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467 lines
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Plaintext
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WebP Container Specification
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============================
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_Working Draft, v0.5, 20120713_
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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). This document describes
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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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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 a still image (i.e., an encoded matrix of pixels) and,
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optionally, transparency information. In case we need to refer only to the
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matrix of pixels, we will call it the _canvas_ of the image.
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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 'ALPH' chunk with transparency information.
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* The image bitstream contained in either a 'VP8 ' or 'VP8L' chunk.
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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 'ALPH' chunk does not appear in its required
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position, a decoder can choose to stop searching for it. The rule of
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ignoring late chunks should make programs that need to do a full search
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give the same results as the ones stopping early.
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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| Rsv | 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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: SHOULD be `0`.
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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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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 Minus One: 24 bits
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: _1-based_ width of the canvas in pixels.
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The actual canvas width is '1 + Canvas Width Minus One'
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Canvas Height Minus One: 24 bits
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: _1-based_ height of the canvas in pixels.
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The actual canvas height is '1 + Canvas Height Minus One'
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The product of _Canvas Width_ and _Canvas Height_ MUST 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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#### 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`: Compressed using the WebP lossless format.
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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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For each pixel, filtering is performed using the following calculations.
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Assume the alpha values surrounding the current `X` position are labeled as:
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C | B |
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---+---+
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A | X |
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We seek to compute the alpha value at position `X`. First, a prediction is
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made depending on the filtering method:
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* Method `0`: predictor = 0
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* Method `1`: predictor = A
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* Method `2`: predictor = B
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* Method `3`: predictor = clip(A + B - C)
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where `clip(v)` is equal to:
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* 0 if v < 0
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* 255 if v > 255
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* v otherwise
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The final value is derived by adding the decompressed value `X` to the
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predictor and using modulo-256 arithmetic to wrap the [256-511] range
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into the [0-255] one:
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`alpha = (predictor + X) % 256`
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There are special cases for left-most and top-most pixel positions:
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* Top-left value at location (0,0) uses 0 as predictor value. Otherwise,
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* For horizontal or gradient filtering methods, the left-most pixels at
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location (0, y) are predicted using the location (0, y-1) just above.
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* For vertical or gradient filtering methods, the top-most pixels at
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location (x, 0) are predicted using the location (x-1, 0) on the left.
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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 frame.
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An image containing a 'VP8L' chunk SHOULD NOT contain this chunk.
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**Rationale**: the transparency information of an image is already part of
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the 'VP8L' chunk.
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The alpha channel data is stored as uncompressed 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 8-bit 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 [WebP Lossless Bitstream Format]
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[webpllspec]) of implicit dimension width x height. That is, this
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image-stream 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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#### Bitstream (VP8/VP8L)
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This chunk contains compressed image data.
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A bitstream chunk may be either (i) a VP8 chunk, using "VP8 " (note the
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significant fourth-character space) as its tag _or_ (ii) a VP8L chunk, using
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"VP8L" as its tag.
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The formats of VP8 and VP8L chunks are as described in sections
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[Simple file format (lossy)](#simple-file-format-lossy)
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and [Simple file format (lossless)](#simple-file-format-lossless) respectively.
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#### Unknown Chunks
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A file MAY contain other unknown chunks. Readers SHOULD be ignore these chunks.
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Writers SHOULD preserve them in their original order.
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Example file layouts
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--------------------
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A lossy encoded image with alpha may 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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+- ALPH (alpha bitstream)
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+- VP8 (bitstream)
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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A losslessly encoded image may 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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+- XYZW (unknown chunk)
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+- VP8L (lossless bitstream)
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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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[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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