Metadata specification in container spec
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Urvang Joshi
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@ -13,7 +13,7 @@ end of this file.
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WebP Container Specification
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============================
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_Working Draft, v0.6, 20121031_
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_Working Draft, v0.7, 20121101_
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* TOC placeholder
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@ -27,8 +27,9 @@ WebP is an image format that uses either (i) the VP8 key frame encoding
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to compress image data in a lossy way, or (ii) the WebP lossless encoding
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(and possibly other encodings in the future). These encoding schemes should
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make it more efficient than currently used formats. It is optimized for fast
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image transfer over the network (e.g., for websites). This document describes
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the structure of a WebP file.
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image transfer over the network (e.g., for websites). The WebP format has
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feature parity (color profile, metadata etc) with other formats as well. This
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document describes the structure of a WebP file.
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The WebP container (i.e., RIFF container for WebP) allows feature support over
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and above the basic use case of WebP (i.e., a file containing a single image
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@ -38,6 +39,8 @@ for:
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* **Lossless compression.** An image can be losslessly compressed, using the
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WebP Lossless Format.
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* **Metadata.** An image may have metadata stored in EXIF or XMP formats.
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* **Transparency.** An image may have transparency, i.e., an alpha channel.
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* **Color Profile.** An image may have an embedded ICC profile as described
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@ -258,6 +261,10 @@ An extended format file consists of:
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* The image bitstream contained in either a 'VP8 ' or 'VP8L' chunk.
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* An optional 'EXIF' chunk with EXIF metadata.
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* An optional 'XMP ' chunk with XMP metadata.
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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 come too late.
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@ -277,17 +284,25 @@ Extended WebP file header:
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+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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| ChunkHeader('VP8X') |
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+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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| R |I|L| Rsv | Reserved |
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|Rsv|I|L|E|X| R | Reserved |
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+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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| Canvas Width Minus One | ...
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+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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... Canvas Height Minus One |
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+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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Reserved (Rsv): 4 bits
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Reserved (R): 2 bits
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: SHOULD be `0`.
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XMP metadata (X): 1 bit
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: Set if the file contains XMP metadata.
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EXIF metadata (E): 1 bit
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: Set if the file contains EXIF metadata.
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Alpha (L): 1 bit
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: Set if the file contains some (or all) images with transparency information
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@ -297,7 +312,7 @@ ICC profile (I): 1 bit
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: Set if the file contains an ICC profile.
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Reserved (R): 2 bits
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Reserved (Rsv): 2 bits
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: SHOULD be `0`.
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@ -466,6 +481,48 @@ See the [ICC Specification][iccspec] for details.
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If this chunk is not present, sRGB SHOULD be assumed.
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#### Metadata
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Metadata can be stored in 'EXIF' or 'XMP ' chunks.
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There SHOULD be at most one chunk of each type ('EXIF' and 'XMP '). If there
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are more such chunks, readers MAY ignore all except the first one. Also, a file
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may possibly contain both 'EXIF' and 'XMP ' chunks.
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The chunks are defined as follows:
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EXIF chunk:
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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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| ChunkHeader('EXIF') |
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+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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| EXIF Metadata |
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+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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EXIF Metadata: _Chunk Size_ bytes
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: image metadata in EXIF format.
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XMP chunk:
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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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| ChunkHeader('XMP ') |
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+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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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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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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#### Unknown Chunks
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A file MAY contain other unknown chunks. Readers SHOULD ignore these chunks.
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@ -492,16 +549,19 @@ RIFF/WEBP
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+- VP8L (lossless bitstream)
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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A lossless image with ICC profile may look as follows:
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A lossless image with ICC profile and XMP metadata may
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look as follows:
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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RIFF/WEBP
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+- VP8X (descriptions of features used)
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+- ICCP (color profile)
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+- VP8L (lossless bitstream)
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+- XMP (metadata)
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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[vp8spec]: http://tools.ietf.org/html/rfc6386
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[webpllspec]: https://gerrit.chromium.org/gerrit/gitweb?p=webm/libwebp.git;a=blob;f=doc/webp-lossless-bitstream-spec.txt;hb=master
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[iccspec]: http://www.color.org/icc_specs2.xalter
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[metadata]: http://www.metadataworkinggroup.org/pdf/mwg_guidance.pdf
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[rfc 2119]: http://tools.ietf.org/html/rfc2119
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