>Image metadata in <xref target="Exif"/> format.</dd>		<dd>Image metadata in <xref target="Exif"/> format.</dd>
	</dl>		</dl>
	<t>
	'XMP ' Chunk:
	</t>
	<figure>		<figure>
	<name>'XMP ' Chunk</name>		<name>'XMP ' Chunk</name>
	<artwork name="" type="" align="left" alt=""><![CDATA[		<artwork name="" type="" align="left" alt=""><![CDATA[
	0 1 2 3		0 1 2 3
	0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1		0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
	+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+		+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
	| ChunkHeader('XMP ') |		| ChunkHeader('XMP ') |
	| |		| |
	+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+		+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
	: XMP Metadata :		: XMP Metadata :
	+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+		+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
	]]></artwork>		]]></artwork>
	</figure>		</figure>
	<dl newline="true" spacing="normal" indent="4">		<dl newline="true" spacing="normal" indent="4">
	<dt>XMP Metadata: <em>Chunk Size</em> bytes</dt>		<dt>XMP Metadata: <em>Chunk Size</em> bytes</dt>
	<dd>Image metadata in <xref target="XMP"/> format.</dd>		<dd>Image metadata in <xref target="XMP"/> format.</dd>
	</dl>		</dl>
	<t>Note that the fourth character in the 'XMP ' FourCC is an ASCII sp		<aside><t>Note that the fourth character in the 'XMP ' FourCC is an A
	ace		SCII space
	(0x20).</t>		(0x20).</t></aside>
	<t>Additional guidance about handling metadata can be found in the		<t>Additional guidance about handling metadata can be found in the
	Metadata Working Group's <xref target="MWG">"Guidelines For		Metadata Working Group's <xref target="MWG">"Guidelines For
	Handling Image Metadata"</xref>.</t>		Handling Image Metadata"</xref>.</t>
	</section>		</section>
	<section anchor="unknown-chunks" numbered="true" toc="default">		<section anchor="unknown-chunks" numbered="true" toc="default">
	<name>Unknown Chunks</name>		<name>Unknown Chunks</name>
	<t>A RIFF chunk (described in <xref		<t>A RIFF chunk (described in <xref
	target="riff-file-format"/>) whose <em>FourCC</em> is		target="riff-file-format"/>) whose <em>FourCC</em> is
	different from any of the chunks described in this section is		different from any of the chunks described in this section is
	considered an <em>unknown chunk</em>.</t>		considered an <em>unknown chunk</em>.</t>
	skipping to change at line 918 ¶		skipping to change at line 892 ¶
	displayed. A new loop iteration is then begun, or the canvas is left		displayed. A new loop iteration is then begun, or the canvas is left
	in its final state if all iterations have been completed.</t>		in its final state if all iterations have been completed.</t>
	<t>The following pseudocode illustrates the rendering process. The		<t>The following pseudocode illustrates the rendering process. The
	notation <em>VP8X.field</em> means the field in the 'VP8X' Chunk		notation <em>VP8X.field</em> means the field in the 'VP8X' Chunk
	with the same description.</t>		with the same description.</t>
	<sourcecode type="pseudocode"><![CDATA[		<sourcecode type="pseudocode"><![CDATA[
	VP8X.flags.hasAnimation MUST be TRUE		VP8X.flags.hasAnimation MUST be TRUE
	canvas <- new image of size VP8X.canvasWidth x VP8X.canvasHeight with		canvas <- new image of size VP8X.canvasWidth x VP8X.canvasHeight with
	background color ANIM.background_color.		background color ANIM.background_color or
			application-defined color.
	loop_count <- ANIM.loopCount		loop_count <- ANIM.loopCount
	dispose_method <- Dispose to background color		dispose_method <- Dispose to background color
	if loop_count == 0:		if loop_count == 0:
	loop_count = inf		loop_count = inf
	frame_params <- nil		frame_params <- nil
	next chunk in image_data is ANMF MUST be TRUE		next chunk in image_data is ANMF MUST be TRUE
	for loop = 0..loop_count - 1		for loop = 0..loop_count - 1
	clear canvas to ANIM.background_color or application-defined color		clear canvas to ANIM.background_color or application-defined color
	until eof or non-ANMF chunk		until eof or non-ANMF chunk
	frame_params.frameX = Frame X		frame_params.frameX = Frame X
	skipping to change at line 946 ¶		skipping to change at line 921 ¶
	VP8X.canvasHeight >= frame_bottom MUST be TRUE		VP8X.canvasHeight >= frame_bottom MUST be TRUE
	for subchunk in 'Frame Data':		for subchunk in 'Frame Data':
	if subchunk.tag == "ALPH":		if subchunk.tag == "ALPH":
	alpha subchunks not found in 'Frame Data' earlier MUST be		alpha subchunks not found in 'Frame Data' earlier MUST be
	TRUE		TRUE
	frame_params.alpha = alpha_data		frame_params.alpha = alpha_data
	else if subchunk.tag == "VP8 " OR subchunk.tag == "VP8L":		else if subchunk.tag == "VP8 " OR subchunk.tag == "VP8L":
	bitstream subchunks not found in 'Frame Data' earlier MUST		bitstream subchunks not found in 'Frame Data' earlier MUST
	be TRUE		be TRUE
	frame_params.bitstream = bitstream_data		frame_params.bitstream = bitstream_data
			apply dispose_method.
	render frame with frame_params.alpha and frame_params.bitstream		render frame with frame_params.alpha and frame_params.bitstream
	on canvas with top-left corner at (frame_params.frameX,		on canvas with top-left corner at (frame_params.frameX,
	frame_params.frameY), using Blending method		frame_params.frameY), using Blending method
	frame_params.blendingMethod.		frame_params.blendingMethod.
	canvas contains the decoded image.		canvas contains the decoded image.
