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Merge changes I96bc063c,I45880467,If9e18e5a,I6ee938e4,I0a410b28, ... into main
* changes: webp-container-spec: add prose for rendering process webp-container-spec: note reserved fields MUST be ignored webp-lossless-bitstream-spec: improve 'small' color table stmt webp-container-spec: remove redundant sentence doc/webp-*: fix some punctuation, grammar webp-container-spec: clarify background color note webp-container-spec: come too late -> out of order webp-container-spec: prefer hex literals webp-container-spec: change SHOULD to MUST w/ANIM chunk webp-container-spec: add unknown fields MUST be ignored webp-container-spec: make padding byte=0 a MUST webp-container-spec: update note on trailing data webp-container-spec: clarify Chunk Size is in bytes
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James Zern
Gerrit Code Review
commit
dc05b4db2a
@ -25,7 +25,7 @@ to compress image data in a lossy way, or (ii) the WebP lossless encoding
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(and possibly other encodings in the future). These encoding schemes should
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make it more efficient than currently used formats. It is optimized for fast
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image transfer over the network (e.g., for websites). The WebP format has
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feature parity (color profile, metadata, animation etc) with other formats as
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feature parity (color profile, metadata, animation, etc.) with other formats as
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well. This document describes the structure of a WebP file.
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The WebP container (i.e., RIFF container for WebP) allows feature support over
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@ -84,8 +84,8 @@ _uint32_
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_FourCC_
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: A _FourCC_ (four-character code) is a _uint32_ created by concatenating four
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ASCII characters in little-endian order. This means 'aaaa' (%x61.61.61.61) and
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'AAAA' (%x41.41.41.41) are treated as different _FourCCs_.
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ASCII characters in little-endian order. This means 'aaaa' (0x61616161) and
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'AAAA' (0x41414141) are treated as different _FourCCs_.
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_1-based_
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@ -123,14 +123,13 @@ Chunk FourCC: 32 bits
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Chunk Size: 32 bits (_uint32_)
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: The size of the chunk not including this field, the chunk identifier or
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padding.
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: The size of the chunk in bytes, not including this field, the chunk
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identifier or padding.
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Chunk Payload: _Chunk Size_ bytes
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: The data payload. If _Chunk Size_ is odd, a single padding byte -- that
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SHOULD be `0` to conform with RIFF -- is added. Applications MAY use
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another value, but readers may fail to parse the file.
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: The data payload. If _Chunk Size_ is odd, a single padding byte -- that MUST
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be `0` to conform with RIFF -- is added.
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**Note:** RIFF has a convention that all-uppercase chunk FourCCs are standard
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chunks that apply to any RIFF file format, while FourCCs specific to a file
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@ -166,10 +165,11 @@ File Size: 32 bits (_uint32_)
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A WebP file MUST begin with a RIFF header with the FourCC 'WEBP'. The file size
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in the header is the total size of the chunks that follow plus `4` bytes for
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the 'WEBP' FourCC. The file SHOULD NOT contain anything after it. Readers MAY
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parse such files, ignoring the trailing data. As the size of any chunk is even,
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the size given by the RIFF header is also even. The contents of individual
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chunks will be described in the following sections.
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the 'WEBP' FourCC. The file SHOULD NOT contain any data after the data
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specified by _File Size_. Readers MAY parse such files, ignoring the trailing
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data. As the size of any chunk is even, the size given by the RIFF header is
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also even. The contents of individual chunks will be described in the following
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sections.
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Simple File Format (Lossy)
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@ -206,7 +206,7 @@ VP8 data: _Chunk Size_ bytes
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: VP8 bitstream data.
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Note the fourth character in the 'VP8 ' FourCC is an ASCII space (%x20).
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Note the fourth character in the 'VP8 ' FourCC is an ASCII space (0x20).
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The VP8 bitstream format specification can be found at [VP8 Data Format and
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Decoding Guide][vp8spec]. Note that the VP8 frame header contains the VP8 frame
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@ -292,7 +292,7 @@ details about frames can be found in the [Animation](#animation) 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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file, ignoring the chunks that are out of order.
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**Rationale:** Setting the order of chunks should allow quicker file
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parsing. For example, if an 'ALPH' chunk does not appear in its required
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@ -322,7 +322,7 @@ Extended WebP file header:
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Reserved (Rsv): 2 bits
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: MUST be `0`.
