mirror of
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Update the WebP Container Spec.
- Add details about the VP8L chunk support. - Also add new example bitsteams containing VP8L chunk. - Add back a section describing the VP8 chunk. - Restrict some fields to 16 or 24 bits instead of 32 bits. - Fields whose values are always positive are stored 1-based instead of 0-based. - Unknown chunks can only occur at certain places in the file. - Remove the restriction for some fields to be divisible by 32 or 16. Instead they are restricted to be even. - Add a restriction for (canvas_width * canvas_height) product. - Add 3 bits for rotation & symmetry in VP8X flags. - Add some new example layouts. - Add/clarify some nitty-gritties throughout the doc. - Use the terms frame/tile more consistently and logically. - Update related TODOs. Change-Id: I611c1f58ecc3ee87546ca31bad1609376fad251e
This commit is contained in:
Urvang Joshi
parent
31b68fe639
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11
doc/TODO
11
doc/TODO
@ -3,10 +3,6 @@
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* Determine that normative RFC 2119 terms (MUST, SHOULD, MAY, etc.) are
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truly intended in all cases where capitalized.
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* Document hierarchy WRT headings has a flaw, in that topics related to
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animated WebPs are discussed under subheads of "Single-image WebP
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Files".
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* Several passages could be made clearer.
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* Overall edit for scope. Portions are phrased as an introduction to
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@ -15,10 +11,3 @@
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* To wit, suggest s/[spec|specification]/guide/g . "Spec" can imply a
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standards track; in any case it's too formal for a work in progress.
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* Sections and passages re "multi-image" should likely be suppressed
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until multi-image drops.
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* Improve the term "Mux-Container".
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@ -13,7 +13,7 @@ end of this file.
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WebP Container Specification
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============================
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_Working Draft, v0.3, 20120523_
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_Working Draft, v0.4, 20120613_
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* TOC placeholder
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@ -23,40 +23,42 @@ _Working Draft, v0.3, 20120523_
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Introduction
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------------
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WebP is a still image format that uses the VP8 key frame encoding, and
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possibly other encodings in the future, to compress image data in a
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lossy way. The VP8 encoding should make it more efficient than currently
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used formats. It is optimized for fast image transfer over the network
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(e.g., for websites). However, it also aims for feature parity
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(color profile, XMP metadata, animation, etc.) with other formats. This
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document describes the structure of a WebP file.
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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 first version of WebP handled only the basic use case: a file
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containing a single image (being one VP8 key frame), with no metadata.
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The use of a RIFF container permits additional feature support. This
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document describes additional support for:
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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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* **Metadata and color profiles.** We specify chunks that can contain
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this information, as other popular formats do.
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* **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 512kB limit on the size of the first
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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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* **Animation.** An image may have pauses between frames, making it
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an animation.
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* **Transparency.** An image may have transparency, i.e., an alpha channel for
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each frame/tile.
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Files not using these new features are backward compatible with the
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original format. Use of these features will produce files that are not
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compatible with older programs.
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The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
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"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
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document are to be interpreted as described in [RFC 2119][].
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@ -71,9 +73,9 @@ metadata, etc. In case we need to refer only to the matrix of pixels,
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we will call it the _canvas_ of the image.
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The canvas of an image is built from one or multiple tiles. Each tile
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is a separately encoded VP8 key frame (other encodings are possible in
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the future). Building an image from several tiles allows us to overcome
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the size limitations of a single VP8 frame. Tiles are an internal detail
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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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@ -85,10 +87,18 @@ _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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@ -180,15 +190,28 @@ File Size: 32 bits (_uint32_)
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: The ASCII characters 'W' 'E' 'B' 'P'.
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Simple file format
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------------------
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Simple WebP file header:
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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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@ -198,18 +221,50 @@ VP8 data: _Chunk Size_ bytes
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: VP8 bitstream data.
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The content of a 'VP8 ' chunk (note the last character is a space) MUST be one
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VP8 key frame (with optional padding).
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The current [VP8 Data Format and Decoding Guide][vp8spec] can be found
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at the IETF website, <http://www.ietf.org/>.
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The VP8 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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This layout SHOULD be used if the image does not require advanced
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features: color profiles, XMP metadata, animation or tiling. Files with
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this layout are smaller and supported by older software.
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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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@ -222,14 +277,14 @@ An extended format file consists of:
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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 XMP metadata.
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* Some other chunk types may be defined by future specifications and
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placed anywhere in the file.
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* 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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@ -242,18 +297,21 @@ multiple frames. Data for each frame consists of:
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present in non-animated images.
