Internet DRAFT - draft-zern-webp
draft-zern-webp
Network Working Group J. Zern
Internet-Draft P. Massimino
Intended status: Informational J. Alakuijala
Expires: 28 April 2024 Google LLC
October 2023
WebP Image Format
draft-zern-webp-13
Abstract
This document defines the WebP image format and registers a media
type supporting its use.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on 3 April 2024.
Copyright Notice
Copyright (c) 2023 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents (https://trustee.ietf.org/
license-info) in effect on the date of publication of this document.
Please review these documents carefully, as they describe your rights
and restrictions with respect to this document. Code Components
extracted from this document must include Revised BSD License text as
described in Section 4.e of the Trust Legal Provisions and are
provided without warranty as described in the Revised BSD License.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. WebP Container Specification . . . . . . . . . . . . . . . . 3
2.1. Introduction (from "WebP Container Specification") . . . 3
2.2. Terminology & Basics . . . . . . . . . . . . . . . . . . 4
2.3. RIFF File Format . . . . . . . . . . . . . . . . . . . . 5
2.4. WebP File Header . . . . . . . . . . . . . . . . . . . . 6
2.5. Simple File Format (Lossy) . . . . . . . . . . . . . . . 6
2.6. Simple File Format (Lossless) . . . . . . . . . . . . . . 7
2.7. Extended File Format . . . . . . . . . . . . . . . . . . 8
2.7.1. Chunks . . . . . . . . . . . . . . . . . . . . . . . 10
2.7.1.1. Animation . . . . . . . . . . . . . . . . . . . . 10
2.7.1.2. Alpha . . . . . . . . . . . . . . . . . . . . . . 14
2.7.1.3. Bitstream (VP8/VP8L) . . . . . . . . . . . . . . 17
2.7.1.4. Color Profile . . . . . . . . . . . . . . . . . . 17
2.7.1.5. Metadata . . . . . . . . . . . . . . . . . . . . 17
2.7.1.6. Unknown Chunks . . . . . . . . . . . . . . . . . 18
2.7.2. Canvas Assembly from Frames . . . . . . . . . . . . . 19
2.7.3. Example File Layouts . . . . . . . . . . . . . . . . 20
3. Specification for WebP Lossless Bitstream . . . . . . . . . . 21
3.1. Abstract (from "Specification for WebP Lossless
Bitstream") . . . . . . . . . . . . . . . . . . . . . . . 21
3.2. Introduction (from "Specification for WebP Lossless
Bitstream") . . . . . . . . . . . . . . . . . . . . . . . 22
3.3. Nomenclature . . . . . . . . . . . . . . . . . . . . . . 23
3.4. RIFF Header . . . . . . . . . . . . . . . . . . . . . . . 24
3.5. Transforms . . . . . . . . . . . . . . . . . . . . . . . 25
3.5.1. Predictor Transform . . . . . . . . . . . . . . . . . 26
3.5.2. Color Transform . . . . . . . . . . . . . . . . . . . 30
3.5.3. Subtract Green Transform . . . . . . . . . . . . . . 32
3.5.4. Color Indexing Transform . . . . . . . . . . . . . . 33
3.6. Image Data . . . . . . . . . . . . . . . . . . . . . . . 35
3.6.1. Roles of Image Data . . . . . . . . . . . . . . . . . 35
3.6.2. Encoding of Image Data . . . . . . . . . . . . . . . 35
3.6.2.1. Prefix-Coded Literals . . . . . . . . . . . . . . 36
3.6.2.2. LZ77 Backward Reference . . . . . . . . . . . . . 36
3.6.2.3. Color Cache Coding . . . . . . . . . . . . . . . 39
3.7. Entropy Code . . . . . . . . . . . . . . . . . . . . . . 40
3.7.1. Overview . . . . . . . . . . . . . . . . . . . . . . 40
3.7.2. Details . . . . . . . . . . . . . . . . . . . . . . . 40
3.7.2.1. Decoding and Building the Prefix Codes . . . . . 41
3.7.2.2. Decoding of Meta Prefix Codes . . . . . . . . . . 43
3.7.2.3. Decoding Entropy-Coded Image Data . . . . . . . . 45
3.8. Overall Structure of the Format . . . . . . . . . . . . . 46
3.8.1. Basic Structure . . . . . . . . . . . . . . . . . . . 46
3.8.2. Structure of Transforms . . . . . . . . . . . . . . . 46
3.8.3. Structure of the Image Data . . . . . . . . . . . . . 46
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4. Security Considerations . . . . . . . . . . . . . . . . . . . 47
5. Interoperability Considerations . . . . . . . . . . . . . . . 48
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 48
6.1. The 'image/webp' Media Type . . . . . . . . . . . . . . . 48
6.1.1. Registration Details . . . . . . . . . . . . . . . . 48
7. References . . . . . . . . . . . . . . . . . . . . . . . . . 49
7.1. Normative References . . . . . . . . . . . . . . . . . . 49
7.2. Informative References . . . . . . . . . . . . . . . . . 51
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 52
1. Introduction
WebP is an image file format based on the Resource Interchange File
Format (RIFF) [RIFF-spec] (Section 2) that supports lossless and
lossy compression as well as alpha (transparency) and animation. It
covers use cases similar to JPEG [JPEG-spec], PNG [RFC2083], and the
Graphics Interchange Format (GIF) [GIF-spec].
WebP consists of two compression algorithms used to reduce the size
of image pixel data, including alpha (transparency) information.
Lossy compression is achieved using VP8 intra-frame encoding
[RFC6386]. The lossless algorithm (Section 3) stores and restores
the pixel values exactly, including the color values for fully
transparent pixels. A universal algorithm for sequential data
compression [LZ77], prefix coding [Huffman], and a color cache are
used for compression of the bulk data.
2. WebP Container Specification
Note that this section is based on the documentation in the libwebp
source repository [webp-riff-src].
2.1. Introduction (from "WebP Container Specification")
WebP is an image format that uses either (i) the VP8 intra-frame
encoding [RFC6386] to compress image data in a lossy way or (ii) the
WebP lossless encoding (Section 3). These encoding schemes should
make it more efficient than older formats, such as JPEG, GIF, and
PNG. It is optimized for fast image transfer over the network (for
example, for websites). The WebP format has feature parity (color
profile, metadata, animation, etc.) with other formats as well. This
section describes the structure of a WebP file.
The WebP container (that is, the RIFF container for WebP) allows
feature support over and above the basic use case of WebP (that is, a
file containing a single image encoded as a VP8 key frame). The WebP
container provides additional support for the following:
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* Lossless Compression: An image can be losslessly compressed, using
the WebP lossless format.
* Metadata: An image may have metadata stored in Exchangeable Image
File Format [Exif] or Extensible Metadata Platform [XMP] format.
* Transparency: An image may have transparency, that is, an alpha
channel.
* Color Profile: An image may have an embedded ICC profile (ICCP)
[ICC].
* Animation: An image may have multiple frames with pauses between
them, making it an animation.
2.2. Terminology & Basics
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in
BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.
A WebP file contains either a still image (that is, an encoded matrix
of pixels) or an animation (Section 2.7.1.1). Optionally, it can
also contain transparency information, a color profile, and metadata.
We refer to the matrix of pixels as the _canvas_ of the image.
Bit numbering in chunk diagrams starts at 0 for the most significant
bit ('MSB 0'), as described in [RFC1166].
Below are additional terms used throughout this section:
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.
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FourCC
A four-character code (FourCC) is a uint32 created by
concatenating four ASCII characters in little-endian order. This
means 'aaaa' (0x61616161) and 'AAAA' (0x41414141) are treated as
different FourCCs.
1-based
An unsigned integer field storing values offset by -1, for
example, such a field would store value _25_ as _24_.
ChunkHeader('ABCD')
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.
2.3. RIFF File Format
The WebP file format is based on the RIFF [RIFF-spec] 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 :
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 1: 'RIFF' Chunk Structure
Chunk FourCC: 32 bits
ASCII four-character code used for chunk identification.
Chunk Size: 32 bits (_uint32_)
The size of the chunk in bytes, 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
-- which MUST be 0 to conform with RIFF [RIFF-spec] -- is added.
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| 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.
2.4. 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' |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 2: WebP File Header Chunk
'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 4 GiB 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 any data after the data specified by _File Size_. Readers
MAY parse such files, ignoring the trailing data. As the size of any
chunk is even, the size given by the RIFF header is also even. The
contents of individual chunks are described in the following
sections.
