mirror of
https://github.com/webmproject/libwebp.git
synced 2024-12-26 05:38:22 +01:00
135ca69eb7
Clarify that a file must contain at least one frame. Change-Id: I7ecc97084498adc108275585d0c7d0aaa9f5c6ed
774 lines
29 KiB
Plaintext
774 lines
29 KiB
Plaintext
<!--
|
|
|
|
Although you may be viewing an alternate representation, this document
|
|
is sourced in Markdown, a light-duty markup scheme, and is optimized for
|
|
the [kramdown](http://kramdown.rubyforge.org/) transformer.
|
|
|
|
See the accompanying README. External link targets are referenced at the
|
|
end of this file.
|
|
|
|
-->
|
|
|
|
|
|
WebP Container Specification
|
|
============================
|
|
|
|
_Working Draft, v0.4, 20120613_
|
|
|
|
|
|
* 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). However, it also aims
|
|
for feature parity (color profile, metadata, animation, etc.) with other
|
|
formats. 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.
|
|
|
|
* **Transparency.** An image may have transparency, i.e., an alpha channel
|
|
for each frame/tile.
|
|
|
|
* **Metadata.** An image can have metadata stored in any of the popular
|
|
metadata formats.
|
|
|
|
* **Color profiles.** An image can have an ICC profile characterizing a color
|
|
input or output device.
|
|
|
|
* **Animation.** An image may have pauses between frames, making it
|
|
an animation.
|
|
|
|
* **Tiling.** A single VP8 frame has an inherent limitation for width
|
|
or height of 2^14 pixels, and a 512 KiB limit on the size of the first
|
|
compressed partition. To support larger images, we support images
|
|
that are composed of multiple tiles, each encoded as a separate VP8
|
|
frame. All tiles form logically a single image: they have common
|
|
metadata, color profile, etc. Tiling may also improve efficiency for
|
|
larger images, e.g., grass can be encoded differently than sky.
|
|
|
|
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][].
|
|
|
|
|
|
Terminology & Basics
|
|
------------------------
|
|
|
|
A WebP file contains either a still image (i.e., an encoded matrix of
|
|
pixels) or an animation (see below), with possibly a color profile,
|
|
metadata, etc. In case we need to refer only to the matrix of pixels,
|
|
we will call it the _canvas_ of the image.
|
|
|
|
The canvas of an image is built from one or multiple tiles. Each tile
|
|
is a separately encoded VP8 key frame or a WebP lossless bitstream. Building
|
|
an image from several tiles allows the format to overcome the size limitations
|
|
of a single VP8 frame / WebP lossless bitstream. Tiles are an internal detail
|
|
of the file: they are not supposed to be exposed to the user.
|
|
|
|
Below are additional terms used throughout this document:
|
|
|
|
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.
|
|
|
|
_1-based_
|
|
: An unsigned integer field storing values offset by `-1`. e.g., Such a field
|
|
would store value _25_ as _24_.
|
|
|
|
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 or _chunk tag_ used for chunk identification.
|
|
|
|
Chunk Size: 32 bits (_uint32_)
|
|
|
|
: The size of the chunk (_ckSize_) not including this field, the chunk
|
|
identifier and 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 size header of individual
|
|
chunks, where 'ABCD' is the fourcc for the chunk. This element's
|
|
size is 8 bytes.
|
|
|
|
: Note that, in this specification, all chunk tag characters are in
|
|
file order, not in byte order of a uint32 of any particular
|
|
architecture.
|
|
|
|
_list of chunks_
|
|
|
|
: A concatenation of multiple chunks.
|
|
|
|
: We will refer to the first chunk as having _position_ 0, the second
|
|
as position 1, etc. By _chunk with index 0 among "ABCD"_ we mean
|
|
the first chunk among the chunks of type "ABCD" in the list, the
|
|
_chunk with index 1 among "ABCD"_ is the second such chunk, etc.
|
|
|
|
A WebP file MUST begin with a single chunk with a tag 'RIFF'. All
|
|
other defined chunks are contained within this chunk. The file SHOULD
|
|
NOT contain anything after it.
|
|
|
|
The maximum size of RIFF's _ckSize_ is 2^32 minus 10 bytes. The size
|
|
of the whole file is at most 4GiB minus 2 bytes.
|
|
|
|
**Note:** some RIFF libraries are said to have bugs when handling files
|
|
larger than 1GiB or 2GiB. If you are using an existing library, check
|
|
that it handles large files correctly.
|
|
|
|
The first four bytes of the RIFF chunk contents (i.e., bytes 8-11 of the file)
|
|
MUST be the ASCII string "WEBP". They are followed by a list of chunks. As the
|
|
size of any chunk is even, the size of the RIFF chunk is also even. The
|
|
contents of the chunks in that list will be described in the following sections.
