690 lines
25 KiB
Plaintext
690 lines
25 KiB
Plaintext
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WebP Container Specification
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============================
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_Working Draft, v0.3, 20120523_
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* TOC placeholder
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{:toc}
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Introduction
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------------
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WebP is a still image format that uses the VP8 key frame encoding, and
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possibly other encodings in the future, to compress image data in a
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lossy way. The VP8 encoding should make it more efficient than currently
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used formats. It is optimized for fast image transfer over the network
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(e.g., for websites). However, it also aims for feature parity
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(color profile, XMP metadata, animation, etc.) with other formats. This
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document describes the structure of a WebP file.
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The first version of WebP handled only the basic use case: a file
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containing a single image (being one VP8 key frame), with no metadata.
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The use of a RIFF container permits additional feature support. This
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document describes additional support for:
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* **Metadata and color profiles.** We specify chunks that can contain
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this information, as other popular formats do.
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* **Tiling.** A single VP8 frame has an inherent limitation for width
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or height of 2^14 pixels, and a 512kB limit on the size of the first
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compressed partition. To support larger images, we support images
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that are composed of multiple tiles, each encoded as a separate VP8
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frame. All tiles form logically a single image: they have common
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metadata, color profile, etc. Tiling may also improve efficiency for
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larger images, e.g., grass can be encoded differently than sky.
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* **Animation.** An image may have pauses between frames, making it
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an animation.
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* **Transparency.** An image may have transparency, i.e., an alpha channel for
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each frame/tile.
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Files not using these new features are backward compatible with the
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original format. Use of these features will produce files that are not
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compatible with older programs.
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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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Terminology & Basics
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------------------------
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A WebP file contains either a still image (i.e., an encoded matrix of
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pixels) or an animation (see below), with possibly a color profile,
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metadata, etc. In case we need to refer only to the matrix of pixels,
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we will call it the _canvas_ of the image.
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The canvas of an image is built from one or multiple tiles. Each tile
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is a separately encoded VP8 key frame (other encodings are possible in
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the future). Building an image from several tiles allows us to overcome
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the size limitations of a single VP8 frame. Tiles are an internal detail
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of the file: they are not supposed to be exposed to the user.
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Below are additional terms used throughout this document:
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Code that reads WebP files is referred to as a _reader_, while
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code that writes them is referred to as a _writer_.
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_uint16_
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: A 16-bit, little-endian, unsigned integer.
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_uint32_
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: A 32-bit, little-endian, unsigned integer.
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The basic element of a RIFF file is a _chunk_. It consists of:
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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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| Chunk FourCC |
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+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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| Chunk Size |
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+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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| Chunk Payload |
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+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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Chunk FourCC: 32 bits
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: ASCII four character code or _chunk tag_ used for chunk identification.
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Chunk Size: 32 bits (_uint32_)
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: The size of the chunk (_ckSize_) not including this field, the chunk
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identifier and padding.
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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 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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: Note that, in this specification, all chunk tag characters are in
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file order, not in byte order of a uint32 of any particular
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architecture.
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_list of chunks_
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: A concatenation of multiple chunks.
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: We will refer to the first chunk as having _position_ 0, the second
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as position 1, etc. By _chunk with index 0 among "ABCD"_ we mean
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the first chunk among the chunks of type "ABCD" in the list, the
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_chunk with index 1 among "ABCD"_ is the second such chunk, etc.
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A WebP file MUST begin with a single chunk with a tag 'RIFF'. All
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other defined chunks are contained within this chunk. The file SHOULD
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NOT contain anything after it.
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The maximum size of RIFF's _ckSize_ is 2^32 minus 10 bytes. The size
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of the whole file is at most 4GiB minus 2 bytes.
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**Note:** some RIFF libraries are said to have bugs when handling files
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larger than 1GiB or 2GiB. If you are using an existing library, check
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that it handles large files correctly.
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The first four bytes of the RIFF chunk contents (i.e., bytes 8-11 of the file)
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MUST be the ASCII string "WEBP". They are followed by a list of chunks. As the
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size of any chunk is even, the size of the RIFF chunk is also even. The
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contents of the chunks in that list will be described in the following sections.