2.5. 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:
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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 :
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 3: Simple WebP (Lossy) File Format
'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 :
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 4: 'VP8 ' Chunk
VP8 data: _Chunk Size_ bytes
VP8 bitstream data.
Note that the fourth character in the 'VP8 ' FourCC is an ASCII space
(0x20).
The VP8 bitstream format specification is described in [RFC6386].
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, Recommendation 601 [rec601] SHOULD be
used. Applications MAY use another conversion method, but visual
results may differ among decoders.
2.6. 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.
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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 :
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 5: Simple WebP (Lossless) File Format
'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 :
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 6: 'VP8L' Chunk
VP8L data: _Chunk Size_ bytes
VP8L bitstream data.
The specification of the VP8L bitstream can be found in Section 3.
Note that the VP8L header contains the VP8L image width and height.
That is assumed to be the width and height of the canvas.
2.7. 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 a color profile.
* An optional 'ANIM' Chunk with animation control data.
* Image data.
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* An optional 'EXIF' Chunk with Exif metadata.
* An optional 'XMP ' Chunk with XMP metadata.
* An optional list of unknown chunks (Section 2.7.1.6).
For a _still image_, the _image data_ consists of a single frame,
which is made up of:
* An optional alpha subchunk (Section 2.7.1.2).
* A bitstream subchunk (Section 2.7.1.3).
For an _animated image_, the _image data_ consists of multiple
frames. More details about frames can be found in Section 2.7.1.1.
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 are out of order.
| 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:
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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 7: Extended WebP File Header
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Reserved (Rsv): 2 bits
MUST be 0. Readers MUST ignore this field.
ICC profile (I): 1 bit
Set if the file contains an 'ICCP' Chunk.
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
MUST be 0. Readers MUST ignore this field.
Reserved: 24 bits
MUST be 0. Readers MUST ignore this field.
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. Unknown fields MUST be
ignored.
2.7.1. Chunks
2.7.1.1. Animation
An animation is controlled by 'ANIM' and 'ANMF' Chunks.
'ANIM' Chunk:
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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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 8: 'ANIM' Chunk
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. The background color is also used
when the Disposal method is 1.
Note:
* The background color MAY contain a nonopaque alpha value, even
if the _Alpha_ flag in the 'VP8X' Chunk (Figure 7) 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. If it is 0, this
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 MUST 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.
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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 :
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 9: 'ANMF' Chunk
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 that the interpretation of the Frame
Duration of 0 (and often <= 10) is defined by the implementation.
Many tools and browsers assign a minimum duration similar to GIF.
Reserved: 6 bits
MUST be 0. Readers MUST ignore this field.
Blending method (B): 1 bit
Indicates how transparent pixels of _the current frame_ are to be
blended with corresponding pixels of the previous canvas:
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* 0: Use alpha-blending. After disposing of the previous frame,
render the current frame on the canvas using alpha-blending
(Section 2.7.1.1, Paragraph 10, Item 16.4.2). If the current
frame does not have an alpha channel, assume the alpha value
is 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 the background color. Fill the _rectangle_ on
the canvas covered by the _current frame_ with the background
color specified in the 'ANIM' Chunk (Section 2.7.1.1,
Paragraph 2).
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:
Given that each of the R, G, B, and A channels is 8 bits 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 (Section 2.7.1.4) of the image.
If the color profile is not present, standard RGB (sRGB) is to
be assumed. (Note that sRGB also needs to be linearized due
to a gamma of ~2.2.)
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Frame Data: _Chunk Size_ - 16 bytes
Consists of:
* An optional alpha subchunk (Section 2.7.1.2) for the frame.
* A bitstream subchunk (Section 2.7.1.3) for the frame.
* An optional list of unknown chunks (Section 2.7.1.6).
Note: The 'ANMF' payload, _Frame Data_ above, consists of individual
_padded_ chunks, as described by the RIFF file format (Section 2.3).
2.7.1.2. 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... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 10: 'ALPH' Chunk
Reserved (Rsv): 2 bits
MUST be 0. Readers MUST ignore this field.
Preprocessing (P): 2 bits
These informative bits are used to signal the preprocessing that
has been performed during compression. The decoder can use this
information to, for example, dither the values or smooth the
gradients prior to display.
* 0: No preprocessing.
* 1: Level reduction.
Decoders are not required to use this information in any
specified way.
Filtering method (F): 2 bits
The filtering methods used are described as follows:
* 0: None.
* 1: Horizontal filter.
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* 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 |
Figure 11
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, or
* 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 the left-most and top-most pixel
positions.
For example, the top-left value at location (0, 0) uses 0 as the
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.
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* 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.
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 the
compression method is '0') or compressed using the lossless format
(when the compression method is '1').
* Raw data: This 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 Section 3) of implicit dimensions
width x height. That is, this image-stream does NOT contain any
headers describing the image dimensions.
Rationale: The dimensions are already known from other sources, so
storing them again would be redundant and prone to errors.
Once the image-stream is decoded into Alpha, Red, Green, Blue
(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.
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2.7.1.3. 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 FourCC, _or_
(ii) a 'VP8L' Chunk, using 'VP8L' as its FourCC.
The formats of' VP8 ' and 'VP8L' Chunks are as described in Sections
2.5 and 2.6, respectively.
2.7.1.4. 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 :
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 12: 'ICCP' Chunk
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 [ICC] for details.
If this chunk is not present, sRGB SHOULD be assumed.
2.7.1.5. 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.
The chunks are defined as follows:
'EXIF' Chunk:
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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 :
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 13: 'EXIF' Chunk
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 :
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 14: 'XMP ' Chunk
XMP Metadata: _Chunk Size_ bytes
Image metadata in [XMP] format.
Note that the fourth character in the 'XMP ' FourCC is an ASCII space
(0x20).
Additional guidance about handling metadata can be found in the
Metadata Working Group's "Guidelines For Handling Image Metadata"
[MWG].
2.7.1.6. Unknown Chunks
A RIFF chunk (described in Section 2.3) whose _FourCC_ is different
from any of the chunks described in this section 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:
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* at the end of the file, as described in Section 2.7, or
* at the end of 'ANMF' Chunks, as described in Section 2.7.1.1.
Readers SHOULD ignore these chunks. Writers SHOULD preserve them in
their original order (unless they specifically intend to modify these
chunks).
2.7.2. Canvas Assembly from Frames
Here, we provide an overview of how a reader MUST assemble a canvas
in the case of an animated image.
The process begins with creating a canvas using the dimensions given
in the 'VP8X' Chunk, Canvas Width Minus One + 1 pixels wide by Canvas
Height Minus One + 1 pixels high. The Loop Count field from the
'ANIM' Chunk controls how many times the animation process is
repeated. This is Loop Count - 1 for nonzero Loop Count values or
infinite if the Loop Count is zero.
At the beginning of each loop iteration, the canvas is filled using
the background color from the 'ANIM' Chunk or an application-defined
color.
'ANMF' Chunks contain individual frames given in display order.
Before rendering each frame, the previous frame's Disposal method is
applied.
The rendering of the decoded frame begins at the Cartesian
coordinates (2 * Frame X, 2 * Frame Y), using the top-left corner of
the canvas as the origin. Frame Width Minus One + 1 pixels wide by
Frame Height Minus One + 1 pixels high are rendered onto the canvas
using the Blending method.
The canvas is displayed for Frame Duration milliseconds. This
continues until all frames given by 'ANMF' Chunks have been
displayed. A new loop iteration is then begun, or the canvas is left
in its final state if all iterations have been completed.
The following pseudocode illustrates the rendering process. The
notation _VP8X.field_ means the field in the 'VP8X' Chunk with the
same description.
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VP8X.flags.hasAnimation MUST be TRUE
canvas <- new image of size VP8X.canvasWidth x VP8X.canvasHeight with
background color ANIM.background_color.
loop_count <- ANIM.loopCount
dispose_method <- Dispose to background color
if loop_count == 0:
loop_count = inf
frame_params <- nil
next chunk in image_data is ANMF MUST be TRUE
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
VP8X.canvasWidth >= frame_right MUST be TRUE
VP8X.canvasHeight >= frame_bottom MUST be TRUE
for subchunk in 'Frame Data':
if subchunk.tag == "ALPH":
alpha subchunks not found in 'Frame Data' earlier MUST be
TRUE
frame_params.alpha = alpha_data
else if subchunk.tag == "VP8 " OR subchunk.tag == "VP8L":
bitstream subchunks not found in 'Frame Data' earlier MUST
be TRUE
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 Blending method
frame_params.blendingMethod.
canvas contains the decoded image.
