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More aggressive copy-edit; add TODO; validate HTML5

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Change-Id: I45e7fde3eb33067274b5d454451f1bf8785511fd
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Lou Quillio 2011-10-04 15:11:43 -07:00
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5
doc/README
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@ -9,6 +9,7 @@ automatically.
[1]: http://kramdown.rubyforge.org/ [1]: http://kramdown.rubyforge.org/
[2]: http://rubygems.org/ [2]: http://rubygems.org/
HTML generation can then be done from the project root, like so: HTML generation can then be done from the project root:
$ kramdown doc/webp-container-spec.txt --template doc/template.html > doc/output/webp-container-spec.html $ kramdown doc/webp-container-spec.txt --template doc/template.html > \
doc/output/webp-container-spec.html

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@ -0,0 +1,24 @@
<louquillio@google.com>, 20111004
* Determine that normative RFC 2119 terms (MUST, SHOULD, MAY, etc.) are
truly intended in all cases where capitalized.
* Document hierarchy WRT headings has a flaw, in that topics related to
animated WebPs are discussed under subheads of "Single-image WebP
Files".
* Several passages could be made clearer.
* Overall edit for scope. Portions are phrased as an introduction to
the 0.1.3 RIFF container additions, rather than a holistic guide to
WebP.
* To wit, suggest s/[spec|specification]/guide/g . "Spec" can imply a
standards track; in any case it's too formal for a work in progress.
* Sections and passages re "multi-image" should likely be suppressed
until multi-image drops.
* Improve the term "Mux-Container".

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doc/template.html
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@ -1,5 +1,7 @@
<html> <!DOCTYPE html>
<html lang="en">
<head> <head>
<meta charset="utf-8">
<title>WebP Container Specification</title> <title>WebP Container Specification</title>
<meta name="generator" content="kramdown <%= ::Kramdown::VERSION %>" /> <meta name="generator" content="kramdown <%= ::Kramdown::VERSION %>" />
<style type="text/css"> <style type="text/css">

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doc/webp-container-spec.txt
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@ -1,8 +1,8 @@
<!-- <!--
This document is encoded in Markdown, a light-duty markup scheme, and is Although you may be viewing an alternate representation, this document
optimized for the [kramdown](http://kramdown.rubyforge.org/) is sourced in Markdown, a light-duty markup scheme, and is optimized for
transformer. the [kramdown](http://kramdown.rubyforge.org/) transformer.
See the accompanying README. External link targets are referenced at the See the accompanying README. External link targets are referenced at the
end of this file. end of this file.
@ -13,7 +13,7 @@ end of this file.
WebP Container Specification WebP Container Specification
============================ ============================
_Working Draft, v0.1, 20110926_ _Working Draft, v0.1, 20111004_
* TOC placeholder * TOC placeholder
@ -23,59 +23,56 @@ _Working Draft, v0.1, 20110926_
Introduction Introduction
------------ ------------
WebP is a still image format that uses the VP8 key frame encoding (and, WebP is a still image format that uses the VP8 key frame encoding, and
possibly, other codecs in the future) to compress image data in a lossy possibly other encodings in the future, to compress image data in a
way. The VP8 encoding should make it more efficient than currently used lossy way. The VP8 encoding should make it more efficient than currently
formats. It is optimized for fast image transfer over the network used formats. It is optimized for fast image transfer over the network
(e.g., for websites). However, it also aims for feature parity (like (e.g., for websites). However, it also aims for feature parity (like
Color Profile, XMP Metadata, Animation, etc.) with other formats. This Color Profile, XMP Metadata, Animation, etc.) with other formats. This
document describes the structure of such a file. document describes the structure of a WebP file.
The first version of WebP handled only the basic use-case -- a file The first version of WebP handled only the basic use case: a file
having a single image (being one VP8 key frame) with no metadata. containing a single image (being one VP8 key frame), with no metadata.