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**Note:** RIFF has a convention that all-uppercase chunks are standard
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chunks that apply to any RIFF file format, while chunks specific to a
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file format are all lowercase. WebP does not follow this convention.
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WebP file header
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----------------
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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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| 'R' | 'I' | 'F' | 'F' |
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+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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| File Size |
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+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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| 'W' | 'E' | 'B' | 'P' |
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+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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'RIFF': 32 bits
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: The ASCII characters 'R' 'I' 'F' 'F'.
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File Size: 32 bits (_uint32_)
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: The size of the file in bytes starting at offset 8.
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'WEBP': 32 bits
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: The ASCII characters 'W' 'E' 'B' 'P'.
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Simple file format
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------------------
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Simple 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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| WebP file header (12 bytes) |
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+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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| ChunkHeader('VP8 ') |
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+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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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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The content of a 'VP8 ' chunk (note the last character is a space) MUST be one
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VP8 key frame (with optional padding).
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The current [VP8 Data Format and Decoding Guide][vp8spec] can be found
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at the IETF website, <http://www.ietf.org/>.
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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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This layout SHOULD be used if the image does not require advanced
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features: color profiles, XMP metadata, animation or tiling. Files with
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this layout are smaller and supported by older software.
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Extended file format
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--------------------
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**Note:** Older readers may not support files using the extended format.
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An extended format file consists of:
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* A 'VP8X' chunk with information about features used in the file.
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* An optional 'ICCP' chunk with color profile.
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* An optional 'LOOP' chunk with animation control data.
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* Data for all the frames.
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* An optional 'META' chunk with XMP metadata.
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* Some other chunk types may be defined by future specifications and
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placed anywhere in the file.
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As will be described in the 'VP8X' chunk description, by checking a
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flag one can distinguish animated and non-animated images. A
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non-animated image has exactly one frame. An animated one may have
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multiple frames. Data for each frame consists of:
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* An optional 'FRM ' (fourth character is a significant space) chunk
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with animation frame metadata. It MUST be present in animated
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images at the beginning of data for that frame. It MUST NOT be
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present in non-animated images.
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* An optional 'TILE' chunk with tile position metadata. It MUST be
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present at the beginning of data for an image that's represented as
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multiple tile images.
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* An optional 'ALPH' chunk with alpha bitstream of the tile. It MUST be
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present for an image containing transparency. It MUST NOT be present
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in non-transparent images.
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* A 'VP8 ' chunk with the bitstream of the tile.
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All chunks SHOULD be placed in the same order as listed above (except
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for unknown chunks, which MAY appear anywhere). If a chunk appears
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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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**Rationale:** Setting the order of chunks should allow quicker file
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parsing. For example, if an ICCP 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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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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| WebP file header (12 bytes) |
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+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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| ChunkHeader('VP8X') |
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+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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| Rsv |L|M|I|A|T| Reserved |
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+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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| Canvas Width |
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+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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| Canvas Height |
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+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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Tiling (T): 1 bit
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: Set if the image is represented by tiles.
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Animation (A): 1 bit
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: Set if the file is an animation. Data in 'LOOP' and 'FRM ' chunks
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should be used to control the animation.
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ICC profile (I): 1 bit
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: Set if the file contains an 'ICCP' chunk.
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Metadata (M): 1 bit
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: Set if the file contains a 'META' chunk.
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Alpha (L): 1 bit
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: Set if the file contains images with transparency information ("alpha").
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Reserved (Rsv): 3 bits
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: SHOULD be `0`.
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Reserved: 24 bits
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: SHOULD be `0`.
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Canvas Width: 32 bits
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: Width of the canvas in pixels.
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Canvas Height: 32 bits
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: Height of the canvas in pixels.
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Future specifications MAY add more fields. If a chunk of larger size is found,
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programs MUST ignore the extra bytes but SHOULD preserve them when modifying
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the file.
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### Chunks
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#### Animation
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Loop 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('LOOP') |
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+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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| Loop Count |
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+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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Loop Count: 16 bits (_uint16_)
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: The number of times to loop the animation. `0` means infinitely.
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For images that are animations, this chunk contains the global
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parameters of the animation.
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This chunk MUST appear if the _Animation_ flag in chunk VP8X is set.
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If the _Animation_ flag is not set and this chunk is present, it
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SHOULD be ignored.
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Per-frame parameters of the animation:
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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('FRM ') |
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+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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| Frame X |
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+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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| Frame Y |
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+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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| Frame Width |
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+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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| Frame Height |
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+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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| Frame Duration |
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+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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Frame X: 32 bits (_uint32_)
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: The X coordinate of the upper left corner of the frame.
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Frame Y: 32 bits (_uint32_)
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: The Y coordinate of the upper left corner of the frame.
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Frame Width: 32 bits (_uint32_)
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: The width of the frame.
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Frame Height: 32 bits (_uint32_)
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: The height of the frame.
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Frame Duration: 16 bits (_uint16_)
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: Time to wait before displaying the next tile, in 1 millisecond units.
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Notes for frames containing VP8 data:
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* _Frame X_ and _Frame Y_ values MUST be divisible by `32`.
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**Rationale:** This ensures that pixels on U and V planes are aligned to a
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16-byte boundary (even after a rotation), which may help with vector
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instructions on some architectures. This also makes the tiles align to
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16-pixel macroblock boundaries.
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* _Frame Width_ MUST be divisible by `16` or
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`Frame X + Frame Width == Canvas Width` MUST be true.
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* _Frame Height_ MUST be divisible by `16` or
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`Frame Y + Frame Height == Canvas Height` MUST be true.
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**Rationale:** The width and height constraints simplify the handling of
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macroblocks that are on the edge of a tile. VP8 decoders can overwrite
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pixels outside the boundary in such a macroblock, and this guarantees they
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won't overwrite any data.
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#### Tiling
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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('TILE') |
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+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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| Tile Canvas X |
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+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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| Tile Canvas Y |
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+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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Tile Canvas X: 32 bits (_uint32_)
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: X coordinate of the upper left corner of the tile.
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Tile Canvas Y: 32 bits (_uint32_)
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: Y coordinate of the upper left corner of the tile.
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This chunk contains information about a single tile and describes the
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(optional) alpha chunk and bitstream chunk that follows it.
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Notes for tiles containing VP8 data:
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* _Tile Canvas X_ and _Tile Canvas Y_ values MUST be
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divisible by `32`.
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* The _Tile Width_ and _Tile Height_ can be extracted from the VP8 data.
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See 'Section 9' in the [VP8 RFC][vp8spec].
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* The width of a tile MUST be divisible by `16` or
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`Tile Canvas X + Tile Width == Canvas Width` MUST be true.
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* The height of a tile MUST be divisible by `16` or
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`Tile Canvas Y + Tile Height == Canvas Height` MUST be true.
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#### Alpha
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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('ALPH') |
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+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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|Rsv| P | F | C | Alpha Bitstream... |
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+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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Compression method (C): 2 bits
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: The compression method used:
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* `0`: No compression.
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* `1`: Backward reference counts encoded with arithmetic encoder.
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Filtering method (F): 2 bits
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: The filtering method used:
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* `0`: None.
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* `1`: Horizontal filter.
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* `2`: Vertical filter.
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* `3`: Gradient filter.
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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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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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* `0`: no pre-processing
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* `1`: level reduction
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Decoders are not required to use this information in any specified way.
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Reserved (Rsv): 2 bits
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: SHOULD be `0`.
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Alpha bitstream: _Chunk Size_ - `1` bytes
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: Encoded alpha bitstream.
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This optional chunk contains encoded alpha data for a single tile.
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Either **ALL or NONE** of the tiles must contain this chunk.
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The alpha channel data is losslessly stored as raw data (when
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compression method is '0') or compressed using the lossless format
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(when the compression method is '1').
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* raw data: consists of a byte sequence of length width * height,
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containing all the 8bit transparency values in scan order.
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* lossless format compression: the byte sequence is a compressed
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image-stream (as described in the specification for lossless format)
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of implicit dimension width x height. Note that this image-stream byte
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sequence does NOT contain any headers describing the image dimension.
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**Rationale**: the dimension is already known from other sources,
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so storing it again would be redundant and error-prone.
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Once the image-stream is decoded into ARGB color values, following
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the process described in the lossless format specification, the
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transparency information must be extracted from the *green* channel
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of the ARGB quadruplet.
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**Rationale**: the green channel is allowed extra transformation
|
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steps in the specification -unlike the other channels- that can
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improve compression.
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#### Color profile
|
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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('ICCP') |
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+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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| Compression | Color Profile |
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+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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|
|
Compression: 8 bits
|
|
|
|
: Compression method used:
|
|
|
|
* `0`: None.
|
|
* `1`: Deflate/inflate.
|
|
|
|
Color Profile: _Chunk Size_ - `1` bytes
|
|
|
|
: ICC profile.
|
|
|
|
There SHOULD be at most one 'ICCP' chunk.
|
|
See <http://www.color.org> for specifications.
|
|
|
|
If this chunk is not present, sRGB SHOULD be assumed.
|
|
|
|
#### Metadata
|
|
|
|
0 1 2 3
|
|
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
|
|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|
|
| ChunkHeader('META') |
|
|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|
|
| Compression | XMP Metadata |
|
|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|
|
|
|
Compression: 8 bits
|
|
|
|
: Compression method used:
|
|
|
|
* `0`: None.
|
|
* `1`: Deflate/inflate.
|
|
|
|
XMP Metadata: _Chunk Size_ - `1` bytes
|
|
|
|
: XMP metadata.
|
|
|
|
There SHOULD be at most one such chunk. If there are more such chunks, readers
|
|
MAY ignore all except the first one.
|
|
|
|
XMP packets are XML text as specified in the [XMP Specification Part
|
|
1][xmpspec]. The chunk tag is different from the one specified by Adobe
|
|
for WAV and AVI (also RIFF formats), because we have the option of
|
|
compression.
|
|
|
|
Additional guidance about handling metadata can be found in the
|
|
Metadata Working Group's [Guidelines for Handling Metadata][metadata].
|
|
Note that the sections of the document about reconciliation of EXIF,
|
|
XMP and IPTC-IIM don't apply to WebP. As WebP supports only XMP, no
|
|
reconciliation is necessary.
|
|
|
|
#### Other Chunks
|
|
|
|
A file MAY contain other chunks. Readers SHOULD be ignore these chunks. Writers
|
|
SHOULD preserve them in their original order.
|
|
|
|
### Assembling the Canvas from Tiles and Animation
|
|
|
|
Here we provide an overview of how 'TILE' chunks and 'FRM '/'LOOP' chunks are
|
|
used to assemble the canvas. The notation _VP8X.field_ means the field in
|
|
the 'VP8X' chunk with the same description.
|
|
|
|
Decoding a non-animated canvas MUST be equivalent to the following
|
|
pseudocode:
|
|
|
|
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
|
|
assert not VP8X.flags.haveAnimation
|
|
canvas ← new black image of size
|
|
VP8X.canvasWidth x VP8X.canvasHeight.
|
|
tile_params.tileCanvasX = tile_params.tileCanvasY = 0
|
|
for chunk in data_for_all_frames:
|
|
if chunk.tag == "TILE":
|
|
assert No other TILE chunk after the last "VP8 " chunk
|
|
assert No ALPH chunk after the last "VP8 " chunk
|
|
tile_params = chunk
|
|
if chunk.tag == "ALPH":
|
|
assert No other ALPH chunk after the last "VP8 " chunk
|
|
tile_params.alpha = alpha_data
|
|
if chunk.tag == "VP8 ":
|
|
render image in chunk in canvas with top-left corner in
|
|
(tile_params.tileCanvasX, tile_params.tileCanvasY).
