From b3ec18c5561cfc703cea7ab31dc4ba115feb3904 Mon Sep 17 00:00:00 2001 From: Lou Quillio Date: Fri, 1 Jun 2012 12:02:41 -0700 Subject: [PATCH] Provide for code-block syntax highlighting. modified: doc/README modified: doc/webp-lossless-bitstream-spec.txt Change-Id: I5cbc9c0a4fbbcc049a4d792e1fac367d28acf4a6 --- doc/README | 24 +++++++++++++++++++-- doc/webp-lossless-bitstream-spec.txt | 31 ++++++++++++++++++++++++++-- 2 files changed, 51 insertions(+), 4 deletions(-) diff --git a/doc/README b/doc/README index e6b14b08..159ed0ec 100644 --- a/doc/README +++ b/doc/README @@ -1,6 +1,6 @@ -Generate HTML Container Spec Doc from Text Source -================================================= +Generate libwebp Container Spec Docs from Text Source +===================================================== HTML generation requires kramdown [1], easily installed as a rubygem [2]. Rubygems installation should satisfy dependencies @@ -13,3 +13,23 @@ HTML generation can then be done from the project root: $ kramdown doc/webp-container-spec.txt --template doc/template.html > \ doc/output/webp-container-spec.html + +kramdown can optionally syntax highlight code blocks, using CodeRay [3], +a dependency of kramdown that rubygems will install automatically. The +following will apply inline CSS styling, and then use perl to strip +line numbers and unwanted leading whitespace. + +$ kramdown doc/webp-lossless-bitstream-spec.txt --template \ + doc/template.html --coderay-css style | \ + perl -p -e 's|.*?\s||gm' > \ + doc/output/webp-lossless-bitstream-spec.html + +Note that code blocks must be immediately followed by a language +identifier for syntax highlighting to succeed. Example: + +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ +int block_index = (y >> size_bits) * block_xsize + + (x >> size_bits); +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ +{:lang='c'} + diff --git a/doc/webp-lossless-bitstream-spec.txt b/doc/webp-lossless-bitstream-spec.txt index 18a33c89..d614f694 100644 --- a/doc/webp-lossless-bitstream-spec.txt +++ b/doc/webp-lossless-bitstream-spec.txt @@ -112,6 +112,7 @@ significant bits of the original data. Thus the statement ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ b = ReadBits(2); ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ +{:lang='c'} is equivalent with the two statements below: @@ -119,6 +120,7 @@ is equivalent with the two statements below: b = ReadBits(1); b |= ReadBits(1) << 1; ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ +{:lang='c'} We assume that each color component (e.g. alpha, red, blue and green) is represented using an 8-bit byte. We define the corresponding type as uint8. @@ -161,6 +163,7 @@ Width and height are decoded as 14-bit integers as follows: int image_width = ReadBits(14) + 1; int image_height = ReadBits(14) + 1; ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ +{:lang='c'} The 14-bit dynamics for image size limit the maximum size of a WebP lossless image to 16384✕16384 pixels. @@ -193,6 +196,7 @@ while (ReadBits(1)) { // Transform present. // Decode actual image data (section 4). ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ +{:lang='c'} If a transform is present then the next two bits specify the transform type. There are four types of transforms. @@ -205,6 +209,7 @@ enum TransformType { COLOR_INDEXING_TRANSFORM = 3, }; ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ +{:lang='c'} The transform type is followed by the transform data. Transform data contains the required information to apply the inverse transform and @@ -233,6 +238,7 @@ int block_height = (1 << size_bits); #define DIV_ROUND_UP(num, den) ((num) + (den) - 1) / (den)) int block_xsize = DIV_ROUND_UP(image_width, 1 << size_bits); ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ +{:lang='c'} The transform data contains the prediction mode for each block of the image. All the block_width * block_height pixels of a block use same @@ -245,6 +251,7 @@ For a pixel x, y, one can compute the respective filter block address by: int block_index = (y >> size_bits) * block_xsize + (x >> size_bits); ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ +{:lang='c'} There are 14 different prediction modes. In each prediction mode, the current pixel value is predicted from one or more neighboring pixels whose @@ -295,6 +302,7 @@ uint8 Average2(uint8 a, uint8 b) { return (a + b) / 2; } ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ +{:lang='c'} The Select predictor is defined as follows: @@ -322,6 +330,7 @@ uint32 Select(uint32 L, uint32 T, uint32 TL) { } } ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ +{:lang='c'} The function ClampedAddSubstractFull and ClampedAddSubstractHalf are performed for each ARGB component as follows: @@ -332,18 +341,21 @@ int Clamp(int a) { return (a < 0) ? 0 : (a > 255) ? 