However, the use of a RIFF container allowed to extend it. This The use of a RIFF container permits additional feature support. This
document extends it by additionally introducing support for: document describes additional support for:
* **Metadata and color profiles.** We specify chunks that can contain * **Metadata and color profiles.** We specify chunks that can contain
this information, like can other popular formats. this information, as other popular formats do.
* **Tiling.** A single VP8 frame has an inherent limitation for width * **Tiling.** A single VP8 frame has an inherent limitation for width
or height of 2^14 pixels and a 512kB limit on the size of first or height of 2^14 pixels, and a 512kB limit on the size of the first
compressed partition. To support larger images, we support images compressed partition. To support larger images, we support images
that are composed of multiple tiles, each encoded as a separate VP8 that are composed of multiple tiles, each encoded as a separate VP8
frame. All tiles form logically a single image -- they have common frame. All tiles form logically a single image: they have common
metadata, color profile etc. Tiling may also improve efficiency for metadata, color profile, etc. Tiling may also improve efficiency for
larger images -- grass can be encoded differently than sky. larger images, e.g., grass can be encoded differently than sky.
* **Animation.** An image may have pauses between frames, making it * **Animation.** An image may have pauses between frames, making it
an animation. Files not using these new features are backward an animation.
compatible with the original format. Using these features will
produce files that are not compatible with older programs. Files not using these new features are backward compatible with the
original format. Use of these features will produce files that are not
compatible with older programs.
Terminology Terminology &amp; Basics
----------- ------------------------
A WebP file contains either a still image (i.e. an encoded matrix of 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, 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 metadata, etc. In case we need to refer only to the matrix of pixels,
will call is the **_canvas_** of the image. we will call it the **_canvas_** of the image.
The canvas of an image is built from one or multiple tiles. Each tile The canvas of an image is built from one or multiple tiles. Each tile
is a separately encoded VP8 key frame (other codec are possible in the is a separately encoded VP8 key frame (other encodings are possible in
future). Building an image from several tiles allows to overcome the the future). Building an image from several tiles allows us to overcome
size limitations of a single VP8 frame. Tiling is supposed to be an the size limitations of a single VP8 frame. Tiles are an internal detail
internal detail of the file -- they are not supposed to be exposed to of the file: they are not supposed to be exposed to the user.
the user.
Below are additional terms used throughout this document:
Basics Code that reads WebP files is referred to as a **_reader_**, while
------ code that writes them is referred to as a **_writer_**.
This section introduces basic terms used throughout the document.
Code reading WebP files will be referred to as **_readers_**, while
code writing them will be referred as **_writers_**.
A 16-bit, little-endian, unsigned integer will be denoted as A 16-bit, little-endian, unsigned integer will be denoted as
**_uint16_**. **_uint16_**.
@ -96,66 +93,66 @@ The basic element of a RIFF file is a **_chunk_**. It consists of:
A chunk with a tag "ABCD" will be also called a **_chunk of type_** A chunk with a tag "ABCD" will be also called a **_chunk of type_**
"ABCD". Note that, in this specification, all chunk tag characters are "ABCD". Note that, in this specification, all chunk tag characters are
in file order, not in byte order of an uint32 of any particular in file order, not in byte order of a uint32 of any particular
architecture. architecture.
Note that the padding **MUST** be also added to the last chunk of the Note that the padding **MUST** be added to the last chunk of the file.
file.
A **_list of chunks_** is a concatenation of multiple chunks. We will A **_list of chunks_** is a concatenation of multiple chunks. We will
call the first chunk as having _position_ 0, the second as position 1 refer to the first chunk as having _position_ 0, the second as position
etc. By _chunk with index 0 among "ABCD"_ we mean the first chunk among 1, etc. By _chunk with index 0 among "ABCD"_ we mean the first chunk
the chunks of type "ABCD" in the list, the _chunk with index 1 among among the chunks of type "ABCD" in the list, the _chunk with index 1
"ABCD"_ is the second such chunk, etc. among "ABCD"_ is the second such chunk, etc.
