mirror of
https://github.com/webmproject/libwebp.git
synced 2024-11-20 12:28:26 +01:00
d5bad03328
Marking certain frames as "do not blend" helps avoiding alpha-blending at decode/render time. It also helps inserting I-frames (frames which can be independently decoded) into the animation. Change-Id: Iaa222805db88d2f1c81104ce9d882e7c7ff8cfdb
910 lines
34 KiB
Plaintext
910 lines
34 KiB
Plaintext
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WebP Container Specification
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============================
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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). The WebP format has
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feature parity (color profile, metadata, animation etc) with other formats as
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well. 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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* **Metadata.** An image may have metadata stored in EXIF or XMP formats.
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* **Transparency.** An image may have transparency, i.e., an alpha channel.
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* **Color Profile.** An image may have an embedded ICC profile as described
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by the [International Color Consortium][iccspec].
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* **Animation.** An image may have multiple frames with pauses between them,
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making it an animation.
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* **Image Fragmentation.** A single bitstream in WebP has an inherent
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limitation for width or height of 2^14 pixels, and, when using VP8, a 512
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KiB limit on the size of the first compressed partition. To support larger
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images, the format supports images that are composed of multiple fragments,
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each encoded as a separate bitstream. All fragments logically form a single
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image: they have common metadata, color profile, etc. Image fragmentation
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may also improve efficiency for larger images, e.g., grass can be encoded
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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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**Note:** Out of the features mentioned above, lossy compression, lossless
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compression, transparency, metadata, color profile and animation are finalized
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and are to be considered stable. On the other hand, image fragmentation is
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experimental as of now, and is open to discussion, feedback and comments.
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The same is indicated using annotation "_status: experimental_" in the relevant
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sections of this document.
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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 pixels)
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or an [animation](#animation). Optionally, it can also contain transparency
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information, color profile and metadata. 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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_Reader/Writer_
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: Code that reads WebP files is referred to as a _reader_, while code that
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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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_FourCC_
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: A _FourCC_ (four-character code) is a _uint32_ created by concatenating four
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ASCII characters in little-endian order.
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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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RIFF file format
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----------------
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The WebP file format is based on the RIFF (resource interchange file format)
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document format.
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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 used for chunk identification.
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Chunk Size: 32 bits (_uint32_)
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: The size of the chunk not including this field, the chunk identifier or
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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 _Chunk 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:** RIFF has a convention that all-uppercase chunk FourCCs are standard
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chunks that apply to any RIFF file format, while FourCCs specific to a file
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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. The maximum value of
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this field is 2^32 minus 10 bytes and thus the size of the whole file is at
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most 4GiB minus 2 bytes.
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'WEBP': 32 bits
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: The ASCII characters 'W' 'E' 'B' 'P'.
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A WebP file MUST begin with a RIFF header with the FourCC 'WEBP'. The file size
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in the header is the total size of the chunks that follow plus `4` bytes for
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the 'WEBP' FourCC. The file SHOULD NOT contain anything after it. As the size
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of any chunk is even, the size given by the RIFF header is also even. The
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contents of individual chunks will be described in the following sections.
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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 format:
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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 format:
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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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* An optional 'ANIM' chunk with animation control data.
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* Image data.
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* An optional 'EXIF' chunk with EXIF metadata.
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* An optional 'XMP ' chunk with XMP metadata.
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* An optional list of [unknown chunks](#unknown-chunks). _\[status: experimental\]_
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For a _still image_, the _image data_ consists of a single frame, whereas for
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an _animated image_, it consists of multiple frames. More details about frames
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can be found in the [Animation](#animation) section.
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Moreover, each frame can be fragmented or non-fragmented, as will be described
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in the [Extended WebP file header](#extended_header) section. More details about
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fragments can be found in the [Fragments](#fragments) section.
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All chunks SHOULD be placed in the same order as listed above. If a chunk
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appears in the wrong place, the file is invalid, but readers MAY parse the
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file, ignoring the chunks that 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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{:#extended_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|I|L|E|X|A|F| 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): 2 bits
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: SHOULD be `0`.
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ICC profile (I): 1 bit
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: Set if the file contains an ICC profile.
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Alpha (L): 1 bit
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: Set if any of the frames of the image contain transparency information
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("alpha").
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EXIF metadata (E): 1 bit
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: Set if the file contains EXIF metadata.
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XMP metadata (X): 1 bit
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: Set if the file contains XMP metadata.
