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webp-container-spec: clarify chunk order requirements

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based on comments from:
https://datatracker.ietf.org/doc/draft-zern-webp/ballot/#draft-zern-webp_robert-wilton

Change-Id: If2cd956b607c51b714e82e8eee7a1c4375c3c632
This commit is contained in:
James Zern 2024-03-11 14:01:40 -07:00
parent 8a7e911231
commit a16d30cb57
1 changed files with 11 additions and 8 deletions
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19
doc/webp-container-spec.txt
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@ -290,15 +290,18 @@ up of:
For an _animated image_, the _image data_ consists of multiple frames. More
details about frames can be found in the [Animation](#animation) section.
All chunks SHOULD be placed in the same order as listed above. If a chunk
appears in the wrong place, the file is invalid, but readers MAY parse the
file, ignoring the chunks that are out of order.
All chunks necessary for reconstruction and color correction, that is 'VP8X',
'ICCP', 'ANIM', 'ANMF', 'ALPH', 'VP8' and 'VP8L', MUST appear in the order
described above. Readers SHOULD fail when chunks necessary for reconstruction
and color correction are out of order.
**Rationale:** Setting the order of chunks should allow quicker file
parsing. For example, if an 'ALPH' Chunk does not appear in its required
position, a decoder can choose to stop searching for it. The rule of
ignoring late chunks should make programs that need to do a full search
give the same results as the ones stopping early.
[Metadata](#metadata) and [unknown](#unknown-chunks) chunks MAY appear out of
order.
**Rationale:** The chunks necessary for reconstruction should 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 varying the order of metadata and
custom chunks to suit the implementation.
Extended WebP file header:
{:#extended_header}