A WebP file **MUST** begin with a single chunk with a tag "RIFF". All A WebP file **MUST** begin with a single chunk with a tag "RIFF". All
other defined chunks are within this chunk. It **SHOULD NOT** contain other defined chunks are contained within this chunk. The file **SHOULD
anything after it. NOT** contain anything after it.
The maximum size of RIFF's _ckSize_ is 2^32 -- 10 bytes (i.e. the size The maximum size of RIFF's _ckSize_ is 2^32 minus 10 bytes. The size
of the whole file is at most 4GiB -- 2 bytes). of the whole file is at most 4GiB minus 2 bytes.
**Note:** some RIFF libraries are said to have bugs when handling files **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 larger than 1GiB or 2GiB. If you are using an existing library, check
that it handles large files correctly. that it handles large files correctly.
The first four bytes of the RIFF chunk contents (i.e. bytes 8-11 of the 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 file) **MUST** be the ASCII string "WEBP". They are followed by a list
of chunks. Note that as the size of any chunk is even, the size of the of chunks. Note that as the size of any chunk is even, the size of the
RIFF chunk is also even. RIFF chunk is also even.
The content of the chunks in that list will be described in the The contents of the chunks in that list will be described in the
following sections. following sections.
**Note:** RIFF has a convention that all-uppercase chunks are standard **Note:** RIFF has a convention that all-uppercase chunks are standard
chunks that apply to any RIFF file format, while chunks specific for a chunks that apply to any RIFF file format, while chunks specific to a
file format are all-lowercase. WebP doesn't follow this convention. file format are all-lowercase. WebP doesn't follow this convention.
Single-image WebP Files Single-image WebP Files
----------------------- -----------------------
First, we will describe a subset of WebP files -- files containing only First, we will describe a subset of WebP files: files containing only
one image (later, we will use it to define multi-image files -- file one image. Later, we will define multi-image files, which contain
having several different images). several images.
### Chunks Layout ### Chunks Layout
This section describes what chunks and in what order may appear in a This section describes which chunks may appear in a single-image WebP
single-image WebP file. The content of these chunks will be described file, and their order. The contents of these chunks will be described
in subsequent sections. in subsequent sections.
The first chunk inside the RIFF chunk **MUST** be with a tag of "VP8 " The first chunk inside the RIFF chunk **MUST** have a tag of "VP8 "
(note the space as the last character) or "VP8X". Other tags for the (note that the fourth character is a space, and is significant) or
first chunk **MAY** be introduced by future specifications if we add "VP8X". Other tags for the first chunk **MAY** be introduced by future
new codecs. This tag of the first chunk determines which of the two specifications if new encodings are added. This tag of the first chunk
possible layouts is used. determines which of the two possible layouts is used.
**Rationale:** we fix the possible tags of the first chunk so that it **Rationale:** We fix the possible tags of the first chunk so that it
is possible to introduce other codecs, to keep the "WEBP" signature at is possible to introduce other codecs, to keep the "WEBP" signature at
the beginning of RIFF chunk, while still being able to check the codec the beginning of the RIFF chunk while still being able to check the
used by the image by inspecting the byte stream at a fixed position. codec used by the image by inspecting the byte stream at a fixed
position.
The two possible layouts will be called _images without special layout_ The two possible layouts will be called _images without special layout_
and _images with special layout_. and _images with special layout_.
@ -174,7 +171,7 @@ Such images consist of:
* A "VP8 " chunk with the bitstream of the single tile. * A "VP8 " chunk with the bitstream of the single tile.
**Example:** An example layout of such a file looks as follows: **Example:** An example layout of such a file is as follows:
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
RIFF/WEBP RIFF/WEBP
@ -184,16 +181,14 @@ RIFF/WEBP
#### Images With Special Layout #### Images With Special Layout
If the first subchunk of RIFF has the tag "VP8X" (other tags may be If the first subchunk of RIFF has the tag "VP8X", the file contains an
introduced by future specifications, if new codecs are added), the file _image with special layout_.
contains an _image with special layout_.
