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Edit for consistency, usage and grammar.
Substantial edit, though less than 100% thorough. This makes changes that are clearly safe, but avoids others where my domain knowledge is incomplete and accuracy might be compromised. modified: doc/webp-lossless-bitstream-spec.txt Change-Id: I89361a2e1157b8d2e44a8b4f4603f65833f0c1e6
This commit is contained in:
Lou Quillio
@@ -12,7 +12,7 @@ end of this file.
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Specification for WebP Lossless Bitstream
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=========================================
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_2012-06-08_
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_2012-06-19_
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Abstract
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@@ -26,8 +26,8 @@ itself, for storing statistical data about the images, such as the used
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entropy codes, spatial predictors, color space conversion, and color
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table. LZ77, Huffman coding, and a color cache are used for compression
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of the bulk data. Decoding speeds faster than PNG have been
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demonstrated, as well as 25 % denser compression than what can be
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achieved using today's PNG format.
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demonstrated, as well as 25% denser compression than can be achieved
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using today's PNG format.
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* TOC placeholder
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@@ -44,53 +44,52 @@ ARGB image
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: A two-dimensional array containing ARGB pixels.
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color cache
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: A small hash-addressed array to store recently used colors
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and to be able to recall them with shorter codes.
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: A small hash-addressed array to store recently used colors, to be able
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to recall them with shorter codes.
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color indexing image
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: A one-dimensional image of colors that can be
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indexed using a small integer (up to 256 within WebP lossless).
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: A one-dimensional image of colors that can be indexed using a small
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integer (up to 256 within WebP lossless).
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color transform image
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: A two-dimensional subresolution image containing
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data about correlations of color components.
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: A two-dimensional subresolution image containing data about
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correlations of color components.
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distance mapping
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: Changes LZ77 distances to have the smallest values for
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pixels in 2d proximity.
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: Changes LZ77 distances to have the smallest values for pixels in 2D
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proximity.
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entropy image
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: A two-dimensional subresolution image indicating which
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entropy coding should be used in a respective square in the image,
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i.e., each pixel is a meta Huffman code.
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: A two-dimensional subresolution image indicating which entropy coding
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should be used in a respective square in the image, i.e., each pixel
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is a meta Huffman code.
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Huffman code
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: A classic way to do entropy coding where a smaller number of
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bits are used for more frequent codes.
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: A classic way to do entropy coding where a smaller number of bits are
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used for more frequent codes.
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LZ77
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: Dictionary-based sliding window compression algorithm that either
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emits symbols or describes them as sequences of past symbols.
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meta Huffman code
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: A small integer (up to 16 bits) that indexes an element
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in the meta Huffman table.
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: A small integer (up to 16 bits) that indexes an element in the meta
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Huffman table.
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predictor image
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: A two-dimensional subresolution image indicating which
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spatial predictor is used for a particular square in the image.
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: A two-dimensional subresolution image indicating which spatial
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predictor is used for a particular square in the image.
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prefix coding
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: A way to entropy code larger integers that codes a few bits
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of the integer using an entropy code and codifies the remaining bits
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raw. This allows for the descriptions of the entropy codes to remain
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: A way to entropy code larger integers that codes a few bits of the
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integer using an entropy code and codifies the remaining bits raw.
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This allows for the descriptions of the entropy codes to remain
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relatively small even when the range of symbols is large.
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scan-line order
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: A processing order of pixels, left-to-right, top-to-
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bottom, starting from the left-hand-top pixel, proceeding towards
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right. Once a row is completed, continue from the left-hand column of
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the next row.
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: A processing order of pixels, left-to-right, top-to-bottom, starting
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from the left-hand-top pixel, proceeding to the right. Once a row is
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completed, continue from the left-hand column of the next row.
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1 Introduction
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@@ -100,15 +99,14 @@ This document describes the compressed data representation of a WebP
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lossless image. It is intended as a detailed reference for WebP lossless
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encoder and decoder implementation.
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In this document, we use extensively the syntax of the C programming
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language to describe the bitstream, and assume the existence of a
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function for reading bits, `ReadBits(n)`. The bytes are read in the
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natural order of the stream containing them, and bits of each byte are
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read in the least-significant-bit-first order. When multiple bits are
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read at the same time the integer is constructed from the original data
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in the original order, the most significant bits of the returned
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integer are also the most significant bits of the original data. Thus
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the statement
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In this document, we extensively use C programming language syntax to
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describe the bitstream, and assume the existence of a function for
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reading bits, `ReadBits(n)`. The bytes are read in the natural order of
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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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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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b = ReadBits(2);
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@@ -130,35 +128,32 @@ bits 23..16, green in bits 15..8 and blue in bits 7..0, but
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implementations of the format are free to use another representation
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internally.
