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webp-container-spec: remove references to fragments
this portion of the format was never finalized
(cherry picked from commit a66e66c79d)
Change-Id: I80aa1b27457a0e52b047c7284df2f58b181ca5d8
This commit is contained in:
James Zern
parent
c5ceea4899
commit
5c1eeda922
@ -46,25 +46,13 @@ for:
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* **Animation.** An image may have multiple frames with pauses between them,
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making it an animation.
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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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**Note:** Out of the features mentioned above, lossy compression, lossless
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compression, transparency, metadata, color profile and animation are finalized
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and are to be considered stable. On the other hand, image fragmentation is
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experimental as of now, and is open to discussion, feedback and comments.
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The same is indicated using annotation "_status: experimental_" in the relevant
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sections of this document.
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and are to be considered stable.
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Terminology & Basics
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------------------------
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@ -278,10 +266,6 @@ For a _still image_, the _image data_ consists of a single frame, whereas for
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an _animated image_, it consists of multiple frames. More details about frames
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can be found in the [Animation](#animation) section.
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Moreover, each frame can be fragmented or non-fragmented, as will be described
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in the [Extended WebP file header](#extended_header) section. More details about
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fragments can be found in the [Fragments](#fragments) section.
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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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@ -302,7 +286,7 @@ 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|A|F| Reserved |
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|Rsv|I|L|E|X|A|R| Reserved |
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+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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| Canvas Width Minus One | ...
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+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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@ -335,9 +319,9 @@ Animation (A): 1 bit
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: Set if this is an animated image. Data in 'ANIM' and 'ANMF' chunks should be
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used to control the animation.
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Image Fragmentation (F): 1 bit _\[status: experimental\]_
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Reserved (R): 1 bit
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: Set if any of the frames in the image are represented by fragments.
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: SHOULD be `0`.
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Reserved: 24 bits
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@ -506,9 +490,7 @@ Disposal method (D): 1 bit
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Frame Data: _Chunk Size_ - `16` bytes
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: For a fragmented frame, it consists of multiple [fragment chunks](#fragments).
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: For a non-fragmented frame, it consists of:
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: Consists of:
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* An optional [alpha subchunk](#alpha) for the frame.
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@ -519,49 +501,6 @@ Frame Data: _Chunk Size_ - `16` bytes
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**Note**: The 'ANMF' payload, _Frame Data_ above, consists of individual
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_padded_ chunks as described by the [RIFF file format](#riff-file-format).
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#### Fragments _\[status: experimental\]_
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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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* An optional list of [unknown chunks](#unknown-chunks).
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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 a frame SHOULD have the following properties:
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* They collectively cover the whole frame.
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* No pair of fragments have any overlapping region on the frame.
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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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@ -643,8 +582,8 @@ 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 this frame/fragment. A
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frame/fragment containing a 'VP8L' chunk SHOULD NOT contain this chunk.
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This optional chunk contains encoded alpha data for this frame. A
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frame containing a 'VP8L' chunk SHOULD NOT contain this chunk.
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**Rationale**: The transparency information is already part of the 'VP8L'
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chunk.
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@ -675,7 +614,7 @@ compression method is '0') or compressed using the lossless format
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#### Bitstream (VP8/VP8L)
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This chunk contains compressed bitstream data for a single frame/fragment.
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This chunk contains compressed bitstream data for a single frame.
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A bitstream chunk may be either (i) a VP8 chunk, using "VP8 " (note the
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significant fourth-character space) as its tag _or_ (ii) a VP8L chunk, using
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@ -762,47 +701,17 @@ A file MAY contain unknown chunks:
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* At the end of the file as described in [Extended WebP file
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header](#extended_header) section.
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* At the end of FRGM and ANMF chunks as described in [Fragments](#fragments)
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and [Animation](#animation) sections.
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* At the end of ANMF chunks as described in the
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[Animation](#animation) section.
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Readers SHOULD ignore these chunks. Writers SHOULD preserve them in their
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original order (unless they specifically intend to modify these chunks).
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### Assembling the Canvas from fragments/frames
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### Assembling the Canvas from frames
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Here we provide an overview of how a reader should assemble a canvas in case
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of a fragmented-image and in case of an animated image. The notation
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_VP8X.field_ means the field 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: _\[status: experimental\]_
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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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Here we provide an overview of how a reader should assemble a canvas in the
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case of an animated image. The notation _VP8X.field_ means the field in the
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'VP8X' chunk with the same description.
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Displaying an _animated image_ canvas MUST be equivalent to the following
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pseudocode:
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@ -825,13 +734,6 @@ for loop = 0, ..., loop_count - 1
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frame_params.frameDuration = Frame Duration
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assert VP8X.canvasWidth >= frame_params.frameX + frame_params.frameWidth
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assert VP8X.canvasHeight >= frame_params.frameY + frame_params.frameHeight
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if VP8X.flags.hasFragments and first subchunk in 'Frame Data' is FRGM
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// Fragmented frame.
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frame_params.{bitstream,alpha} = canvas decoded from subchunks in
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'Frame Data' as per the pseudocode for
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_fragmented image_ above.
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else
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// Non-fragmented frame.
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for subchunk in 'Frame Data':
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if subchunk.tag == "ALPH":
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assert alpha subchunks not found in 'Frame Data' earlier
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@ -878,17 +780,6 @@ 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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An animated image with EXIF metadata may look as follows:
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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