|
|
|
|
**Note:** RIFF has a convention that all-uppercase chunks are standard
|
|
chunks that apply to any RIFF file format, while chunks 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.
|
|
|
|
'WEBP': 32 bits
|
|
|
|
: The ASCII characters 'W' 'E' 'B' 'P'.
|
|
|
|
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 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) |
|
|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|
|
| 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 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) |
|
|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|
|
| 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.
|
|
|
|
* Optionally, some other unknown chunk types that may be defined by future
|
|
specifications.
|
|
|
|
* An optional 'LOOP' chunk with animation control data.
|
|
|
|
* Data for all the frames.
|
|
|
|
* An optional 'META' chunk with metadata.
|
|
|
|
A file MUST contain at least one frame. As will be described in the 'VP8X'
|
|
chunk description, by checking a flag one can distinguish animated and
|
|
non-animated images. A non-animated image has exactly one frame. An animated
|
|
one may have multiple frames. Data for each frame consists of:
|
|
|
|
* An optional 'FRM ' (fourth character is a significant space) chunk
|
|
with animation frame metadata. It MUST be present in animated
|
|
images at the beginning of data for that frame. It MUST NOT be
|
|
present in non-animated images.
|
|
|
|
* An optional 'TILE' chunk with tile position metadata. It MUST be
|
|
present at the beginning of each tile for a frame that is represented as
|
|
multiple tile images.
|
|
|
|
* An optional 'ALPH' chunk with alpha bitstream of the frame/tile.
|
|
|
|
* A 'VP8 ' or a 'VP8L' chunk containing compressed image data of the
|
|
frame/tile.
|
|
|
|
* An optional unknown chunk type that may be defined by future
|
|
specifications.
|
|
|
|
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 ICCP 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:
|
|
|
|
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') |
|
|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|
|
| R |L|M|I|A|T| Reserved |
|
|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|
|
| Canvas Width Minus One | ...
|
|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|
|
... Canvas Height Minus One |
|
|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|
|
|
|
Tiling (T): 1 bit
|
|
|
|
: Set if the image is represented by tiles.
|
|
|
|
Animation (A): 1 bit
|
|
|
|
: Set if the file is an animation. Data in 'LOOP' and 'FRM ' chunks
|
|
should be used to control the animation.
|
|
|
|
ICC profile (I): 1 bit
|
|
|
|
: Set if the file contains an 'ICCP' chunk.
|
|
|
|
Metadata (M): 1 bit
|
|
|
|
: Set if the file contains a 'META' chunk.
|
|
|
|
Alpha (L): 1 bit
|
|
|
|
: Set if the file contains some (or all) images with transparency information
|
|
("alpha").
|
|
|
|
Rotation and Symmetry (R): 3 bits
|
|
|
|
: Specify an isometry to be applied to every bitstream chunk decoded.
|
|
|
|
The table below specifies into what coordinates a point (x,y) in the original
|
|
coordinate system has to be transformed into:
|
|
|
|
| Value | Name | New coordinates |
|
|
|:------|:--------------------------:|:--------------------------------------: |
|
|
| 0 | Identify | (x,y) |
|
|
|-------
|
|
| 1 | Horizontal symmetry | (x, CanvasHeight-1-y) |
|
|
|-------
|
|
| 2 | Vertical symmetry | (CanvasWidth-1-x, y) |
|
|
|-------
|
|
| 3 | Rotation 180 degrees | (CanvasWidth-1-x, CanvasHeight-1-y) |
|
|
|-------
|
|
| 4 | Diagonal symmetry 1 | (y, x) |
|
|
|-------
|
|
| 5 | Rotation clockwise | (CanvasHeight-1-y, x) |
|
|
|-------
|
|
| 6 | Rotation counter-clockwise | (y, CanvasWidth-1-x) |
|
|
|-------
|
|
| 7 | Diagonal symmetry 2 | (CanvasHeight-1-y, CanvasWidth-1-x) |
|
|
|-------
|
|
{: rules="groups"}
|
|
|
|
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
|
|
|
|
Loop 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('LOOP') |
|
|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|
|
| Loop Count |
|
|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|
|
|
|
Loop Count: 16 bits (_uint16_)
|
|
|
|
: The number of times to loop the animation. `0` means infinitely.
|
|
|
|
For images that are animations, this chunk contains the global
|
|
parameters of the animation.
|
|
|
|
This chunk MUST appear if the _Animation_ flag in chunk VP8X is set.