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Broadly a WebP lossless image contains header data, transform
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Broadly, a WebP lossless image contains header data, transform
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information and actual image data. Headers contain width and height of
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the image. A WebP lossless image can go through five different types of
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transformation before being entropy encoded. The transform information
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in the bitstream contains the required data to apply the respective
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in the bitstream contains the data required to apply the respective
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inverse transforms.
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2 RIFF Header
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-------------
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The beginning of the header has the RIFF container. This consist of the
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The beginning of the header has the RIFF container. This consists of the
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following 21 bytes:
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1. String "RIFF"
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2. A little-endian 32 bit value of the block length, the whole size
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of the block controlled by the RIFF header. Normally this equals
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the payload size (file size subtracted by 8 bytes, i.e., 4 bytes
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for 'RIFF' identifier and 4 bytes for storing this value itself).
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the payload size (file size minus 8 bytes: 4 bytes for the 'RIFF'
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identifier and 4 bytes for storing the value itself).
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3. String "WEBP" (RIFF container name).
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4. String "VP8L" (chunk tag for lossless encoded image data).
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5. A little-endian 32-bit value of the number of bytes in the
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lossless stream.
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6. One byte signature 0x64. Decoders need to accept also 0x65 as a
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valid stream, it has a planned future use. Today, a solid white
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image of the specified size should be shown for images having a
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0x2f signature.
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6. One byte signature 0x2f.
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First 28 bits of the bitstream specify the width and height of the
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The first 28 bits of the bitstream specify the width and height of the
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image. Width and height are decoded as 14-bit integers as follows:
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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@@ -169,6 +164,21 @@ int image_height = ReadBits(14) + 1;
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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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The alpha_is_used bit is a hint only, and should not impact decoding.
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It should be set to 0 when all alpha values are 255 in the picture, and
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1 otherwise.
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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int alpha_is_used = ReadBits(1);
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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The version_number is a 3 bit code that must be discarded by the decoder
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at this time. Complying encoders write a 3-bit value 0.
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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int version_number = ReadBits(3);
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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3 Transformations
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-----------------
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@@ -177,9 +187,9 @@ Transformations are reversible manipulations of the image data that can
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reduce the remaining symbolic entropy by modeling spatial and color
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correlations. Transformations can make the final compression more dense.
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An image can go through four types of transformations. A 1 bit indicates
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the presence of a transform. Every transform is allowed to be used only
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once. The transformations are used only for the main level ARGB image --
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An image can go through four types of transformation. A 1 bit indicates
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the presence of a transform. Each transform is allowed to be used only
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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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@@ -195,7 +205,7 @@ while (ReadBits(1)) { // Transform present.
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...
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}
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// Decode actual image data (section 4).
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// Decode actual image data (Section 4).
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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If a transform is present then the next two bits specify the transform
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@@ -211,12 +221,12 @@ enum TransformType {
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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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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contains the information required to apply the inverse transform and
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depends on the transform type. Next we describe the transform data for
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different types.
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### Predictor transform
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### Predictor Transform
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The predictor transform can be used to reduce entropy by exploiting the
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fact that neighboring pixels are often correlated. In the predictor
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@@ -227,11 +237,11 @@ prediction to use. We divide the image into squares and all the pixels
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in a square use same prediction mode.
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The first 4 bits of prediction data define the block width and height in
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number of bits. The number of block columns, _block_xsize_, is used in
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number of bits. The number of block columns, `block_xsize`, is used in
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indexing two-dimensionally.
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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int size_bits = ReadBits(4);
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int size_bits = ReadBits(3) + 2;
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int block_width = (1 << size_bits);
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int block_height = (1 << size_bits);
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#define DIV_ROUND_UP(num, den) ((num) + (den) - 1) / (den))
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@@ -241,7 +251,8 @@ int block_xsize = DIV_ROUND_UP(image_width, 1 << size_bits);
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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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prediction mode. The prediction modes are treated as pixels of an image
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and encoded using the same techniques described in chapter 4.