	Show the contents of the canvas for		Show the contents of the canvas for
	frame_params.frameDuration * 1 ms.		frame_params.frameDuration * 1 ms.
	dispose_method = frame_params.disposeMethod		dispose_method = frame_params.disposeMethod
	]]></sourcecode>		]]></sourcecode>
	</section>		</section>
	skipping to change at line 1010 ¶		skipping to change at line 986 ¶
	+- ANMF (frame4 parameters + data)		+- ANMF (frame4 parameters + data)
	+- EXIF (metadata)		+- EXIF (metadata)
	]]></artwork>		]]></artwork>
	</figure>		</figure>
	</section>		</section>
	</section>		</section>
	</section>		</section>
	<section anchor="webp-lossless" numbered="true" toc="default">		<section anchor="webp-lossless" numbered="true" toc="default">
	<name>Specification for WebP Lossless Bitstream</name>		<name>Specification for WebP Lossless Bitstream</name>
	<t>Note that this section is based on the documentation in the <xref		<aside><t>Note that this section is based on the documentation in the <xref
	target="webp-lossless-src">libwebp source repository</xref>.</t>		target="webp-lossless-src">libwebp source repository</xref>.</t></aside>
	<section numbered="true" toc="default">		<section numbered="true" toc="default">
	<name>Abstract (from "Specification for WebP Lossless Bitstream")</name>		<name>Abstract (from "Specification for WebP Lossless Bitstream")</name>
	<t>WebP lossless is an image format for lossless compression of ARGB		<t>WebP lossless is an image format for lossless compression of ARGB
	images. The lossless format stores and restores the pixel values		images. The lossless format stores and restores the pixel values
	exactly, including the color values for pixels whose alpha value is 0.		exactly, including the color values for pixels whose alpha value is 0.
	The format uses subresolution images, recursively embedded into the		The format uses subresolution images, recursively embedded into the
	format itself, for storing statistical data about the images, such as		format itself, for storing statistical data about the images, such as
	the used entropy codes, spatial predictors, color space conversion,		the used entropy codes, spatial predictors, color space conversion,
	and color table. A universal algorithm for sequential data compression <xref target="LZ77"/>, prefix coding, and a color cache are used for		and color table. A universal algorithm for sequential data compression <xref target="LZ77"/>, prefix coding, and a color cache are used for
	compression of the bulk data. Decoding speeds faster than PNG have		compression of the bulk data. Decoding speeds faster than PNG have
	skipping to change at line 1196 ¶		skipping to change at line 1172 ¶
	// Decode transform data.		// Decode transform data.
	...		...
	}		}
	// Decode actual image data.		// Decode actual image data.
	]]></sourcecode>		]]></sourcecode>
	<t>If a transform is present, then the next two bits specify the		<t>If a transform is present, then the next two bits specify the
	transform type. There are four types of transforms.</t>		transform type. There are four types of transforms.</t>
	<table align="left">		<table align="left">
			<name>Transform Types</name>
	<thead>		<thead>
	<tr>		<tr>
	<th>Transform</th>		<th>Transform</th>
	<th>Bit</th>		<th>Bit</th>
	</tr>		</tr>
	</thead>		</thead>
	<tbody>		<tbody>
	<tr>		<tr>
	<td>PREDICTOR_TRANSFORM</td>		<td>PREDICTOR_TRANSFORM</td>
	<td>0</td>		<td>0</td>
	skipping to change at line 1275 ¶		skipping to change at line 1252 ¶
	]]></sourcecode>		]]></sourcecode>
	<t>There are 14 different prediction modes. In each prediction mode,		<t>There are 14 different prediction modes. In each prediction mode,
	the current pixel value is predicted from one or more neighboring		the current pixel value is predicted from one or more neighboring
	pixels whose values are already known.</t>		pixels whose values are already known.</t>
	<t>We chose the neighboring pixels (TL, T, TR, and L) of the current		<t>We chose the neighboring pixels (TL, T, TR, and L) of the current
	pixel (P) as follows:</t>		pixel (P) as follows:</t>
	<figure>		<figure>
			<name>Neighboring Pixels of the Current Pixel (P)</name>
	<artwork name="" type="ascii-art" align="left" alt=""><![CDATA[		<artwork name="" type="ascii-art" align="left" alt=""><![CDATA[
	O O O O O O O O O O O		O O O O O O O O O O O
	O O O O O O O O O O O		O O O O O O O O O O O
	O O O O TL T TR O O O O		O O O O TL T TR O O O O
	O O O O L P X X X X X		O O O O L P X X X X X
	X X X X X X X X X X X		X X X X X X X X X X X
	X X X X X X X X X X X		X X X X X X X X X X X
	]]></artwork>		]]></artwork>
	</figure>		</figure>
	<t>where TL means top-left, T means top, TR means top-right, and L mean s left. At the		<t>where TL means top-left, T means top, TR means top-right, and L mean s left. At the
	time of predicting a value for P, all O, TL, T, TR, and L pixels have		time of predicting a value for P, all O, TL, T, TR, and L pixels have
	already been processed, and the P pixel and all X pixels are		already been processed, and the P pixel and all X pixels are
	unknown.</t>		unknown.</t>
	<t>Given the preceding neighboring pixels, the different prediction mod es		<t>Given the preceding neighboring pixels, the different prediction mod es
	are defined as follows.</t>		are defined as follows.</t>
	<table align="left">		<table align="left">
			<name>Prediction Modes</name>
	<thead>		<thead>
	<tr>		<tr>
	<th>Mode</th>		<th>Mode</th>
	<th>Predicted Value of Each Channel of the Current Pixel</th>		<th>Predicted Value of Each Channel of the Current Pixel</th>
	</tr>		</tr>
	</thead>		</thead>
	<tbody>		<tbody>
	<tr>		<tr>
	<td>0</td>		<td>0</td>
	<td>0xff000000 (represents solid black color in ARGB)</td>		<td>0xff000000 (represents solid black color in ARGB)</td>
	skipping to change at line 1415 ¶		skipping to change at line 1394 ¶
	int ClampAddSubtractFull(int a, int b, int c) {		int ClampAddSubtractFull(int a, int b, int c) {
	return Clamp(a + b - c);		return Clamp(a + b - c);
	}		}
	int ClampAddSubtractHalf(int a, int b) {		int ClampAddSubtractHalf(int a, int b) {
	return Clamp(a + (a - b) / 2);		return Clamp(a + (a - b) / 2);
	}		}
	]]></sourcecode>		]]></sourcecode>
	<t>There are special handling rules for some border pixels. If there		<t>There are special handling rules for some border pixels. If there
	is a prediction transform, regardless of the mode [0..13] for these		is a predictor transform, regardless of the mode [0..13] for these
	pixels, the predicted value for the left-topmost pixel of the image		pixels, the predicted value for the left-topmost pixel of the image
	is 0xff000000, all pixels on the top row are L-pixel, and		is 0xff000000, all pixels on the top row are L-pixel, and
	all pixels on the leftmost column are T-pixel.</t>		all pixels on the leftmost column are T-pixel.</t>
	<t>Addressing the TR-pixel for pixels on the rightmost column is		<t>Addressing the TR-pixel for pixels on the rightmost column is
	exceptional. The pixels on the rightmost column are predicted by		exceptional. The pixels on the rightmost column are predicted by
	using the modes [0..13], just like pixels not on the border, but the		using the modes [0..13], just like pixels not on the border, but the
	leftmost pixel on the same row as the current pixel is instead used		leftmost pixel on the same row as the current pixel is instead used
	as the TR-pixel.</t>		as the TR-pixel.</t>
	<t>The final pixel value is obtained by adding each channel of the		<t>The final pixel value is obtained by adding each channel of the
	skipping to change at line 1507 ¶		skipping to change at line 1486 ¶
	<t>A conversion from the 8-bit unsigned representation		<t>A conversion from the 8-bit unsigned representation
	(<tt>uint8</tt>) to the 8-bit signed one (<tt>int8</tt>) is required		(<tt>uint8</tt>) to the 8-bit signed one (<tt>int8</tt>) is required
	before calling <tt>ColorTransformDelta()</tt>.		before calling <tt>ColorTransformDelta()</tt>.
	The signed value should be interpreted as an 8-bit two's complement number (that is: uint8 range [128..255] is mapped		The signed value should be interpreted as an 8-bit two's complement number (that is: uint8 range [128..255] is mapped
	to the [-128..-1] range of its converted int8 value).</t>		to the [-128..-1] range of its converted int8 value).</t>
	<t>The multiplication is to be done using more precision (with at		<t>The multiplication is to be done using more precision (with at
	least 16-bit precision). The sign extension property of the shift		least 16-bit precision). The sign extension property of the shift
	operation does not matter here; only the lowest 8 bits are used from		operation does not matter here; only the lowest 8 bits are used from
	the result, and there the sign extension shifting and unsigned		the result, and in these bits, the sign extension shifting and
	shifting are consistent with each other.</t>		unsigned shifting are consistent with each other.</t>
	<t>Now, we describe the contents of color transform data so that		<t>Now, we describe the contents of color transform data so that
	decoding can apply the inverse color transform and recover the		decoding can apply the inverse color transform and recover the
	original red and blue values. The first 3 bits of the color		original red and blue values. The first 3 bits of the color
	transform data contain the width and height of the image block in		transform data contain the width and height of the image block in
	number of bits, just like the predictor transform:</t>		number of bits, just like the predictor transform:</t>
	<sourcecode type="c"><![CDATA[		<sourcecode type="c"><![CDATA[
	int size_bits = ReadBits(3) + 2;		int size_bits = ReadBits(3) + 2;
	int block_width = 1 << size_bits;		int block_width = 1 << size_bits;
	skipping to change at line 1643 ¶		skipping to change at line 1622 ¶
	packs several (2, 4, or 8) pixels into a single pixel, reducing the		packs several (2, 4, or 8) pixels into a single pixel, reducing the
	image width respectively.</t>		image width respectively.</t>
	<aside><t>Pixel bundling allows for a more efficient		<aside><t>Pixel bundling allows for a more efficient
	joint distribution entropy coding of neighboring pixels and gives		joint distribution entropy coding of neighboring pixels and gives
	some arithmetic coding-like benefits to the entropy code, but it can		some arithmetic coding-like benefits to the entropy code, but it can
	only be used when there are 16 or fewer unique values.</t></aside>		only be used when there are 16 or fewer unique values.</t></aside>
	<t><tt>color_table_size</tt> specifies how many pixels are		<t><tt>color_table_size</tt> specifies how many pixels are
	combined:</t>		combined:</t>
	<table align="left">		<table align="left">
			<name>Color Table Size to Bundled Pixel Bit Width Mapping</name>
	<thead>		<thead>
	<tr>		<tr>
	<th>color_table_size</th>		<th>color_table_size</th>
	<th>width_bits value</th>		<th>width_bits value</th>
	</tr>		</tr>
	</thead>		</thead>
	<tbody>		<tbody>
	<tr>		<tr>
	<td>1..2</td>		<td>1..2</td>
	<td>3</td>		<td>3</td>
	skipping to change at line 1723 ¶		skipping to change at line 1703 ¶
	<li>ARGB image: Stores the actual pixels of the image.</li>		<li>ARGB image: Stores the actual pixels of the image.</li>
	<li>Entropy image: Stores the meta prefix		<li>Entropy image: Stores the meta prefix
	codes (see <xref		codes (see <xref
	target="decoding-of-meta-prefix-codes">"Decoding of Meta Prefix Cod es"</xref>).</li>		target="decoding-of-meta-prefix-codes">"Decoding of Meta Prefix Cod es"</xref>).</li>
	<li>Predictor image: Stores the metadata for the predictor transform (see <xref		<li>Predictor image: Stores the metadata for the predictor transform (see <xref
	target="predictor-transform">"Predictor Transform"</xref>).</li>		target="predictor-transform">"Predictor Transform"</xref>).</li>
	<li>Color transform image: Created by		<li>Color transform image: Created by
	<tt>ColorTransformElement</tt> values (defined in <xref		<tt>ColorTransformElement</tt> values (defined in <xref
	target="color-transform">"Color Transform"</xref>) for different		target="color-transform">"Color Transform"</xref>) for different
	blocks of the image.</li>		blocks of the image.</li>
	<li>Color indexing image: An array of size		<li>Color indexing image: An array of the size of
	<tt>color_table_size</tt> (up to 256 ARGB values) storing the		<tt>color_table_size</tt> (up to 256 ARGB values) that stores the
	metadata for the color		metadata for the color
	indexing transform (see <xref target="color-indexing-transform" >"Color Indexing Transform"</xref>).</li>		indexing transform (see <xref target="color-indexing-transform" >"Color Indexing Transform"</xref>).</li>
	</ol>		</ol>
	</section>		</section>
	<section numbered="true" toc="default">		<section numbered="true" toc="default">
	<name>Encoding of Image Data</name>		<name>Encoding of Image Data</name>
	<t>The encoding of image data is independent of its role.</t>		<t>The encoding of image data is independent of its role.</t>
	<t>The image is first divided into a set of fixed-size blocks		<t>The image is first divided into a set of fixed-size blocks
	skipping to change at line 1799 ¶		skipping to change at line 1779 ¶
	stored as they are (without an entropy code).</t>		stored as they are (without an entropy code).</t>
	<aside><t>Rationale: This approach reduces the storage requirement		<aside><t>Rationale: This approach reduces the storage requirement
	for the entropy code. Also, large values are usually rare, so		for the entropy code. Also, large values are usually rare, so
	extra bits would be used for very few values in the image. Thus,		extra bits would be used for very few values in the image. Thus,
	this approach results in better compression overall.</t></aside>		this approach results in better compression overall.</t></aside>
	<t>The following table denotes the prefix codes and extra bits used		<t>The following table denotes the prefix codes and extra bits used
	for storing different ranges of values.</t>		for storing different ranges of values.</t>
	<t>Note: The maximum backward reference length is limited to 4096.		<aside><t>Note: The maximum backward reference length is limited to 4 096.
	Hence, only the first 24 prefix codes (with the respective extra		Hence, only the first 24 prefix codes (with the respective extra
	bits) are meaningful for length values. For distance values,		bits) are meaningful for length values. For distance values,
	however, all the 40 prefix codes are valid.</t>		however, all the 40 prefix codes are valid.</t></aside>
	<table align="left">		<table align="left">
			<name>Value to Prefix Code and Extra Bits Mapping</name>
	<thead>		<thead>
	<tr>		<tr>
	<th>Value Range</th>		<th>Value Range</th>
	<th>Prefix Code</th>		<th>Prefix Code</th>
	<th>Extra Bits</th>		<th>Extra Bits</th>
	</tr>		</tr>
	</thead>		</thead>
	<tbody>		<tbody>
	<tr>		<tr>
	<td>1</td>		<td>1</td>
	skipping to change at line 1920 ¶		skipping to change at line 1901 ¶
	112</tt>].</li>		112</tt>].</li>
	<li>Pixels that are in the same row as the current pixel and are up		<li>Pixels that are in the same row as the current pixel and are up
	to 8 columns to the left of the current pixel [<tt>8</tt> such		to 8 columns to the left of the current pixel [<tt>8</tt> such
	pixels].</li>		pixels].</li>
	</ul>		</ul>
	<t>The mapping between distance code <tt>distance_code</tt> and the n eighboring		<t>The mapping between distance code <tt>distance_code</tt> and the n eighboring
	pixel offset <tt>(xi, yi)</tt> is as follows:</t>		pixel offset <tt>(xi, yi)</tt> is as follows:</t>
	<figure>		<figure>
			<name>Distance Code to Neighboring Pixel Offset Mapping</name>
	<artwork name="" type="ascii-art" align="left" alt=""><![CDATA[		<artwork name="" type="ascii-art" align="left" alt=""><![CDATA[
	(0, 1), (1, 0), (1, 1), (-1, 1), (0, 2), (2, 0), (1, 2),		(0, 1), (1, 0), (1, 1), (-1, 1), (0, 2), (2, 0), (1, 2),
	(-1, 2), (2, 1), (-2, 1), (2, 2), (-2, 2), (0, 3), (3, 0),		(-1, 2), (2, 1), (-2, 1), (2, 2), (-2, 2), (0, 3), (3, 0),
	(1, 3), (-1, 3), (3, 1), (-3, 1), (2, 3), (-2, 3), (3, 2),		(1, 3), (-1, 3), (3, 1), (-3, 1), (2, 3), (-2, 3), (3, 2),
	(-3, 2), (0, 4), (4, 0), (1, 4), (-1, 4), (4, 1), (-4, 1),		(-3, 2), (0, 4), (4, 0), (1, 4), (-1, 4), (4, 1), (-4, 1),
	(3, 3), (-3, 3), (2, 4), (-2, 4), (4, 2), (-4, 2), (0, 5),		(3, 3), (-3, 3), (2, 4), (-2, 4), (4, 2), (-4, 2), (0, 5),
	(3, 4), (-3, 4), (4, 3), (-4, 3), (5, 0), (1, 5), (-1, 5),		(3, 4), (-3, 4), (4, 3), (-4, 3), (5, 0), (1, 5), (-1, 5),
	(5, 1), (-5, 1), (2, 5), (-2, 5), (5, 2), (-5, 2), (4, 4),		(5, 1), (-5, 1), (2, 5), (-2, 5), (5, 2), (-5, 2), (4, 4),
	(-4, 4), (3, 5), (-3, 5), (5, 3), (-5, 3), (0, 6), (6, 0),		(-4, 4), (3, 5), (-3, 5), (5, 3), (-5, 3), (0, 6), (6, 0),
	(1, 6), (-1, 6), (6, 1), (-6, 1), (2, 6), (-2, 6), (6, 2),		(1, 6), (-1, 6), (6, 1), (-6, 1), (2, 6), (-2, 6), (6, 2),
	skipping to change at line 2112 ¶		skipping to change at line 2094 ¶
	code_lengths[symbol0] = 1;		code_lengths[symbol0] = 1;
	if (num_symbols == 2) {		if (num_symbols == 2) {
	symbol1 = ReadBits(8);		symbol1 = ReadBits(8);
	code_lengths[symbol1] = 1;		code_lengths[symbol1] = 1;
	}		}
	]]></sourcecode>		]]></sourcecode>
	<aside><t>The two symbols should be different. Duplicate symbols are		<aside><t>The two symbols should be different. Duplicate symbols are
	allowed, but inefficient.</t></aside>		allowed, but inefficient.</t></aside>
	<t>Note: Another special case is when <em>all</em> prefix code		<aside><t>Note: Another special case is when <em>all</em> prefix code
	lengths are <em>zeros</em> (an empty prefix code). For example, a		lengths are <em>zeros</em> (an empty prefix code). For example, a
	prefix code for distance can be empty if there are no backward		prefix code for distance can be empty if there are no backward
	references. Similarly, prefix codes for alpha, red, and blue can		references. Similarly, prefix codes for alpha, red, and blue can
	be empty if all pixels within the same meta prefix code are		be empty if all pixels within the same meta prefix code are
	produced using the color cache. However, this case doesn't need		produced using the color cache. However, this case doesn't need
	special handling, as empty prefix codes can be coded as those		special handling, as empty prefix codes can be coded as those
	containing a single symbol <tt>0</tt>.</t>		containing a single symbol <tt>0</tt>.</t></aside>
	</section>		</section>
	<section anchor="normal-code-length">		<section anchor="normal-code-length">
	<name>Normal Code Length Code</name>		<name>Normal Code Length Code</name>
	<t>The code lengths of the prefix code fit in 8 bits and are read		<t>The code lengths of the prefix code fit in 8 bits and are read
	as follows. First, <tt>num_code_lengths</tt> specifies the number		as follows. First, <tt>num_code_lengths</tt> specifies the number
	of code lengths.</t>		of code lengths.</t>
	<sourcecode type="c"><![CDATA[		<sourcecode type="c"><![CDATA[
	int num_code_lengths = 4 + ReadBits(4);		int num_code_lengths = 4 + ReadBits(4);
	skipping to change at line 2295 ¶		skipping to change at line 2277 ¶
	</section>		</section>
	<section anchor="decoding-entropy-coded-image-data" numbered="true"		<section anchor="decoding-entropy-coded-image-data" numbered="true"
	toc="default">		toc="default">
	<name>Decoding Entropy-Coded Image Data</name>		<name>Decoding Entropy-Coded Image Data</name>
	<t>For the current position (x, y) in the image, the decoder first		<t>For the current position (x, y) in the image, the decoder first
	identifies the corresponding prefix code group (as explained in		identifies the corresponding prefix code group (as explained in
	the last section). Given the prefix code group, the pixel is read		the last section). Given the prefix code group, the pixel is read
	and decoded as follows.</t>		and decoded as follows.</t>
	<t>Next, read symbol S from the bitstream using prefix code #1.		<t>Next, read symbol S from the bitstream using prefix code #1.</t>
	Note that S is any integer in the range <tt>0</tt> to		<aside><t>Note that S is any integer in the range <tt>0</tt> to
	<tt>(256 + 24 + </tt> <xref		<tt>(256 + 24 + color_cache_size - 1)</tt>. See <xref
	target="color-cache-coding"><tt>color_cache_size</tt></xref><tt> -		target="color-cache-coding"/> for details about
	1)</tt>.</t>		<tt>color_cache_size</tt>.</t></aside>
	<t>The interpretation of S depends on its value:</t>		<t>The interpretation of S depends on its value:</t>
	<ol spacing="normal" type="1">		<ol spacing="normal" type="1">
	<li><t>If S &lt; 256</t>		<li><t>If S &lt; 256</t>
	<ol spacing="normal" type="i">		<ol spacing="normal" type="i">
	<li>Use S as the green component.</li>		<li>Use S as the green component.</li>
	<li>Read red from the bitstream using prefix code #2.</li>		<li>Read red from the bitstream using prefix code #2.</li>
	<li>Read blue from the bitstream using prefix code #3.</li>		<li>Read blue from the bitstream using prefix code #3.</li>
	<li>Read alpha from the bitstream using prefix code #4.</li>		<li>Read alpha from the bitstream using prefix code #4.</li>
	</ol>		</ol>
	skipping to change at line 2343 ¶		skipping to change at line 2325 ¶
	</section>		</section>
	</section>		</section>
	</section>		</section>
	<section numbered="true" toc="default">		<section numbered="true" toc="default">
	<name>Overall Structure of the Format</name>		<name>Overall Structure of the Format</name>
	<t>Below is a view into the format in Augmented Backus-Naur Form <xref		<t>Below is a view into the format in Augmented Backus-Naur Form <xref
	target="RFC5234"/> <xref target="RFC7405"/>. It does not cover all deta ils. The end-of-image (EOI)		target="RFC5234"/> <xref target="RFC7405"/>. It does not cover all deta ils. The end-of-image (EOI)
	is only implicitly coded into the number of pixels (image_width *		is only implicitly coded into the number of pixels (image_width *
	image_height).</t>		image_height).</t>
	<t>Note that <tt>*element</tt> means <tt>element</tt> can be repeated 0 o		<aside><t>Note that <tt>*element</tt> means <tt>element</tt> can be repea