Show the contents of the canvas for
frame_params.frameDuration * 1 ms.
dispose_method = frame_params.disposeMethod
2.7.3. 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)
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Figure 15: A Lossy-Encoded Image with Alpha
A lossless-encoded image may look as follows:
RIFF/WEBP
+- VP8X (descriptions of features used)
+- VP8L (lossless bitstream)
+- XYZW (unknown chunk)
Figure 16: A Lossless-Encoded Image
A lossless image with an ICC profile and XMP metadata may look as
follows:
RIFF/WEBP
+- VP8X (descriptions of features used)
+- ICCP (color profile)
+- VP8L (lossless bitstream)
+- XMP (metadata)
Figure 17: A Lossless Image with an ICC Profile and 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)
Figure 18: An Animated Image with Exif Metadata
3. Specification for WebP Lossless Bitstream
Note that this section is based on the documentation in the libwebp
source repository [webp-lossless-src].
3.1. Abstract (from "Specification for WebP Lossless Bitstream")
WebP lossless is an image format for lossless compression of ARGB
images. The lossless format stores and restores the pixel values
exactly, including the color values for pixels whose alpha value is
0. The format uses subresolution images, recursively embedded into
the format itself, for storing statistical data about the images,
such as the used entropy codes, spatial predictors, color space
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conversion, and color table. A universal algorithm for sequential
data compression [LZ77], prefix coding, and a color cache are used
for compression of the bulk data. Decoding speeds faster than PNG
have been demonstrated, as well as 25% denser compression than can be
achieved using today's PNG format [webp-lossless-study].
3.2. Introduction (from "Specification for WebP Lossless Bitstream")
This section describes the compressed data representation of a WebP
lossless image.
In this section, we extensively use C programming language syntax
[ISO.9899.2018] to describe the bitstream and assume the existence of
a function for reading bits, ReadBits(n). The bytes are read in the
natural order of the stream containing them, and bits of each byte
are read in least-significant-bit-first order. When multiple bits
are read at the same time, the integer is constructed from the
original data in the original order. The most significant bits of
the returned integer are also the most significant bits of the
original data. Thus, the statement
b = ReadBits(2);
is equivalent with the two statements below:
b = ReadBits(1);
b |= ReadBits(1) << 1;
We assume that each color component (that is, alpha, red, blue, and
green) is represented using an 8-bit byte. We define the
corresponding type as uint8. A whole ARGB pixel is represented by a
type called uint32, which is an unsigned integer consisting of 32
bits. In the code showing the behavior of the transforms, these
values are codified in the following bits: alpha in bits 31..24, red
in bits 23..16, green in bits 15..8, and blue in bits 7..0; however,
implementations of the format are free to use another representation
internally.
Broadly, a WebP lossless image contains header data, transform
information, and actual image data. Headers contain the width and
height of the image. A WebP lossless image can go through four
different types of transforms before being entropy encoded. The
transform information in the bitstream contains the data required to
apply the respective inverse transforms.
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3.3. Nomenclature
ARGB
A pixel value consisting of alpha, red, green, and blue values.
ARGB image
A two-dimensional array containing ARGB pixels.
color cache
A small hash-addressed array to store recently used colors to be
able to recall them with shorter codes.
color indexing image
A one-dimensional image of colors that can be indexed using a
small integer (up to 256 within WebP lossless).
color transform image
A two-dimensional subresolution image containing data about
correlations of color components.
distance mapping
Changes LZ77 distances to have the smallest values for pixels in
two-dimensional proximity.
entropy image
A two-dimensional subresolution image indicating which entropy
coding should be used in a respective square in the image, that
is, each pixel is a meta prefix code.
[LZ77]
A dictionary-based sliding window compression algorithm that
either emits symbols or describes them as sequences of past
symbols.
meta prefix code
A small integer (up to 16 bits) that indexes an element in the
meta prefix table.
predictor image
A two-dimensional subresolution image indicating which spatial
predictor is used for a particular square in the image.
prefix code
A classic way to do entropy coding where a smaller number of bits
are used for more frequent codes.
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prefix coding
A way to entropy code larger integers, which codes a few bits of
the integer using an entropy code and codifies the remaining bits
raw. This allows for the descriptions of the entropy codes to
remain relatively small even when the range of symbols is large.
scan-line order
A processing order of pixels (left to right and top to bottom),
starting from the left-hand-top pixel. Once a row is completed,
continue from the left-hand column of the next row.
3.4. RIFF Header
The beginning of the header has the RIFF container. This consists of
the following 21 bytes:
1. String 'RIFF'.
2. A little-endian, 32-bit value of the chunk length, which is the
whole size of the chunk controlled by the RIFF header. Normally,
this equals the payload size (file size minus 8 bytes: 4 bytes
for the 'RIFF' identifier and 4 bytes for storing the value
itself).
3. String 'WEBP' (RIFF container name).
4. String 'VP8L' (FourCC for lossless-encoded image data).
5. A little-endian, 32-bit value of the number of bytes in the
lossless stream.
6. 1-byte signature 0x2f.
The first 28 bits of the bitstream specify the width and height of
the image. Width and height are decoded as 14-bit integers as
follows:
int image_width = ReadBits(14) + 1;
int image_height = ReadBits(14) + 1;
The 14-bit precision for image width and height limits the maximum
size of a WebP lossless image to 16384x16384 pixels.
The alpha_is_used bit is a hint only and SHOULD NOT impact decoding.
It SHOULD be set to 0 when all alpha values are 255 in the picture
and 1 otherwise.
int alpha_is_used = ReadBits(1);
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The version_number is a 3-bit code that MUST be set to 0. Any other
value MUST be treated as an error.
int version_number = ReadBits(3);
3.5. Transforms
The transforms are reversible manipulations of the image data that
can reduce the remaining symbolic entropy by modeling spatial and
color correlations. They can make the final compression more dense.
An image can go through four types of transforms. A 1 bit indicates
the presence of a transform. Each transform is allowed to be used
only once. The transforms are used only for the main-level ARGB
image; the subresolution images (color transform image, entropy
image, and predictor image) have no transforms, not even the 0 bit
indicating the end of transforms.
| Typically, an encoder would use these transforms to reduce the
| Shannon entropy in the residual image. Also, the transform
| data can be decided based on entropy minimization.
while (ReadBits(1)) { // Transform present.
// Decode transform type.
enum TransformType transform_type = ReadBits(2);
// Decode transform data.
...
}
// Decode actual image data.
If a transform is present, then the next two bits specify the
transform type. There are four types of transforms.
+==========================+=====+
| Transform | Bit |
+==========================+=====+
| PREDICTOR_TRANSFORM | 0 |
+--------------------------+-----+
| COLOR_TRANSFORM | 1 |
+--------------------------+-----+
| SUBTRACT_GREEN_TRANSFORM | 2 |
+--------------------------+-----+
| COLOR_INDEXING_TRANSFORM | 3 |
+--------------------------+-----+
Table 1
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The transform type is followed by the transform data. Transform data
contains the information required to apply the inverse transform and
depends on the transform type. The inverse transforms are applied in
the reverse order that they are read from the bitstream, that is,
last one first.
Next, we describe the transform data for different types.
3.5.1. Predictor Transform
The predictor transform can be used to reduce entropy by exploiting
the fact that neighboring pixels are often correlated. In the
predictor transform, the current pixel value is predicted from the
pixels already decoded (in scan-line order) and only the residual
value (actual - predicted) is encoded. The green component of a
pixel defines which of the 14 predictors is used within a particular
block of the ARGB image. The _prediction mode_ determines the type
of prediction to use. We divide the image into squares, and all the
pixels in a square use the same prediction mode.
The first 3 bits of prediction data define the block width and height
in number of bits.
int size_bits = ReadBits(3) + 2;
int block_width = (1 << size_bits);
int block_height = (1 << size_bits);
#define DIV_ROUND_UP(num, den) (((num) + (den) - 1) / (den))
int transform_width = DIV_ROUND_UP(image_width, 1 << size_bits);
The transform data contains the prediction mode for each block of the
image. It is a subresolution image where the green component of a
pixel defines which of the 14 predictors is used for all the
block_width * block_height pixels within a particular block of the
ARGB image. This subresolution image is encoded using the same
techniques described in Section 3.6.
