mirror of
https://github.com/webmproject/libwebp.git
synced 2025-07-19 15:29:48 +02:00
Removed CodeRay syntax declarations ...
... as they became unnecessary when upstream (kramdown)
implemented LQ feature request:
17625c8082
Also updated (and simplified) syntax-highlighting instructions.
modified: doc/README
modified: doc/webp-lossless-bitstream-spec.txt
Change-Id: I6f02b0d0a69a4d1d96cb0f771936cbe9e2e6bbec
This commit is contained in:
Lou Quillio
@ -112,7 +112,6 @@ significant bits of the original data. Thus the statement
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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b = ReadBits(2);
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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{:lang='c'}
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is equivalent with the two statements below:
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@ -120,7 +119,6 @@ is equivalent with the two statements below:
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b = ReadBits(1);
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b |= ReadBits(1) << 1;
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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{:lang='c'}
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We assume that each color component (e.g. alpha, red, blue and green) is
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represented using an 8-bit byte. We define the corresponding type as uint8.
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@ -163,7 +161,6 @@ Width and height are decoded as 14-bit integers as follows:
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int image_width = ReadBits(14) + 1;
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int image_height = ReadBits(14) + 1;
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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{:lang='c'}
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The 14-bit dynamics for image size limit the maximum size of a WebP
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lossless image to 16384✕16384 pixels.
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@ -196,7 +193,6 @@ while (ReadBits(1)) { // Transform present.
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// Decode actual image data (section 4).
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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{:lang='c'}
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If a transform is present then the next two bits specify the transform
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type. There are four types of transforms.
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@ -209,7 +205,6 @@ enum TransformType {
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COLOR_INDEXING_TRANSFORM = 3,
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};
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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{:lang='c'}
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The transform type is followed by the transform data. Transform data
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contains the required information to apply the inverse transform and
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@ -238,7 +233,6 @@ int block_height = (1 << size_bits);
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#define DIV_ROUND_UP(num, den) ((num) + (den) - 1) / (den))
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int block_xsize = DIV_ROUND_UP(image_width, 1 << size_bits);
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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{:lang='c'}
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The transform data contains the prediction mode for each block of the
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image. All the block_width * block_height pixels of a block use same
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@ -251,7 +245,6 @@ For a pixel x, y, one can compute the respective filter block address by:
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int block_index = (y >> size_bits) * block_xsize +
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(x >> size_bits);
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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{:lang='c'}
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There are 14 different prediction modes. In each prediction mode, the
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current pixel value is predicted from one or more neighboring pixels whose
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@ -302,7 +295,6 @@ uint8 Average2(uint8 a, uint8 b) {
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return (a + b) / 2;
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}
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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{:lang='c'}
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The Select predictor is defined as follows:
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@ -330,7 +322,6 @@ uint32 Select(uint32 L, uint32 T, uint32 TL) {
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}
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}
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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{:lang='c'}
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The function ClampedAddSubstractFull and ClampedAddSubstractHalf are
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performed for each ARGB component as follows:
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@ -341,21 +332,18 @@ int Clamp(int a) {
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return (a < 0) ? 0 : (a > 255) ? 255 : a;
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}
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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{:lang='c'}
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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int ClampAddSubtractFull(int a, int b, int c) {
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return Clamp(a + b - c);
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}
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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{:lang='c'}
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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int ClampAddSubtractHalf(int a, int b) {
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return Clamp(a + (a - b) / 2);
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}
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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{:lang='c'}
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There are special handling rules for some border pixels. If there is a
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prediction transform, regardless of the mode [0..13] for these pixels, the
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@ -388,7 +376,6 @@ typedef struct {
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uint8 red_to_blue;
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} ColorTransformElement;
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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{:lang='c'}
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The actual color transformation is done by defining a color transform
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delta. The color transform delta depends on the ColorTransformElement which
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@ -425,7 +412,6 @@ void ColorTransform(uint8 red, uint8 blue, uint8 green,
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*new_blue = tmp_blue & 0xff;
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}
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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{:lang='c'}
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ColorTransformDelta is computed using a signed 8-bit integer representing a
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3.5-fixed-point number, and a signed 8-bit RGB color channel (c) [-
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@ -436,7 +422,6 @@ int8 ColorTransformDelta(int8 t, int8 c) {
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return (t * c) >> 5;
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}
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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{:lang='c'}
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The multiplication is to be done using more precision (with at least 16 bit
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dynamics). The sign extension property of the shift operation does not
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@ -455,7 +440,6 @@ int size_bits = ReadStream(4);
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int block_width = 1 << size_bits;
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int block_height = 1 << size_bits;
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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{:lang='c'}
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The remaining part of the color transform data contains
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ColorTransformElement instances corresponding to each block of the image.
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@ -490,7 +474,6 @@ void InverseTransform(uint8 red, uint8 green, uint8 blue,
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*new_blue = blue & 0xff;
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}
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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{:lang='c'}
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### Subtract Green Transform
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@ -506,7 +489,6 @@ void AddGreenToBlueAndRed(uint8 green, uint8 *red, uint8 *blue) {
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*blue = (*blue + green) & 0xff;
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}
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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{:lang='c'}
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This transform is redundant as it can be modeled using the color transform.
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This transform is still often useful, and since it can extend the dynamics
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@ -537,7 +519,6 @@ table. The decoder reads the color indexing transform data as follow:
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// 8 bit value for color table size
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int color_table_size = ReadStream(8) + 1;
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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{:lang='c'}
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The color table is stored using the image storage format itself. The color
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table can be obtained by reading an image, without the RIFF header, image
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@ -558,7 +539,6 @@ The indexing is done based on the green component of the ARGB color.
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// Inverse transform
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argb = color_table[GREEN(argb)];
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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{:lang='c'}
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When the color table is of a small size (equal to or less than 16 colors),
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several pixels are bundled into a single pixel. The pixel bundling packs
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@ -580,7 +560,6 @@ if (color_table_size <= 2) {
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width_bits = 1;
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}
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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{:lang='c'}
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The 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
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@ -689,7 +668,6 @@ uint32 extra_bits = (prefix_code - 2) >> 1;
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uint32 offset = (2 + (prefix_code & 1)) << extra_bits;
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return offset + ReadBits(extra_bits) + 1;
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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{:lang='c'}
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### LZ77 backward reference entropy coding
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@ -750,7 +728,6 @@ is 1, the color cache size is read:
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int color_cache_code_bits = ReadBits(br, 4);
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int color_cache_size = 1 << color_cache_code_bits;
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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{:lang='c'}
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color_cache_code_bits defines the size of the color_cache by (1 <<
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color_cache_code_bits). The range of allowed values for
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@ -833,7 +810,6 @@ The first bit indicates the number of codes:
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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int num_symbols = ReadBits(1) + 1;
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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{:lang='c'}
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The first symbol is stored either using a 1-bit code for values of 0 and 1,
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or using a 8-bit code for values in range [0, 255]. The second symbol, when
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@ -846,7 +822,6 @@ if (num_symbols == 2) {
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symbols[1] = ReadBits(8);
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}
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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{:lang='c'}
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Empty trees can be coded as trees that contain one 0 symbol, and can be
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codified using four bits. For example, a distance tree can be empty if
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@ -871,7 +846,6 @@ for (i = 0; i < num_codes; ++i) {
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code_lengths[kCodeLengthCodeOrder[i]] = ReadBits(3);
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}
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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{:lang='c'}
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* Code length code [0..15] indicate literal code lengths.
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* Value 0 means no symbols have been coded,
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@ -912,7 +886,6 @@ int huffman_bits = ReadBits(4);
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int huffman_xsize = DIV_ROUND_UP(xsize, 1 << huffman_bits);
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int huffman_ysize = DIV_ROUND_UP(ysize, 1 << huffman_bits);
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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{:lang='c'}
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huffman_bits gives the amount of subsampling in the entropy image.
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@ -925,7 +898,6 @@ code, is coded only by the number of codes:
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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int num_meta_codes = max(entropy_image) + 1;
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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{:lang='c'}
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Now, we can obtain the five Huffman codes for green, alpha, red, blue and
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distance for a given (x, y) by the following expression:
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@ -934,7 +906,6 @@ distance for a given (x, y) by the following expression:
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meta_codes[(entropy_image[(y >> huffman_bits) * huffman_xsize +
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(x >> huffman_bits)] >> 8) & 0xffff]
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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{:lang='c'}
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The huffman_code[5 * meta_code + k], codes with k == 0 are for the green &
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length code, k == 4 for the distance code, and the codes at k == 1, 2, and
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