|
|
tile_params.tileCanvasX = tile_params.tileCanvasY = 0
|
|
Ignore unknown chunks
|
|
canvas contains the decoded canvas.
|
|
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
|
|
|
|
Decoding an animated canvas MUST be equivalent to the following
|
|
pseudocode:
|
|
|
|
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
|
|
assert VP8X.flags.haveAnimation
|
|
canvas ← new black image of size
|
|
VP8X.canvasWidth x VP8X.canvasHeight.
|
|
if LOOP.loopCount==0:
|
|
LOOP.loopCount=∞
|
|
current_FRM ← nil
|
|
for LOOP.loop = 0, ..., LOOP.loopCount-1
|
|
assert First chunk in data_for_all_frames is a FRM
|
|
for chunk in data_for_all_frames:
|
|
if chunk.tag == "FRM ":
|
|
assert No other FRM chunk after the last "VP8 " chunk
|
|
assert No ALPH chunk after the last "VP8 " chunk
|
|
if current_FRM != nil:
|
|
Show the contents of canvas for
|
|
current_FRM.frameDuration * 1ms.
|
|
current_FRM = chunk
|
|
if chunk.tag == "ALPH":
|
|
assert No other ALPH chunk after the last "VP8 " chunk
|
|
tile_params.alpha = alpha_data
|
|
if chunk.tag == "VP8 ":
|
|
assert tile_params.tileCanvasX >= current_FRM.frameX
|
|
assert tile_params.tileCanvasY >= current_FRM.frameY
|
|
assert tile_params.tileCanvasX + chunk.tileWidth >=
|
|
current_FRM.frameX + current_FRM.frameWidth
|
|
assert tile_params.tileCanvasY + chunk.tileHeight >=
|
|
current_FRM.frameX + current_FRM.frameHeight
|
|
render image in chunk in canvas with top-left corner in
|
|
(tile_params.tileCanvasX, tile_params.tileCanvasY).
|
|
tile_params.tileCanvasX = tile_params.tileCanvasY = 0
|
|
Ignore unknown chunks
|
|
canvas contains the decoded canvas.
|
|
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
|
|
|
|
As described earlier, if an assert related to chunk ordering fails, the reader
|
|
MAY ignore the badly-ordered chunks instead of failing to decode the file.
|
|
|
|
Example file layouts
|
|
--------------------
|
|
|
|
A non-animated, tiled image without transparency may look as follows:
|
|
|
|
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
|
|
RIFF/WEBP
|
|
+- VP8X (descriptions of features used)
|
|
+- ICCP (color profile)
|
|
+- TILE (First tile parameters)
|
|
+- VP8 (bitstream - first tile)
|
|
+- TILE (Second tile parameters)
|
|
+- VP8 (bitstream - second tile)
|
|
+- TILE (third tile parameters)
|
|
+- VP8 (bitstream - third tile)
|
|
+- TILE (fourth tile parameters)
|
|
+- VP8 (bitstream - fourth tile)
|
|
+- META (XMP metadata)
|
|
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
|
|
|
|
An animated image with transparency may look as follows:
|
|
|
|
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
|
|
RIFF/WEBP
|
|
+- VP8X (descriptions of features used)
|
|
+- LOOP (animation control parameters)
|
|
+- FRM (first animation frame parameters)
|
|
+- ALPH (alpha bitstream - first image frame)
|
|
+- VP8 (bitstream - first image frame)
|
|
+- FRM (second animation frame parameters)
|
|
+- ALPH (alpha bitstream - second image frame)
|
|
+- VP8 (bitstream - second image frame)
|
|
+- META (XMP metadata)
|
|
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
|
|
|
|
[vp8spec]: http://tools.ietf.org/html/rfc6386
|
|
[xmpspec]: http://www.adobe.com/content/dam/Adobe/en/devnet/xmp/pdfs/XMPSpecificationPart1.pdf
|
|
[metadata]: http://www.metadataworkinggroup.org/pdf/mwg_guidance.pdf
|
|
[rfc 2119]: http://tools.ietf.org/html/rfc2119
|