The number of block columns, transform_width, is used in two-
dimensional indexing. For a pixel (x, y), one can compute the
respective filter block address by:
int block_index = (y >> size_bits) * transform_width +
(x >> size_bits);
There are 14 different prediction modes. In each prediction mode,
the current pixel value is predicted from one or more neighboring
pixels whose values are already known.
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We chose the neighboring pixels (TL, T, TR, and L) of the current
pixel (P) as follows:
O O O O O O O O O O O
O O O O O O O O O O O
O O O O TL T TR O O O O
O O O O L P X X X X X
X X X X X X X X X X X
X X X X X X X X X X X
Figure 19
where TL means top-left, T means top, TR means top-right, and L means
left. At the time of predicting a value for P, all O, TL, T, TR, and
L pixels have already been processed, and the P pixel and all X
pixels are unknown.
Given the preceding neighboring pixels, the different prediction
modes are defined as follows.
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+======+======================================================+
| Mode | Predicted Value of Each Channel of the Current Pixel |
+======+======================================================+
| 0 | 0xff000000 (represents solid black color in ARGB) |
+------+------------------------------------------------------+
| 1 | L |
+------+------------------------------------------------------+
| 2 | T |
+------+------------------------------------------------------+
| 3 | TR |
+------+------------------------------------------------------+
| 4 | TL |
+------+------------------------------------------------------+
| 5 | Average2(Average2(L, TR), T) |
+------+------------------------------------------------------+
| 6 | Average2(L, TL) |
+------+------------------------------------------------------+
| 7 | Average2(L, T) |
+------+------------------------------------------------------+
| 8 | Average2(TL, T) |
+------+------------------------------------------------------+
| 9 | Average2(T, TR) |
+------+------------------------------------------------------+
| 10 | Average2(Average2(L, TL), Average2(T, TR)) |
+------+------------------------------------------------------+
| 11 | Select(L, T, TL) |
+------+------------------------------------------------------+
| 12 | ClampAddSubtractFull(L, T, TL) |
+------+------------------------------------------------------+
| 13 | ClampAddSubtractHalf(Average2(L, T), TL) |
+------+------------------------------------------------------+
Table 2
Average2 is defined as follows for each ARGB component:
uint8 Average2(uint8 a, uint8 b) {
return (a + b) / 2;
}
The Select predictor is defined as follows:
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uint32 Select(uint32 L, uint32 T, uint32 TL) {
// L = left pixel, T = top pixel, TL = top-left pixel.
// ARGB component estimates for prediction.
int pAlpha = ALPHA(L) + ALPHA(T) - ALPHA(TL);
int pRed = RED(L) + RED(T) - RED(TL);
int pGreen = GREEN(L) + GREEN(T) - GREEN(TL);
int pBlue = BLUE(L) + BLUE(T) - BLUE(TL);
// Manhattan distances to estimates for left and top pixels.
int pL = abs(pAlpha - ALPHA(L)) + abs(pRed - RED(L)) +
abs(pGreen - GREEN(L)) + abs(pBlue - BLUE(L));
int pT = abs(pAlpha - ALPHA(T)) + abs(pRed - RED(T)) +
abs(pGreen - GREEN(T)) + abs(pBlue - BLUE(T));
// Return either left or top, the one closer to the prediction.
if (pL < pT) {
return L;
} else {
return T;
}
}
The functions ClampAddSubtractFull and ClampAddSubtractHalf are
performed for each ARGB component as follows:
// Clamp the input value between 0 and 255.
int Clamp(int a) {
return (a < 0) ? 0 : (a > 255) ? 255 : a;
}
int ClampAddSubtractFull(int a, int b, int c) {
return Clamp(a + b - c);
}
int ClampAddSubtractHalf(int a, int b) {
return Clamp(a + (a - b) / 2);
}
There are special handling rules for some border pixels. If there is
a prediction transform, regardless of the mode [0..13] for these
pixels, the predicted value for the left-topmost pixel of the image
is 0xff000000, all pixels on the top row are L-pixel, and all pixels
on the leftmost column are T-pixel.
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Addressing the TR-pixel for pixels on the rightmost column is
exceptional. The pixels on the rightmost column are predicted by
using the modes [0..13], just like pixels not on the border, but the
leftmost pixel on the same row as the current pixel is instead used
as the TR-pixel.
The final pixel value is obtained by adding each channel of the
predicted value to the encoded residual value.
void PredictorTransformOutput(uint32 residual, uint32 pred,
uint8* alpha, uint8* red,
uint8* green, uint8* blue) {
*alpha = ALPHA(residual) + ALPHA(pred);
*red = RED(residual) + RED(pred);
*green = GREEN(residual) + GREEN(pred);
*blue = BLUE(residual) + BLUE(pred);
}
3.5.2. Color Transform
The goal of the color transform is to decorrelate the R, G, and B
values of each pixel. The color transform keeps the green (G) value
as it is, transforms the red (R) value based on the green value, and
transforms the blue (B) value based on the green value and then on
the red value.
As is the case for the predictor transform, first the image is
divided into blocks, and the same transform mode is used for all the
pixels in a block. For each block, there are three types of color
transform elements.
typedef struct {
uint8 green_to_red;
uint8 green_to_blue;
uint8 red_to_blue;
} ColorTransformElement;
The actual color transform is done by defining a color transform
delta. The color transform delta depends on the
ColorTransformElement, which is the same for all the pixels in a
particular block. The delta is subtracted during the color
transform. The inverse color transform then is just adding those
deltas.
The color transform function is defined as follows:
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void ColorTransform(uint8 red, uint8 blue, uint8 green,
ColorTransformElement *trans,
uint8 *new_red, uint8 *new_blue) {
// Transformed values of red and blue components
int tmp_red = red;
int tmp_blue = blue;
// Applying the transform is just subtracting the transform deltas
tmp_red -= ColorTransformDelta(trans->green_to_red, green);
tmp_blue -= ColorTransformDelta(trans->green_to_blue, green);
tmp_blue -= ColorTransformDelta(trans->red_to_blue, red);
*new_red = tmp_red & 0xff;
*new_blue = tmp_blue & 0xff;
}
ColorTransformDelta is computed using a signed 8-bit integer
representing a 3.5-fixed-point number and a signed 8-bit RGB color
channel (c) [-128..127] and is defined as follows:
int8 ColorTransformDelta(int8 t, int8 c) {
return (t * c) >> 5;
}
A conversion from the 8-bit unsigned representation (uint8) to the
8-bit signed one (int8) is required before calling
ColorTransformDelta(). The signed value should be interpreted as an
8-bit two's complement number (that is: uint8 range [128..255] is
mapped to the [-128..-1] range of its converted int8 value).
The multiplication is to be done using more precision (with at least
16-bit precision). The sign extension property of the shift
operation does not matter here; only the lowest 8 bits are used from
the result, and there the sign extension shifting and unsigned
shifting are consistent with each other.
Now, we describe the contents of color transform data so that
decoding can apply the inverse color transform and recover the
original red and blue values. The first 3 bits of the color
transform data contain the width and height of the image block in
number of bits, just like the predictor transform:
int size_bits = ReadBits(3) + 2;
int block_width = 1 << size_bits;
int block_height = 1 << size_bits;
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The remaining part of the color transform data contains
ColorTransformElement instances, corresponding to each block of the
image. Each ColorTransformElement 'cte' is treated as a pixel in a
subresolution image whose alpha component is 255, red component is
cte.red_to_blue, green component is cte.green_to_blue, and blue
component is cte.green_to_red.
During decoding, ColorTransformElement instances of the blocks are
decoded and the inverse color transform is applied on the ARGB values
of the pixels. As mentioned earlier, that inverse color transform is
just adding ColorTransformElement values to the red and blue
channels. The alpha and green channels are left as is.
void InverseTransform(uint8 red, uint8 green, uint8 blue,
ColorTransformElement *trans,
uint8 *new_red, uint8 *new_blue) {
// Transformed values of red and blue components
int tmp_red = red;
int tmp_blue = blue;
// Applying the inverse transform is just adding the
// color transform deltas
tmp_red += ColorTransformDelta(trans->green_to_red, green);
tmp_blue += ColorTransformDelta(trans->green_to_blue, green);
tmp_blue +=
ColorTransformDelta(trans->red_to_blue, tmp_red & 0xff);
*new_red = tmp_red & 0xff;
*new_blue = tmp_blue & 0xff;
}
3.5.3. Subtract Green Transform
The subtract green transform subtracts green values from red and blue
values of each pixel. When this transform is present, the decoder
needs to add the green value to both the red and blue values. There
is no data associated with this transform. The decoder applies the
inverse transform as follows:
void AddGreenToBlueAndRed(uint8 green, uint8 *red, uint8 *blue) {
*red = (*red + green) & 0xff;
*blue = (*blue + green) & 0xff;
}
This transform is redundant, as it can be modeled using the color
transform, but since there is no additional data here, the subtract
green transform can be coded using fewer bits than a full-blown color
transform.