**Note:** older readers are not supporting images with special layout **Note:** Older readers may not support images with special layout.
and will fail for images having them.
Such an image consists of: Such an image consists of:
* A "VP8X" chunk with information about features used in this file. * A "VP8X" chunk with information about features used in the file.
* An optional "ICCP" chunk with color profile. * An optional "ICCP" chunk with color profile.
@ -203,34 +198,35 @@ Such an image consists of:
* An optional "META" chunk with XMP metadata. * An optional "META" chunk with XMP metadata.
* Some other chunks may be defined by future specifications and placed * Some other chunk types may be defined by future specifications and
anywhere in the file. placed anywhere in the file.
As will be described in the "VP8X" chunk description, by checking a As will be described in the "VP8X" chunk description, by checking a
flag one can distinguish animated and non-animated images. A flag one can distinguish animated and non-animated images. A
non-animated image has exactly one frame. An animated one may have non-animated image has exactly one frame. An animated one may have
multiple frames. Data for each frame consists of: multiple frames. Data for each frame consists of:
* An optional "FRM " (note the space as the last character) chunk with * An optional "FRM " (fourth character is a significant space) chunk
animation frame metadata. It **MUST** be present in animated images with animation frame metadata. It **MUST** be present in animated
at the beginning of data for that frame. It **MUST NOT** be present images at the beginning of data for that frame. It **MUST NOT** be
in non-animated images. present in non-animated images.
* An optional "TILE" chunk with tile position metadata. It **MUST** be * An optional "TILE" chunk with tile position metadata. It **MUST** be
present at the beginning of data of image that's represented as present at the beginning of data for an image that's represented as
multiple tile images. multiple tile images.
* A "VP8 " chunk with the bitstream of the tile. * A "VP8 " chunk with the bitstream of the tile.
All chunks **MUST** be placed in the same order as listed above (except All chunks **MUST** be placed in the same order as listed above (except
for unknown chunks, that **MAY** appear anywhere). If a chunk appears for unknown chunks, which **MAY** appear anywhere). If a chunk appears
in a wrong place, the file is invalid, but readers **MAY** parse the in the wrong place, the file is invalid, but readers **MAY** parse the
file ignoring the chunks that come too late. file, ignoring the chunks that come too late.
**Rationale:** setting the order of chunks should allow to quickly stop **Rationale:** Setting the order of chunks should allow quicker file
the search for e.g., the ICCP if it is not present in the file. The parsing. For example, if an ICCP chunk does not appear in its required
rule of ignoring late chunks should make programs that needs to do a position, a decoder can choose to stop searching for it. The rule of
full search give the same results as the ones stopping early. ignoring late chunks should make programs that need to do a full search
give the same results as the ones stopping early.
**Example:** An example layout of a non-animated, tiled image may look **Example:** An example layout of a non-animated, tiled image may look
as follows: as follows:
@ -266,9 +262,9 @@ RIFF/WEBP
### Assembling the Canvas from Tiles and Animation ### Assembling the Canvas from Tiles and Animation
Contents of the chunks will be described in details in subsequent Contents of the chunks will be described in subsequent sections. Here we
section. Here, we provide an overview how they are used to assemble the provide an overview of how they are used to assemble the canvas. The
canvas. The notation _VP8X.canvasWidth_ means the field in the "VP8X" notation _VP8X.canvasWidth_ means the field in the "VP8X"
described as _canvasWidth_. described as _canvasWidth_.
