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doc: remove non-finalized chunk references
tiles, animation, etc. Change-Id: Ie83135be576f516fe475c7be21171f9d12a17ce4
This commit is contained in:
James Zern
parent
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commit
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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, v0.4, 20120613_
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_Working Draft, v0.5, 20120713_
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* TOC placeholder
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@ -27,9 +27,8 @@ 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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image transfer over the network (e.g., for websites). This document describes
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the structure of a WebP file.
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The WebP container (i.e., RIFF container for WebP) allows feature support over
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and above the basic use case of WebP (i.e., a file containing a single image
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@ -39,25 +38,7 @@ for:
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* **Lossless compression.** An image can be losslessly compressed, using the
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WebP Lossless Format.
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* **Transparency.** An image may have transparency, i.e., an alpha channel
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for each frame/tile.
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* **Metadata.** An image can have metadata stored in any of the popular
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metadata formats.
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* **Color profiles.** An image can have an ICC profile characterizing a color
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input or output device.
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* **Animation.** An image may have pauses between frames, making it
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an animation.
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* **Tiling.** A single VP8 frame has an inherent limitation for width
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or height of 2^14 pixels, and a 512 KiB limit on the size of the first
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compressed partition. To support larger images, we support images
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that are composed of multiple tiles, each encoded as a separate VP8
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frame. All tiles form logically a single image: they have common
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metadata, color profile, etc. Tiling may also improve efficiency for
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larger images, e.g., grass can be encoded differently than sky.
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* **Transparency.** An image may have transparency, i.e., an alpha channel.
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The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
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"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
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@ -67,16 +48,9 @@ document are to be interpreted as described in [RFC 2119][].
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Terminology & Basics
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------------------------
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A WebP file contains either a still image (i.e., an encoded matrix of
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pixels) or an animation (see below), with possibly a color profile,
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metadata, etc. In case we need to refer only to the matrix of pixels,
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we will call it the _canvas_ of the image.
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The canvas of an image is built from one or multiple tiles. Each tile
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is a separately encoded VP8 key frame or a WebP lossless bitstream. Building
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an image from several tiles allows the format to overcome the size limitations
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of a single VP8 frame / WebP lossless bitstream. Tiles are an internal detail
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of the file: they are not supposed to be exposed to the user.
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A WebP file contains a still image (i.e., an encoded matrix of pixels) and,
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optionally, transparency information. In case we need to refer only to the
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matrix of pixels, we will call it the _canvas_ of the image.
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Below are additional terms used throughout this document:
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@ -275,46 +249,17 @@ 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 'ALPH' chunk with transparency information.
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* Optionally, some other unknown chunk types that may be defined by future
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specifications.
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* An optional 'LOOP' chunk with animation control data.
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* Data for all the frames.
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* An optional 'META' chunk with metadata.
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A file MUST contain at least one frame. As will be described in the 'VP8X'
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chunk description, by checking a flag one can distinguish animated and
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non-animated images. A non-animated image has exactly one frame. An animated
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one may have multiple frames. Data for each frame consists of:
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* An optional 'FRM ' (fourth character is a significant space) chunk
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with animation frame metadata. It MUST be present in animated
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images at the beginning of data for that frame. It MUST NOT be
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present in non-animated images.
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* An optional 'TILE' chunk with tile position metadata. It MUST be
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present at the beginning of each tile for a frame that is represented as
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multiple tile images.
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* An optional 'ALPH' chunk with alpha bitstream of the frame/tile.
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* A 'VP8 ' or a 'VP8L' chunk containing compressed image data of the
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frame/tile.
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* An optional unknown chunk type that may be defined by future
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specifications.
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* The image bitstream contained in either a 'VP8 ' or 'VP8L' chunk.
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All chunks SHOULD be placed in the same order as listed above. If a chunk
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appears in the wrong place, the file is invalid, but readers MAY parse the
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file, ignoring the chunks that come too late.