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3.5.4. Color Indexing Transform
If there are not many unique pixel values, it may be more efficient
to create a color index array and replace the pixel values by the
array's indices. The color indexing transform achieves this. (In
the context of WebP lossless, we specifically do not call this a
palette transform because a similar but more dynamic concept exists
in WebP lossless encoding: color cache.)
The color indexing transform checks for the number of unique ARGB
values in the image. If that number is below a threshold (256), it
creates an array of those ARGB values, which is then used to replace
the pixel values with the corresponding index: the green channel of
the pixels are replaced with the index, all alpha values are set to
255, and all red and blue values are set to 0.
The transform data contains the color table size and the entries in
the color table. The decoder reads the color indexing transform data
as follows:
// 8-bit value for the color table size
int color_table_size = ReadBits(8) + 1;
The color table is stored using the image storage format itself. The
color table can be obtained by reading an image, without the RIFF
header, image size, and transforms, assuming the height of 1 pixel
and the width of color_table_size. The color table is always
subtraction-coded to reduce image entropy. The deltas of palette
colors contain typically much less entropy than the colors
themselves, leading to significant savings for smaller images. In
decoding, every final color in the color table can be obtained by
adding the previous color component values by each ARGB component
separately and storing the least significant 8 bits of the result.
The inverse transform for the image is simply replacing the pixel
values (which are indices to the color table) with the actual color
table values. The indexing is done based on the green component of
the ARGB color.
// Inverse transform
argb = color_table[GREEN(argb)];
If the index is equal to or larger than color_table_size, the argb
color value should be set to 0x00000000 (transparent black).
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When the color table is small (equal to or less than 16 colors),
several pixels are bundled into a single pixel. The pixel bundling
packs several (2, 4, or 8) pixels into a single pixel, reducing the
image width respectively.
| Pixel bundling allows for a more efficient joint distribution
| entropy coding of neighboring pixels and gives some arithmetic
| coding-like benefits to the entropy code, but it can only be
| used when there are 16 or fewer unique values.
color_table_size specifies how many pixels are combined:
+==================+==================+
| color_table_size | width_bits value |
+==================+==================+
| 1..2 | 3 |
+------------------+------------------+
| 3..4 | 2 |
+------------------+------------------+
| 5..16 | 1 |
+------------------+------------------+
| 17..256 | 0 |
+------------------+------------------+
Table 3
width_bits has a value of 0, 1, 2, or 3. A value of 0 indicates no
pixel bundling is to be done for the image. A value of 1 indicates
that two pixels are combined, and each pixel has a range of [0..15].
A value of 2 indicates that four pixels are combined, and each pixel
has a range of [0..3]. A value of 3 indicates that eight pixels are
combined, and each pixel has a range of [0..1], that is, a binary
value.
The values are packed into the green component as follows:
* width_bits = 1: For every x value, where x = 2k + 0, a green value
at x is positioned into the 4 least significant bits of the green
value at x / 2, and a green value at x + 1 is positioned into the
4 most significant bits of the green value at x / 2.
* width_bits = 2: For every x value, where x = 4k + 0, a green value
at x is positioned into the 2 least significant bits of the green
value at x / 4, and green values at x + 1 to x + 3 are positioned
in order to the more significant bits of the green value at x / 4.
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* width_bits = 3: For every x value, where x = 8k + 0, a green value
at x is positioned into the least significant bit of the green
value at x / 8, and green values at x + 1 to x + 7 are positioned
in order to the more significant bits of the green value at x / 8.
After reading this transform, image_width is subsampled by
width_bits. This affects the size of subsequent transforms. The new
size can be calculated using DIV_ROUND_UP, as defined in
Section 3.5.1.
image_width = DIV_ROUND_UP(image_width, 1 << width_bits);
3.6. Image Data
Image data is an array of pixel values in scan-line order.
3.6.1. Roles of Image Data
We use image data in five different roles:
1. ARGB image: Stores the actual pixels of the image.
2. Entropy image: Stores the meta prefix codes (see "Decoding of
Meta Prefix Codes" (Section 3.7.2.2)).
3. Predictor image: Stores the metadata for the predictor transform
(see "Predictor Transform" (Section 3.5.1)).
4. Color transform image: Created by ColorTransformElement values
(defined in "Color Transform" (Section 3.5.2)) for different
blocks of the image.
5. Color indexing image: An array of size color_table_size (up to
256 ARGB values) storing the metadata for the color indexing
transform (see "Color Indexing Transform" (Section 3.5.4)).
3.6.2. Encoding of Image Data
The encoding of image data is independent of its role.
The image is first divided into a set of fixed-size blocks (typically
16x16 blocks). Each of these blocks are modeled using their own
entropy codes. Also, several blocks may share the same entropy
codes.
| Rationale: Storing an entropy code incurs a cost. This cost
| can be minimized if statistically similar blocks share an
| entropy code, thereby storing that code only once. For
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| example, an encoder can find similar blocks by clustering them
| using their statistical properties or by repeatedly joining a
| pair of randomly selected clusters when it reduces the overall
| amount of bits needed to encode the image.
Each pixel is encoded using one of the three possible methods:
1. Prefix-coded literals: Each channel (green, red, blue, and alpha)
is entropy-coded independently.
2. LZ77 backward reference: A sequence of pixels are copied from
elsewhere in the image.
3. Color cache code: Using a short multiplicative hash code (color
cache index) of a recently seen color.
The following subsections describe each of these in detail.
3.6.2.1. Prefix-Coded Literals
The pixel is stored as prefix-coded values of green, red, blue, and
alpha (in that order). See Section 3.7.2.3 for details.
3.6.2.2. LZ77 Backward Reference
Backward references are tuples of _length_ and _distance code_:
* Length indicates how many pixels in scan-line order are to be
copied.
* Distance code is a number indicating the position of a previously
seen pixel, from which the pixels are to be copied. The exact
mapping is described below (Section 3.6.2.2.1).
The length and distance values are stored using *LZ77 prefix coding*.
LZ77 prefix coding divides large integer values into two parts: the
_prefix code_ and the _extra bits_. The prefix code is stored using
an entropy code, while the extra bits are stored as they are (without
an entropy code).
| Rationale: This approach reduces the storage requirement for
| the entropy code. Also, large values are usually rare, so
| extra bits would be used for very few values in the image.
| Thus, this approach results in better compression overall.
The following table denotes the prefix codes and extra bits used for
storing different ranges of values.
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Note: The maximum backward reference length is limited to 4096.
Hence, only the first 24 prefix codes (with the respective extra
bits) are meaningful for length values. For distance values,
however, all the 40 prefix codes are valid.
+=================+=============+============+
| Value Range | Prefix Code | Extra Bits |
+=================+=============+============+
| 1 | 0 | 0 |
+-----------------+-------------+------------+
| 2 | 1 | 0 |
+-----------------+-------------+------------+
| 3 | 2 | 0 |
+-----------------+-------------+------------+
| 4 | 3 | 0 |
+-----------------+-------------+------------+
| 5..6 | 4 | 1 |
+-----------------+-------------+------------+
| 7..8 | 5 | 1 |
+-----------------+-------------+------------+
| 9..12 | 6 | 2 |
+-----------------+-------------+------------+
| 13..16 | 7 | 2 |
+-----------------+-------------+------------+
| ... | ... | ... |
+-----------------+-------------+------------+
| 3072..4096 | 23 | 10 |
+-----------------+-------------+------------+
| ... | ... | ... |
+-----------------+-------------+------------+
| 524289..786432 | 38 | 18 |
+-----------------+-------------+------------+
| 786433..1048576 | 39 | 18 |
+-----------------+-------------+------------+
Table 4
The pseudocode to obtain a (length or distance) value from the prefix
code is as follows:
if (prefix_code < 4) {
return prefix_code + 1;
}
int extra_bits = (prefix_code - 2) >> 1;
int offset = (2 + (prefix_code & 1)) << extra_bits;
return offset + ReadBits(extra_bits) + 1;
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3.6.2.2.1. Distance Mapping
As noted previously, a distance code is a number indicating the
position of a previously seen pixel, from which the pixels are to be
copied. This subsection defines the mapping between a distance code
and the position of a previous pixel.