Decoding a non-animated canvas **MUST** be equivalent to the following Decoding a non-animated canvas **MUST** be equivalent to the following
@ -310,7 +306,8 @@ for LOOP.loop = 0, ..., LOOP.loopCount-1
assert tile_params.tileCanvasY >= current_FRM.frameY assert tile_params.tileCanvasY >= current_FRM.frameY
assert tile_params.tileCanvasX + chunk.tileWidth >= current_FRM.frameX + current_FRM.frameWidth assert tile_params.tileCanvasX + chunk.tileWidth >= current_FRM.frameX + current_FRM.frameWidth
assert tile_params.tileCanvasY + chunk.tileHeight >= current_FRM.frameX + current_FRM.frameHeight assert tile_params.tileCanvasY + chunk.tileHeight >= current_FRM.frameX + current_FRM.frameHeight
render image in chunk in canvas with top-left corner in (tile_params.tileCanvasX, tile_params.tileCanvasY) using the isometry in VP8X.flags.rotationAndSymmetry. tile_params.tileCanvasX = tile_params.tileCanvasY = 0 render image in chunk in canvas with top-left corner in (tile_params.tileCanvasX, tile_params.tileCanvasY) using the isometry in VP8X.flags.rotationAndSymmetry.
tile_params.tileCanvasX = tile_params.tileCanvasY = 0
Ignore unknown chunks Ignore unknown chunks
canvas contains the decoded canvas. canvas contains the decoded canvas.
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
@ -323,27 +320,28 @@ decode the file.
### Bitstream Chunks (VP8) ### Bitstream Chunks (VP8)
These chunks contain compressed image data. Currently, the only allowed These chunks contain compressed image data. Currently, the only allowed
bitstream is VP8 and uses "VP8 " (note the space as the last character) bitstream is VP8, using "VP8 " (note the significant fourth-character
as its tag. We will refer to all chunks with this tag as **_bitstream space) as its tag. We will refer to all chunks with this tag as
chunks_**. As described earlier, images without special layout have a **_bitstream chunks_**. As described earlier, images without special
single bitstream chunk as the first subchunk of RIFF, while images with layout have a single bitstream chunk as the first subchunk of RIFF,
special layout may contain several of them -- one for each tile. while images with special layout may contain several of them, one for
each tile.
The content of a "VP8 " chunk **MUST** be one VP8 key frame (with The content of a "VP8 " chunk **MUST** be one VP8 key frame (with
optional padding -- see below). optional padding. See below).
The current draft of a VP8 specification can be found at The current [VP8 Data Format and Decoding Guide][vp8spec] can be found
[IETF.org][vp8spec]. Note that the VP8 frame header contains the VP8 at the IETF website, <http://www.ietf.org/>. Note that the VP8 frame
frame width and height. It is assumed to be the width and height of the header contains the VP8 frame width and height. That is assumed to be
tile. the width and height of the tile.
The VP8 specification specifies how to decode the image into Y'CbCr The VP8 specification describes how to decode the image into Y'CbCr
format. To convert to RGB, Rec. 601 **SHOULD** be used. format. To convert to RGB, Rec. 601 **SHOULD** be used.
For compatibility with older readers, if the size of the frame is odd, For compatibility with older readers, if the size of the frame is odd,
writers **SHOULD** append a padding byte (preferably `0`) inside the writers **SHOULD** append a padding byte (preferably `0`) inside the
chunk contents, making the chunk's _ckSize_ even. Newer readers chunk contents, making the chunk's _ckSize_ even. Newer readers
**MUST** support odd-sized tile chunks. **MUST** support odd-sized bitstream chunks.
### VP8X Chunk (Special Layout) ### VP8X Chunk (Special Layout)
@ -354,33 +352,33 @@ WebP.
The content of the chunk is as follows: The content of the chunk is as follows:
* **uint32** _flags_. The following bits are currently used (with `0` * **uint32** flags. The following bits are currently used (with `0`
being the least significant bit): being the least significant bit):
* bit 0: _haveTile_: set if the image is represented by Tiles. * bit 0: _hasTile_: Set if the image is represented by Tiles.
* bit 1: _haveAnimation_: set if the file is an animation. Data in * bit 1: _hasAnimation_: Set if the file is an animation. Data in
"LOOP" and "FRM " chunks should be used to control the animation. "LOOP" and "FRM " chunks should be used to control the animation.