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* An optional 'TILE' chunk with tile position metadata. It MUST be
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present at the beginning of data for an image that's represented as
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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 tile. It MUST be
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present for an image containing transparency. It MUST NOT be present
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in non-transparent images.
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* An optional 'ALPH' chunk with alpha bitstream of the frame/tile. If the
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file contains transparency, this chunk MUST be present for each frame/tile
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containing a 'VP8 ' chunk. It MUST NOT be present otherwise.
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* A 'VP8 ' chunk with the bitstream of the 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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All chunks SHOULD be placed in the same order as listed above (except
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for unknown chunks, which MAY appear anywhere). If a chunk appears
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in the wrong place, the file is invalid, but readers MAY parse the
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* 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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@ -271,12 +329,12 @@ Extended WebP file header:
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+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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| ChunkHeader('VP8X') |
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+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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| Rsv |L|M|I|A|T| Reserved |
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| 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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| 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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@ -299,25 +357,48 @@ Alpha (L): 1 bit
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: Set if the file contains images with transparency information ("alpha").
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Reserved (Rsv): 3 bits
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Rotation and Symmetry (R): 3 bits
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: SHOULD be `0`.
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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: 32 bits
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Canvas Width: 24 bits
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: Width of the canvas in pixels.
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: _1-based_ width of the canvas in pixels.
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Canvas Height: 32 bits
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Canvas Height: 24 bits
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: Height of the canvas in pixels.
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: _1-based_ height of the canvas in pixels.
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Future specifications MAY add more fields. If a chunk of larger size is found,
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programs MUST ignore the extra bytes but SHOULD preserve them when modifying
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the file.
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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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@ -344,63 +425,48 @@ This chunk MUST appear if the _Animation_ flag in chunk VP8X is set.
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If the _Animation_ flag is not set and this chunk is present, it
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SHOULD be ignored.
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Per-frame parameters of the animation:
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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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| Frame X | ...
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+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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| Frame Y |
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... Frame Y | Frame Width ...
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+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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| Frame Width |
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+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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| Frame Height |
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... | Frame Height |
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+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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| Frame Duration |
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+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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Frame X: 32 bits (_uint32_)
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Frame X: 24 bits (_uint24_)
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: The X coordinate of the upper left corner of the frame.
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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: 32 bits (_uint32_)
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Frame Y: 24 bits (_uint24_)
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: The Y coordinate of the upper left corner of the frame.
|
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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: 32 bits (_uint32_)
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Frame Width: 24 bits (_uint24_)
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: The width of the frame.
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: The _1-based_ width of the frame.
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Frame Height: 32 bits (_uint32_)
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Frame Height: 24 bits (_uint24_)
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: The height of the frame.
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: The _1-based_ height of the frame.
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|
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Frame Duration: 16 bits (_uint16_)
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Frame Duration: 24 bits (_uint24_)
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: Time to wait before displaying the next tile, in 1 millisecond units.
|
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: Time to wait before displaying the next frame, in 1 millisecond units
|
||||
(_1-based_).
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Notes for frames containing VP8 data:
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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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* _Frame X_ and _Frame Y_ values MUST be divisible by `32`.
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**Rationale:** This ensures that pixels on U and V planes are aligned to a
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16-byte boundary (even after a rotation), which may help with vector
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||||
instructions on some architectures. This also makes the tiles align to
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||||
16-pixel macroblock boundaries.
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* _Frame Width_ MUST be divisible by `16` or
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||||
`Frame X + Frame Width == Canvas Width` MUST be true.
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* _Frame Height_ MUST be divisible by `16` or
|
||||
`Frame Y + Frame Height == Canvas Height` MUST be true.
|
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|
||||
**Rationale:** The width and height constraints simplify the handling of
|
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macroblocks that are on the edge of a tile. VP8 decoders can overwrite
|
||||
pixels outside the boundary in such a macroblock, and this guarantees they
|
||||
won't overwrite any data.
|
||||
|
||||
#### Tiling
|
||||
|
||||
@ -409,35 +475,23 @@ Notes for frames containing VP8 data:
|
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+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|
||||
| ChunkHeader('TILE') |
|
||||
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|
||||
| Tile Canvas X |
|
||||
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|
||||
| Tile Canvas Y |
|
||||
| Tile X | ...
|
||||
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|
||||
... Tile Y |
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||||
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|
||||
|
||||
Tile Canvas X: 32 bits (_uint32_)
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Tile X: 24 bits (_uint24_)
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|
||||
: X coordinate of the upper left corner of the tile.
|
||||
: X coordinate of the upper left corner of the tile is `Tile X * 2`
|
||||
|
||||
Tile Canvas Y: 32 bits (_uint32_)
|
||||
Tile Y: 24 bits (_uint24_)
|
||||
|
||||
: Y coordinate of the upper left corner of the tile.
