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Plaintext
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WebP Container Specification
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============================
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_Working Draft, v0.4, 20120613_
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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 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). However, it also aims
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for feature parity (color profile, metadata, animation, etc.) with other
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formats. This 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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encoded as a VP8 key frame). The WebP container provides additional support
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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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* **Transparency.** An image may have transparency, i.e., an alpha channel
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for each frame/tile.
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* **Metadata.** An image can have metadata stored in any of the popular
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metadata formats.
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* **Color profiles.** An image can have an ICC profile characterizing a color
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input or output device.
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* **Animation.** An image may have pauses between frames, making it
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an animation.
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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 512 KiB 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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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 or a WebP lossless bitstream. Building
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an image from several tiles allows the format to overcome the size limitations
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of a single VP8 frame / WebP lossless bitstream. 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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_uint24_
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: A 24-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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_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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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 (lossy)
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--------------------------
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This layout SHOULD be used if the image requires _lossy_ encoding and does not
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require transparency or other advanced features provided by the extended format.
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Files with this layout are smaller and supported by older software.
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Simple WebP (lossy) 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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| VP8 chunk |
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+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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VP8 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('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 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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Simple file format (lossless)
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-----------------------------
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**Note:** Older readers may not support files using the lossless format.
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This layout SHOULD be used if the image requires _lossless_ encoding (with an
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optional transparency channel) and does not require advanced features provided
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by the extended format.
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Simple WebP (lossless) 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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| VP8L chunk |
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+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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VP8L 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('VP8L') |
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+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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| VP8L data |
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+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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VP8L data: _Chunk Size_ bytes
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: VP8L bitstream data.
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The current specification of the VP8L bitstream can be found at
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[WebP Lossless Bitstream Format][webpllspec]. Note that the VP8L header
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contains the VP8L image width and height. That is assumed to be the width
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and height of the canvas.
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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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* Optionally, some other unknown chunk types that may be defined by future
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specifications.
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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 metadata.
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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 each tile for a frame that is represented as
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multiple tile images.
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* An optional 'ALPH' chunk with alpha bitstream of the frame/tile.
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* A 'VP8 ' or a 'VP8L' chunk containing compressed image data of the
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frame/tile.
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* An optional unknown chunk type that may be defined by future
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specifications.
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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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**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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| R |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 some (or all) images with transparency information
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("alpha").
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Rotation and Symmetry (R): 3 bits
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: Specify an isometry to be applied to every bitstream chunk decoded.
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The table below specifies into what coordinates a point (x,y) in the original
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coordinate system has to be transformed into:
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| Value | Name | New coordinates |
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|:------|:--------------------------:|:--------------------------------------: |
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| 0 | Identify | (x,y) |
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|-------
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| 1 | Horizontal symmetry | (x, CanvasHeight-1-y) |
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|-------
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| 2 | Vertical symmetry | (CanvasWidth-1-x, y) |
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|-------
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| 3 | Rotation 180 degrees | (CanvasWidth-1-x, CanvasHeight-1-y) |
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|-------
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| 4 | Diagonal symmetry 1 | (y, x) |
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|-------
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| 5 | Rotation clockwise | (CanvasHeight-1-y, x) |
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|-------
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| 6 | Rotation counter-clockwise | (y, CanvasWidth-1-x) |
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|-------
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| 7 | Diagonal symmetry 2 | (CanvasHeight-1-y, CanvasWidth-1-x) |
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|-------
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{: rules="groups"}
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Reserved: 24 bits
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: SHOULD be `0`.
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Canvas Width: 24 bits
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: _1-based_ width of the canvas in pixels.
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Canvas Height: 24 bits
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: _1-based_ height of the canvas in pixels.
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The product of _Canvas Width_ and _Canvas Height_ SHOULD be at most `2^32 - 1`.
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Future specifications MAY add more fields.
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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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Frame 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('FRM ') |
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+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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| Frame X | ...
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+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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... Frame Y | 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: 24 bits (_uint24_)
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: The X coordinate of the upper left corner of the frame is `Frame X * 2`
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Frame Y: 24 bits (_uint24_)
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: The Y coordinate of the upper left corner of the frame is `Frame Y * 2`
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Frame Width: 24 bits (_uint24_)
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: The _1-based_ width of the frame.
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Frame Height: 24 bits (_uint24_)
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: The _1-based_ height of the frame.
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Frame Duration: 24 bits (_uint24_)
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: Time to wait before displaying the next frame, in 1 millisecond units
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(_1-based_).
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For images that are animations, this chunk contains information about a single
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frame, and describes the (optional) alpha chunk and the bitstream chunk that
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follows it. If the _Animation flag_ is not set and this chunk is present,
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it SHOULD be ignored.
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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 X | ...
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+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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... Tile Y |
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+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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Tile X: 24 bits (_uint24_)
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: X coordinate of the upper left corner of the tile is `Tile X * 2`
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Tile Y: 24 bits (_uint24_)
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: Y coordinate of the upper left corner of the tile is `Tile Y * 2`
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For images that contain tiling, this chunk contains information about a single
|
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tile and describes the (optional) alpha chunk and bitstream chunk that follow
|
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it. If the _Tile flag_ is not set and this chunk is present, it SHOULD be
|
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ignored.
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|
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#### Alpha
|
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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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| 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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|
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: The compression method used:
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* `0`: No compression.
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* `1`: Compressed using the WebP lossless format.
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Filtering method (F): 2 bits
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|
|
: The filtering method used:
|
|
|
|
* `0`: None.
|
|
* `1`: Horizontal filter.
|
|
* `2`: Vertical filter.
|
|
* `3`: Gradient filter.
|
|
|
|
Pre-processing (P): 2 bits
|
|
|
|
: These INFORMATIVE bits are used to signal the pre-processing that has
|
|
been performed during compression. The decoder can use this information to
|
|
e.g. dither the values or smooth the gradients prior to display.
|
|
|
|
* `0`: no pre-processing
|
|
* `1`: level reduction
|
|
|
|
Decoders are not required to use this information in any specified way.
|
|
|
|
Reserved (Rsv): 2 bits
|
|
|
|
: SHOULD be `0`.
|
|
|
|
Alpha bitstream: _Chunk Size_ - `1` bytes
|
|
|
|
: Encoded alpha bitstream.
|
|
|
|
This optional chunk contains encoded alpha data for a single frame/tile.
|
|
For images with transparency, some of the frames/tiles may contain this chunk.
|
|
However, a frame/tile containing a 'VP8L' chunk SHOULD NOT contain this chunk.
|
|
**Rationale**: the transparency information of a frame/tile is already part of
|
|
the 'VP8L' chunk.
|
|
|
|
The alpha channel data is losslessly stored as raw data (when
|
|
compression method is '0') or compressed using the lossless format
|
|
(when the compression method is '1').
|
|
|
|
* Raw data: consists of a byte sequence of length width * height,
|
|
containing all the 8bit transparency values in scan order.
|
|
|
|
* Lossless format compression: the byte sequence is a compressed
|
|
image-stream (as described in the [WebP Lossless Bitstream Format]
|
|
[webpllspec]) of implicit dimension width x height. (That is, this
|
|
image-stream does NOT contain any headers describing the image dimension).
|
|
|
|
**Rationale**: the dimension is already known from other sources,
|
|
so storing it again would be redundant and error-prone.
|
|
|
|
Once the image-stream is decoded into ARGB color values, following
|
|
the process described in the lossless format specification, the
|
|
transparency information must be extracted from the *green* channel
|
|
of the ARGB quadruplet.
|
|
|
|
**Rationale**: the green channel is allowed extra transformation
|
|
steps in the specification -unlike the other channels- that can
|
|
improve compression.
|
|
|
|
#### Bitstream (VP8/VP8L)
|
|
|
|
This chunk contains compressed image data. As described earlier, images
|
|
with a simple file format (lossy/lossless) have a single bitstream chunk
|
|
as the first subchunk of RIFF, while images with extended file format may
|
|
contain several of them, one for each frame/tile.
|
|
|
|
A bitstream chunk may be either (i) a VP8 chunk, using "VP8 " (note the
|
|
significant fourth-character space) as its tag _or_ (ii) a VP8L chunk , using
|
|
"VP8L" as its tag.
|
|
|
|
The formats of VP8 and VP8L chunks are as described in sections _Simple file
|
|
format (lossy)_ and _Simple file format (lossless)_ respectively.
|
|
|
|
#### Color profile
|
|
|
|
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('ICCP') |
|
|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|
|
| Color Profile |
|
|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|
|
|
|
Color Profile: _Chunk Size_ bytes
|
|
|
|
: ICC profile.
|
|
|
|
This chunk MUST appear before data for all the frames.
|
|
|
|
There SHOULD be at most one such chunk. If there are more such chunks, readers
|
|
MAY ignore all except the first one.