|
|
If the _Animation_ flag is not set and this chunk is present, it
|
|
SHOULD be ignored.
|
|
|
|
Frame 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('FRM ') |
|
|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|
|
| Frame X | ...
|
|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|
|
... Frame Y | Frame Width Minus One ...
|
|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|
|
... | Frame Height Minus One |
|
|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|
|
| Frame Duration Minus One |
|
|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|
|
|
|
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 Minus One: 24 bits (_uint24_)
|
|
|
|
: _1-based_ time to wait before displaying the next frame, in 1 millisecond
|
|
units. The actual duration is '1 + Frame Duration Minus One' milliseconds.
|
|
|
|
For images that are animations, this chunk contains information about a single
|
|
frame, and describes the (optional) alpha chunk and the bitstream chunk that
|
|
follows it. If the _Animation flag_ is not set and this chunk is present,
|
|
it SHOULD be ignored.
|
|
|
|
|
|
#### Tiling
|
|
|
|
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('TILE') |
|
|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|
|
| Tile X | ...
|
|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|
|
... Tile Y |
|
|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|
|
|
|
Tile X: 24 bits (_uint24_)
|
|
|
|
: X coordinate of the upper left corner of the tile is `Tile X * 2`
|
|
|
|
Tile Y: 24 bits (_uint24_)
|
|
|
|
: Y coordinate of the upper left corner of the tile is `Tile Y * 2`
|
|
|
|
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
|
|
|
|
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... |
|
|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|
|
|
|
Compression method (C): 2 bits
|
|
|
|
: The compression method used:
|
|
|
|
* `0`: No compression.
|
|
* `1`: Compressed using the WebP lossless format.
|
|
|
|
Filtering method (F): 2 bits
|
|
|
|
: The filtering method used:
|
|
|
|
* `0`: None.
|
|
* `1`: Horizontal filter.
|
|
* `2`: Vertical filter.
|
|
* `3`: Gradient filter.
|
|
|
|
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
|
|
|
|
Decoders are not required to use this information in any specified way.
|
|
|
|
Reserved (Rsv): 2 bits
|
|
|
|
: SHOULD be `0`.
|
|
|
|
Alpha bitstream: _Chunk Size_ - `1` bytes
|
|
|
|
: Encoded alpha bitstream.
|
|
|
|
This optional chunk contains encoded alpha data for a single frame/tile.
|
|
For images with transparency, some of the frames/tiles may contain this chunk.
|
|
However, 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,
|
|
containing all the 8bit 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 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
|
|
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 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.
|
|
|
|
#### Metadata
|
|
|
|
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('META') |
|
|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|
|
| Metadata |
|
|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|
|
|
|
Metadata: _Chunk Size_ bytes
|
|
|
|
: image 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.
|
|
|
|
Additional guidance about handling metadata can be found in the
|
|
Metadata Working Group's [Guidelines for Handling Metadata][metadata].
|
|
|
|
#### Unknown Chunks
|
|
|
|
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
|
|
|
|
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
|
|
pseudocode:
|
|
|
|
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
|
|
assert not VP8X.flags.haveAnimation
|
|
canvas ← new black image of size
|
|
VP8X.canvasWidth x VP8X.canvasHeight.
|
|
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 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 Bitstream chunk
|
|
tile_params.alpha = alpha_data
|
|
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
|
|
pseudocode:
|
|
|
|
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
|
|
assert VP8X.flags.haveAnimation
|
|
canvas ← new black image of size
|
|
VP8X.canvasWidth x VP8X.canvasHeight.
|
|
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 FRM
|
|
for chunk in data_for_all_frames:
|
|
if chunk.tag == "FRM ":
|
|
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 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.
|
|
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
|
|
|
|
As described earlier, if an assert related to chunk ordering fails, the reader
|
|
MAY ignore the badly-ordered chunks instead of failing to decode the file.
|
|
|
|
Example file layouts
|
|
--------------------
|
|
|
|
A tiled image without transparency may look as follows:
|
|
|
|
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
|
|
RIFF/WEBP
|
|
+- VP8X (descriptions of features used)
|
|
+- ICCP (color profile)
|
|
+- TILE (First tile parameters)
|
|
+- VP8 (bitstream - first tile)
|
|
+- TILE (Second tile parameters)
|
|
+- VP8 (bitstream - second tile)
|
|
+- TILE (third tile parameters)
|
|
+- VP8 (bitstream - third tile)
|
|
+- TILE (fourth tile parameters)
|
|
+- VP8 (bitstream - fourth tile)
|
|
+- META (metadata)
|
|
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
|
|
|
|
An animated image with transparency 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
|
|
[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
|