|
||||
: Y coordinate of the upper left corner of the tile is `Tile Y * 2`
|
||||
|
||||
This chunk contains information about a single tile and describes the
|
||||
(optional) alpha chunk and bitstream chunk that follows it.
|
||||
|
||||
Notes for tiles containing VP8 data:
|
||||
|
||||
* _Tile Canvas X_ and _Tile Canvas Y_ values MUST be
|
||||
divisible by `32`.
|
||||
|
||||
* The _Tile Width_ and _Tile Height_ can be extracted from the VP8 data.
|
||||
See 'Section 9' in the [VP8 RFC][vp8spec].
|
||||
|
||||
* The width of a tile MUST be divisible by `16` or
|
||||
`Tile Canvas X + Tile Width == Canvas Width` MUST be true.
|
||||
|
||||
* The height of a tile MUST be divisible by `16` or
|
||||
`Tile Canvas Y + Tile Height == Canvas Height` MUST be true.
|
||||
For images that contain tiling, this chunk contains information about a single
|
||||
tile and describes the (optional) alpha chunk and bitstream chunk that follow
|
||||
it. If the _Tile flag_ is not set and this chunk is present, it SHOULD be
|
||||
ignored.
|
||||
|
||||
|
||||
#### Alpha
|
||||
@ -455,7 +509,7 @@ Compression method (C): 2 bits
|
||||
: The compression method used:
|
||||
|
||||
* `0`: No compression.
|
||||
* `1`: Backward reference counts encoded with arithmetic encoder.
|
||||
* `1`: Compressed using the WebP lossless format.
|
||||
|
||||
Filtering method (F): 2 bits
|
||||
|
||||
@ -485,20 +539,23 @@ Alpha bitstream: _Chunk Size_ - `1` bytes
|
||||
|
||||
: Encoded alpha bitstream.
|
||||
|
||||
This optional chunk contains encoded alpha data for a single tile.
|
||||
Either **ALL or NONE** of the tiles must contain this chunk.
|
||||
This optional chunk contains encoded alpha data for a single frame/tile.
|
||||
Either **ALL or NONE** of the frame/tiles must contain this chunk. However,
|
||||
there is one exception to this rule: 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,
|
||||
* 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 specification for lossless format)
|
||||
of implicit dimension width x height. Note that this image-stream byte
|
||||
sequence does NOT contain any headers describing the image dimension.
|
||||
* 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.
|
||||
@ -512,6 +569,20 @@ compression method is '0') or compressed using the lossless format
|
||||
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
|
||||
@ -519,21 +590,17 @@ compression method is '0') or compressed using the lossless format
|
||||
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|
||||
| ChunkHeader('ICCP') |
|
||||
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|
||||
| Compression | Color Profile |
|
||||
| Color Profile |
|
||||
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|
||||
|
||||
Compression: 8 bits
|
||||
|
||||
: Compression method used:
|
||||
|
||||
* `0`: None.
|
||||
* `1`: Deflate/inflate.
|
||||
|
||||
Color Profile: _Chunk Size_ - `1` bytes
|
||||
Color Profile: _Chunk Size_ bytes
|
||||
|
||||
: ICC profile.
|
||||
|
||||
There SHOULD be at most one 'ICCP' chunk.
|
||||
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.
|
||||
@ -545,38 +612,25 @@ If this chunk is not present, sRGB SHOULD be assumed.
|
||||
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|
||||
| ChunkHeader('META') |
|
||||
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|
||||
| Compression | XMP Metadata |
|
||||
| Metadata |
|
||||
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|
||||
|
||||
Compression: 8 bits
|
||||
Metadata: _Chunk Size_ bytes
|
||||
|
||||
: Compression method used:
|
||||
: image metadata.
|
||||
|
||||
* `0`: None.
|
||||
* `1`: Deflate/inflate.
|
||||
|
||||
XMP Metadata: _Chunk Size_ - `1` bytes
|
||||
|
||||
: XMP 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.
|
||||
|
||||
XMP packets are XML text as specified in the [XMP Specification Part
|
||||
1][xmpspec]. The chunk tag is different from the one specified by Adobe
|
||||
for WAV and AVI (also RIFF formats), because we have the option of
|
||||
compression.
|
||||
|
||||
Additional guidance about handling metadata can be found in the
|
||||
Metadata Working Group's [Guidelines for Handling Metadata][metadata].
|
||||
Note that the sections of the document about reconciliation of EXIF,
|
||||
XMP and IPTC-IIM don't apply to WebP. As WebP supports only XMP, no
|
||||
reconciliation is necessary.