255 : a; } ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ +{:lang='c'} ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ int ClampAddSubtractFull(int a, int b, int c) { return Clamp(a + b - c); } ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ +{:lang='c'} ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ int ClampAddSubtractHalf(int a, int b) { return Clamp(a + (a - b) / 2); } ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ +{:lang='c'} There are special handling rules for some border pixels. If there is a prediction transform, regardless of the mode [0..13] for these pixels, the @@ -376,6 +388,7 @@ typedef struct { uint8 red_to_blue; } ColorTransformElement; ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ +{:lang='c'} The actual color transformation is done by defining a color transform delta. The color transform delta depends on the ColorTransformElement which @@ -412,6 +425,7 @@ void ColorTransform(uint8 red, uint8 blue, uint8 green, *new_blue = tmp_blue & 0xff; } ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ +{:lang='c'} ColorTransformDelta is computed using a signed 8-bit integer representing a 3.5-fixed-point number, and a signed 8-bit RGB color channel (c) [- @@ -422,6 +436,7 @@ int8 ColorTransformDelta(int8 t, int8 c) { return (t * c) >> 5; } ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ +{:lang='c'} The multiplication is to be done using more precision (with at least 16 bit dynamics). The sign extension property of the shift operation does not @@ -440,6 +455,7 @@ int size_bits = ReadStream(4); int block_width = 1 << size_bits; int block_height = 1 << size_bits; ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ +{:lang='c'} The remaining part of the color transform data contains ColorTransformElement instances corresponding to each block of the image. @@ -474,6 +490,7 @@ void InverseTransform(uint8 red, uint8 green, uint8 blue, *new_blue = blue & 0xff; } ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ +{:lang='c'} ### Subtract Green Transform @@ -489,6 +506,7 @@ void AddGreenToBlueAndRed(uint8 green, uint8 *red, uint8 *blue) { *blue = (*blue + green) & 0xff; } ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ +{:lang='c'} This transform is redundant as it can be modeled using the color transform. This transform is still often useful, and since it can extend the dynamics @@ -519,6 +537,7 @@ table. The decoder reads the color indexing transform data as follow: // 8 bit value for color table size int color_table_size = ReadStream(8) + 1; ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ +{:lang='c'} The color table is stored using the image storage format itself. The color table can be obtained by reading an image, without the RIFF header, image @@ -539,6 +558,7 @@ The indexing is done based on the green component of the ARGB color. // Inverse transform argb = color_table[GREEN(argb)]; ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ +{:lang='c'} When the color table is of a small size (equal to or less than 16 colors), several pixels are bundled into a single pixel. The pixel bundling packs @@ -560,6 +580,7 @@ if (color_table_size <= 2) { width_bits = 1; } ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ +{:lang='c'} The width_bits has a value of 0, 1, 2 or 3. A value of 0 indicates no pixel bundling to be done for the image. A value of 1 indicates that two pixels @@ -668,6 +689,7 @@ uint32 extra_bits = (prefix_code - 2) >> 1; uint32 offset = (2 + (prefix_code & 1)) << extra_bits; return offset + ReadBits(extra_bits) + 1; ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ +{:lang='c'} ### LZ77 backward reference entropy coding @@ -728,6 +750,7 @@ is 1, the color cache size is read: int color_cache_code_bits = ReadBits(br, 4); int color_cache_size = 1 << color_cache_code_bits; ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ +{:lang='c'} color_cache_code_bits defines the size of the color_cache by (1 << color_cache_code_bits). The range of allowed values for @@ -810,6 +833,7 @@ The first bit indicates the number of codes: ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ int num_symbols = ReadBits(1) + 1; ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ +{:lang='c'} The first symbol is stored either using a 1-bit code for values of 0 and 1, or using a 8-bit code for values in range [0, 255]. The second symbol, when @@ -822,6 +846,7 @@ if (num_symbols == 2) { symbols[1] = ReadBits(8); } ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ +{:lang='c'} Empty trees can be coded as trees that contain one 0 symbol, and can be codified using four bits. For example, a distance tree can be empty if @@ -846,6 +871,7 @@ for (i = 0; i < num_codes; ++i) { code_lengths[kCodeLengthCodeOrder[i]] = ReadBits(3); } ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ +{:lang='c'} * Code length code [0..15] indicate literal code lengths. * Value 0 means no symbols have been coded, @@ -886,6 +912,7 @@ int huffman_bits = ReadBits(4); int huffman_xsize = DIV_ROUND_UP(xsize, 1 << huffman_bits); int huffman_ysize = DIV_ROUND_UP(ysize, 1 << huffman_bits); ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ +{:lang='c'} huffman_bits gives the amount of subsampling in the entropy image. @@ -898,6 +925,7 @@ code, is coded only by the number of codes: ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ int num_meta_codes = max(entropy_image) + 1; ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ +{:lang='c'} Now, we can obtain the five Huffman codes for green, alpha, red, blue and distance for a given (x, y) by the following expression: @@ -906,6 +934,7 @@ distance for a given (x, y) by the following expression: meta_codes[(entropy_image[(y >> huffman_bits) * huffman_xsize + (x >> huffman_bits)] >> 8) & 0xffff] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ +{:lang='c'} The huffman_code[5 * meta_code + k], codes with k == 0 are for the green & length code, k == 4 for the distance code, and the codes at k == 1, 2, and @@ -979,5 +1008,3 @@ A possible example sequence ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ - -