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Merge "webp-lossless-bitstream-spec: convert BNF to ABNF" into main

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James Zern 2022-10-15 02:01:48 +00:00 committed by Gerrit Code Review
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doc/webp-lossless-bitstream-spec.txt
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@ -1075,33 +1075,41 @@ The interpretation of S depends on its value:
7 Overall Structure of the Format
---------------------------------
Below is a view into the format in Backus-Naur form. It does not cover
all details. End-of-image (EOI) is only implicitly coded into the number
of pixels (xsize * ysize).
Below is a view into the format in Augmented Backus-Naur Form ([ABNF]). It does
not cover all details. End-of-image (EOI) is only implicitly coded into the
number of pixels (xsize * ysize).
#### 7.1 Basic Structure
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
<format> ::= <RIFF header><image size><image stream>
<image stream> ::= <optional-transform><spatially-coded image>
format = RIFF-header image-size image-stream
RIFF-header = "RIFF" 4OCTET "WEBP" "VP8L" 4OCTET %x2F
image-size = 14BIT 14BIT ; width - 1, height - 1
image-stream = optional-transform spatially-coded-image
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
#### 7.2 Structure of Transforms
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
<optional-transform> ::=
(1-bit value 1; <transform> <optional-transform>) | 1-bit value 0
<transform> ::= <predictor-tx> | <color-tx> | <subtract-green-tx> |
<color-indexing-tx>
<predictor-tx> ::= 2-bit value 0; <predictor image>
<predictor image> ::= 3-bit sub-pixel code ; <entropy-coded image>
<color-tx> ::= 2-bit value 1; <color image>
<color image> ::= 3-bit sub-pixel code ; <entropy-coded image>
<subtract-green-tx> ::= 2-bit value 2
<color-indexing-tx> ::= 2-bit value 3; <color-indexing image>
<color-indexing image> ::= 8-bit color count; <entropy-coded image>
optional-transform = (%b1 transform optional-transform) / %b0
transform = predictor-tx / color-tx / subtract-green-tx
transform =/ color-indexing-tx
predictor-tx = %b00 predictor-image
predictor-image = 3BIT ; sub-pixel code
entropy-coded-image
color-tx = %b01 color-image
color-image = 3BIT ; sub-pixel code
entropy-coded-image
subtract-green-tx = %b10
color-indexing-tx = %b11 color-indexing-image
color-indexing-image = 8BIT ; color count
entropy-coded-image
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
@ -1110,35 +1118,40 @@ of pixels (xsize * ysize).
\[AMENDED2\]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
<spatially-coded image> ::= <color cache info><meta prefix><data>
<entropy-coded image> ::= <color cache info><data>
<color cache info> ::=
1 bit value 0 | (1-bit value 1; 4-bit value for color cache size)
<meta prefix> ::= 1-bit value 0 |
(1-bit value 1; <entropy image>)
<data> ::= <prefix codes><lz77-coded image>
<entropy image> ::= 3-bit subsample value; <entropy-coded image>
<prefix codes> ::= <prefix code group> |
<prefix code group><prefix codes>
<prefix code group> ::= <prefix code><prefix code><prefix code>
<prefix code><prefix code>
See "Interpretation of Meta Prefix Codes" to
understand what each of these five prefix
codes are for.
<prefix code> ::= <simple prefix code> | <normal prefix code>
<simple prefix code> ::= see "Simple code length code" for details
<normal prefix code> ::= <code length code>; encoded code lengths
<code length code> ::= see section "Normal code length code"
<lz77-coded image> ::= ((<argb-pixel> | <lz77-copy> |
<color-cache-code>) <lz77-coded image>) | ""
spatially-coded-image = color-cache-info meta-prefix data
entropy-coded-image = color-cache-info data
color-cache-info = %b0
color-cache-info =/ (%b1 4BIT) ; 1 followed by color cache size
meta-prefix = %b0 / (%b1 entropy-image)
data = prefix-codes lz77-coded-image
entropy-image = 3BIT ; subsample value
entropy-coded-image
prefix-codes = prefix-code-group *prefix-codes
prefix-code-group =
5prefix-code ; See "Interpretation of Meta Prefix Codes" to
; understand what each of these five prefix
; codes are for.
prefix-code = simple-prefix-code / normal-prefix-code
simple-prefix-code = ; see "Simple code length code" for details
normal-prefix-code = code-length-code encoded-code-lengths
code-length-code = ; see section "Normal code length code"
lz77-coded-image =
*((argb-pixel / lz77-copy / color-cache-code) lz77-coded-image)
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
A possible example sequence:
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
<RIFF header><image size>1-bit value 1<subtract-green-tx>
1-bit value 1<predictor-tx>1-bit value 0<color cache info>
1-bit value 0<prefix codes><lz77-coded image>
RIFF-header image-size %b1 subtract-green-tx
%b1 predictor-tx %b0 color-cache-info
%b0 prefix-codes lz77-coded-image
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
[ABNF]: https://www.rfc-editor.org/rfc/rfc5234
[canonical_huff]: https://en.wikipedia.org/wiki/Canonical_Huffman_code