|
||||
|
||||
#### Other Chunks
|
||||
#### Unknown Chunks
|
||||
|
||||
A file MAY contain other chunks. Readers SHOULD be ignore these chunks. Writers
|
||||
SHOULD preserve them in their original order.
|
||||
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
|
||||
|
||||
@ -584,63 +638,62 @@ 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
|
||||
Decoding a _non-animated_ canvas MUST be equivalent to the following
|
||||
pseudocode:
|
||||
|
||||
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
|
||||
assert not VP8X.flags.haveAnimation
|
||||
canvas ← new black image of size
|
||||
VP8X.canvasWidth x VP8X.canvasHeight.
|
||||
tile_params.tileCanvasX = tile_params.tileCanvasY = 0
|
||||
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 "VP8 " chunk
|
||||
assert No ALPH chunk after the last "VP8 " chunk
|
||||
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 "VP8 " chunk
|
||||
assert No other ALPH chunk after the last Bitstream chunk
|
||||
tile_params.alpha = alpha_data
|
||||
if chunk.tag == "VP8 ":
|
||||
render image in chunk in canvas with top-left corner in
|
||||
(tile_params.tileCanvasX, tile_params.tileCanvasY).
|
||||
tile_params.tileCanvasX = tile_params.tileCanvasY = 0
|
||||
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
|
||||
Decoding an _animated_ canvas MUST be equivalent to the following
|
||||
pseudocode:
|
||||
|
||||
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
|
||||
assert VP8X.flags.haveAnimation
|
||||
canvas ← new black image of size
|
||||
VP8X.canvasWidth x VP8X.canvasHeight.
|
||||
if LOOP.loopCount==0:
|
||||
LOOP.loopCount=∞
|
||||
current_FRM ← nil
|
||||
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 a FRM
|
||||
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 "VP8 " chunk
|
||||
assert No ALPH chunk after the last "VP8 " chunk
|
||||
if current_FRM != nil:
|
||||
Show the contents of canvas for
|
||||
current_FRM.frameDuration * 1ms.
|
||||
current_FRM = chunk
|
||||
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 "VP8 " chunk
|
||||
tile_params.alpha = alpha_data
|
||||
if chunk.tag == "VP8 ":
|
||||
assert tile_params.tileCanvasX >= current_FRM.frameX
|
||||
assert tile_params.tileCanvasY >= current_FRM.frameY
|
||||
assert tile_params.tileCanvasX + chunk.tileWidth >=
|
||||
current_FRM.frameX + current_FRM.frameWidth
|
||||
assert tile_params.tileCanvasY + chunk.tileHeight >=
|
||||
current_FRM.frameX + current_FRM.frameHeight
|
||||
render image in chunk in canvas with top-left corner in
|
||||
(tile_params.tileCanvasX, tile_params.tileCanvasY).
|
||||
tile_params.tileCanvasX = tile_params.tileCanvasY = 0
|
||||
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.
|
||||
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
|
||||
@ -651,7 +704,7 @@ MAY ignore the badly-ordered chunks instead of failing to decode the file.
|
||||
Example file layouts
|
||||
--------------------
|
||||
|
||||
A non-animated, tiled image without transparency may look as follows:
|
||||
A tiled image without transparency may look as follows:
|
||||
|
||||
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
|
||||
RIFF/WEBP
|
||||
@ -665,7 +718,7 @@ RIFF/WEBP
|
||||
+- VP8 (bitstream - third tile)
|
||||
+- TILE (fourth tile parameters)
|
||||
+- VP8 (bitstream - fourth tile)
|
||||
+- META (XMP metadata)
|
||||
+- META (metadata)
|
||||
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
|
||||
|
||||
An animated image with transparency may look as follows:
|
||||
@ -675,15 +728,43 @@ RIFF/WEBP
|
||||
+- VP8X (descriptions of features used)
|
||||
+- LOOP (animation control parameters)
|
||||
+- FRM (first animation frame parameters)
|
||||
+- ALPH (alpha bitstream - first image frame)
|
||||
+- VP8 (bitstream - first image frame)
|
||||
+- ALPH (alpha bitstream - first frame)
|
||||
+- VP8 (bitstream - first frame)
|
||||
+- FRM (second animation frame parameters)
|
||||
+- ALPH (alpha bitstream - second image frame)
|
||||
+- VP8 (bitstream - second image frame)
|
||||
+- META (XMP metadata)
|
||||
+- 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
|
||||
[xmpspec]: http://www.adobe.com/content/dam/Adobe/en/devnet/xmp/pdfs/XMPSpecificationPart1.pdf
|
||||
[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
|
||||
|
||||
Loading…
Reference in New Issue
Block a user