	r more times. <tt>5element</tt>		ted 0 or more times. <tt>5element</tt>
	means <tt>element</tt> is repeated exactly 5 times. <tt>%b</tt> represents a bin		means <tt>element</tt> is repeated exactly 5 times. <tt>%b</tt> represents a bin
	ary value.</t>		ary value.</t></aside>
	<section numbered="true" toc="default">		<section numbered="true" toc="default">
	<name>Basic Structure</name>		<name>Basic Structure</name>
	<sourcecode type="abnf"><![CDATA[		<sourcecode type="abnf"><![CDATA[
	format = RIFF-header image-header image-stream		format = RIFF-header image-header image-stream
	RIFF-header = %s"RIFF" 4OCTET %s"WEBPVP8L" 4OCTET		RIFF-header = %s"RIFF" 4OCTET %s"WEBPVP8L" 4OCTET
	image-header = %x2F image-size alpha-is-used version		image-header = %x2F image-size alpha-is-used version
	image-size = 14BIT 14BIT ; width - 1, height - 1		image-size = 14BIT 14BIT ; width - 1, height - 1
	alpha-is-used = 1BIT		alpha-is-used = 1BIT
	skipping to change at line 2414 ¶		skipping to change at line 2396 ¶
	prefix-code = simple-prefix-code / normal-prefix-code		prefix-code = simple-prefix-code / normal-prefix-code
	simple-prefix-code = ; see "Simple Code Length Code" for details		simple-prefix-code = ; see "Simple Code Length Code" for details
	normal-prefix-code = ; see "Normal Code Length Code" for details		normal-prefix-code = ; see "Normal Code Length Code" for details
	lz77-coded-image =		lz77-coded-image =
	*((argb-pixel / lz77-copy / color-cache-code) lz77-coded-image)		*((argb-pixel / lz77-copy / color-cache-code) lz77-coded-image)
	]]></sourcecode>		]]></sourcecode>
	<t>The following is a possible example sequence:</t>		<t>The following is a possible example sequence:</t>
	<sourcecode><![CDATA[		<sourcecode type="abnf"><![CDATA[
	RIFF-header image-size %b1 subtract-green-tx		RIFF-header image-size %b1 subtract-green-tx
	%b1 predictor-tx %b0 color-cache-info		%b1 predictor-tx %b0 color-cache-info
	%b0 prefix-codes lz77-coded-image		%b0 prefix-codes lz77-coded-image
	]]></sourcecode>		]]></sourcecode>
	</section>		</section>
	</section>		</section>
	</section>		</section>
	<section anchor="Security" numbered="true" toc="default">		<section anchor="Security" numbered="true" toc="default">
	<name>Security Considerations</name>		<name>Security Considerations</name>
	skipping to change at line 2437 ¶		skipping to change at line 2419 ¶
	uninitialized data usage, null pointer dereferences, resource (disk or		uninitialized data usage, null pointer dereferences, resource (disk or
	memory) exhaustion, and extended resource usage (long running time) as		memory) exhaustion, and extended resource usage (long running time) as
	part of the demuxing and decoding process. In particular,		part of the demuxing and decoding process. In particular,
	implementations reading this format are likely to take input from		implementations reading this format are likely to take input from
	unknown and possibly unsafe sources -- both clients (for example, web bro wsers or		unknown and possibly unsafe sources -- both clients (for example, web bro wsers or
	email clients) and servers (for example, applications that accept uploade d		email clients) and servers (for example, applications that accept uploade d
	images). These may result in arbitrary code execution, information		images). These may result in arbitrary code execution, information
	leakage (memory layout and contents), or crashes and thereby allow a		leakage (memory layout and contents), or crashes and thereby allow a
	device to be compromised or cause a denial of service to an application		device to be compromised or cause a denial of service to an application
	using the format <xref target="cve.mitre.org-libwebp"/> <xref		using the format <xref target="cve.mitre.org-libwebp"/> <xref
	target="crbug-security"/>.</t>		target="issues-security"/>.</t>
	<t>The format does not employ "active content" but does allow metadata		<t>The format does not employ "active content" but does allow metadata
	(for example, <xref target="XMP"/> and <xref target="Exif"/>) and custom ch unks to be		(for example, <xref target="XMP"/> and <xref target="Exif"/>) and custom ch unks to be
	embedded in a file. Applications that interpret these chunks may be		embedded in a file. Applications that interpret these chunks may be
	subject to security considerations for those formats.</t>		subject to security considerations for those formats.</t>
	</section>		</section>
	<section anchor="Interop" numbered="true" toc="default">		<section anchor="Interop" numbered="true" toc="default">
	<name>Interoperability Considerations</name>		<name>Interoperability Considerations</name>
	<t>The format is defined using little-endian byte ordering (see <xref		<t>The format is defined using little-endian byte ordering (see <xref
	target="RFC2781" section="3.1"/>), but demuxing and decoding are		target="RFC2781" section="3.1"/>), but demuxing and decoding are
	possible on platforms using a different ordering with the appropriate		possible on platforms using a different ordering with the appropriate
	conversion.		conversion.
	The container is based on RIFF and allows extension via user-defined		The container is based on RIFF and allows extension via user-defined
	chunks, but nothing beyond the chunks defined by the container format		chunks, but nothing beyond the chunks defined by the container format
	(<xref target="webp-container"/>) are required for decoding of the image.		(<xref target="webp-container"/>) are required for decoding of the image.