Distance codes larger than 120 denote the pixel distance in scan-line
order, offset by 120.
The smallest distance codes [1..120] are special and are reserved for
a close neighborhood of the current pixel. This neighborhood
consists of 120 pixels:
* Pixels that are 1 to 7 rows above the current pixel and are up to
8 columns to the left or up to 7 columns to the right of the
current pixel [Total such pixels = 7 * (8 + 1 + 7) = 112].
* Pixels that are in the same row as the current pixel and are up to
8 columns to the left of the current pixel [8 such pixels].
The mapping between distance code distance_code and the neighboring
pixel offset (xi, yi) is as follows:
(0, 1), (1, 0), (1, 1), (-1, 1), (0, 2), (2, 0), (1, 2),
(-1, 2), (2, 1), (-2, 1), (2, 2), (-2, 2), (0, 3), (3, 0),
(1, 3), (-1, 3), (3, 1), (-3, 1), (2, 3), (-2, 3), (3, 2),
(-3, 2), (0, 4), (4, 0), (1, 4), (-1, 4), (4, 1), (-4, 1),
(3, 3), (-3, 3), (2, 4), (-2, 4), (4, 2), (-4, 2), (0, 5),
(3, 4), (-3, 4), (4, 3), (-4, 3), (5, 0), (1, 5), (-1, 5),
(5, 1), (-5, 1), (2, 5), (-2, 5), (5, 2), (-5, 2), (4, 4),
(-4, 4), (3, 5), (-3, 5), (5, 3), (-5, 3), (0, 6), (6, 0),
(1, 6), (-1, 6), (6, 1), (-6, 1), (2, 6), (-2, 6), (6, 2),
(-6, 2), (4, 5), (-4, 5), (5, 4), (-5, 4), (3, 6), (-3, 6),
(6, 3), (-6, 3), (0, 7), (7, 0), (1, 7), (-1, 7), (5, 5),
(-5, 5), (7, 1), (-7, 1), (4, 6), (-4, 6), (6, 4), (-6, 4),
(2, 7), (-2, 7), (7, 2), (-7, 2), (3, 7), (-3, 7), (7, 3),
(-7, 3), (5, 6), (-5, 6), (6, 5), (-6, 5), (8, 0), (4, 7),
(-4, 7), (7, 4), (-7, 4), (8, 1), (8, 2), (6, 6), (-6, 6),
(8, 3), (5, 7), (-5, 7), (7, 5), (-7, 5), (8, 4), (6, 7),
(-6, 7), (7, 6), (-7, 6), (8, 5), (7, 7), (-7, 7), (8, 6),
(8, 7)
Figure 20
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For example, the distance code 1 indicates an offset of (0, 1) for
the neighboring pixel, that is, the pixel above the current pixel (0
pixel difference in the X direction and 1 pixel difference in the Y
direction). Similarly, the distance code 3 indicates the top-left
pixel.
The decoder can convert a distance code distance_code to a scan-line
order distance dist as follows:
(xi, yi) = distance_map[distance_code - 1]
dist = xi + yi * image_width
if (dist < 1) {
dist = 1
}
where distance_map is the mapping noted above, and image_width is the
width of the image in pixels.
3.6.2.3. Color Cache Coding
Color cache stores a set of colors that have been recently used in
the image.
| Rationale: This way, the recently used colors can sometimes be
| referred to more efficiently than emitting them using the other
| two methods (described in Sections 3.6.2.1 and 3.6.2.2).
Color cache codes are stored as follows. First, there is a 1-bit
value that indicates if the color cache is used. If this bit is 0,
no color cache codes exist, and they are not transmitted in the
prefix code that decodes the green symbols and the length prefix
codes. However, if this bit is 1, the color cache size is read next:
int color_cache_code_bits = ReadBits(4);
int color_cache_size = 1 << color_cache_code_bits;
color_cache_code_bits defines the size of the color cache (1 <<
color_cache_code_bits). The range of allowed values for
color_cache_code_bits is [1..11]. Compliant decoders MUST indicate a
corrupted bitstream for other values.
A color cache is an array of size color_cache_size. Each entry
stores one ARGB color. Colors are looked up by indexing them by
(0x1e35a7bd * color) >> (32 - color_cache_code_bits). Only one
lookup is done in a color cache; there is no conflict resolution.
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In the beginning of decoding or encoding of an image, all entries in
all color cache values are set to zero. The color cache code is
converted to this color at decoding time. The state of the color
cache is maintained by inserting every pixel, be it produced by
backward referencing or as literals, into the cache in the order they
appear in the stream.
3.7. Entropy Code
3.7.1. Overview
Most of the data is coded using a canonical prefix code [Huffman].
Hence, the codes are transmitted by sending the _prefix code
lengths_, as opposed to the actual _prefix codes_.
In particular, the format uses *spatially variant prefix coding*. In
other words, different blocks of the image can potentially use
different entropy codes.
| Rationale: Different areas of the image may have different
| characteristics. So, allowing them to use different entropy
| codes provides more flexibility and potentially better
| compression.
3.7.2. Details
The encoded image data consists of several parts:
1. Decoding and building the prefix codes.
2. Meta prefix codes.
3. Entropy-coded image data.
For any given pixel (x, y), there is a set of five prefix codes
associated with it. These codes are (in bitstream order):
* *Prefix code #1*: Used for green channel, backward-reference
length, and color cache.
* *Prefix code #2, #3, and #4*: Used for red, blue, and alpha
channels, respectively.
* *Prefix code #5*: Used for backward-reference distance.
From here on, we refer to this set as a *prefix code group*.
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3.7.2.1. Decoding and Building the Prefix Codes
This section describes how to read the prefix code lengths from the
bitstream.
The prefix code lengths can be coded in two ways. The method used is
specified by a 1-bit value.
* If this bit is 1, it is a _simple code length code_.
* If this bit is 0, it is a _normal code length code_.
In both cases, there can be unused code lengths that are still part
of the stream. This may be inefficient, but it is allowed by the
format. The described tree must be a complete binary tree. A single
leaf node is considered a complete binary tree and can be encoded
using either the simple code length code or the normal code length
code. When coding a single leaf node using the _normal code length
code_, all but one code length are zeros, and the single leaf node
value is marked with the length of 1 -- even when no bits are
consumed when that single leaf node tree is used.
3.7.2.1.1. Simple Code Length Code
This variant is used in the special case when only 1 or 2 prefix
symbols are in the range [0..255] with code length 1. All other
prefix code lengths are implicitly zeros.
The first bit indicates the number of symbols:
int num_symbols = ReadBits(1) + 1;
The following are the symbol values. This first symbol is coded
using 1 or 8 bits, depending on the value of is_first_8bits. The
range is [0..1] or [0..255], respectively. The second symbol, if
present, is always assumed to be in the range [0..255] and coded
using 8 bits.
int is_first_8bits = ReadBits(1);
symbol0 = ReadBits(1 + 7 * is_first_8bits);
code_lengths[symbol0] = 1;
if (num_symbols == 2) {
symbol1 = ReadBits(8);
code_lengths[symbol1] = 1;
}
| The two symbols should be different. Duplicate symbols are
| allowed, but inefficient.
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Note: Another special case is when _all_ prefix code lengths are
_zeros_ (an empty prefix code). For example, a prefix code for
distance can be empty if there are no backward references.
Similarly, prefix codes for alpha, red, and blue can be empty if all
pixels within the same meta prefix code are produced using the color
cache. However, this case doesn't need special handling, as empty
prefix codes can be coded as those containing a single symbol 0.
3.7.2.1.2. Normal Code Length Code
The code lengths of the prefix code fit in 8 bits and are read as
follows. First, num_code_lengths specifies the number of code
lengths.
int num_code_lengths = 4 + ReadBits(4);
The code lengths are themselves encoded using prefix codes; lower-
level code lengths, code_length_code_lengths, first have to be read.
