libwebp/doc/webp-container-spec.txt

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WebP Container Specification
============================

_Working Draft, v0.2, 20120207_


* TOC placeholder
{:toc}


Introduction
------------

WebP is a still image format that uses the VP8 key frame encoding, and
possibly other encodings in the future, to compress image data in a
lossy way. The VP8 encoding should make it more efficient than currently
used formats. It is optimized for fast image transfer over the network
(e.g., for websites). However, it also aims for feature parity
(color profile, XMP metadata, animation, etc.) with other formats. This
document describes the structure of a WebP file.

The first version of WebP handled only the basic use case: a file
containing a single image (being one VP8 key frame), with no metadata.
The use of a RIFF container permits additional feature support. This
document describes additional support for:

  * **Metadata and color profiles.** We specify chunks that can contain
    this information, as other popular formats do.

  * **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 the first
    compressed partition. To support larger images, we support images
    that are composed of multiple tiles, each encoded as a separate VP8
    frame. All tiles form logically a single image:  they have common
    metadata, color profile, etc. Tiling may also improve efficiency for
    larger images, e.g., grass can be encoded differently than sky.

  * **Animation.** An image may have pauses between frames, making it
    an animation.

  * **Transparency.** An image may have transparency, i.e., an alpha channel for
    each frame/tile.

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.

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC 2119][].


Terminology &amp; Basics
------------------------

A WebP file contains either a still image (i.e., an encoded matrix of
pixels) or an animation (see below), with possibly a color profile,
metadata, etc. In case we need to refer only to the matrix of pixels,
we will call it the _canvas_ of the image.

The canvas of an image is built from one or multiple tiles. Each tile
is a separately encoded VP8 key frame (other encodings are possible in
the future). Building an image from several tiles allows us to overcome
the size limitations of a single VP8 frame. Tiles are an internal detail
of the file:  they are not supposed to be exposed to the user.

Below are additional terms used throughout this document:

Code that reads WebP files is referred to as a _reader_, while
code that writes them is referred to as a _writer_.

_uint16_

: A 16-bit, little-endian, unsigned integer.

_uint32_

: A 32-bit, little-endian, unsigned integer.

The basic element of a RIFF file is a _chunk_. It consists of:

     0                   1                   2                   3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                         Chunk FourCC                          |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                          Chunk Size                           |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                         Chunk Payload                         |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Chunk FourCC: 32 bits

: ASCII four character code or _chunk tag_ used for chunk identification.

Chunk Size: 32 bits (_uint32_)

: The size of the chunk (_ckSize_) not including this field, the chunk
  identifier and padding.

Chunk Payload: _Chunk Size_ bytes

: The data payload. If _Chunk Size_ is odd a single padding byte that
  SHOULD be `0` is added.

_ChunkHeader('ABCD')_

: This is used to describe the fourcc and size header of individual
  chunks, where 'ABCD' is the fourcc for the chunk. This element's
  size is 8 bytes.

_chunk of type_

: A chunk with a tag "ABCD".

: Note that, in this specification, all chunk tag characters are in
  file order, not in byte order of a uint32 of any particular
  architecture.

_list of chunks_

: A concatenation of multiple chunks.

: We will refer to the first chunk as having _position_ 0, the second
  as position 1, etc. By _chunk with index 0 among "ABCD"_ we mean
  the first chunk among the chunks of type "ABCD" in the list, the
  _chunk with index 1 among "ABCD"_ is the second such chunk, etc.

A WebP file MUST begin with a single chunk with a tag 'RIFF'. All
other defined chunks are contained within this chunk. The file SHOULD
NOT contain anything after it.

The maximum size of RIFF's _ckSize_ is 2^32 minus 10 bytes. The size
of the whole file is at most 4GiB minus 2 bytes.

**Note:** some RIFF libraries are said to have bugs when handling files
larger than 1GiB or 2GiB. If you are using an existing library, check
that it handles large files correctly.

The first four bytes of the RIFF chunk contents (i.e., bytes 8-11 of the file)
MUST be the ASCII string "WEBP". They are followed by a list of chunks. As the
size of any chunk is even, the size of the RIFF chunk is also even.  The
contents of the chunks in that list will be described in the following sections.

**Note:** RIFF has a convention that all-uppercase chunks are standard
chunks that apply to any RIFF file format, while chunks specific to a
file format are all lowercase. WebP does not follow this convention.