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: MUST be `0`. Readers MUST ignore this field.
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ICC profile (I): 1 bit
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@ -348,11 +348,11 @@ Animation (A): 1 bit
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Reserved (R): 1 bit
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: MUST be `0`.
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: MUST be `0`. Readers MUST ignore this field.
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Reserved: 24 bits
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: MUST be `0`.
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: MUST be `0`. Readers MUST ignore this field.
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Canvas Width Minus One: 24 bits
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@ -366,7 +366,7 @@ Canvas Height Minus One: 24 bits
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The product of _Canvas Width_ and _Canvas Height_ MUST be at most `2^32 - 1`.
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Future specifications MAY add more fields.
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Future specifications may add more fields. Unknown fields MUST be ignored.
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### Chunks
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@ -400,8 +400,8 @@ Background Color: 32 bits (_uint32_)
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**Note**:
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* Background color MAY contain a transparency value (alpha), even if the
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_Alpha_ flag in [VP8X chunk](#extended_header) is unset.
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* Background color MAY contain a non-opaque alpha value, even if the _Alpha_
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flag in [VP8X chunk](#extended_header) is unset.
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* Viewer applications SHOULD treat the background color value as a hint, and
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are not required to use it.
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@ -414,8 +414,8 @@ Loop Count: 16 bits (_uint16_)
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: The number of times to loop the animation. `0` means infinitely.
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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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If the _Animation_ flag is not set and this chunk is present, it MUST be
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ignored.
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ANMF chunk:
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@ -466,7 +466,7 @@ Frame Duration: 24 bits (_uint24_)
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Reserved: 6 bits
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: MUST be 0.
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: MUST be `0`. Readers MUST ignore this field.
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Blending method (B): 1 bit
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@ -547,7 +547,7 @@ _padded_ chunks as described by the [RIFF file format](#riff-file-format).
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Reserved (Rsv): 2 bits
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: MUST be `0`.
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: MUST be `0`. Readers MUST ignore this field.
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Pre-processing (P): 2 bits
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@ -686,8 +686,7 @@ If this chunk is not present, sRGB SHOULD be assumed.
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Metadata can be stored in 'EXIF' or 'XMP ' chunks.
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There SHOULD be at most one chunk of each type ('EXIF' and 'XMP '). If there
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are more such chunks, readers MAY ignore all except the first one. Also, a file
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may possibly contain both 'EXIF' and 'XMP ' chunks.
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are more such chunks, readers MAY ignore all except the first one.
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The chunks are defined as follows:
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@ -721,7 +720,7 @@ XMP Metadata: _Chunk Size_ bytes
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: image metadata in XMP format.
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Note the fourth character in the 'XMP ' FourCC is an ASCII space (%x20).
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Note the fourth character in the 'XMP ' FourCC is an ASCII space (0x20).
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Additional guidance about handling metadata can be found in the
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Metadata Working Group's [Guidelines for Handling Metadata][metadata].
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@ -747,19 +746,40 @@ original order (unless they specifically intend to modify these chunks).
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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 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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Here we provide an overview of how a reader MUST assemble a canvas in the case
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of an animated image.
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Displaying an _animated image_ canvas MUST be equivalent to the following
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pseudocode:
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The process begins with creating a canvas using the dimensions given in the
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'VP8X' chunk, `Canvas Width Minus One + 1` pixels wide by `Canvas Height Minus
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One + 1` pixels high. The `Loop Count` field from the 'ANIM' chunk controls how
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many times the animation process is repeated. This is `Loop Count - 1` for
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non-zero `Loop Count` values or infinitely if `Loop Count` is zero.
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At the beginning of each loop iteration the canvas is filled using the
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background color from the 'ANIM' chunk or an application defined color.
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'ANMF' chunks contain individual frames given in display order. Before rendering
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each frame, the previous frame's `Disposal method` is applied.
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The rendering of the decoded frame begins at the Cartesian coordinates (`2 *
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Frame X`, `2 * Frame Y`) using the top-left corner of the canvas as the origin.
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`Frame Width Minus One + 1` pixels wide by `Frame Height Minus One + 1` pixels
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high are rendered onto the canvas using the `Blending method`.
