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doc/webp-container-spec: cosmetics
partially normalize indent, vertical whitespace and capitalization with the copy used on developers.google.com/speed/webp Change-Id: I8044418eeb9eaf5bd5c799675c74f6f845d503d6
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James Zern
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@ -67,7 +67,7 @@ Below are additional terms used throughout this document:
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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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writes them is referred to as a _writer_.
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_uint16_
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@ -89,10 +89,12 @@ _FourCC_
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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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would store value _25_ as _24_.
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RIFF file format
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RIFF File Format
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----------------
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The WebP file format is based on the RIFF (resource interchange file format)
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document format.
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@ -132,7 +134,8 @@ _ChunkHeader('ABCD')_
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chunks that apply to any RIFF file format, while FourCCs specific to a file
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format are all lowercase. WebP does not follow this convention.
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WebP file header
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WebP File Header
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----------------
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0 1 2 3
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@ -152,8 +155,8 @@ WebP file header
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File Size: 32 bits (_uint32_)
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: The size of the file in bytes starting at offset 8. The maximum value of
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this field is 2^32 minus 10 bytes and thus the size of the whole file is at
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most 4GiB minus 2 bytes.
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this field is 2^32 minus 10 bytes and thus the size of the whole file is at
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most 4GiB minus 2 bytes.
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'WEBP': 32 bits
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@ -165,7 +168,8 @@ the 'WEBP' FourCC. The file SHOULD NOT contain anything after it. As the size
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of any chunk is even, the size given by the RIFF header is also even. The
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contents of individual chunks will be described in the following sections.
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Simple file format (lossy)
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Simple File Format (Lossy)
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--------------------------
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This layout SHOULD be used if the image requires _lossy_ encoding and does not
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@ -203,7 +207,8 @@ width and height. That is assumed to be the width and height of the canvas.
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The VP8 specification describes how to decode the image into Y'CbCr
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format. To convert to RGB, Rec. 601 SHOULD be used.
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Simple file format (lossless)
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Simple File Format (Lossless)
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-----------------------------
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**Note:** Older readers may not support files using the lossless format.
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@ -241,7 +246,8 @@ The current specification of the VP8L bitstream can be found at
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contains the VP8L image width and height. That is assumed to be the width
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and height of the canvas.
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Extended file format
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Extended File Format
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--------------------
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**Note:** Older readers may not support files using the extended format.
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@ -304,7 +310,7 @@ ICC profile (I): 1 bit
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Alpha (L): 1 bit
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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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("alpha").
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EXIF metadata (E): 1 bit
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@ -317,7 +323,7 @@ XMP metadata (X): 1 bit
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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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used to control the animation.
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Reserved (R): 1 bit
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@ -366,9 +372,9 @@ animation.
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Background Color: 32 bits (_uint32_)
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: The default background color of the canvas in \[Blue, Green, Red, Alpha\]
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byte order. This color MAY be used to fill the unused space on the canvas around
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the frames, as well as the transparent pixels of the first frame. Background
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color is also used when disposal method is `1`.
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byte order. This color MAY be used to fill the unused space on the canvas
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around the frames, as well as the transparent pixels of the first frame.
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Background color is also used when disposal method is `1`.
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**Note**:
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@ -389,7 +395,6 @@ This chunk MUST appear if the _Animation_ flag in the VP8X chunk is set.
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If the _Animation_ flag is not set and this chunk is present, it
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SHOULD be ignored.
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ANMF chunk:
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For animated images, this chunk contains information about a _single_ frame.
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@ -432,8 +437,8 @@ Frame Height Minus One: 24 bits (_uint24_)
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Frame Duration: 24 bits (_uint24_)
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: The time to wait before displaying the next frame, in 1 millisecond units.
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In particular, frame duration of 0 is useful when one wants to update multiple
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areas of the canvas at once during the animation.
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In particular, frame duration of 0 is useful when one wants to update
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multiple areas of the canvas at once during the animation.
