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MUX API Updates
- Add alpha support in mux. - Remove WebPMuxAddNamedData() and WebPMuxGetNamedData() APIs. Add WebPMuxSetImage(), WebPmuxGetImage() and WebPMuxDeleteImage() APIs instead. - Refactor code using WebPImage struct. - Corresponding changes in webpmux binary. - WebPMuxSetImage()/AddFrame()/AddTile() can now work with data which starts from "RIFF...". This simplifies reading a single-image webp file and adding it as an image/frame/tile in mux. Change-Id: I7d98a6407dfe55c84a682ef7e46bc622f5a6f8d9
This commit is contained in:
Urvang Joshi
@ -50,6 +50,9 @@ document describes additional support for:
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* **Animation.** An image may have pauses between frames, making it
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an animation.
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* **Transparency.** An image may have transparency, i.e., an alpha channel for
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each frame/tile.
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Files not using these new features are backward compatible with the
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original format. Use of these features will produce files that are not
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compatible with older programs.
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@ -215,6 +218,10 @@ multiple frames. Data for each frame consists of:
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present at the beginning of data for an image that's represented as
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multiple tile images.
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* An optional "ALPH" chunk with alpha bitstream of the tile. It **MUST** be
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present for an image containing transparency. It **MUST NOT** be present
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in non-transparent images.
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* A "VP8 " chunk with the bitstream of the tile.
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All chunks **MUST** be placed in the same order as listed above (except
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@ -228,8 +235,8 @@ position, a decoder can choose to stop searching for it. The rule of
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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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**Example:** An example layout of a non-animated, tiled image may look
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as follows:
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**Example:** An example layout of a non-animated, tiled image without
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transparency may look as follows:
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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RIFF/WEBP
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@ -246,17 +253,20 @@ RIFF/WEBP
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+- META (XMP metadata)
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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**Example:** An example layout of an animated image may look as follows:
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**Example:** An example layout of an animated image with transparency may look
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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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+- LOOP (animation control parameters)
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+- FRM (first animation frame parameters)
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+- ALPH (alpha bitstream - first image frame)
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+- VP8 (bitstream - first image frame)
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+- FRM (second animation frame parameters)
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+- ALPH (alpha bitstream - second image frame)
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+- VP8 (bitstream - second image frame)
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+- META(XMP metadata)
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+- META (XMP metadata)
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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@ -272,14 +282,20 @@ pseudocode:
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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assert not VP8X.flags.haveAnimation
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canvas ← new black image of size VP8X.canvasWidth x VP8X.canvasHeight.
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canvas ← new black image of size
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VP8X.canvasWidth x VP8X.canvasHeight.
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tile_params.tileCanvasX = tile_params.tileCanvasY = 0
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for chunk in data_for_all_frames:
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if chunk.tag == "TILE":
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assert No other TILE chunk after the last "VP8 " chunk
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assert No ALPH chunk after the last "VP8 " chunk
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tile_params = chunk
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if chunk.tag == "ALPH":
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assert No other ALPH chunk after the last "VP8 " chunk
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tile_params.alpha = alpha_data
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if chunk.tag == "VP8 ":
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render image in chunk in canvas with top-left corner in (tile_params.tileCanvasX, tile_params.tileCanvasY) using the isometry in VP8X.flags.rotationAndSymmetry.
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render image in chunk in canvas with top-left corner in
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(tile_params.tileCanvasX, tile_params.tileCanvasY).
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tile_params.tileCanvasX = tile_params.tileCanvasY = 0
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Ignore unknown chunks
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canvas contains the decoded canvas.
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@ -290,7 +306,8 @@ pseudocode:
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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assert VP8X.flags.haveAnimation
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canvas ← new black image of size VP8X.canvasWidth x VP8X.canvasHeight.
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canvas ← new black image of size
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VP8X.canvasWidth x VP8X.canvasHeight.
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if LOOP.loopCount==0:
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LOOP.loopCount=∞
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current_FRM ← nil
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@ -298,15 +315,24 @@ for LOOP.loop = 0, ..., LOOP.loopCount-1
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assert First chunk in data_for_all_frames is a FRM
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for chunk in data_for_all_frames:
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if chunk.tag == "FRM ":
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assert No other FRM chunk after the last "VP8 " chunk
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assert No ALPH chunk after the last "VP8 " chunk
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if current_FRM != nil:
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Show the contents of canvas for current_FRM.frameDuration*10ms.
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Show the contents of canvas for
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current_FRM.frameDuration * 1ms.
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current_FRM = chunk
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if chunk.tag == "ALPH":
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assert No other ALPH chunk after the last "VP8 " chunk
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tile_params.alpha = alpha_data
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if chunk.tag == "VP8 ":
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assert tile_params.tileCanvasX >= current_FRM.frameX
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assert tile_params.tileCanvasY >= current_FRM.frameY
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assert tile_params.tileCanvasX + chunk.tileWidth >= current_FRM.frameX + current_FRM.frameWidth
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assert tile_params.tileCanvasY + chunk.tileHeight >= current_FRM.frameX + current_FRM.frameHeight
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render image in chunk in canvas with top-left corner in (tile_params.tileCanvasX, tile_params.tileCanvasY) using the isometry in VP8X.flags.rotationAndSymmetry.
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assert tile_params.tileCanvasX + chunk.tileWidth >=
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current_FRM.frameX + current_FRM.frameWidth
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assert tile_params.tileCanvasY + chunk.tileHeight >=
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current_FRM.frameX + current_FRM.frameHeight
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render image in chunk in canvas with top-left corner in
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(tile_params.tileCanvasX, tile_params.tileCanvasY).
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tile_params.tileCanvasX = tile_params.tileCanvasY = 0
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Ignore unknown chunks
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canvas contains the decoded canvas.
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@ -482,6 +508,31 @@ constraints:
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_tileCanvasY + tileHeight == canvasHeight_ **MUST** be true.
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### ALPH Chunks (Alpha Bitstreams)
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This optional chunk contains encoded alpha data for a single tile. Either
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**ALL or NONE** of the tiles must contain this chunk.
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The alpha channel can be encoded either losslessly or with lossy preprocessing
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(quantization). After the optional preprocessing, the alpha values are encoded
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with a lossless compression method like zlib. Work is in progress to improve the
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compression gain further by exploring alternate compression methods and hence,
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the bit-stream for the Alpha-chunk is still experimental and expected to change.
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The contents of such a chunk are as follows:
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* Byte 0: The _compression method_ used. Currently two methods are
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supported:
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* 0 --> No compression
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* 1 --> Zlib compression.
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* Byte 1: _Reserved_. **Should** be 0.
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* Byte 2 onwards: _Encoded alpha bitstream_.
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### ICCP Chunk (Color Profile)
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An optional "ICCP" chunk contains an ICC profile. There **SHOULD** be
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@ -525,4 +576,4 @@ specification. Such chunks **MUST** be ignored, but preserved. Writers
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[vp8spec]: http://tools.ietf.org/html/draft-bankoski-vp8-bitstream
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[xmpspec]: http://www.adobe.com/content/dam/Adobe/en/devnet/xmp/pdfs/XMPSpecificationPart1.pdf
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[metadata]: http://www.metadataworkinggroup.org/pdf/mwg_guidance.pdf
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[metadata]: http://www.metadataworkinggroup.org/pdf/mwg_guidance.pdf
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