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Animation (A): 1 bit
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: Set if this is an animated image. Data in 'ANIM' and 'ANMF' chunks should be
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used to control the animation.
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Image Fragmentation (F): 1 bit _\[status: experimental\]_
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: Set if any of the frames in the image are represented by fragments.
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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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#### Animation
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An animation is controlled by ANIM and ANMF chunks.
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ANIM Chunk:
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{:#anim_chunk}
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For an animated image, this chunk contains the _global parameters_ of the
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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('ANIM') |
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+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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| Background Color |
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+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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| Loop Count |
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+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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Background Color: 32 bits (_uint32_)
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: The default background color of the canvas in \[Blue, Green, Red, Alpha\]
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byte order. This color MAY be used to fill the unused space on the canvas around
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the frames, as well as the transparent pixels of the first frame. Background
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color is also used when disposal method is `1`.
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**Note**:
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* Background color MAY contain a transparency value (alpha), even if the
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_Alpha_ flag in [VP8X chunk](#extended_header) is unset.
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* Viewer applications SHOULD treat the background color value as a hint, and
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are not required to use it.
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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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This chunk MUST appear if the _Animation_ flag in the VP8X chunk 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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ANMF chunk:
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For animated images, this chunk contains information about a _single_ frame.
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If the _Animation flag_ is not set, then this chunk SHOULD NOT be present.
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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('ANMF') |
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+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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| Frame X | ...
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+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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... Frame Y | Frame Width Minus One ...
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+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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... | Frame Height Minus One |
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+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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| Frame Duration | Reserved |B|D|
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+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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| Frame Data |
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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 Minus One: 24 bits (_uint24_)
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: The _1-based_ width of the frame.
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The frame width is `1 + Frame Width Minus One`
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Frame Height Minus One: 24 bits (_uint24_)
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: The _1-based_ height of the frame.
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The frame height is `1 + Frame Height Minus One`
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Frame Duration: 24 bits (_uint24_)
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: The time to wait before displaying the next frame, in 1 millisecond units.
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In particular, frame duration of 0 is useful when one wants to update multiple
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areas of the canvas at once during the animation.
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Reserved: 6 bits
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: SHOULD be 0.
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Blending method (B): 1 bit
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: Indicates how transparent pixels of _the current frame_ are to be blended with
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corresponding pixels of the previous canvas:
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* `0`: Use alpha blending. After disposing of the previous frame, render the
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current frame on the canvas using [alpha-blending](#alpha-blending). If the
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current frame does not have an alpha channel, assume alpha value of 255,
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effectively replacing the rectangle.
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* `1`: Do not blend. After disposing of the previous frame, render the
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current frame on the canvas by overwriting the rectangle covered by the
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current frame.
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Disposal method (D): 1 bit
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: Indicates how _the current frame_ is to be treated after it has been displayed
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(before rendering the next frame) on the canvas:
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* `0`: Do not dispose. Leave the canvas as is.
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* `1`: Dispose to background color. Fill the _rectangle_ on the canvas covered
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by the _current frame_ with background color specified in the
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[ANIM chunk](#anim_chunk).
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**Notes**:
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* The frame disposal only applies to the _frame rectangle_, that is, the
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rectangle defined by _Frame X_, _Frame Y_, _frame width_ and _frame height_.
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It may or may not cover the whole canvas.
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{:#alpha-blending}
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* **Alpha-blending**:
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Given that each of the R, G, B and A channels is 8-bit, and the RGB
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channels are _not premultiplied_ by alpha, the formula for blending
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'dst' onto 'src' is:
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~~~~~
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blend.A = src.A + dst.A * (1 - src.A / 255)
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if blend.A = 0 then
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blend.RGB = 0
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else
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blend.RGB = (src.RGB * src.A +
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dst.RGB * dst.A * (1 - src.A / 255)) / blend.A
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~~~~~
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* Alpha-blending SHOULD be done in linear color space, by taking into account
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the [color profile](#color-profile) of the image. If the color profile is
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not present, sRGB is to be assumed. (Note that sRGB also needs to be
|
|
linearized due to a gamma of ~2.2).
|
|
|
|
Frame Data: _Chunk Size_ - `16` bytes
|
|
|
|
: For a fragmented frame, it consists of multiple [fragment chunks](#fragments).
|
|
|
|
: For a non-fragmented frame, it consists of:
|
|
|
|
* An optional [alpha subchunk](#alpha) for the frame.
|
|
|
|
* A [bitstream subchunk](#bitstream-vp8vp8l) for the frame.