	These have been finalized but were extended in the format's early		These have been finalized, but they were extended in the format's early
	stages, so some older readers may not support lossless or animated image		stages, so some older readers may not support lossless or animated image
	decoding.</t>		decoding.</t>
	</section>		</section>
	<section anchor="IANA" numbered="true" toc="default">		<section anchor="IANA" numbered="true" toc="default">
	<name>IANA Considerations</name>		<name>IANA Considerations</name>
	<t>IANA has registered the 'image/webp' media type <xref		<t>IANA has registered the 'image/webp' media type <xref
	target="RFC2046"/>.</t>		target="RFC2046"/>.</t>
	<section anchor="webp-media-type" numbered="true" toc="default">		<section anchor="webp-media-type" numbered="true" toc="default">
	<name>The 'image/webp' Media Type</name>		<name>The 'image/webp' Media Type</name>
	<t>This section contains the media type registration details per <xref		<t>This section contains the media type registration details per <xref
	target="RFC6838"/>.</t>		target="RFC6838"/>.</t>
	<section numbered="true" toc="default">		<section numbered="true" toc="default">
	<name>Registration Details</name>		<name>Registration Details</name>
	<!-- RFC Editor Note: Remove this text element after updating. -->
	<t><strong>RFC Editor Note:</strong> Replace RFC XXXX / rfcXXXX with
	the published RFC number.</t>
	<dl newline="false" spacing="normal">		<dl newline="false" spacing="normal">
	<dt>Type name:</dt> <dd>image</dd>		<dt>Type name:</dt> <dd>image</dd>
	<dt>Subtype name:</dt> <dd>webp</dd>		<dt>Subtype name:</dt> <dd>webp</dd>
	<dt>Required parameters:</dt> <dd>N/A</dd>		<dt>Required parameters:</dt> <dd>N/A</dd>
	<dt>Optional parameters:</dt> <dd>N/A</dd>		<dt>Optional parameters:</dt> <dd>N/A</dd>
	<dt>Encoding considerations:</dt> <dd>Binary. The <xref target="RFC46 48">Base64		<dt>Encoding considerations:</dt> <dd>Binary. The <xref target="RFC46 48">Base64
	encoding</xref> should be used on transports that cannot accommodate		encoding</xref> should be used on transports that cannot accommodate
	binary data directly.</dd>		binary data directly.</dd>
	<dt>Security considerations:</dt> <dd>See RFC XXXX, <xref target="Sec		<dt>Security considerations:</dt> <dd>See RFC 9649, <xref target="Sec
	urity"/>.</dd>		urity"/>.</dd>
	<dt>Interoperability considerations:</dt> <dd>See RFC XXXX, <xref tar		<dt>Interoperability considerations:</dt> <dd>See RFC 9649, <xref tar
	get="Interop"/>.</dd>		get="Interop"/>.</dd>
	<dt>Published specification:</dt> <dd>RFC XXXX</dd>		<dt>Published specification:</dt> <dd>RFC 9649</dd>
	<dt>Applications that use this media type:</dt> <dd>Applications that are used to		<dt>Applications that use this media type:</dt> <dd>Applications that are used to
	display and process images, especially when smaller image file sizes		display and process images, especially when smaller image file sizes
	are important.</dd>		are important.</dd>
	<dt>Fragment identifier considerations:</dt> <dd>N/A</dd>		<dt>Fragment identifier considerations:</dt> <dd>N/A</dd>
	<dt>Additional information:</dt>		<dt>Additional information:</dt>
	<dd><t><br/></t>		<dd><t><br/></t>
	<dl spacing="compact">		<dl spacing="compact">
	<dt>Deprecated alias names for this type:</dt> <dd>N/A</dd>		<dt>Deprecated alias names for this type:</dt> <dd>N/A</dd>
	<dt>Magic number(s):</dt> <dd>The first 4 bytes are 0x52, 0x49, 0x46, 0x46		<dt>Magic number(s):</dt> <dd>The first 4 bytes are 0x52, 0x49, 0x46, 0x46
	skipping to change at line 2506 ¶		skipping to change at line 2485 ¶
	('WEBPVP8').</dd>		('WEBPVP8').</dd>
	<dt>File extension(s):</dt> <dd>webp</dd>		<dt>File extension(s):</dt> <dd>webp</dd>
	<dt>Apple Uniform Type Identifier:</dt> <dd>org.webmproject.webp conf orms to		<dt>Apple Uniform Type Identifier:</dt> <dd>org.webmproject.webp conf orms to
	public.image</dd>		public.image</dd>
	<dt>Object Identifiers:</dt> <dd>N/A</dd>		<dt>Object Identifiers:</dt> <dd>N/A</dd>
	</dl></dd>		</dl></dd>
	<dt>Person &amp; email address to contact for further information:</dt>		<dt>Person &amp; email address to contact for further information:</dt>
	<dd>James Zern &lt;jzern@google.com&gt;</dd></dl>		<dd>James Zern &lt;jzern@google.com&gt;</dd></dl>
	<dl newline="false" spacing="normal">		<dl newline="false" spacing="normal">
			<dt>Intended usage:</dt> <dd>COMMON</dd>
	<dt>Restrictions on usage:</dt> <dd>N/A</dd>		<dt>Restrictions on usage:</dt> <dd>N/A</dd>
	<dt>Author:</dt>		<dt>Author:</dt>
	<dd>James Zern &lt;jzern@google.com&gt;</dd>		<dd>James Zern &lt;jzern@google.com&gt;</dd>
	</dl>		<dt>Change controller:</dt><dd>IETF</dd>
	<dl newline="false" spacing="compact">		<dt>Provisional registration?:</dt><dd>No</dd>
	<dt>Change controller:</dt><dd></dd>
	<dt></dt><dd>IETF</dd>
	</dl>
	<dl newline="false" spacing="normal">
	<dt>Intended usage:</dt> <dd>COMMON</dd>
	</dl>		</dl>
	</section>		</section>
	</section>		</section>
	</section>		</section>
	</middle>		</middle>
	<back>		<back>
	<references>		<references>
	<name>References</name>		<name>References</name>
	<references>		<references>
	<name>Normative References</name>		<name>Normative References</name>
	skipping to change at line 2612 ¶		skipping to change at line 2586 ¶
	<author>		<author>
	<organization>Adobe Inc.</organization>		<organization>Adobe Inc.</organization>
	</author>		</author>
	</front>		</front>
	</reference>		</reference>
	</references>		</references>
	<references>		<references>
	<name>Informative References</name>		<name>Informative References</name>
	<reference anchor="crbug-security" target="https://bugs.chromium.