* bit 2: _haveIccp_: set if the file contains a "ICCP" chunk with a * bit 2: _hasIccp_: Set if the file contains an "ICCP" chunk with a
color profile. If a file contains an "ICCP" chunk but this bit is color profile. If a file contains an "ICCP" chunk but this bit is
not set, the error is flagged while constructing the not set, the error is flagged while constructing the
Mux-Container. Mux-Container.
* bit 3: _haveMetadata_: set if the file contains a "META" chunk * bit 3: _hasMetadata_: Set if the file contains a "META" chunk
with a XMP metadata. If a file contains an "META" chunk but this with a XMP metadata. If a file contains an "META" chunk but this
bit is not set, the error is flagged while constructing the bit is not set, the error is flagged while constructing the
Mux-Container. FIXME Mux-Container.
Future specification **MAY** define other bits in flags. Bits not Future specifications **MAY** define other bits in flags. Bits not
defined by this specification **MUST** be preserved when modifying the defined by this specification **MUST** be preserved when modifying the
file. file.
* **uint32** _canvasWidth_: width of the canvas in pixels (after the * **uint32** _canvasWidth_: Width of the canvas in pixels (after the
optional rotation or symmetry -- see below). optional rotation or symmetry; see below).
* **uint32** _canvasHeight_: height of the canvas in pixels (after the * **uint32** _canvasHeight_: Height of the canvas in pixels (after
optional rotation or symmetry see below). the optional rotation or symmetry; see below).
Future specifications **MAY** add more fields. If a chunk of larger size Future specifications **MAY** add more fields. If a chunk of larger size
is found, programs **MUST** ignore the extra bytes but **MUST** preserve is found, programs **MUST** ignore the extra bytes but **MUST** preserve
@ -392,14 +390,14 @@ them when modifying the file.
For images that are animations, this chunk contains the global For images that are animations, this chunk contains the global
parameters of the animation. parameters of the animation.
This chunk **MUST** appear if the _haveAnimation_ flag in chunk VP8X is This chunk **MUST** appear if the _hasAnimation_ flag in chunk VP8X is
set. If the _haveAnimation_ flag is not set and this chunk is present, set. If the _hasAnimation_ flag is not set and this chunk is present,
it **MUST** be ignored. it **MUST** be ignored.
The content of the chunk is as follows: The content of the chunk is as follows:
* **uint16** _loopCount_: For animations, the number of times to loop * **uint16** _loopCount_: For animations, the number of times to loop
this animation. `0` means infinite. the animation. `0` means infinitely.
Future specifications **MAY** add more fields. If a chunk of larger Future specifications **MAY** add more fields. If a chunk of larger
size is found, programs **MUST** ignore the extra bytes but **MUST** size is found, programs **MUST** ignore the extra bytes but **MUST**
@ -413,39 +411,39 @@ parameters of the animation.
The content of the chunk is as follows: The content of the chunk is as follows:
* **uint32** _frameX_: x coordinate of the upper left corner of the * **uint32** _frameX_: X coordinate of the upper left corner of the
frame. For images using the VP8 codec, it **MUST** be divisible by frame. For images using the VP8 codec, this value **MUST** be
`32`. Other codecs **MAY** specify other constraints. Described in divisible by `32`. Other codecs **MAY** specify other constraints.
more details later. Described in more detail later.
* **uint32** _frameY_: y coordinate of the upper left corner of the * **uint32** _frameY_: Y coordinate of the upper left corner of the
frame. For images using the VP8 codec, it **MUST** be divisible by frame. For images using the VP8 codec, this value **MUST** be
`32`. Other codecs **MAY** specify other constraints. Described in divisible by `32`. Other codecs **MAY** specify other constraints.
more details later. Described in more detail later.