WebP file header
----------------

     0                   1                   2                   3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |      'R'      |      'I'      |      'F'      |      'F'      |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                           File Size                           |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |      'W'      |      'E'      |      'B'      |      'P'      |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

'RIFF': 32 bits

: The ASCII characters 'R' 'I' 'F' 'F'.

File Size: 32 bits (_uint32_)

: The size of the file in bytes starting at offset 8.

'WEBP': 32 bits

: The ASCII characters 'W' 'E' 'B' 'P'.

Simple file format
------------------
Simple 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('VP8 ')                      |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                           VP8 data                            |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

VP8 data: _Chunk Size_ bytes

: VP8 bitstream data.

The content of a 'VP8 ' chunk (note the last character is a space) MUST be one
VP8 key frame (with optional padding).

The current [VP8 Data Format and Decoding Guide][vp8spec] can be found
at the IETF website, <http://www.ietf.org/>.

The VP8 specification describes how to decode the image into Y'CbCr
format. To convert to RGB, Rec. 601 SHOULD be used.

This layout SHOULD be used if the image does not require advanced
features: color profiles, XMP metadata, animation or tiling. Files with
this layout are smaller and supported by older software.

Extended file format
--------------------

**Note:** Older readers may not support files using the extended format.

An extended format file consists of:

  * A 'VP8X' chunk with information about features used in the file.

  * An optional 'ICCP' chunk with color profile.

  * An optional 'LOOP' chunk with animation control data.

  * Data for all the frames.

  * An optional 'META' chunk with XMP metadata.

  * Some other chunk types may be defined by future specifications and
    placed anywhere in the file.

As will be described in the 'VP8X' chunk description, by checking a
flag one can distinguish animated and non-animated images. A
non-animated image has exactly one frame. An animated one may have
multiple frames. Data for each frame consists of:

  * An optional 'FRM ' (fourth character is a significant space) chunk
    with animation frame metadata. It MUST be present in animated
    images at the beginning of data for that frame. It MUST NOT be
    present in non-animated images.

  * An optional 'TILE' chunk with tile position metadata. It MUST be
    present at the beginning of data for an image that's represented as
    multiple tile images.

  * An optional 'ALPH' chunk with alpha bitstream of the tile. It MUST be
    present for an image containing transparency. It MUST NOT be present
    in non-transparent images.

  * A 'VP8 ' chunk with the bitstream of the tile.

All chunks SHOULD be placed in the same order as listed above (except
for unknown chunks, which MAY appear anywhere). If a chunk appears
in the wrong place, the file is invalid, but readers MAY parse the
file, ignoring the chunks that come too late.

**Rationale:** Setting the order of chunks should allow quicker file
parsing. For example, if an ICCP chunk does not appear in its required
position, a decoder can choose to stop searching for it.  The rule of
ignoring late chunks should make programs that need to do a full search
give the same results as the ones stopping early.

Extended WebP file header:

     0                   1                   2                   3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                   WebP file header (12 bytes)                 |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                      ChunkHeader('VP8X')                      |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |  Rsrv |M|I|A|T|                   Reserved                    |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                          Canvas Width                         |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                          Canvas Height                        |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Tiling (T): 1 bit

: Set if the image is represented by tiles.

Animation (A): 1 bit

: Set if the file is an animation. Data in 'LOOP' and 'FRM ' chunks
  should be used to control the animation.

ICC profile (I): 1 bit

: Set if the file contains an 'ICCP' chunk.

Metadata (M): 1 bit

: Set if the file contains a 'META' chunk.

Reserved (Rsrv): 4 bits

: SHOULD be `0`.

Reserved: 16 bits

: SHOULD be `0`.

Canvas Width: 32 bits

: Width of the canvas in pixels.

Canvas Height: 32 bits

: Height of the canvas in pixels.

Future specifications MAY add more fields. If a chunk of larger size is found,
programs MUST ignore the extra bytes but SHOULD preserve them when modifying
the file.

### Chunks

#### Animation

Loop Chunk:

     0                   1                   2                   3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                      ChunkHeader('LOOP')                      |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |          Loop Count           |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Loop Count: 16 bits (_uint16_)

: The number of times to loop the animation. `0` means infinitely.

For images that are animations, this chunk contains the global
parameters of the animation.

This chunk MUST appear if the _Animation_ flag in chunk VP8X is set.
If the _Animation_ flag is not set and this chunk is present, it
SHOULD be ignored.

