rfc9649v2.txt		rfc9649.txt
	Internet Engineering Task Force (IETF) J. Zern		Internet Engineering Task Force (IETF) J. Zern
	Request for Comments: 9649 P. Massimino		Request for Comments: 9649 P. Massimino
	Category: Informational J. Alakuijala		Category: Informational J. Alakuijala
	ISSN: 2070-1721 Google LLC		ISSN: 2070-1721 Google LLC
	September 2024		October 2024
	WebP Image Format		WebP Image Format
	Abstract		Abstract
	This document defines the WebP image format and registers a media		This document defines the WebP image format and registers a media
	type supporting its use.		type supporting its use.
	Status of This Memo		Status of This Memo
	skipping to change at line 493 ¶		skipping to change at line 493 ¶
	For an animated image, this chunk contains the _global parameters_ of		For an animated image, this chunk contains the _global parameters_ of
	the animation.		the animation.
	Background Color: 32 bits (_uint32_)		Background Color: 32 bits (_uint32_)
	The default background color of the canvas in [Blue, Green, Red,		The default background color of the canvas in [Blue, Green, Red,
	Alpha] byte order. This color MAY be used to fill the unused		Alpha] byte order. This color MAY be used to fill the unused
	space on the canvas around the frames, as well as the transparent		space on the canvas around the frames, as well as the transparent
	pixels of the first frame. The background color is also used		pixels of the first frame. The background color is also used
	when the Disposal method is 1.		when the Disposal method is 1.
	Note:		Notes:
	* The background color MAY contain a nonopaque alpha value, even		* The background color MAY contain a nonopaque alpha value, even
	if the _Alpha_ flag in the 'VP8X' Chunk (Figure 7) is unset.		if the _Alpha_ flag in the 'VP8X' Chunk (Figure 7) is unset.
	* Viewer applications SHOULD treat the background color value as		* Viewer applications SHOULD treat the background color value as
	a hint and are not required to use it.		a hint and are not required to use it.
	* The canvas is cleared at the start of each loop. The		* The canvas is cleared at the start of each loop. The
	background color MAY be used to achieve this.		background color MAY be used to achieve this.
	skipping to change at line 742 ¶		skipping to change at line 742 ¶
	* Raw data: This consists of a byte sequence of length = width *		* 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.		order.
	* Lossless format compression: The byte sequence is a compressed		* Lossless format compression: The byte sequence is a compressed
	image-stream (as described in Section 3) of implicit dimensions		image-stream (as described in Section 3) of implicit dimensions
	width x height. That is, this image-stream does NOT contain any		width x height. That is, this image-stream does NOT contain any
	headers describing the image dimensions.		headers describing the image dimensions.
	Rationale: The dimensions are already known from other sources, so		| Rationale: The dimensions are already known from other sources,
	storing them again would be redundant and prone to errors.		| so storing them again would be redundant and prone to errors.
	Once the image-stream is decoded into Alpha, Red, Green, Blue		Once the image-stream is decoded into Alpha, Red, Green, Blue
	(ARGB) color values, following the process described in the		(ARGB) color values, following the process described in the
	lossless format specification, the transparency information must		lossless format specification, the transparency information must
	be extracted from the green channel of the ARGB quadruplet.		be extracted from the green channel of the ARGB quadruplet.
	Rationale: The green channel is allowed extra transformation steps		| Rationale: The green channel is allowed extra transformation
	in the specification -- unlike the other channels -- that can		| steps in the specification -- unlike the other channels -- that
	improve compression.		| can improve compression.
	2.7.1.3. Bitstream (VP8/VP8L)		2.7.1.3. Bitstream (VP8/VP8L)
	This chunk contains compressed bitstream data for a single frame.		This chunk contains compressed bitstream data for a single frame.
	A bitstream chunk may be either (i) a 'VP8 ' Chunk, using 'VP8 '		A bitstream chunk may be either (i) a 'VP8 ' Chunk, using 'VP8 '
	(note the significant fourth-character space) as its FourCC, _or_		(note the significant fourth-character space) as its FourCC, _or_
	(ii) a 'VP8L' Chunk, using 'VP8L' as its FourCC.		(ii) a 'VP8L' Chunk, using 'VP8L' as its FourCC.
	The formats of' VP8 ' and 'VP8L' Chunks are as described in Sections		The formats of' VP8 ' and 'VP8L' Chunks are as described in Sections
	skipping to change at line 2193 ¶		skipping to change at line 2193 ¶
	security considerations for those formats.		security considerations for those formats.
	5. Interoperability Considerations		5. Interoperability Considerations
	The format is defined using little-endian byte ordering (see		The format is defined using little-endian byte ordering (see
	Section 3.1 of [RFC2781]), but demuxing and decoding are possible on		Section 3.1 of [RFC2781]), but demuxing and decoding are possible on
	platforms using a different ordering with the appropriate conversion.		platforms using a different ordering with the appropriate 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
	(Section 2) are required for decoding of the image. These have been		(Section 2) are required for decoding of the image. These have been
	finalized but were extended in the format's early stages, so some		finalized, but they were extended in the format's early stages, so
	older readers may not support lossless or animated image decoding.		some older readers may not support lossless or animated image
			decoding.
	6. IANA Considerations		6. IANA Considerations
	IANA has registered the 'image/webp' media type [RFC2046].		IANA has registered the 'image/webp' media type [RFC2046].
	6.1. The 'image/webp' Media Type		6.1. The 'image/webp' Media Type
	This section contains the media type registration details per		This section contains the media type registration details per
	[RFC6838].		[RFC6838].
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