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| WebP Container Specification |
| ============================ |
| |
| * TOC placeholder |
| {:toc} |
| |
| |
| Introduction |
| ------------ |
| |
| WebP is an image format that uses either (i) the VP8 key frame encoding |
| to compress image data in a lossy way, or (ii) the WebP lossless encoding |
| (and possibly other encodings in the future). These encoding schemes should |
| make it more efficient than currently used formats. It is optimized for fast |
| image transfer over the network (e.g., for websites). The WebP format has |
| feature parity (color profile, metadata, animation etc) with other formats as |
| well. This document describes the structure of a WebP file. |
| |
| The WebP container (i.e., RIFF container for WebP) allows feature support over |
| and above the basic use case of WebP (i.e., a file containing a single image |
| encoded as a VP8 key frame). The WebP container provides additional support |
| for: |
| |
| * **Lossless compression.** An image can be losslessly compressed, using the |
| WebP Lossless Format. |
| |
| * **Metadata.** An image may have metadata stored in EXIF or XMP formats. |
| |
| * **Transparency.** An image may have transparency, i.e., an alpha channel. |
| |
| * **Color Profile.** An image may have an embedded ICC profile as described |
| by the [International Color Consortium][iccspec]. |
| |
| * **Animation.** An image may have multiple frames with pauses between them, |
| making it an animation. |
| |
| The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", |
| "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this |
| document are to be interpreted as described in [RFC 2119][]. |
| |
| Bit numbering in chunk diagrams starts at `0` for the most significant bit |
| ('MSB 0') as described in [RFC 1166][]. |
| |
| **Note:** Out of the features mentioned above, lossy compression, lossless |
| compression, transparency, metadata, color profile and animation are finalized |
| and are to be considered stable. |
| |
| Terminology & Basics |
| ------------------------ |
| |
| A WebP file contains either a still image (i.e., an encoded matrix of pixels) |
| or an [animation](#animation). Optionally, it can also contain transparency |
| information, color profile and metadata. In case we need to refer only to the |
| matrix of pixels, we will call it the _canvas_ of the image. |
| |
| Below are additional terms used throughout this document: |
| |
| _Reader/Writer_ |
| |
| : Code that reads WebP files is referred to as a _reader_, while code that |
| writes them is referred to as a _writer_. |
| |
| _uint16_ |
| |
| : A 16-bit, little-endian, unsigned integer. |
| |
| _uint24_ |
| |
| : A 24-bit, little-endian, unsigned integer. |
| |
| _uint32_ |
| |
| : A 32-bit, little-endian, unsigned integer. |
| |
| _FourCC_ |
| |
| : A _FourCC_ (four-character code) is a _uint32_ created by concatenating four |
| ASCII characters in little-endian order. |
| |
| _1-based_ |
| |
| : An unsigned integer field storing values offset by `-1`. e.g., Such a field |
| would store value _25_ as _24_. |
| |
| |
| RIFF File Format |
| ---------------- |
| |
| The WebP file format is based on the RIFF (resource interchange file format) |
| document format. |
| |
| The basic element of a RIFF file is a _chunk_. It consists of: |
| |
| 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 |
| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
| | Chunk FourCC | |
| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
| | Chunk Size | |
| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
| | Chunk Payload | |
| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
| |
| Chunk FourCC: 32 bits |
| |
| : ASCII four-character code used for chunk identification. |
| |
| Chunk Size: 32 bits (_uint32_) |
| |
| : The size of the chunk not including this field, the chunk identifier or |
| padding. |
| |
| Chunk Payload: _Chunk Size_ bytes |
| |
| : The data payload. If _Chunk Size_ is odd, a single padding byte -- that |
| SHOULD be `0` -- is added. |
| |
| _ChunkHeader('ABCD')_ |
| |
| : This is used to describe the _FourCC_ and _Chunk Size_ header of individual |
| chunks, where 'ABCD' is the FourCC for the chunk. This element's |
| size is 8 bytes. |
| |
| **Note:** RIFF has a convention that all-uppercase chunk FourCCs are standard |
| chunks that apply to any RIFF file format, while FourCCs specific to a file |
| format are all lowercase. WebP does not follow this convention. |
| |
| |
| WebP File Header |
| ---------------- |
| |
