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Image fragment specification in container spec
Change-Id: If8cad296738465df4327dfb6b44efa7e0356070e
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Urvang Joshi
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@ -13,7 +13,7 @@ end of this file.
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WebP Container Specification
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============================
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_Working Draft, v0.7, 20121102_
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_Working Draft, v0.8, 20121102_
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* TOC placeholder
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@ -46,6 +46,15 @@ for:
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* **Color Profile.** An image may have an embedded ICC profile as described
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by the [International Color Consortium][iccspec].
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* **Image Fragmentation.** A single bitstream in WebP has an inherent
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limitation for width or height of 2^14 pixels, and, when using VP8, a 512
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KiB limit on the size of the first compressed partition. To support larger
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images, the format supports images that are composed of multiple fragments,
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each encoded as a separate bitstream. All fragments logically form a single
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image: they have common metadata, color profile, etc. Image fragmentation
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may also improve efficiency for larger images, e.g., grass can be encoded
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differently than sky.
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The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
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"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
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document are to be interpreted as described in [RFC 2119][].
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@ -60,8 +69,10 @@ matrix of pixels, we will call it the _canvas_ of the image.
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Below are additional terms used throughout this document:
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Code that reads WebP files is referred to as a _reader_, while
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code that writes them is referred to as a _writer_.
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_Reader/Writer_
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: Code that reads WebP files is referred to as a _reader_, while code that
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writes them is referred to as a _writer_.
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_uint16_
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@ -76,6 +87,7 @@ _uint32_
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: A 32-bit, little-endian, unsigned integer.
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_1-based_
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: An unsigned integer field storing values offset by `-1`. e.g., Such a field
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would store value _25_ as _24_.
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@ -257,14 +269,30 @@ An extended format file consists of:
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* An optional 'ICCP' chunk with color profile.
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* An optional 'ALPH' chunk with transparency information.
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* The image bitstream contained in either a 'VP8 ' or 'VP8L' chunk.
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* Image data (described below).
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* An optional 'EXIF' chunk with EXIF metadata.
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* An optional 'XMP ' chunk with XMP metadata.
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The image can be fragmented or non-fragmented, as will be described in the
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[Extended WebP file header](#extended_header) section.
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For a _non-fragmented_ image, the _image data_ consists of:
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* An optional 'ALPH' chunk with transparency information.
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* The image bitstream contained in either a 'VP8 ' or 'VP8L' chunk.
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For a _fragmented_ image, the _image data_ consists of multiple fragments,
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where each fragment consists of:
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* A 'FRGM' chunk with the fragment information.
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* An optional 'ALPH' chunk with transparency information.
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* The bitstream for the fragment contained in either a 'VP8 ' or 'VP8L' chunk.
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All chunks SHOULD be placed in the same order as listed above. If a chunk
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appears in the wrong place, the file is invalid, but readers MAY parse the
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file, ignoring the chunks that come too late.
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@ -276,6 +304,7 @@ ignoring late chunks should make programs that need to do a full search
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give the same results as the ones stopping early.
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Extended WebP file header:
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{:#extended_header}
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0 1 2 3
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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
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@ -284,7 +313,7 @@ Extended WebP file header:
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+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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| ChunkHeader('VP8X') |
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+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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|Rsv|I|L|E|X| R | Reserved |
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|Rsv|I|L|E|X|R|F| Reserved |
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+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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| Canvas Width Minus One | ...
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+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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@ -301,7 +330,7 @@ ICC profile (I): 1 bit
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Alpha (L): 1 bit
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: Set if the file contains some (or all) images with transparency information
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: Set if any of the frames of the image contain transparency information
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("alpha").
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EXIF metadata (E): 1 bit
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@ -312,10 +341,14 @@ XMP metadata (X): 1 bit
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: Set if the file contains XMP metadata.
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Reserved (R): 2 bits
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Reserved (R): 1 bit
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: SHOULD be `0`.
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Image Fragmentation (F): 1 bit
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: Set if the image is represented by fragments.
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Reserved: 24 bits
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: SHOULD be `0`.
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@ -336,6 +369,48 @@ Future specifications MAY add more fields.
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### Chunks
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#### Image Fragments
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For images that are represented by fragments, this chunk contains data for
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a single fragment. If the _Image Fragmentation Flag_ is not set, then this chunk
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SHOULD NOT be present.
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0 1 2 3
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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
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+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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| ChunkHeader('FRGM') |
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+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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| Fragment X | ...