* **uint32** _frameWidth_: width of the frame. For images using the * **uint32** _frameWidth_: Width of the frame. For images using the
VP8 codec, it **MUST** be divisible by `16` or such that VP8 codec, this value **MUST** be divisible by `16`, or be such that
_frameX + frameWidth == canvasWidth_. Other codecs **MAY** specify _frameX + frameWidth == canvasWidth_. Other codecs **MAY** specify
other constraints. Desribed in more details later. other constraints. Described in more detail later.
* **uint32** _frameHeight_: height. For images using the VP8 codec, it * **uint32** _frameHeight_: Height. For images using the VP8 codec,
**MUST** be divisible by `16` or such that _frameY + frameHeight == this value **MUST** be divisible by `16`, or be such that _frameY +
canvasHeight_. Other codecs **MAY** specify other constraints. frameHeight == canvasHeight_. Other codecs **MAY** specify other
Desribed in more details later. constraints. Described in more detail later.
* **uint16** _frameDuration_: Time to wait before displaying the next * **uint16** _frameDuration_: Time to wait before displaying the next
tile, in 1ms units. tile, in 1ms units.
**Rationale:** The requirement for corner coordinates to be divisible **Rationale:** The requirement for corner coordinates to be divisible
by `32` means that pixels on U and V planes are aligned to 16-byte by `32` means that pixels on U and V planes are aligned to a 16-byte
boundary (even after a rotation), what may help with vector boundary (even after a rotation), which may help with vector
instructions on some architectures. Also, this makes the tiles also instructions on some architectures. This makes the tiles also align to
aligned to 16-pixel macroblock boundaries. 16-pixel macroblock boundaries.
**Rationale:** The requirement for the width and height to be **Rationale:** The requirement for the width and height to be
divisible by `16` or touching the edge of the canvas simplifies the divisible by `16` or touching the edge of the canvas simplifies the
handling of macroblocks that are on the edge of a tile -- VP8 decoders handling of macroblocks that are on the edge of a tile. VP8 decoders
can overwrite pixels outside the boundary in such a macroblock and this can overwrite pixels outside the boundary in such a macroblock, and this
guarantees they won't overwrite any data. guarantees they won't overwrite any data.
Future specifications **MAY** add more fields. If a chunk of larger Future specifications **MAY** add more fields. If a chunk of larger
@ -456,41 +454,41 @@ preserve them when modifying the file.
### TILE Chunks (Tile Parameters) ### TILE Chunks (Tile Parameters)
This chunk contains information about a single tile and describes the This chunk contains information about a single tile and describes the
bitstream chunk that proceeds it. bitstream chunk that follows it.
The content of such a chunk is as follows: The contents of such a chunk are as follows:
* **uint32** _tileCanvasX_: x coordinate of the upper left corner of * **uint32** _tileCanvasX_: X coordinate of the upper left corner of
the tile. For VP8 tiles, it **MUST** be divisible by `32`. Other the tile. For VP8 tiles, this value **MUST** be divisible by `32`.
codecs **MAY** specify other constraints. Other codecs **MAY** specify other constraints.
* **uint32** _tileCanvasY_: y coordinate of the upper left corner of * **uint32** _tileCanvasY_: Y coordinate of the upper left corner of
the tile. For VP8 tiles, it **MUST** be divisible by `32`. Other the tile. For VP8 tiles, this value **MUST** be divisible by `32`.
codecs **MAY** specify other constraints. Other codecs **MAY** specify other constraints.
Future specifications **MAY** add more fields. If a chunk of larger size Future specifications **MAY** add more fields. If a chunk of larger size
is found, programs **MUST** ignore the extra bytes but **MUST** preserve is found, programs **MUST** ignore the extra bytes but **MUST** preserve
them when modifying the file. them when modifying the file.