| 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 |
| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
| | 'R' | 'I' | 'F' | 'F' | |
| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
| | File Size | |
| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
| | 'W' | 'E' | 'B' | 'P' | |
| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
| |
| 'RIFF': 32 bits |
| |
| : The ASCII characters 'R' 'I' 'F' 'F'. |
| |
| File Size: 32 bits (_uint32_) |
| |
| : The size of the file in bytes starting at offset 8. The maximum value of |
| this field is 2^32 minus 10 bytes and thus the size of the whole file is at |
| most 4GiB minus 2 bytes. |
| |
| 'WEBP': 32 bits |
| |
| : The ASCII characters 'W' 'E' 'B' 'P'. |
| |
| A WebP file MUST begin with a RIFF header with the FourCC 'WEBP'. The file size |
| in the header is the total size of the chunks that follow plus `4` bytes for |
| the 'WEBP' FourCC. The file SHOULD NOT contain anything after it. As the size |
| of any chunk is even, the size given by the RIFF header is also even. The |
| contents of individual chunks will be described in the following sections. |
| |
| |
| Simple File Format (Lossy) |
| -------------------------- |
| |
| This layout SHOULD be used if the image requires _lossy_ encoding and does not |
| require transparency or other advanced features provided by the extended format. |
| Files with this layout are smaller and supported by older software. |
| |
| Simple WebP (lossy) file format: |
| |
| 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 |
| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
| | WebP file header (12 bytes) | |
| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
| | VP8 chunk | |
| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
| |
| VP8 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('VP8 ') | |
| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
| | VP8 data | |
| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
| |
| VP8 data: _Chunk Size_ bytes |
| |
| : VP8 bitstream data. |
| |
| The VP8 bitstream format specification can be found at [VP8 Data Format and |
| Decoding Guide][vp8spec]. Note that the VP8 frame header contains the VP8 frame |
| width and height. That is assumed to be the width and height of the canvas. |
| |
| The VP8 specification describes how to decode the image into Y'CbCr |
| format. To convert to RGB, Rec. 601 SHOULD be used. |
| |
| |
| Simple File Format (Lossless) |
| ----------------------------- |
| |
| **Note:** Older readers may not support files using the lossless format. |
| |
| This layout SHOULD be used if the image requires _lossless_ encoding (with an |
| optional transparency channel) and does not require advanced features provided |
| by the extended format. |
| |
| Simple WebP (lossless) file format: |
| |
| 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 |
| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
| | WebP file header (12 bytes) | |
| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
| | VP8L chunk | |
| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
| |
| VP8L 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('VP8L') | |
| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
| | VP8L data | |
| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
| |
| VP8L data: _Chunk Size_ bytes |
| |
| : VP8L bitstream data. |
| |
| The current specification of the VP8L bitstream can be found at |
| [WebP Lossless Bitstream Format][webpllspec]. Note that the VP8L header |
| contains the VP8L image width and height. That is assumed to be the width |
| and height of the canvas. |
| |
| |
| Extended File Format |
| -------------------- |
| |
| **Note:** Older readers may not support files using the extended format. |
| |
| An extended format file consists of: |
| |
| * A 'VP8X' chunk with information about features used in the file. |
| |
| * An optional 'ICCP' chunk with color profile. |
| |
| * An optional 'ANIM' chunk with animation control data. |
| |
| * Image data. |
| |
| * An optional 'EXIF' chunk with EXIF metadata. |
| |
| * An optional 'XMP ' chunk with XMP metadata. |
| |
| * An optional list of [unknown chunks](#unknown-chunks). _\[status: experimental\]_ |
| |
| For a _still image_, the _image data_ consists of a single frame, which is made |
| up of: |
| |
| * An optional [alpha subchunk](#alpha). |
| |
| * A [bitstream subchunk](#bitstream-vp8vp8l). |
| |
| For an _animated image_, the _image data_ consists of multiple frames. More |
| details about frames can be found in the [Animation](#animation) section. |
| |
| All chunks SHOULD be placed in the same order as listed above. If a chunk |