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+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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... Fragment Y | Fragment Data |
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+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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Fragment X: 24 bits (_uint24_)
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: The X coordinate of the upper left corner of the fragment is `Fragment X * 2`
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Fragment Y: 24 bits (_uint24_)
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: The Y coordinate of the upper left corner of the fragment is `Fragment Y * 2`
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Fragment Data: _Chunk Size_ - `6` bytes
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: It contains:
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* An optional [alpha subchunk](#alpha) for the fragment.
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* The [bitstream subchunk](#bitstream-vp8vp8l) for the fragment.
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Note: The width and height of the fragment is obtained from the bitstream
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subchunk.
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The fragments of an image SHOULD have the following properties:
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* They collectively cover the whole canvas.
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* No pair of fragments have any overlapping region on the canvas.
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* No portion of any fragment should be located outside of the canvas.
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#### Alpha
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0 1 2 3
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@ -390,8 +465,8 @@ where `clip(v)` is equal to:
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* v otherwise
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The final value is derived by adding the decompressed value `X` to the
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predictor and using modulo-256 arithmetic to wrap the [256-511] range
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into the [0-255] one:
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predictor and using modulo-256 arithmetic to wrap the \[256-511\] range
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into the \[0-255\] one:
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`alpha = (predictor + X) % 256`
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@ -417,10 +492,10 @@ Alpha bitstream: _Chunk Size_ - `1` bytes
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: Encoded alpha bitstream.
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This optional chunk contains encoded alpha data for the image. An image
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This optional chunk contains encoded alpha data for this frame. A frame
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containing a 'VP8L' chunk SHOULD NOT contain this chunk.
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**Rationale**: The transparency information of the image is already part
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**Rationale**: The transparency information of the frame is already part
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of the 'VP8L' chunk.
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The alpha channel data is stored as uncompressed raw data (when
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@ -528,6 +603,42 @@ Metadata Working Group's [Guidelines for Handling Metadata][metadata].
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A file MAY contain other unknown chunks. Readers SHOULD ignore these chunks.
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Writers SHOULD preserve them in their original order.
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### Assembling the Canvas from fragments
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Here we provide an overview of how 'FRGM' chunks are used to assemble the
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canvas in case of a fragmented-image. The notation _VP8X.field_ means the field
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in the 'VP8X' chunk with the same description.
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Displaying a _fragmented image_ canvas MUST be equivalent to the following
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pseudocode:
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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assert VP8X.flags.hasFragments
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canvas ← new black image of size VP8X.canvasWidth x VP8X.canvasHeight.
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frgm_params ← nil
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for chunk in image_data:
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assert chunk.tag is "FRGM"
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frgm_params.fragmentX = Fragment X
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frgm_params.fragmentY = Fragment Y
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for subchunk in 'Fragment Data':
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if subchunk.tag == "ALPH":
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assert alpha subchunks not found in 'Fragment Data' earlier
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frgm_params.alpha = alpha_data
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else if subchunk.tag == "VP8 " OR subchunk.tag == "VP8L":
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assert bitstream subchunks not found in 'Fragment Data' earlier
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frgm_params.bitstream = bitstream_data
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frgm_params.fragmentWidth = Width extracted from bitstream subchunk
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frgm_params.fragmentHeight = Height extracted from bitstream subchunk
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assert VP8X.canvasWidth >=
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frgm_params.fragmentX + frgm_params.fragmentWidth
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assert VP8X.canvasHeight >=
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frgm_params.fragmentY + frgm_params.fragmentHeight
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assert fragment has the properties mentioned in "Image Fragments" section.
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render fragment with frame_params.alpha and frame_params.bitstream on canvas
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with top-left corner in (frgm_params.fragmentX, frgm_params.fragmentY).
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canvas contains the decoded canvas.
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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Example file layouts
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--------------------
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@ -560,6 +671,17 @@ RIFF/WEBP
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+- XMP (metadata)
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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A fragmented image may look as follows:
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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RIFF/WEBP
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+- VP8X (descriptions of features used)
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+- FRGM (fragment1 parameters + data)
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+- FRGM (fragment2 parameters + data)
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+- FRGM (fragment3 parameters + data)
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+- FRGM (fragment4 parameters + data)
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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[vp8spec]: http://tools.ietf.org/html/rfc6386
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[webpllspec]: https://gerrit.chromium.org/gerrit/gitweb?p=webm/libwebp.git;a=blob;f=doc/webp-lossless-bitstream-spec.txt;hb=master
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[iccspec]: http://www.color.org/icc_specs2.xalter
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