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**Rationale:** Setting the order of chunks should allow quicker file
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parsing. For example, if an ICCP chunk does not appear in its required
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position, a decoder can choose to stop searching for it. The rule of
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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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@ -327,61 +272,25 @@ Extended WebP file header:
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+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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| ChunkHeader('VP8X') |
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+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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| R |L|M|I|A|T| Reserved |
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| Rsv |L| Rsv | Reserved |
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+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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| Canvas Width Minus One | ...
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+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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... Canvas Height Minus One |
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+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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Tiling (T): 1 bit
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Reserved (Rsv): 4 bits
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: Set if the image is represented by tiles.
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Animation (A): 1 bit
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: Set if the file is an animation. Data in 'LOOP' and 'FRM ' chunks
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should be used to control the animation.
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ICC profile (I): 1 bit
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: Set if the file contains an 'ICCP' chunk.
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Metadata (M): 1 bit
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: Set if the file contains a 'META' chunk.
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: SHOULD be `0`.
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Alpha (L): 1 bit
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: Set if the file contains some (or all) images with transparency information
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("alpha").
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Rotation and Symmetry (R): 3 bits
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Reserved (Rsv): 3 bits
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: Specify an isometry to be applied to every bitstream chunk decoded.
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The table below specifies into what coordinates a point (x,y) in the original
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coordinate system has to be transformed into:
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| Value | Name | New coordinates |
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|:------|:--------------------------:|:--------------------------------------: |
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| 0 | Identify | (x,y) |
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|-------
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| 1 | Horizontal symmetry | (x, CanvasHeight-1-y) |
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|-------
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| 2 | Vertical symmetry | (CanvasWidth-1-x, y) |
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|-------
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| 3 | Rotation 180 degrees | (CanvasWidth-1-x, CanvasHeight-1-y) |
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|-------
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| 4 | Diagonal symmetry 1 | (y, x) |
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|-------
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| 5 | Rotation clockwise | (CanvasHeight-1-y, x) |
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|-------
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| 6 | Rotation counter-clockwise | (y, CanvasWidth-1-x) |
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|-------
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| 7 | Diagonal symmetry 2 | (CanvasHeight-1-y, CanvasWidth-1-x) |
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|-------
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{: rules="groups"}
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: SHOULD be `0`.
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Reserved: 24 bits
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@ -403,100 +312,6 @@ Future specifications MAY add more fields.
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### Chunks
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#### Animation
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Loop Chunk:
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0 1 2 3
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0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
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+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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| ChunkHeader('LOOP') |
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+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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| Loop Count |
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+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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Loop Count: 16 bits (_uint16_)
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: The number of times to loop the animation. `0` means infinitely.
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For images that are animations, this chunk contains the global
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parameters of the animation.
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This chunk MUST appear if the _Animation_ flag in chunk VP8X is set.
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If the _Animation_ flag is not set and this chunk is present, it
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SHOULD be ignored.
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Frame chunk:
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0 1 2 3
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0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
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+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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| ChunkHeader('FRM ') |
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+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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| Frame X | ...
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+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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... Frame Y | Frame Width Minus One ...
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+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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... | Frame Height Minus One |
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+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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| Frame Duration Minus One |
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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 Minus One: 24 bits (_uint24_)
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: The _1-based_ time to wait before displaying the next frame, in 1 millisecond
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units. The actual duration is '1 + Frame Duration Minus One' milliseconds.
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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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follow it. If the _Animation flag_ is not set and this chunk is present,
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it SHOULD be ignored.
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#### Tiling
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0 1 2 3
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0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
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+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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| ChunkHeader('TILE') |
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+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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| Tile X | ...
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+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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... Tile Y |
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+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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Tile X: 24 bits (_uint24_)
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: The X coordinate of the upper left corner of the tile is `Tile X * 2`
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Tile Y: 24 bits (_uint24_)
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: The Y coordinate of the upper left corner of the tile is `Tile Y * 2`
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For images that contain tiling, this chunk contains information about a single
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tile and describes the optional alpha chunk and the bitstream chunk that follow
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it. If the _Tile flag_ is not set and this chunk is present, it SHOULD be
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ignored.
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#### Alpha
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0 1 2 3
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@ -578,10 +393,9 @@ Alpha bitstream: _Chunk Size_ - `1` bytes
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: Encoded alpha bitstream.