|
|
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') |
|
|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|
|
| Metadata |
|
|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|
|
|
|
Metadata: _Chunk Size_ bytes
|
|
|
|
: image metadata.
|
|
|
|
This chunk MUST appear after data for all the frames.
|
|
|
|
There SHOULD be at most one such chunk. If there are more such chunks, readers
|
|
MAY ignore all except the first one.
|
|
|
|
Additional guidance about handling metadata can be found in the
|
|
Metadata Working Group's [Guidelines for Handling Metadata][metadata].
|
|
|
|
#### Unknown Chunks
|
|
|
|
A file MAY contain other unknown 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.tileX = tile_params.tileY = 0
|
|
for chunk in data_for_all_frames:
|
|
if chunk.tag == "TILE":
|
|
assert No other TILE chunk after the last Bitstream chunk
|
|
assert No ALPH chunk after the last Bitstream chunk
|
|
tile_params = chunk
|
|
assert VP8X.canvasWidth >=
|
|
tile_params.tileX + tile_params.tileWidth
|
|
assert VP8X.canvasHeight >=
|
|
tile_params.tileY + tile_params.tileHeight
|
|
if chunk.tag == "ALPH":
|
|
assert No other ALPH chunk after the last Bitstream chunk
|
|
tile_params.alpha = alpha_data
|
|
if chunk.tag == "VP8 " OR chunk.tag == "VP8L":
|
|
render image in chunk on canvas with top-left corner in
|
|
(tile_params.tileX, tile_params.tileY).
|
|
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.
|
|
loop_count ← LOOP.loopCount
|
|
if loop_count == 0:
|
|
loop_count = ∞
|
|
frame_params ← nil
|
|
for LOOP.loop = 0, ..., LOOP.loopCount-1
|
|
assert First chunk in data_for_all_frames is FRM
|
|
for chunk in data_for_all_frames:
|
|
if chunk.tag == "FRM ":
|
|
assert No other FRM chunk after the last Bitstream chunk
|
|
assert No ALPH chunk after the last Bitstream chunk
|
|
frame_params = chunk
|
|
assert VP8X.canvasWidth >=
|
|
frame_params.frameX + frame_params.frameWidth
|
|
assert VP8X.canvasHeight >=
|
|
frame_params.frameY + frame_params.frameHeight
|
|
if chunk.tag == "ALPH":
|
|
assert No other ALPH chunk after the last Bitstream chunk
|
|
frame_params.alpha = alpha_data
|
|
if chunk.tag == "VP8 " OR chunk.tag == "VP8L":
|
|
render image in chunk on canvas with top-left corner in
|
|
(frame_params.frameX, frame_params.frameY). Show the contents
|
|
of the image for frame_params.frameDuration * 1ms.
|
|
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 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 (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 frame)
|
|
+- VP8 (bitstream - first frame)
|
|
+- FRM (second animation frame parameters)
|
|
+- ALPH (alpha bitstream - second frame)
|
|
+- VP8 (bitstream - second frame)
|
|
+- META (metadata)
|
|
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
|
|
|
|
A losslessly encoded non-animated non-tiled image may
|
|
look as follows:
|
|
|
|
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
|
|
RIFF/WEBP
|
|
+- VP8X (descriptions of features used)
|
|
+- ICCP (color profile)
|
|
+- XYZW (unknown chunk)
|
|
+- VP8L (lossless bitstream)
|
|
+- META (metadata)
|
|
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
|
|
|
|
An animated image may have a mix of lossy and lossless
|
|
bitstreams as follows:
|
|
|
|
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
|
|
RIFF/WEBP
|
|
+- VP8X (descriptions of features used)
|
|
+- FRM (first animation frame parameters)
|
|
+- VP8 (lossy bitstream - first frame)
|
|
+- FRM (second animation frame parameters)
|
|
+- VP8L (lossless bitstream - second frame)
|
|
+- ABCD (unknown chunk)
|
|
+- FRM (third animation frame parameters)
|
|
+- VP8 (lossy bitstream - third frame)
|
|
+- EFGH (unknown chunk)
|
|
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
|
|
|
|
[vp8spec]: http://tools.ietf.org/html/rfc6386
|
|
[webpllspec]: https://gerrit.chromium.org/gerrit/gitweb?p=webm/libwebp.git;a=blob;f=doc/webp-lossless-bitstream-spec.txt;hb=master
|
|
[metadata]: http://www.metadataworkinggroup.org/pdf/mwg_guidance.pdf
|
|
[rfc 2119]: http://tools.ietf.org/html/rfc2119
|