Per-frame 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('FRM ')                      |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                            Frame X                            |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                            Frame Y                            |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                          Frame Width                          |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                          Frame Height                         |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |         Frame Duration        |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Frame X: 32 bits (_uint32_)

: The X coordinate of the upper left corner of the frame.

Frame Y: 32 bits (_uint32_)

: The Y coordinate of the upper left corner of the frame.

Frame Width: 32 bits (_uint32_)

: The width of the frame.

Frame Height: 32 bits (_uint32_)

: The height of the frame.

Frame Duration: 16 bits (_uint16_)

: Time to wait before displaying the next tile, in 1 millisecond units.

Notes for frames containing VP8 data:

  * _Frame X_ and _Frame Y_ values MUST be divisible by `32`.

    **Rationale:** This ensures that pixels on U and V planes are aligned to a
    16-byte boundary (even after a rotation), which may help with vector
    instructions on some architectures. This also makes the tiles align to
    16-pixel macroblock boundaries.

  * _Frame Width_ MUST be divisible by `16` or
    `Frame X + Frame Width == Canvas Width` MUST be true.

  * _Frame Height_ MUST be divisible by `16` or
    `Frame Y + Frame Height == Canvas Height` MUST be true.

    **Rationale:** The width and height constraints simplify the 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 guarantees they
    won't overwrite any data.

#### Tiling

     0                   1                   2                   3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                      ChunkHeader('TILE')                      |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                         Tile Canvas X                         |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                         Tile Canvas Y                         |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                           Tile Data                           |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Tile Canvas X: 32 bits (_uint32_)

: X coordinate of the upper left corner of the tile.

Tile Canvas Y: 32 bits (_uint32_)

: Y coordinate of the upper left corner of the tile.

Tile Data: _Chunk Size_ - `8` bytes

: VP8 data.

This chunk contains information about a single tile and describes the
bitstream chunk that follows it.

Notes for tiles containing VP8 data:

  * _Tile Canvas X_ and _Tile Canvas Y_ values MUST be
    divisible by `32`.

  * The _Tile Width_ and _Tile Height_ can be extracted from the VP8 data.
    See 'Section 9' in the [VP8 RFC][vp8spec].

  * The width of a tile MUST be divisible by `16` or
    `Tile Canvas X + Tile Width == Canvas Width` MUST be true.

  * The height of a tile MUST be divisible by `16` or
    `Tile Canvas Y + Tile Height == Canvas Height` MUST be true.


#### 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')                      |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |   F   |   C   |    Reserved   |        Alpha Bitstream        |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Filtering method (F): 4 bits

: The filtering method used:

  * `0`: None.
  * `1`: Horizontal filter.
  * `2`: Vertical filter.
  * `3`: Gradient filter.

Compression method (C): 4 bits

: The compression method used:

  * `0`: No compression.
  * `1`: Backward reference counts encoded with arithmetic encoder.

Reserved: 8 bits

: SHOULD be `0`.

Alpha bitstream: _Chunk Size_ - `2` bytes

: Encoded alpha bitstream.

This optional chunk contains encoded alpha data for a single tile.
Either **ALL or NONE** of the tiles must contain this chunk.

The alpha channel can be encoded either losslessly or with lossy
preprocessing (quantization). After the optional preprocessing, the
alpha values are encoded with a lossless compression method like
zlib. Work is in progress to improve the compression gain further by
exploring alternate compression methods and hence, the bitstream for
the Alpha-chunk is still experimental and expected to change.

#### 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')                      |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |  Compression  |                Color Profile                  |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Compression: 8 bits

: Compression method used:

  * `0`: None.
  * `1`: Deflate/inflate.

Color Profile: _Chunk Size_ - `1` bytes

: ICC profile.

There SHOULD be at most one 'ICCP' chunk.
See <http://www.color.org> for specifications.

If this chunk is not present, sRGB SHOULD be assumed.

#### Metadata

     0                   1                   2                   3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                      ChunkHeader('META')                      |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |  Compression  |                 XMP Metadata                  |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Compression: 8 bits

: Compression method used:

  * `0`: None.
  * `1`: Deflate/inflate.

XMP Metadata: _Chunk Size_ - `1` bytes

: XMP metadata.

There SHOULD be at most one such chunk. If there are more such chunks, readers
MAY ignore all except the first one.

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
for WAV and AVI (also RIFF formats), because we have the option of
compression.