As described earlier, the TILE chunk is followed by a VP8 data. From As described earlier, the TILE chunk is followed by VP8 data. From that
that chun, we can read the height and width of the tile, that we will chunk we can read the height and width of the tile. These we denote as
denote by _tileWidth_ and _tileHeight_. In the case of VP8, we have the _tileWidth_ and _tileHeight_. In the case of VP8, we have the following
following constraints: constraints:
* The width of a tile **MUST** be divisible by `16` or there **MUST** * The width of a tile **MUST** be divisible by `16`, or _tileCanvasX +
be _tileCanvasX + tileWidth == canvasWidth_. tileWidth == canvasWidth_ **MUST** be true.
* The height of a tile **MUST** be divisible by `16` or there **MUST** * The height of a tile **MUST** be divisible by `16`, or
be _tileCanvasY + tileHeight == canvasHeight_. _tileCanvasY + tileHeight == canvasHeight_ **MUST** be true.
### ICCP Chunk (Color Profile) ### ICCP Chunk (Color Profile)
An optional "ICCP" chunk contains an ICC profile. There **SHOULD** be An optional "ICCP" chunk contains an ICC profile. There **SHOULD** be
at most one such chunk. The first byte of the chunk is the compression at most one such chunk. The first byte of the chunk is the compression
type. Two values are currently defined: a value of `0` means no type. Two values are currently defined: a value of `0` means no
compression, while a value of `1` means deflate/inflate compression. It compression, while a value of `1` means deflate/inflate compression. It
is followed by a compressed or non-compressed ICC profile -- see is followed by a compressed or non-compressed ICC profile. See
<http://www.color.org> for specifications. <http://www.color.org> for specifications.
The color profile can be a v2 or v4 profile. If this chunk is missing, The color profile can be a v2 or v4 profile. If this chunk is missing,
@ -499,31 +497,30 @@ sRGB **SHOULD** be assumed.
### META Chunk (Compressed XMP Metadata) ### META Chunk (Compressed XMP Metadata)
Such a chunk (if present) contains XMP metadata. There SHOULD be at Such a chunk (if present) contains XMP metadata. There **SHOULD** be at
most one such chunk. If there are more such chunks, readers **SHOULD** most one such chunk. If there are more such chunks, readers **SHOULD**
ignore all except the first one. The first byte specifies compression ignore all except the first one. The first byte specifies compression
type. Two values are currently defined: a value of `0` means no type. Two values are currently defined: a value of `0` means no
compression, while a value of `1` means deflate/inflate compression. It compression, while a value of `1` means deflate/inflate compression. It
is followed by a compressed or non-compressed XMP metadata packet. is followed by a compressed or non-compressed XMP metadata packet.
XMP packets are XML text specified in the [XMP Specification Part XMP packets are XML text as specified in the [XMP Specification Part
1][xmpspec]. The chunk tag is different from the one specified by Adobe 1][xmpspec]. The chunk tag is different from the one specified by Adobe
for WAV and AVI (also RIFF formats) because we have the options of for WAV and AVI (also RIFF formats), because we have the option of
compression. compression.
Additional guidance about handling metadata can be found in the Additional guidance about handling metadata can be found in the
Metadata Working Group's [Guidelines for Handling Metadata][metadata]. Metadata Working Group's [Guidelines for Handling Metadata][metadata].
Note that the sections of the document about reconciliation of EXIF, Note that the sections of the document about reconciliation of EXIF,
XMP and IPTC-IIM don't apply to WebP, as WebP supports only XMP, thus XMP and IPTC-IIM don't apply to WebP. As WebP supports only XMP, no
no reconciliation is necessary. reconciliation is necessary.
### Other Chunks ### Other Chunks
A file **MAY** contain other chunks, defined in some future A file **MAY** contain other chunks, defined in some future
specification. Such chunks **MUST** be ignored, but preserved. Writers specification. Such chunks **MUST** be ignored, but preserved. Writers
**SHOULD** try to preserve them in the original order. **SHOULD** try to preserve them in their original order.
[vp8spec]: http://tools.ietf.org/html/draft-bankoski-vp8-bitstream [vp8spec]: http://tools.ietf.org/html/draft-bankoski-vp8-bitstream