| appears in the wrong place, the file is invalid, but readers MAY parse the |
| file, ignoring the chunks that come too late. |
| |
| **Rationale:** Setting the order of chunks should allow quicker file |
| parsing. For example, if an 'ALPH' chunk does not appear in its required |
| position, a decoder can choose to stop searching for it. The rule of |
| ignoring late chunks should make programs that need to do a full search |
| give the same results as the ones stopping early. |
| |
| Extended WebP file header: |
| {:#extended_header} |
| |
| 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 |
| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
| | WebP file header (12 bytes) | |
| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
| | ChunkHeader('VP8X') | |
| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
| |Rsv|I|L|E|X|A|R| Reserved | |
| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
| | Canvas Width Minus One | ... |
| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
| ... Canvas Height Minus One | |
| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
| |
| Reserved (Rsv): 2 bits |
| |
| : SHOULD be `0`. |
| |
| ICC profile (I): 1 bit |
| |
| : Set if the file contains an ICC profile. |
| |
| Alpha (L): 1 bit |
| |
| : Set if any of the frames of the image contain transparency information |
| ("alpha"). |
| |
| EXIF metadata (E): 1 bit |
| |
| : Set if the file contains EXIF metadata. |
| |
| XMP metadata (X): 1 bit |
| |
| : Set if the file contains XMP metadata. |
| |
| Animation (A): 1 bit |
| |
| : Set if this is an animated image. Data in 'ANIM' and 'ANMF' chunks should be |
| used to control the animation. |
| |
| Reserved (R): 1 bit |
| |
| : SHOULD be `0`. |
| |
| Reserved: 24 bits |
| |
| : SHOULD be `0`. |
| |
| Canvas Width Minus One: 24 bits |
| |
| : _1-based_ width of the canvas in pixels. |
| The actual canvas width is '1 + Canvas Width Minus One' |
| |
| Canvas Height Minus One: 24 bits |
| |
| : _1-based_ height of the canvas in pixels. |
| The actual canvas height is '1 + Canvas Height Minus One' |
| |
| The product of _Canvas Width_ and _Canvas Height_ MUST be at most `2^32 - 1`. |
| |
| Future specifications MAY add more fields. |
| |
| ### Chunks |
| |
| #### Animation |
| |
| An animation is controlled by ANIM and ANMF chunks. |
| |
| ANIM Chunk: |
| {:#anim_chunk} |
| |
| For an animated image, this chunk contains the _global parameters_ of the |
| animation. |
| |
| 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('ANIM') | |
| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
| | Background Color | |
| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
| | Loop Count | |
| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
| |
| Background Color: 32 bits (_uint32_) |
| |
| : The default background color of the canvas in \[Blue, Green, Red, Alpha\] |
| byte order. This color MAY be used to fill the unused space on the canvas |
| around the frames, as well as the transparent pixels of the first frame. |
| Background color is also used when disposal method is `1`. |
| |
| **Note**: |
| |
| * Background color MAY contain a transparency value (alpha), even if the |
| _Alpha_ flag in [VP8X chunk](#extended_header) is unset. |
| |
| * Viewer applications SHOULD treat the background color value as a hint, and |
| are not required to use it. |
| |
| * The canvas is cleared at the start of each loop. The background color MAY be |
| used to achieve this. |
| |
| Loop Count: 16 bits (_uint16_) |
| |
| : The number of times to loop the animation. `0` means infinitely. |
| |
| This chunk MUST appear if the _Animation_ flag in the VP8X chunk is set. |
| If the _Animation_ flag is not set and this chunk is present, it |
| SHOULD be ignored. |
| |
| ANMF chunk: |
| |
| For animated images, this chunk contains information about a _single_ frame. |
| If the _Animation 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('ANMF') | |
| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
| | Frame X | ... |
| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
| ... Frame Y | Frame Width Minus One ... |
| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
| ... | Frame Height Minus One | |
| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
| | Frame Duration | Reserved |B|D| |
| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
| | Frame Data | |
| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
| |
| Frame X: 24 bits (_uint24_) |
| |
| : The X coordinate of the upper left corner of the frame is `Frame X * 2` |
| |