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Reserved: 6 bits
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@ -441,28 +446,28 @@ Reserved: 6 bits
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Blending method (B): 1 bit
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: Indicates how transparent pixels of _the current frame_ are to be blended with
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corresponding pixels of the previous canvas:
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: Indicates how transparent pixels of _the current frame_ are to be blended
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with corresponding pixels of the previous canvas:
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* `0`: Use alpha blending. After disposing of the previous frame, render the
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current frame on the canvas using [alpha-blending](#alpha-blending). If the
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current frame does not have an alpha channel, assume alpha value of 255,
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effectively replacing the rectangle.
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* `0`: Use alpha blending. After disposing of the previous frame, render the
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current frame on the canvas using [alpha-blending](#alpha-blending). If
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the current frame does not have an alpha channel, assume alpha value of
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255, effectively replacing the rectangle.
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* `1`: Do not blend. After disposing of the previous frame, render the
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current frame on the canvas by overwriting the rectangle covered by the
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current frame.
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* `1`: Do not blend. After disposing of the previous frame, render the
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current frame on the canvas by overwriting the rectangle covered by the
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current frame.
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Disposal method (D): 1 bit
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: Indicates how _the current frame_ is to be treated after it has been displayed
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(before rendering the next frame) on the canvas:
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: Indicates how _the current frame_ is to be treated after it has been
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displayed (before rendering the next frame) on the canvas:
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* `0`: Do not dispose. Leave the canvas as is.
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* `0`: Do not dispose. Leave the canvas as is.
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* `1`: Dispose to background color. Fill the _rectangle_ on the canvas covered
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by the _current frame_ with background color specified in the
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[ANIM chunk](#anim_chunk).
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* `1`: Dispose to background color. Fill the _rectangle_ on the canvas
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covered by the _current frame_ with background color specified in the
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[ANIM chunk](#anim_chunk).
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**Notes**:
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@ -521,20 +526,20 @@ Reserved (Rsv): 2 bits
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Pre-processing (P): 2 bits
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: These INFORMATIVE bits are used to signal the pre-processing that has
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been performed during compression. The decoder can use this information to
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e.g. dither the values or smooth the gradients prior to display.
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been performed during compression. The decoder can use this information to
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e.g. dither the values or smooth the gradients prior to display.
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* `0`: no pre-processing
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* `1`: level reduction
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* `0`: no pre-processing
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* `1`: level reduction
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Filtering method (F): 2 bits
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: The filtering method used:
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* `0`: None.
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* `1`: Horizontal filter.
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* `2`: Vertical filter.
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* `3`: Gradient filter.
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* `0`: None.
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* `1`: Horizontal filter.
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* `2`: Vertical filter.
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* `3`: Gradient filter.
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For each pixel, filtering is performed using the following calculations.
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Assume the alpha values surrounding the current `X` position are labeled as:
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@ -578,15 +583,15 @@ Compression method (C): 2 bits
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: The compression method used:
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* `0`: No compression.
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* `1`: Compressed using the WebP lossless format.
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* `0`: No compression.
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* `1`: Compressed using the WebP lossless format.
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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. A
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frame containing a 'VP8L' chunk SHOULD NOT contain this chunk.
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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 is already part of the 'VP8L'
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chunk.
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@ -624,8 +629,8 @@ significant fourth-character space) as its tag _or_ (ii) a VP8L chunk, using
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"VP8L" as its tag.
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The formats of VP8 and VP8L chunks are as described in sections
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[Simple file format (lossy)](#simple-file-format-lossy)
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and [Simple file format (lossless)](#simple-file-format-lossless) respectively.
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[Simple File Format (Lossy)](#simple-file-format-lossy)
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and [Simple File Format (Lossless)](#simple-file-format-lossless) respectively.
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#### Color profile
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@ -673,7 +678,6 @@ EXIF Metadata: _Chunk Size_ bytes
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: image metadata in EXIF format.
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XMP chunk:
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0 1 2 3
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@ -755,7 +759,8 @@ for loop = 0..loop_count - 1
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Show the contents of the canvas for frame_params.frameDuration * 1ms.
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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Example file layouts
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Example File Layouts
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--------------------
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A lossy encoded image with alpha may look as follows:
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