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This optional chunk contains encoded alpha data for a single frame/tile.
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For images with transparency, some of the frames/tiles may contain this chunk.
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However, a frame/tile containing a 'VP8L' chunk SHOULD NOT contain this chunk.
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**Rationale**: the transparency information of a frame/tile is already part of
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This optional chunk contains encoded alpha data for a single frame.
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An image containing a 'VP8L' chunk SHOULD NOT contain this chunk.
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**Rationale**: the transparency information of an image is already part of
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the 'VP8L' chunk.
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The alpha channel data is stored as uncompressed raw data (when
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@ -610,198 +424,40 @@ compression method is '0') or compressed using the lossless format
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#### Bitstream (VP8/VP8L)
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This chunk contains compressed image data. As described earlier, images
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with a simple file format (lossy/lossless) have a single bitstream chunk
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as the first subchunk of RIFF, while images with extended file format may
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contain several of them, one for each frame/tile.
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This chunk contains compressed image data.
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A bitstream chunk may be either (i) a VP8 chunk, using "VP8 " (note the
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significant fourth-character space) as its tag _or_ (ii) a VP8L chunk , using
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significant fourth-character space) as its tag _or_ (ii) a VP8L chunk, using
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"VP8L" as its tag.
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The formats of VP8 and VP8L chunks are as described in sections
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[Simple file format (lossy)](#simple-file-format-lossy)
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and [Simple file format (lossless)](#simple-file-format-lossless) respectively.
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#### Color profile
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0 1 2 3
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0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
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+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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| ChunkHeader('ICCP') |
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+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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| Color Profile |
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+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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Color Profile: _Chunk Size_ bytes
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: ICC profile.
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This chunk MUST appear before data for all the frames.
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There SHOULD be at most one such chunk. If there are more such chunks, readers
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MAY ignore all except the first one.
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See <http://www.color.org> for specifications.
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If this chunk is not present, sRGB SHOULD be assumed.
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#### Metadata
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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('META') |
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+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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| Metadata |
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+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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Metadata: _Chunk Size_ bytes
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: image metadata.
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This chunk MUST appear after data for all the frames.
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There SHOULD be at most one such chunk. If there are more such chunks, readers
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MAY ignore all except the first one.
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Additional guidance about handling metadata can be found in the
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Metadata Working Group's [Guidelines for Handling Metadata][metadata].
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#### Unknown Chunks
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A file MAY contain other unknown chunks. Readers SHOULD be ignore these chunks.
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Writers SHOULD preserve them in their original order.
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### Assembling the Canvas from Tiles and Animation
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Here we provide an overview of how 'TILE' chunks and 'FRM '/'LOOP' chunks are
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used to assemble the canvas. The notation _VP8X.field_ means the field in
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the 'VP8X' chunk with the same description.
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Decoding a _non-animated_ canvas MUST be equivalent to the following
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pseudocode:
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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assert not VP8X.flags.haveAnimation
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canvas ← new black image of size
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VP8X.canvasWidth x VP8X.canvasHeight.
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tile_params.tileX = tile_params.tileY = 0
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for chunk in data_for_all_frames:
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if chunk.tag == "TILE":
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assert No other TILE chunk after the last Bitstream chunk
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assert No ALPH chunk after the last Bitstream chunk
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tile_params = chunk
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assert VP8X.canvasWidth >=
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tile_params.tileX + tile_params.tileWidth
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assert VP8X.canvasHeight >=
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tile_params.tileY + tile_params.tileHeight
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if chunk.tag == "ALPH":
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assert No other ALPH chunk after the last Bitstream chunk
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tile_params.alpha = alpha_data
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if chunk.tag == "VP8 " OR chunk.tag == "VP8L":
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render image in chunk on canvas with top-left corner in
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(tile_params.tileX, tile_params.tileY).
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Ignore unknown chunks
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canvas contains the decoded canvas.
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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Decoding an _animated_ canvas MUST be equivalent to the following
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pseudocode:
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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assert VP8X.flags.haveAnimation
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canvas ← new black image of size
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VP8X.canvasWidth x VP8X.canvasHeight.
