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Merge changes I7a25b1a6,I51b2c2a0,I87d0cbcf,I6ec60af6,I0a3fe9dc into main
* changes: webp-container-spec.txt: clarify some SHOULDs webp-container-spec.txt: move ChunkHeader to terminology webp-container-spec.txt: clarify 'VP8 '/'XMP ' fourccs webp-container-spec.txt: rightsize table entries webp-container-spec.txt: update 'key words' text
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James Zern
Gerrit Code Review
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@ -46,9 +46,10 @@ for:
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* **Animation.** An image may have multiple frames with pauses between them,
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making it an animation.
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The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
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"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
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document are to be interpreted as described in [RFC 2119][].
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The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD",
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"SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this
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document are to be interpreted as described in BCP 14 [RFC 2119][] [RFC 8174][]
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when, and only when, they appear in all capitals, as shown here.
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Bit numbering in chunk diagrams starts at `0` for the most significant bit
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('MSB 0') as described in [RFC 1166][].
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@ -83,13 +84,20 @@ _uint32_
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_FourCC_
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: A _FourCC_ (four-character code) is a _uint32_ created by concatenating four
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ASCII characters in little-endian order.
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ASCII characters in little-endian order. This means 'aaaa' (%x61.61.61.61) and
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'AAAA' (%x41.41.41.41) are treated as different _FourCCs_.
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_1-based_
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: An unsigned integer field storing values offset by `-1`. e.g., Such a field
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would store value _25_ as _24_.
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_ChunkHeader('ABCD')_
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: This is used to describe the _FourCC_ and _Chunk Size_ header of individual
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chunks, where 'ABCD' is the FourCC for the chunk. This element's size is 8
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bytes.
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RIFF File Format
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----------------
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@ -106,7 +114,7 @@ The basic element of a RIFF file is a _chunk_. It consists of:
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+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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| Chunk Size |
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+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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| Chunk Payload |
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: Chunk Payload :
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+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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Chunk FourCC: 32 bits
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@ -121,13 +129,8 @@ Chunk Size: 32 bits (_uint32_)
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Chunk Payload: _Chunk Size_ bytes
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: The data payload. If _Chunk Size_ is odd, a single padding byte -- that
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SHOULD be `0` -- is added.
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_ChunkHeader('ABCD')_
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: This is used to describe the _FourCC_ and _Chunk Size_ header of individual
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chunks, where 'ABCD' is the FourCC for the chunk. This element's
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size is 8 bytes.
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SHOULD be `0` to conform with RIFF -- is added. Applications MAY use
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another value, but readers may fail to parse the file.
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**Note:** RIFF has a convention that all-uppercase chunk FourCCs are standard
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chunks that apply to any RIFF file format, while FourCCs specific to a file
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@ -163,9 +166,10 @@ File Size: 32 bits (_uint32_)
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A WebP file MUST begin with a RIFF header with the FourCC 'WEBP'. The file size
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in the header is the total size of the chunks that follow plus `4` bytes for
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the 'WEBP' FourCC. The file SHOULD NOT contain anything after it. As the size
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of any chunk is even, the size given by the RIFF header is also even. The
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contents of individual chunks will be described in the following sections.
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the 'WEBP' FourCC. The file SHOULD NOT contain anything after it. Readers MAY
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parse such files, ignoring the trailing data. As the size of any chunk is even,
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the size given by the RIFF header is also even. The contents of individual
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chunks will be described in the following sections.
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Simple File Format (Lossy)
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@ -180,9 +184,11 @@ Simple WebP (lossy) file format:
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0 1 2 3
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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
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+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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| |
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| WebP file header (12 bytes) |
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| |
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+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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| VP8 chunk |
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: VP8 chunk :
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+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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VP8 chunk:
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@ -191,20 +197,24 @@ VP8 chunk:
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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
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+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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| ChunkHeader('VP8 ') |
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| |
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+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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| VP8 data |
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: VP8 data :
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+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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VP8 data: _Chunk Size_ bytes
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: VP8 bitstream data.
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Note the fourth character in the 'VP8 ' fourcc is an ASCII space (%x20).