The rest of those code_length_code_lengths (according to the order in
kCodeLengthCodeOrder) are zeros.
int kCodeLengthCodes = 19;
int kCodeLengthCodeOrder[kCodeLengthCodes] = {
17, 18, 0, 1, 2, 3, 4, 5, 16, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15
};
int code_length_code_lengths[kCodeLengthCodes] = { 0 }; // All zeros
for (i = 0; i < num_code_lengths; ++i) {
code_length_code_lengths[kCodeLengthCodeOrder[i]] = ReadBits(3);
}
Next, if ReadBits(1) == 0, the maximum number of different read
symbols (max_symbol) for each symbol type (A, R, G, B, and distance)
is set to its alphabet size:
* G channel: 256 + 24 + color_cache_size
* Other literals (A, R, and B): 256
* Distance code: 40
Otherwise, it is defined as:
int length_nbits = 2 + 2 * ReadBits(3);
int max_symbol = 2 + ReadBits(length_nbits);
If max_symbol is larger than the size of the alphabet for the symbol
type, the bitstream is invalid.
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A prefix table is then built from code_length_code_lengths and used
to read up to max_symbol code lengths.
* Code [0..15] indicates literal code lengths.
- Value 0 means no symbols have been coded.
- Values [1..15] indicate the bit length of the respective code.
* Code 16 repeats the previous nonzero value [3..6] times, that is,
3 + ReadBits(2) times. If code 16 is used before a nonzero value
has been emitted, a value of 8 is repeated.
* Code 17 emits a streak of zeros of length [3..10], that is, 3 +
ReadBits(3) times.
* Code 18 emits a streak of zeros of length [11..138], that is, 11 +
ReadBits(7) times.
Once code lengths are read, a prefix code for each symbol type (A, R,
G, B, and distance) is formed using their respective alphabet sizes.
3.7.2.2. Decoding of Meta Prefix Codes
As noted earlier, the format allows the use of different prefix codes
for different blocks of the image. _Meta prefix codes_ are indexes
identifying which prefix codes to use in different parts of the
image.
Meta prefix codes may be used _only_ when the image is being used in
the role (Section 3.6.1) of an _ARGB image_.
There are two possibilities for the meta prefix codes, indicated by a
1-bit value:
* If this bit is zero, there is only one meta prefix code used
everywhere in the image. No more data is stored.
* If this bit is one, the image uses multiple meta prefix codes.
These meta prefix codes are stored as an _entropy image_
(described below).
The red and green components of a pixel define a 16-bit meta prefix
code used in a particular block of the ARGB image.
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3.7.2.2.1. Entropy Image
The entropy image defines which prefix codes are used in different
parts of the image.
The first 3 bits contain the prefix_bits value. The dimensions of
the entropy image are derived from prefix_bits:
int prefix_bits = ReadBits(3) + 2;
int prefix_image_width =
DIV_ROUND_UP(image_width, 1 << prefix_bits);
int prefix_image_height =
DIV_ROUND_UP(image_height, 1 << prefix_bits);
where DIV_ROUND_UP is as defined in Section 3.5.1.
The next bits contain an entropy image of width prefix_image_width
and height prefix_image_height.
3.7.2.2.2. Interpretation of Meta Prefix Codes
The number of prefix code groups in the ARGB image can be obtained by
finding the _largest meta prefix code_ from the entropy image:
int num_prefix_groups = max(entropy image) + 1;
where max(entropy image) indicates the largest prefix code stored in
the entropy image.
As each prefix code group contains five prefix codes, the total
number of prefix codes is:
int num_prefix_codes = 5 * num_prefix_groups;
Given a pixel (x, y) in the ARGB image, we can obtain the
corresponding prefix codes to be used as follows:
int position =
(y >> prefix_bits) * prefix_image_width + (x >> prefix_bits);
int meta_prefix_code = (entropy_image[position] >> 8) & 0xffff;
PrefixCodeGroup prefix_group = prefix_code_groups[meta_prefix_code];
where we have assumed the existence of PrefixCodeGroup structure,
which represents a set of five prefix codes. Also,
prefix_code_groups is an array of PrefixCodeGroup (of size
num_prefix_groups).
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The decoder then uses prefix code group prefix_group to decode the
pixel (x, y), as explained in Section 3.7.2.3.
3.7.2.3. Decoding Entropy-Coded Image Data
For the current position (x, y) in the image, the decoder first
identifies the corresponding prefix code group (as explained in the
last section). Given the prefix code group, the pixel is read and
decoded as follows.
Next, read symbol S from the bitstream using prefix code #1. Note
that S is any integer in the range 0 to (256 + 24 + color_cache_size
(Section 3.6.2.3)- 1).
The interpretation of S depends on its value:
1. If S < 256
i. Use S as the green component.
ii. Read red from the bitstream using prefix code #2.
iii. Read blue from the bitstream using prefix code #3.
iv. Read alpha from the bitstream using prefix code #4.
2. If S >= 256 & S < 256 + 24
i. Use S - 256 as a length prefix code.
ii. Read extra bits for the length from the bitstream.
iii. Determine backward-reference length L from length prefix
code and the extra bits read.
iv. Read the distance prefix code from the bitstream using
prefix code #5.
v. Read extra bits for the distance from the bitstream.
vi. Determine backward-reference distance D from the distance
prefix code and the extra bits read.
vii. Copy L pixels (in scan-line order) from the sequence of
pixels starting at the current position minus D pixels.
3. If S >= 256 + 24
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i. Use S - (256 + 24) as the index into the color cache.
ii. Get ARGB color from the color cache at that index.
3.8. Overall Structure of the Format
Below is a view into the format in Augmented Backus-Naur Form
[RFC5234] [RFC7405]. It does not cover all details. The end-of-
image (EOI) is only implicitly coded into the number of pixels
(image_width * image_height).
Note that *element means element can be repeated 0 or more times.
5element means element is repeated exactly 5 times. %b represents a
binary value.
3.8.1. Basic Structure
format = RIFF-header image-header image-stream
RIFF-header = %s"RIFF" 4OCTET %s"WEBPVP8L" 4OCTET
image-header = %x2F image-size alpha-is-used version
image-size = 14BIT 14BIT ; width - 1, height - 1
alpha-is-used = 1BIT
version = 3BIT ; 0
image-stream = optional-transform spatially-coded-image
3.8.2. Structure of Transforms
optional-transform = (%b1 transform optional-transform) / %b0
transform = predictor-tx / color-tx / subtract-green-tx
transform =/ color-indexing-tx
predictor-tx = %b00 predictor-image
predictor-image = 3BIT ; sub-pixel code
entropy-coded-image
color-tx = %b01 color-image
color-image = 3BIT ; sub-pixel code
entropy-coded-image
subtract-green-tx = %b10
color-indexing-tx = %b11 color-indexing-image
color-indexing-image = 8BIT ; color count
entropy-coded-image
3.8.3. Structure of the Image Data
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spatially-coded-image = color-cache-info meta-prefix data
entropy-coded-image = color-cache-info data
color-cache-info = %b0
color-cache-info =/ (%b1 4BIT) ; 1 followed by color cache size
meta-prefix = %b0 / (%b1 entropy-image)
data = prefix-codes lz77-coded-image
entropy-image = 3BIT ; subsample value
entropy-coded-image
prefix-codes = prefix-code-group *prefix-codes
prefix-code-group =
5prefix-code ; See "Interpretation of Meta Prefix Codes" to
; understand what each of these five prefix
; codes are for.
prefix-code = simple-prefix-code / normal-prefix-code
simple-prefix-code = ; see "Simple Code Length Code" for details
normal-prefix-code = ; see "Normal Code Length Code" for details
lz77-coded-image =
*((argb-pixel / lz77-copy / color-cache-code) lz77-coded-image)
The following is a possible example sequence:
RIFF-header image-size %b1 subtract-green-tx
%b1 predictor-tx %b0 color-cache-info
%b0 prefix-codes lz77-coded-image
4. Security Considerations
Implementations of this format face security risks, such as integer
overflows, out-of-bounds reads and writes to both heap and stack,
uninitialized data usage, null pointer dereferences, resource (disk
or memory) exhaustion, and extended resource usage (long running
time) as part of the demuxing and decoding process. In particular,
implementations reading this format are likely to take input from
unknown and possibly unsafe sources -- both clients (for example, web
browsers or email clients) and servers (for example, applications
that accept uploaded images). These may result in arbitrary code
execution, information leakage (memory layout and contents), or
crashes and thereby allow a device to be compromised or cause a
denial of service to an application using the format
[cve.mitre.org-libwebp] [crbug-security].