	org/p/webp/issues/list?q=label%3ASecurity">
	<front>
	<title>libwebp Security Issues</title>
	<author>
	<organization/>
	</author>
	</front>
	</reference>
	<reference anchor="cve.mitre.org-libwebp"		<reference anchor="cve.mitre.org-libwebp"
	target="https://cve.mitre.org/cgi-bin/cvekey.cgi?keyword=libwebp">		target="https://cve.mitre.org/cgi-bin/cvekey.cgi?keyword=libwebp">
	<front>		<front>
	<title>libwebp CVE List</title>		<title>libwebp CVE List</title>
	<author>		<author>
	<organization/>		<organization/>
	</author>		</author>
	</front>		</front>
	</reference>		</reference>
	skipping to change at line 2656 ¶		skipping to change at line 2621 ¶
	Codes</title>		Codes</title>
	<author initials="D." surname="Huffman">		<author initials="D." surname="Huffman">
	<organization/>		<organization/>
	</author>		</author>
	<date month="September" year="1952" />		<date month="September" year="1952" />
	</front>		</front>
	<seriesInfo name="DOI" value="10.1109/JRPROC.1952.273898"/>		<seriesInfo name="DOI" value="10.1109/JRPROC.1952.273898"/>
	<refcontent>Proceedings of the Institute of Radio Engineers, Vol. 40, Issue 9 , pp. 1098-1101</refcontent>		<refcontent>Proceedings of the Institute of Radio Engineers, Vol. 40, Issue 9 , pp. 1098-1101</refcontent>
	</reference>		</reference>
			<reference anchor="issues-security" target="https://issues.webmpr
			oject.org/issues?q=componentid:1618983%2B%20(%22Restrict-View-Security%22%20OR%2
			0type:vulnerability)">
			<front>
			<title>libwebp Security Issues</title>
			<author>
			<organization/>
			</author>
			</front>
			</reference>
	<reference anchor="JPEG-spec"		<reference anchor="JPEG-spec"
	target="https://www.w3.org/Graphics/JPEG/itu-t81.pdf">		target="https://www.w3.org/Graphics/JPEG/itu-t81.pdf">
	<front>		<front>
	<title>Information Technology - Digital Compression and Coding of Con tinuous-Tone Still Images - Requirements and Guidelines</title>		<title>Information Technology - Digital Compression and Coding of Con tinuous-Tone Still Images - Requirements and Guidelines</title>
	<author>		<author>
	<organization/>		<organization/>
	</author>		</author>
	<date month="September" year="1992"/>		<date month="September" year="1992"/>
	</front>		</front>
	<seriesInfo name="ITU-T Recommendation" value="T.81"/>		<seriesInfo name="ITU-T Recommendation" value="T.81"/>
	skipping to change at line 2708 ¶		skipping to change at line 2682 ¶
	target="https://www.loc.gov/preservation/digital/formats/fdd/fdd000025.shtml">		target="https://www.loc.gov/preservation/digital/formats/fdd/fdd000025.shtml">
	<front>		<front>
	<title>RIFF (Resource Interchange File Format)</title>		<title>RIFF (Resource Interchange File Format)</title>
	<author>		<author>
	<organization/>		<organization/>
	</author>		</author>
	</front>		</front>
	</reference>		</reference>
	<reference anchor="webp-lossless-src"		<reference anchor="webp-lossless-src"
	target="https://chromium.googlesource.com/webm/libwebp/+/refs/tags/webp -rfcXXXX/doc/webp-lossless-bitstream-spec.txt">		target="https://chromium.googlesource.com/webm/libwebp/+/refs/tags/webp -rfc9649/doc/webp-lossless-bitstream-spec.txt">
	<front>		<front>
	<title>WebP Lossless Bitstream Specification</title>		<title>WebP Lossless Bitstream Specification</title>
	<author initials="J." surname="Alakuijala"		<author initials="J." surname="Alakuijala"
	fullname="Jyrki Alakuijala">		fullname="Jyrki Alakuijala">
	<organization>Google LLC</organization>		<organization>Google LLC</organization>
	</author>		</author>
	<date month="October" year="2023" />		<date month="July" year="2024" />
	</front>		</front>
	</reference>		</reference>
	<reference anchor="webp-lossless-study" target="https://developer s.google.com/speed/webp/docs/webp_lossless_alpha_study">		<reference anchor="webp-lossless-study" target="https://developer s.google.com/speed/webp/docs/webp_lossless_alpha_study">
	<front>		<front>
	<title>Lossless and Transparency Encoding in WebP</title>		<title>Lossless and Transparency Encoding in WebP</title>
	<author initials="J." surname="Alakuijala"		<author initials="J." surname="Alakuijala"
	fullname="Jyrki Alakuijala">		fullname="Jyrki Alakuijala">
	<organization>Google LLC</organization>		<organization>Google LLC</organization>
	</author>		</author>
	<author initials="V." surname="Rabaud"		<author initials="V." surname="Rabaud"
	fullname="Vincent Rabaud">		fullname="Vincent Rabaud">
	<organization>Google LLC</organization>		<organization>Google LLC</organization>
	</author>		</author>
	<date month="August" year="2017" />		<date month="August" year="2017" />
	</front>		</front>
	</reference>		</reference>
	<reference anchor="webp-riff-src" target="https://chromium.googlesour ce.com/webm/libwebp/+/refs/tags/webp-rfcXXXX/doc/webp-container-spec.txt">		<reference anchor="webp-riff-src" target="https://chromium.googlesour ce.com/webm/libwebp/+/refs/tags/webp-rfc9649/doc/webp-container-spec.txt">
	<front>		<front>
	<title>WebP RIFF Container</title>		<title>WebP RIFF Container</title>
	<author>		<author>
	<organization>Google LLC</organization>		<organization>Google LLC</organization>
	</author>		</author>
	<date month="April" year="2024" />		<date month="July" year="2024" />
	</front>		</front>
	</reference>		</reference>
	</references>		</references>
	</references>		</references>
	</back>		</back>
	</rfc>		</rfc>
End of changes. 66 change blocks.
	125 lines changed or deleted		104 lines changed or added
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