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The VP8 bitstream format specification can be found at [VP8 Data Format and
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Decoding Guide][vp8spec]. Note that the VP8 frame header contains the VP8 frame
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width and height. That is assumed to be the width and height of the canvas.
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The VP8 specification describes how to decode the image into Y'CbCr
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format. To convert to RGB, Rec. 601 SHOULD be used.
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The VP8 specification describes how to decode the image into Y'CbCr format. To
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convert to RGB, Rec. 601 SHOULD be used. Applications MAY use another
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conversion method, but visual results may differ among decoders.
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Simple File Format (Lossless)
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@ -221,9 +231,11 @@ Simple WebP (lossless) file format:
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0 1 2 3
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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
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+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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| |
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| WebP file header (12 bytes) |
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| |
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+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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| VP8L chunk |
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: VP8L chunk :
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+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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VP8L chunk:
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@ -232,8 +244,9 @@ VP8L chunk:
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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
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+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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| ChunkHeader('VP8L') |
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| |
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+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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| VP8L data |
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: VP8L data :
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+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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VP8L data: _Chunk Size_ bytes
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@ -293,9 +306,12 @@ Extended WebP file header:
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0 1 2 3
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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
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+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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| |
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| WebP file header (12 bytes) |
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| |
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+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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| ChunkHeader('VP8X') |
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| |
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+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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|Rsv|I|L|E|X|A|R| Reserved |
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+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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@ -368,6 +384,7 @@ animation.
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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
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+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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| ChunkHeader('ANIM') |
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| |
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+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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| Background Color |
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+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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@ -409,6 +426,7 @@ If the _Animation flag_ is not set, then this chunk SHOULD NOT be present.
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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
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+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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| ChunkHeader('ANMF') |
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| |
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+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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| Frame X | ...
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+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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@ -418,7 +436,7 @@ If the _Animation flag_ is not set, then this chunk SHOULD NOT be present.
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+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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| Frame Duration | Reserved |B|D|
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+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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| Frame Data |
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: Frame Data :
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+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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Frame X: 24 bits (_uint24_)
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@ -521,6 +539,7 @@ _padded_ chunks as described by the [RIFF file format](#riff-file-format).
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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
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+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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| ChunkHeader('ALPH') |
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| |
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+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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|Rsv| P | F | C | Alpha Bitstream... |
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+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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@ -531,7 +550,7 @@ Reserved (Rsv): 2 bits
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Pre-processing (P): 2 bits
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: These INFORMATIVE bits are used to signal the pre-processing that has
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: These _informative_ bits are used to signal the pre-processing that has
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been performed during compression. The decoder can use this information to
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e.g. dither the values or smooth the gradients prior to display.
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@ -644,8 +663,9 @@ and [Simple File Format (Lossless)](#simple-file-format-lossless) respectively.
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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
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+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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| ChunkHeader('ICCP') |
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+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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| Color Profile |
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: Color Profile :
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+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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Color Profile: _Chunk Size_ bytes
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@ -676,8 +696,9 @@ EXIF chunk:
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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
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+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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| ChunkHeader('EXIF') |
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+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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| Exif Metadata |
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: Exif Metadata :
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+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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Exif Metadata: _Chunk Size_ bytes
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@ -690,14 +711,17 @@ XMP chunk:
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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
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+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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| ChunkHeader('XMP ') |
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+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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| XMP Metadata |
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: XMP Metadata :
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+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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XMP Metadata: _Chunk Size_ bytes
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: image metadata in XMP format.
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Note the fourth character in the 'XMP ' fourcc is an ASCII space (%x20).
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Additional guidance about handling metadata can be found in the
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Metadata Working Group's [Guidelines for Handling Metadata][metadata].
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@ -817,3 +841,4 @@ RIFF/WEBP
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[metadata]: https://web.archive.org/web/20180919181934/http://www.metadataworkinggroup.org/pdf/mwg_guidance.pdf
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[rfc 1166]: https://datatracker.ietf.org/doc/html/rfc1166
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[rfc 2119]: https://datatracker.ietf.org/doc/html/rfc2119
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[rfc 8174]: https://datatracker.ietf.org/doc/html/rfc8174
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