Additional guidance about handling metadata can be found in the
Metadata Working Group's [Guidelines for Handling Metadata][metadata].
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, no
reconciliation is necessary.

#### Other Chunks

A file MAY contain other chunks. Readers SHOULD be ignore these chunks. Writers
SHOULD preserve them in their original order.

### Assembling the Canvas from Tiles and Animation

Here we provide an overview of how 'TILE' chunks and 'FRM '/'LOOP' chunks are
used to assemble the canvas. The notation _VP8X.field_ means the field in
the 'VP8X' chunk with the same description.

Decoding a non-animated canvas MUST be equivalent to the following
pseudocode:

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
assert not VP8X.flags.haveAnimation
canvas ← new black image of size
         VP8X.canvasWidth x VP8X.canvasHeight.
tile_params.tileCanvasX = tile_params.tileCanvasY = 0
for chunk in data_for_all_frames:
    if chunk.tag == "TILE":
        assert No other TILE chunk after the last "VP8 " chunk
        assert No ALPH chunk after the last "VP8 " chunk
        tile_params = chunk
    if chunk.tag == "ALPH":
        assert No other ALPH chunk after the last "VP8 " chunk
        tile_params.alpha = alpha_data
    if chunk.tag == "VP8 ":
        render image in chunk in canvas with top-left corner in
            (tile_params.tileCanvasX, tile_params.tileCanvasY).
        tile_params.tileCanvasX = tile_params.tileCanvasY = 0
    Ignore unknown chunks
canvas contains the decoded canvas.
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Decoding an animated canvas MUST be equivalent to the following
pseudocode:

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
assert VP8X.flags.haveAnimation
canvas ← new black image of size
         VP8X.canvasWidth x VP8X.canvasHeight.
if LOOP.loopCount==0:
    LOOP.loopCount=∞
current_FRM ← nil
for LOOP.loop = 0, ..., LOOP.loopCount-1
    assert First chunk in data_for_all_frames is a FRM
    for chunk in data_for_all_frames:
        if chunk.tag == "FRM ":
            assert No other FRM chunk after the last "VP8 " chunk
            assert No ALPH chunk after the last "VP8 " chunk
            if current_FRM != nil:
                Show the contents of canvas for
                    current_FRM.frameDuration * 1ms.
            current_FRM = chunk
        if chunk.tag == "ALPH":
            assert No other ALPH chunk after the last "VP8 " chunk
            tile_params.alpha = alpha_data
        if chunk.tag == "VP8 ":
            assert tile_params.tileCanvasX >= current_FRM.frameX
            assert tile_params.tileCanvasY >= current_FRM.frameY
            assert tile_params.tileCanvasX + chunk.tileWidth >=
                current_FRM.frameX + current_FRM.frameWidth
            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).
            tile_params.tileCanvasX = tile_params.tileCanvasY = 0
        Ignore unknown chunks
canvas contains the decoded canvas.
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

As described earlier, if an assert related to chunk ordering fails, the reader
MAY ignore the badly-ordered chunks instead of failing to decode the file.

Example file layouts
--------------------

A non-animated, tiled image without transparency may look as follows:

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
RIFF/WEBP
+- VP8X (descriptions of features used)
+- ICCP (color profile)
+- TILE (First tile parameters)
+- VP8 (bitstream - first tile)
+- TILE (Second tile parameters)
+- VP8 (bitstream - second tile)
+- TILE (third tile parameters)
+- VP8 (bitstream - third tile)
+- TILE (fourth tile parameters)
+- VP8 (bitstream - fourth tile)
+- META (XMP metadata)
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

An animated image with transparency may look as follows:

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
RIFF/WEBP
+- VP8X (descriptions of features used)
+- LOOP (animation control parameters)
+- FRM (first animation frame parameters)
+- ALPH (alpha bitstream - first image frame)
+- VP8 (bitstream - first image frame)
+- FRM (second animation frame parameters)
+- ALPH (alpha bitstream - second image frame)
+- VP8 (bitstream - second image frame)
+- META (XMP metadata)
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

[vp8spec]:  http://tools.ietf.org/html/rfc6386
[xmpspec]:  http://www.adobe.com/content/dam/Adobe/en/devnet/xmp/pdfs/XMPSpecificationPart1.pdf
[metadata]: http://www.metadataworkinggroup.org/pdf/mwg_guidance.pdf
[rfc 2119]: http://tools.ietf.org/html/rfc2119