| Frame Y: 24 bits (_uint24_) |
| |
| : The Y coordinate of the upper left corner of the frame is `Frame Y * 2` |
| |
| Frame Width Minus One: 24 bits (_uint24_) |
| |
| : The _1-based_ width of the frame. |
| The frame width is `1 + Frame Width Minus One` |
| |
| Frame Height Minus One: 24 bits (_uint24_) |
| |
| : The _1-based_ height of the frame. |
| The frame height is `1 + Frame Height Minus One` |
| |
| Frame Duration: 24 bits (_uint24_) |
| |
| : The time to wait before displaying the next frame, in 1 millisecond units. |
| Note the interpretation of frame duration of 0 (and often <= 10) is |
| implementation defined. Many tools and browsers assign a minimum duration |
| similar to GIF. |
| |
| Reserved: 6 bits |
| |
| : SHOULD be 0. |
| |
| Blending method (B): 1 bit |
| |
| : Indicates how transparent pixels of _the current frame_ are to be blended |
| with corresponding pixels of the previous canvas: |
| |
| * `0`: Use alpha blending. After disposing of the previous frame, render the |
| current frame on the canvas using [alpha-blending](#alpha-blending). If |
| the current frame does not have an alpha channel, assume alpha value of |
| 255, effectively replacing the rectangle. |
| |
| * `1`: Do not blend. After disposing of the previous frame, render the |
| current frame on the canvas by overwriting the rectangle covered by the |
| current frame. |
| |
| Disposal method (D): 1 bit |
| |
| : Indicates how _the current frame_ is to be treated after it has been |
| displayed (before rendering the next frame) on the canvas: |
| |
| * `0`: Do not dispose. Leave the canvas as is. |
| |
| * `1`: Dispose to background color. Fill the _rectangle_ on the canvas |
| covered by the _current frame_ with background color specified in the |
| [ANIM chunk](#anim_chunk). |
| |
| **Notes**: |
| |
| * The frame disposal only applies to the _frame rectangle_, that is, the |
| rectangle defined by _Frame X_, _Frame Y_, _frame width_ and _frame height_. |
| It may or may not cover the whole canvas. |
| |
| {:#alpha-blending} |
| * **Alpha-blending**: |
| |
| Given that each of the R, G, B and A channels is 8-bit, and the RGB |
| channels are _not premultiplied_ by alpha, the formula for blending |
| 'dst' onto 'src' is: |
| |
| ~~~~~ |
| blend.A = src.A + dst.A * (1 - src.A / 255) |
| if blend.A = 0 then |
| blend.RGB = 0 |
| else |
| blend.RGB = (src.RGB * src.A + |
| dst.RGB * dst.A * (1 - src.A / 255)) / blend.A |
| ~~~~~ |
| |
| * Alpha-blending SHOULD be done in linear color space, by taking into account |
| the [color profile](#color-profile) of the image. If the color profile is |
| 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 |
| |
| : 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). |
| |
| #### 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. A frame |
| 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. |
| |
| 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 ANMF chunks as described in the |
| [Animation](#animation) section. |
| |
| 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 frames |
| |
| Here we provide an overview of how a reader should assemble a canvas in the |
| case of an animated image. The notation _VP8X.field_ means the field in the |
| 'VP8X' chunk with the same description. |
| |
| 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 |
| assert next chunk in image_data is ANMF |
| for loop = 0..loop_count - 1 |
| clear canvas to ANIM.background_color or application defined color |
| until eof or non-ANMF chunk |
| 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 |
| frame_right = frame_params.frameX + frame_params.frameWidth |
| frame_bottom = frame_params.frameY + frame_params.frameHeight |
| assert VP8X.canvasWidth >= frame_right |
| assert VP8X.canvasHeight >= frame_bottom |
| 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 at (frame_params.frameX, |
| frame_params.frameY), using dispose method dispose_method. |
| canvas contains the decoded image. |
| Show the contents of the canvas for frame_params.frameDuration * 1ms. |
| ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ |
| |
| |
| 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) |
| ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ |
| |
| 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://chromium.googlesource.com/webm/libwebp/+/master/doc/webp-lossless-bitstream-spec.txt |
| [iccspec]: http://www.color.org/icc_specs2.xalter |
| [metadata]: http://www.metadataworkinggroup.org/pdf/mwg_guidance.pdf |
| [rfc 1166]: http://tools.ietf.org/html/rfc1166 |
| [rfc 2119]: http://tools.ietf.org/html/rfc2119 |