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The format does not employ "active content" but does allow metadata
(for example, [XMP] and [Exif]) and custom chunks to be embedded in a
file. Applications that interpret these chunks may be subject to
security considerations for those formats.
5. Interoperability Considerations
The format is defined using little-endian byte ordering (see
Section 3.1 of [RFC2781]), but demuxing and decoding are possible on
platforms using a different ordering with the appropriate conversion.
The container is based on RIFF and allows extension via user-defined
chunks, but nothing beyond the chunks defined by the container format
(Section 2) are required for decoding of the image. These have been
finalized but were extended in the format's early stages, so some
older readers may not support lossless or animated image decoding.
6. IANA Considerations
IANA has registered the 'image/webp' media type [RFC2046].
6.1. The 'image/webp' Media Type
This section contains the media type registration details per
[RFC6838].
6.1.1. Registration Details
*RFC Editor Note:* Replace RFC XXXX / rfcXXXX with the published RFC
number.
Type name: image
Subtype name: webp
Required parameters: N/A
Optional parameters: N/A
Encoding considerations: Binary. The Base64 encoding [RFC4648]
should be used on transports that cannot accommodate binary data
directly.
Security considerations: See RFC XXXX, Section 4.
Interoperability considerations: See RFC XXXX, Section 5.
Published specification: RFC XXXX
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Applications that use this media type: Applications that are used to
display and process images, especially when smaller image file
sizes are important.
Fragment identifier considerations: N/A
Additional information:
Deprecated alias names for this type: N/A
Magic number(s): The first 4 bytes are 0x52, 0x49, 0x46, 0x46
('RIFF'), followed by 4 bytes for the 'RIFF' Chunk size. The
next 7 bytes are 0x57, 0x45, 0x42, 0x50, 0x56, 0x50, 0x38
('WEBPVP8').
File extension(s): webp
Apple Uniform Type Identifier: org.webmproject.webp conforms to
public.image
Object Identifiers: N/A
Person & email address to contact for further information: James
Zern <jzern@google.com>
Restrictions on usage: N/A
Author: James Zern <jzern@google.com>
Change controller:
IETF
Intended usage: COMMON
7. References
7.1. Normative References
[Exif] Camera & Imaging Products Association (CIPA) and Japan
Electronics and Information Technology Industries
Association (JEITA), "Exchangeable image file format for
digital still cameras: Exif Version 2.3", CIPA DC-
008-2012, JEITA CP-3451C, December 2012,
<https://www.cipa.jp/std/documents/e/DC-008-2012_E.pdf>.
[ICC] International Color Consortium, "Image technology colour
management -- Architecture, profile format, and data
structure", Profile version 4.3.0.0, REVISION of
ICC.1:2004-10, Specification ICC.1:2010, December 2010,
<https://www.color.org/specification/ICC1v43_2010-12.pdf>.
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[ISO.9899.2018]
International Organization for Standardization,
"Information technology -- Programming languages -- C",
Fourth Edition, ISO/IEC 9899:2018, June 2018,
<https://www.iso.org/standard/74528.html>.
[rec601] ITU, "Studio encoding parameters of digital television for
standard 4:3 and wide screen 16:9 aspect ratios", ITU-R
Recommendation BT.601, March 2011,
<https://www.itu.int/rec/R-REC-BT.601/>.
[RFC1166] Kirkpatrick, S., Stahl, M., and M. Recker, "Internet
numbers", RFC 1166, DOI 10.17487/RFC1166, July 1990,
<https://www.rfc-editor.org/info/rfc1166>.
[RFC2046] Freed, N. and N. Borenstein, "Multipurpose Internet Mail
Extensions (MIME) Part Two: Media Types", RFC 2046,
DOI 10.17487/RFC2046, November 1996,
<https://www.rfc-editor.org/info/rfc2046>.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.
[RFC2781] Hoffman, P. and F. Yergeau, "UTF-16, an encoding of ISO
10646", RFC 2781, DOI 10.17487/RFC2781, February 2000,
<https://www.rfc-editor.org/info/rfc2781>.
[RFC4648] Josefsson, S., "The Base16, Base32, and Base64 Data
Encodings", RFC 4648, DOI 10.17487/RFC4648, October 2006,
<https://www.rfc-editor.org/info/rfc4648>.
[RFC5234] Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", STD 68, RFC 5234,
DOI 10.17487/RFC5234, January 2008,
<https://www.rfc-editor.org/info/rfc5234>.
[RFC6386] Bankoski, J., Koleszar, J., Quillio, L., Salonen, J.,
Wilkins, P., and Y. Xu, "VP8 Data Format and Decoding
Guide", RFC 6386, DOI 10.17487/RFC6386, November 2011,
<https://www.rfc-editor.org/info/rfc6386>.
[RFC6838] Freed, N., Klensin, J., and T. Hansen, "Media Type
Specifications and Registration Procedures", BCP 13,
RFC 6838, DOI 10.17487/RFC6838, January 2013,
<https://www.rfc-editor.org/info/rfc6838>.
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[RFC7405] Kyzivat, P., "Case-Sensitive String Support in ABNF",
RFC 7405, DOI 10.17487/RFC7405, December 2014,
<https://www.rfc-editor.org/info/rfc7405>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.
[XMP] Adobe Inc., "XMP Specification",
<https://www.adobe.com/devnet/xmp.html>.
7.2. Informative References
[crbug-security]
"libwebp Security Issues",
<https://bugs.chromium.org/p/webp/issues/
list?q=label%3ASecurity>.
[cve.mitre.org-libwebp]
"libwebp CVE List", <https://cve.mitre.org/cgi-bin/
cvekey.cgi?keyword=libwebp>.
[GIF-spec] CompuServe Incorporated, "Graphics Interchange
Format(sm)", Version 89a, July 1990,
<https://www.w3.org/Graphics/GIF/spec-gif89a.txt>.
[Huffman] Huffman, D., "A Method for the Construction of Minimum-
Redundancy Codes", Proceedings of the Institute of Radio
Engineers, Vol. 40, Issue 9, pp. 1098-1101,
DOI 10.1109/JRPROC.1952.273898, September 1952,
<https://doi.org/10.1109/JRPROC.1952.273898>.
[JPEG-spec]
"Information Technology - Digital Compression and Coding
of Continuous-Tone Still Images - Requirements and
Guidelines", ITU-T Recommendation T.81, ISO/IEC 10918-1,
September 1992,
<https://www.w3.org/Graphics/JPEG/itu-t81.pdf>.
[LZ77] Ziv, J. and A. Lempel, "A Universal Algorithm for
Sequential Data Compression", IEEE Transactions on
Information Theory, Vol. 23, Issue 3, pp. 337-343,
DOI 10.1109/TIT.1977.1055714, May 1977,
<https://doi.org/10.1109/TIT.1977.1055714>.
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[MWG] Metadata Working Group, "Guidelines For Handling Image
Metadata", Version 2.0, November 2010,
<https://web.archive.org/web/20180919181934/
http://www.metadataworkinggroup.org/pdf/mwg_guidance.pdf>.
[RFC2083] Boutell, T., "PNG (Portable Network Graphics)
Specification Version 1.0", RFC 2083,
DOI 10.17487/RFC2083, March 1997,
<https://www.rfc-editor.org/info/rfc2083>.
[RIFF-spec]
"RIFF (Resource Interchange File Format)",
<https://www.loc.gov/preservation/digital/formats/fdd/
fdd000025.shtml>.
[webp-lossless-src]
Alakuijala, J., "WebP Lossless Bitstream Specification",
October 2023,
<https://chromium.googlesource.com/webm/libwebp/+/refs/
tags/webp-rfcXXXX/doc/webp-lossless-bitstream-spec.txt>.
[webp-lossless-study]
Alakuijala, J. and V. Rabaud, "Lossless and Transparency
Encoding in WebP", August 2017,
<https://developers.google.com/speed/webp/docs/
webp_lossless_alpha_study>.
[webp-riff-src]
Google LLC, "WebP RIFF Container", October 2023,
<https://chromium.googlesource.com/webm/libwebp/+/refs/
tags/webp-rfcXXXX/doc/webp-container-spec.txt>.
Authors' Addresses
James Zern
Google LLC
1600 Amphitheatre Parkway
Mountain View, CA 94043
United States of America
Phone: +1 650 253-0000
Email: jzern@google.com
Pascal Massimino
Google LLC
Email: pascal.massimino@gmail.com
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Jyrki Alakuijala
Google LLC
Email: jyrki.alakuijala@gmail.com
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