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webp-container-spec: clarify chunk order requirements
based on comments from: https://datatracker.ietf.org/doc/draft-zern-webp/ballot/#draft-zern-webp_robert-wilton Change-Id: If2cd956b607c51b714e82e8eee7a1c4375c3c632
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@ -290,15 +290,18 @@ up of:
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For an _animated image_, the _image data_ consists of multiple frames. More
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details about frames can be found in the [Animation](#animation) section.
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All chunks SHOULD be placed in the same order as listed above. If a chunk
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appears in the wrong place, the file is invalid, but readers MAY parse the
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file, ignoring the chunks that are out of order.
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All chunks necessary for reconstruction and color correction, that is 'VP8X',
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'ICCP', 'ANIM', 'ANMF', 'ALPH', 'VP8' and 'VP8L', MUST appear in the order
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described above. Readers SHOULD fail when chunks necessary for reconstruction
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and color correction are out of order.
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**Rationale:** Setting the order of chunks should allow quicker file
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parsing. For example, if an 'ALPH' Chunk does not appear in its required
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position, a decoder can choose to stop searching for it. The rule of
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ignoring late chunks should make programs that need to do a full search
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give the same results as the ones stopping early.
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[Metadata](#metadata) and [unknown](#unknown-chunks) chunks MAY appear out of
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order.
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**Rationale:** The chunks necessary for reconstruction should appear first in
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the file to allow a reader to begin decoding an image before receiving all of
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the data. An application may benefit from varying the order of metadata and
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custom chunks to suit the implementation.
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Extended WebP file header:
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{:#extended_header}
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