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loop_count ← LOOP.loopCount
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if loop_count == 0:
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loop_count = ∞
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frame_params ← nil
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for LOOP.loop = 0, ..., LOOP.loopCount-1
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assert First chunk in data_for_all_frames is FRM
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for chunk in data_for_all_frames:
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if chunk.tag == "FRM ":
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assert No other FRM chunk after the last Bitstream chunk
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assert No ALPH chunk after the last Bitstream chunk
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frame_params = chunk
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assert VP8X.canvasWidth >=
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frame_params.frameX + frame_params.frameWidth
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assert VP8X.canvasHeight >=
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frame_params.frameY + frame_params.frameHeight
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if chunk.tag == "ALPH":
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assert No other ALPH chunk after the last Bitstream chunk
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frame_params.alpha = alpha_data
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if chunk.tag == "VP8 " OR chunk.tag == "VP8L":
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render image in chunk on canvas with top-left corner in
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(frame_params.frameX, frame_params.frameY). Show the contents
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of the image for frame_params.frameDuration * 1ms.
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Ignore unknown chunks
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canvas contains the decoded canvas.
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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As described earlier, if an assert related to chunk ordering fails, the reader
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MAY ignore the badly-ordered chunks instead of failing to decode the file.
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Example file layouts
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--------------------
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A tiled image without transparency may look as follows:
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A lossy encoded image with alpha may look as follows:
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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RIFF/WEBP
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+- VP8X (descriptions of features used)
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+- ICCP (color profile)
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+- TILE (First tile parameters)
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+- VP8 (bitstream - first tile)
|
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+- TILE (Second tile parameters)
|
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+- VP8 (bitstream - second tile)
|
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+- TILE (third tile parameters)
|
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+- VP8 (bitstream - third tile)
|
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+- TILE (fourth tile parameters)
|
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+- VP8 (bitstream - fourth tile)
|
||||
+- META (metadata)
|
||||
+- ALPH (alpha bitstream)
|
||||
+- VP8 (bitstream)
|
||||
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
|
||||
|
||||
An animated image with transparency may look as follows:
|
||||
A losslessly encoded image may look as follows:
|
||||
|
||||
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
|
||||
RIFF/WEBP
|
||||
+- VP8X (descriptions of features used)
|
||||
+- LOOP (animation control parameters)
|
||||
+- FRM (first animation frame parameters)
|
||||
+- ALPH (alpha bitstream - first frame)
|
||||
+- VP8 (bitstream - first frame)
|
||||
+- FRM (second animation frame parameters)
|
||||
+- ALPH (alpha bitstream - second frame)
|
||||
+- VP8 (bitstream - second frame)
|
||||
+- META (metadata)
|
||||
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
|
||||
|
||||
A losslessly encoded non-animated non-tiled image may
|
||||
look as follows:
|
||||
|
||||
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
|
||||
RIFF/WEBP
|
||||
+- VP8X (descriptions of features used)
|
||||
+- ICCP (color profile)
|
||||
+- XYZW (unknown chunk)
|
||||
+- VP8L (lossless bitstream)
|
||||
+- META (metadata)
|
||||
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
|
||||
|
||||
An animated image may have a mix of lossy and lossless
|
||||
bitstreams as follows:
|
||||
|
||||
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
|
||||
RIFF/WEBP
|
||||
+- VP8X (descriptions of features used)
|
||||
+- FRM (first animation frame parameters)
|
||||
+- VP8 (lossy bitstream - first frame)
|
||||
+- FRM (second animation frame parameters)
|
||||
+- VP8L (lossless bitstream - second frame)
|
||||
+- ABCD (unknown chunk)
|
||||
+- FRM (third animation frame parameters)
|
||||
+- VP8 (lossy bitstream - third frame)
|
||||
+- EFGH (unknown chunk)
|
||||
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
|
||||
|
||||
[vp8spec]: http://tools.ietf.org/html/rfc6386
|
||||
|
||||
Loading…
Reference in New Issue
Block a user