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The canvas is displayed for `Frame Duration` milliseconds. This continues until
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all frames given by 'ANMF' chunks have been displayed. A new loop iteration is
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then begun or the canvas is left in its final state if all iterations have been
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completed.
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The following pseudocode illustrates the rendering process. The notation
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_VP8X.field_ means the field in the 'VP8X' chunk with the same description.
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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assert VP8X.flags.hasAnimation
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canvas ← new image of size VP8X.canvasWidth x VP8X.canvasHeight with
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background color ANIM.background_color.
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loop_count ← ANIM.loopCount
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dispose_method ← ANIM.disposeMethod
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dispose_method ← Dispose to background color
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if loop_count == 0:
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loop_count = ∞
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frame_params ← nil
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@ -785,10 +805,12 @@ for loop = 0..loop_count - 1
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frame_params.bitstream = bitstream_data
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render frame with frame_params.alpha and frame_params.bitstream
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on canvas with top-left corner at (frame_params.frameX,
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frame_params.frameY), using dispose method dispose_method.
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frame_params.frameY), using blending method
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frame_params.blendingMethod.
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canvas contains the decoded image.
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Show the contents of the canvas for
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frame_params.frameDuration * 1ms.
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dispose_method = frame_params.disposeMethod
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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@ -51,7 +51,7 @@ the stream containing them, and bits of each byte are read in
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least-significant-bit-first order. When multiple bits are read at the
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same time, the integer is constructed from the original data in the
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original order. The most significant bits of the returned integer are
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also the most significant bits of the original data. Thus the statement
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also the most significant bits of the original data. Thus, the statement
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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b = ReadBits(2);
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@ -196,7 +196,7 @@ once. The transformations are used only for the main level ARGB image:
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the subresolution images have no transforms, not even the 0 bit
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indicating the end-of-transforms.
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Typically an encoder would use these transforms to reduce the Shannon
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Typically, an encoder would use these transforms to reduce the Shannon
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entropy in the residual image. Also, the transform data can be decided
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based on entropy minimization.
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@ -566,9 +566,9 @@ pixels are bundled into a single pixel. The pixel bundling packs several
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respectively. Pixel bundling allows for a more efficient joint
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distribution entropy coding of neighboring pixels, and gives some
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arithmetic coding-like benefits to the entropy code, but it can only be
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used when there are a small number of unique values.
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used when there are 16 or fewer unique values.
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`color_table_size` specifies how many pixels are combined together:
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`color_table_size` specifies how many pixels are combined:
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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int width_bits;
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@ -583,13 +583,12 @@ if (color_table_size <= 2) {
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}
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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`width_bits` has a value of 0, 1, 2 or 3. A value of 0 indicates no
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pixel bundling to be done for the image. A value of 1 indicates that two
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pixels are combined together, and each pixel has a range of \[0..15\]. A
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value of 2 indicates that four pixels are combined together, and each
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pixel has a range of \[0..3\]. A value of 3 indicates that eight pixels
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are combined together and each pixel has a range of \[0..1\], i.e., a
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binary value.
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`width_bits` has a value of 0, 1, 2 or 3. A value of 0 indicates no pixel
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bundling to be done for the image. A value of 1 indicates that two pixels are
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combined, and each pixel has a range of \[0..15\]. A value of 2 indicates that
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four pixels are combined, and each pixel has a range of \[0..3\]. A value of 3
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indicates that eight pixels are combined and each pixel has a range of \[0..1\],
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i.e., a binary value.
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The values are packed into the green component as follows:
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@ -659,7 +658,7 @@ Each pixel is encoded using one of the three possible methods:
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3. Color cache code: using a short multiplicative hash code (color cache
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index) of a recently seen color.
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The following sub-sections describe each of these in detail.
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The following subsections describe each of these in detail.
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#### 5.2.1 Prefix Coded Literals
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@ -725,7 +724,7 @@ return offset + ReadBits(extra_bits) + 1;
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{:#distance-mapping}
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As noted previously, distance code is a number indicating the position of a
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previously seen pixel, from which the pixels are to be copied. This sub-section
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previously seen pixel, from which the pixels are to be copied. This subsection
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defines the mapping between a distance code and the position of a previous
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pixel.
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