|
|
|
|
* An optional list of [unknown chunks](#unknown-chunks).
|
|
|
|
**Note**: The 'ANMF' payload, _Frame Data_ above, consists of individual
|
|
_padded_ chunks as described by the [RIFF file format](#riff-file-format).
|
|
|
|
#### Fragments _\[status: experimental\]_
|
|
|
|
For images that are represented by fragments, this chunk contains data for
|
|
a single fragment. If the _Image Fragmentation Flag_ is not set, then this chunk
|
|
SHOULD NOT be present.
|
|
|
|
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('FRGM') |
|
|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|
|
| Fragment X | ...
|
|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|
|
... Fragment Y | Fragment Data |
|
|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|
|
|
|
Fragment X: 24 bits (_uint24_)
|
|
|
|
: The X coordinate of the upper left corner of the fragment is `Fragment X * 2`
|
|
|
|
Fragment Y: 24 bits (_uint24_)
|
|
|
|
: The Y coordinate of the upper left corner of the fragment is `Fragment Y * 2`
|
|
|
|
Fragment Data: _Chunk Size_ - `6` bytes
|
|
|
|
: It contains:
|
|
|
|
* An optional [alpha subchunk](#alpha) for the fragment.
|
|
* The [bitstream subchunk](#bitstream-vp8vp8l) for the fragment.
|
|
* An optional list of [unknown chunks](#unknown-chunks).
|
|
|
|
Note: The width and height of the fragment is obtained from the bitstream
|
|
subchunk.
|
|
|
|
The fragments of a frame SHOULD have the following properties:
|
|
|
|
* They collectively cover the whole frame.
|
|
|
|
* No pair of fragments have any overlapping region on the frame.
|
|
|
|
* No portion of any fragment should be located outside of the canvas.
|
|
|
|
#### Alpha
|
|
|
|
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('ALPH') |
|
|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|
|
|Rsv| P | F | C | Alpha Bitstream... |
|
|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|
|
|
|
Reserved (Rsv): 2 bits
|
|
|
|
: SHOULD be `0`.
|
|
|
|
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
|
|
|
|
Filtering method (F): 2 bits
|
|
|
|
: The filtering method used:
|
|
|
|
* `0`: None.
|
|
* `1`: Horizontal filter.
|
|
* `2`: Vertical filter.
|
|
* `3`: Gradient filter.
|
|
|
|
For each pixel, filtering is performed using the following calculations.
|
|
Assume the alpha values surrounding the current `X` position are labeled as:
|
|
|
|
C | B |
|
|
---+---+
|
|
A | X |
|
|
|
|
We seek to compute the alpha value at position `X`. First, a prediction is
|
|
made depending on the filtering method:
|
|
|
|
* Method `0`: predictor = 0
|
|
* Method `1`: predictor = A
|
|
* Method `2`: predictor = B
|
|
* Method `3`: predictor = clip(A + B - C)
|
|
|
|
where `clip(v)` is equal to:
|
|
|
|
* 0 if v < 0
|
|
* 255 if v > 255
|
|
* v otherwise
|
|
|
|
The final value is derived by adding the decompressed value `X` to the
|
|
predictor and using modulo-256 arithmetic to wrap the \[256-511\] range
|
|
into the \[0-255\] one:
|
|
|
|
`alpha = (predictor + X) % 256`
|
|
|
|
There are special cases for left-most and top-most pixel positions:
|
|
|
|
* Top-left value at location (0,0) uses 0 as predictor value. Otherwise,
|
|
* For horizontal or gradient filtering methods, the left-most pixels at
|
|
location (0, y) are predicted using the location (0, y-1) just above.
|
|
* For vertical or gradient filtering methods, the top-most pixels at
|
|
location (x, 0) are predicted using the location (x-1, 0) on the left.
|
|
|
|
|
|
Decoders are not required to use this information in any specified way.
|
|
|
|
Compression method (C): 2 bits
|
|
|
|
: The compression method used:
|
|
|
|
* `0`: No compression.
|
|
* `1`: Compressed using the WebP lossless format.
|
|
|
|
Alpha bitstream: _Chunk Size_ - `1` bytes
|
|
|
|
: Encoded alpha bitstream.
|
|
|
|
This optional chunk contains encoded alpha data for this frame/fragment. A
|
|
frame/fragment containing a 'VP8L' chunk SHOULD NOT contain this chunk.
|
|
|
|
**Rationale**: The transparency information is already part of the 'VP8L'
|
|
chunk.
|
|
|
|
The alpha channel data is stored as uncompressed 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 8-bit 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 bitstream data for a single frame/fragment.
|
|
|
|
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)](#simple-file-format-lossy)
|
|
and [Simple file format (lossless)](#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 the image data.
|
|
|
|
There SHOULD be at most one such chunk. If there are more such chunks, readers
|
|
MAY ignore all except the first one.
|
|
See the [ICC Specification][iccspec] for details.
|
|
|
|
If this chunk is not present, sRGB SHOULD be assumed.
|
|
|
|
#### Metadata
|
|
|
|
Metadata can be stored in 'EXIF' or 'XMP ' chunks.
|
|
|
|
There SHOULD be at most one chunk of each type ('EXIF' and 'XMP '). If there
|
|
are more such chunks, readers MAY ignore all except the first one. Also, a file
|
|
may possibly contain both 'EXIF' and 'XMP ' chunks.
|
|
|
|
The chunks are defined as follows:
|
|
|
|
EXIF chunk:
|
|
|
|
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('EXIF') |
|
|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|
|
| EXIF Metadata |
|
|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|
|
|
|
EXIF Metadata: _Chunk Size_ bytes
|
|
|
|
: image metadata in EXIF format.
|
|
|
|
|
|
XMP chunk:
|
|
|
|
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('XMP ') |
|
|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|
|
| XMP Metadata |
|
|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|
|
|
|
XMP Metadata: _Chunk Size_ bytes
|
|
|
|
: image metadata in XMP format.
|
|
|
|
Additional guidance about handling metadata can be found in the
|
|
Metadata Working Group's [Guidelines for Handling Metadata][metadata].
|
|
|
|
#### Unknown Chunks _\[status: experimental\]_
|
|
|
|
A RIFF chunk (described in [this](#terminology-amp-basics) section) whose _chunk
|
|
tag_ is different from any of the chunks described in this document, is
|
|
considered an _unknown chunk_.
|
|
|
|
**Rationale**: Allowing unknown chunks gives a provision for future extension
|
|
of the format, and also allows storage of any application-specific data.
|
|
|
|
A file MAY contain unknown chunks:
|
|
|
|
* At the end of the file as described in [Extended WebP file
|
|
header](#extended_header) section.
|
|
* At the end of FRGM and ANMF chunks as described in [Fragments](#fragments)
|
|
and [Animation](#animation) sections.
|
|
|
|
Readers SHOULD ignore these chunks. Writers SHOULD preserve them in their
|
|
original order (unless they specifically intend to modify these chunks).
|
|
|
|
### Assembling the Canvas from fragments/frames
|
|
|
|
Here we provide an overview of how a reader should assemble a canvas in case
|
|
of a fragmented-image and in case of an animated image. The notation
|
|
_VP8X.field_ means the field in the 'VP8X' chunk with the same description.
|
|
|
|
Displaying a _fragmented image_ canvas MUST be equivalent to the following
|
|
pseudocode: _\[status: experimental\]_
|
|
|
|
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
|
|
assert VP8X.flags.hasFragments
|
|
canvas ← new black image of size VP8X.canvasWidth x VP8X.canvasHeight.
|
|
frgm_params ← nil
|
|
for chunk in image_data:
|
|
assert chunk.tag is "FRGM"
|
|
frgm_params.fragmentX = Fragment X
|
|
frgm_params.fragmentY = Fragment Y
|
|
for subchunk in 'Fragment Data':
|
|
if subchunk.tag == "ALPH":
|
|
assert alpha subchunks not found in 'Fragment Data' earlier
|
|
frgm_params.alpha = alpha_data
|
|
else if subchunk.tag == "VP8 " OR subchunk.tag == "VP8L":
|
|
assert bitstream subchunks not found in 'Fragment Data' earlier
|
|
frgm_params.bitstream = bitstream_data
|
|
frgm_params.fragmentWidth = Width extracted from bitstream subchunk
|
|
frgm_params.fragmentHeight = Height extracted from bitstream subchunk
|
|
assert VP8X.canvasWidth >=
|
|
frgm_params.fragmentX + frgm_params.fragmentWidth
|
|
assert VP8X.canvasHeight >=
|
|
frgm_params.fragmentY + frgm_params.fragmentHeight
|
|
assert fragment has the properties mentioned in "Image Fragments" section.
|
|
render fragment with frame_params.alpha and frame_params.bitstream on canvas
|
|
with top-left corner in (frgm_params.fragmentX, frgm_params.fragmentY).
|
|
canvas contains the decoded canvas.
|
|
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
|
|
|
|
Displaying an _animated image_ canvas MUST be equivalent to the following
|
|
pseudocode:
|
|
|
|
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
|
|
assert VP8X.flags.hasAnimation
|
|
canvas ← new image of size VP8X.canvasWidth x VP8X.canvasHeight with
|
|
background color ANIM.background_color.
|
|
loop_count ← ANIM.loopCount
|
|
dispose_method ← ANIM.disposeMethod
|
|
if loop_count == 0:
|
|
loop_count = ∞
|
|
frame_params ← nil
|
|
for loop = 0, ..., loop_count - 1
|
|
assert next chunk in image_data is ANMF
|
|
frame_params.frameX = Frame X
|
|
frame_params.frameY = Frame Y
|
|
frame_params.frameWidth = Frame Width Minus One + 1
|
|
frame_params.frameHeight = Frame Height Minus One + 1
|
|
frame_params.frameDuration = Frame Duration
|
|
assert VP8X.canvasWidth >= frame_params.frameX + frame_params.frameWidth
|
|
assert VP8X.canvasHeight >= frame_params.frameY + frame_params.frameHeight
|
|
if VP8X.flags.hasFragments and first subchunk in 'Frame Data' is FRGM
|
|
// Fragmented frame.
|
|
frame_params.{bitstream,alpha} = canvas decoded from subchunks in
|
|
'Frame Data' as per the pseudocode for
|
|
_fragmented image_ above.
|
|
else
|
|
// Non-fragmented frame.
|
|
for subchunk in 'Frame Data':
|
|
if subchunk.tag == "ALPH":
|
|
assert alpha subchunks not found in 'Frame Data' earlier
|
|
frame_params.alpha = alpha_data
|
|
else if subchunk.tag == "VP8 " OR subchunk.tag == "VP8L":
|
|
assert bitstream subchunks not found in 'Frame Data' earlier
|
|
frame_params.bitstream = bitstream_data
|
|
render frame with frame_params.alpha and frame_params.bitstream on canvas
|
|
with top-left corner in (frame_params.frameX, frame_params.frameY), using
|
|
dispose method dispose_method.
|
|
Show the contents of the image for frame_params.frameDuration * 1ms.
|
|
canvas contains the decoded canvas.
|
|
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
|
|
|
|
Example file layouts
|
|
--------------------
|
|
|
|
A lossy encoded image with alpha may look as follows:
|
|
|
|
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
|
|
RIFF/WEBP
|
|
+- VP8X (descriptions of features used)
|
|
+- ALPH (alpha bitstream)
|
|
+- VP8 (bitstream)
|
|
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
|
|
|
|
A losslessly encoded image may look as follows:
|
|
|
|
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
|
|
RIFF/WEBP
|
|
+- VP8X (descriptions of features used)
|
|
+- XYZW (unknown chunk)
|
|
+- VP8L (lossless bitstream)
|
|
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
|
|
|
|
A lossless image with ICC profile and XMP metadata may
|
|
look as follows:
|
|
|
|
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
|
|
RIFF/WEBP
|
|
+- VP8X (descriptions of features used)
|
|
+- ICCP (color profile)
|
|
+- VP8L (lossless bitstream)
|
|
+- XMP (metadata)
|
|
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
|
|
|
|
A fragmented image may look as follows:
|
|
|
|
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
|
|
RIFF/WEBP
|
|
+- VP8X (descriptions of features used)
|
|
+- FRGM (fragment1 parameters + data)
|
|
+- FRGM (fragment2 parameters + data)
|
|
+- FRGM (fragment3 parameters + data)
|
|
+- FRGM (fragment4 parameters + data)
|
|
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
|
|
|
|
An animated image with EXIF metadata may look as follows:
|
|
|
|
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
|
|
RIFF/WEBP
|
|
+- VP8X (descriptions of features used)
|
|
+- ANIM (global animation parameters)
|
|
+- ANMF (frame1 parameters + data)
|
|
+- ANMF (frame2 parameters + data)
|
|
+- ANMF (frame3 parameters + data)
|
|
+- ANMF (frame4 parameters + data)
|
|
+- EXIF (metadata)
|
|
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
|
|
|
|
[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
|
|
[iccspec]: http://www.color.org/icc_specs2.xalter
|
|
[metadata]: http://www.metadataworkinggroup.org/pdf/mwg_guidance.pdf
|
|
[rfc 2119]: http://tools.ietf.org/html/rfc2119
|