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and encoded using the same techniques described in
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[Chapter 4](#image-data).
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For a pixel _x, y_, one can compute the respective filter block address
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by:
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@@ -258,7 +269,6 @@ whose values are already known.
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We choose the neighboring pixels (TL, T, TR, and L) of the current pixel
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(P) as follows:
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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O O O O O O O O O O O
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O O O O O O O O O O O
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@@ -289,8 +299,8 @@ defined as follows.
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| 9 | Average2(T, TR) |
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| 10 | Average2(Average2(L, TL), Average2(T, TR)) |
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| 11 | Select(L, T, TL) |
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| 12 | ClampedAddSubtractFull(L, T, TL) |
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| 13 | ClampedAddSubtractHalf(Average2(L, T), TL) |
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| 12 | ClampAddSubtractFull(L, T, TL) |
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| 13 | ClampAddSubtractHalf(Average2(L, T), TL) |
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`Average2` is defined as follows for each ARGB component:
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@@ -328,7 +338,7 @@ uint32 Select(uint32 L, uint32 T, uint32 TL) {
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}
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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The function `ClampedAddSubstractFull` and `ClampedAddSubstractHalf` are
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The functions `ClampAddSubtractFull` and `ClampAddSubtractHalf` are
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performed for each ARGB component as follows:
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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@@ -383,24 +393,14 @@ typedef struct {
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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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`
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which is same for all the pixels in a particular block. The delta is
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delta. The color transform delta depends on the `ColorTransformElement`,
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which is the same for all the pixels in a particular block. The delta is
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added during color transform. The inverse color transform then is just
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subtracting those deltas.
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|
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The color transform function is defined as follows:
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||||
|
||||
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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/*
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* Input:
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||||
* red, green, blue values of the pixel
|
||||
* trans: Color transform element of the block where the
|
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* pixel belongs to.
|
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*
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* Output:
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* *new_red = transformed value of red
|
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* *new_blue = transformed value of blue
|
||||
*/
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void ColorTransform(uint8 red, uint8 blue, uint8 green,
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ColorTransformElement *trans,
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uint8 *new_red, uint8 *new_blue) {
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@@ -429,7 +429,7 @@ int8 ColorTransformDelta(int8 t, int8 c) {
|
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
|
||||
|
||||
The multiplication is to be done using more precision (with at least
|
||||
16 bit dynamics). The sign extension property of the shift operation
|
||||
16-bit dynamics). The sign extension property of the shift operation
|
||||
does not matter here: only the lowest 8 bits are used from the result,
|
||||
and there the sign extension shifting and unsigned shifting are
|
||||
consistent with each other.
|
||||
@@ -441,33 +441,26 @@ width and height of the image block in number of bits, just like the
|
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predictor transform:
|
||||
|
||||
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
|
||||
int size_bits = ReadStream(4);
|
||||
int size_bits = ReadStream(3) + 2;
|
||||
int block_width = 1 << size_bits;
|
||||
int block_height = 1 << size_bits;
|
||||
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
|
||||
|
||||
The remaining part of the color transform data contains
|
||||
ColorTransformElement instances corresponding to each block of the
|
||||
image. ColorTransformElement instances are treated as pixels of an image
|
||||
and encoded using the methods described in section 4.
|
||||
`ColorTransformElement` instances corresponding to each block of the
|
||||
image. `ColorTransformElement` instances are treated as pixels of an
|
||||
image and encoded using the methods described in
|
||||
[Chapter 4](#image-data).
|
||||
|
||||
During decoding ColorTransformElement instances of the blocks are
|
||||
During decoding, `ColorTransformElement` instances of the blocks are
|
||||
decoded and the inverse color transform is applied on the ARGB values of
|
||||
the pixels. As mentioned earlier that inverse color transform is just
|
||||
subtracting ColorTransformElement values from the red and blue channels.
|
||||
the pixels. As mentioned earlier, that inverse color transform is just
|
||||
subtracting `ColorTransformElement` values from the red and blue
|
||||
channels.
|
||||
|
||||
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
|
||||
/*
|
||||
* Input:
|
||||
* red, blue and green values in the current state.
|
||||
* trans: Color transform element of the corresponding to the
|
||||
* block of the current pixel.
|
||||
*
|
||||
* Output:
|
||||
* new_red, new_blue: red, blue values after inverse transform.
|
||||
*/
|
||||
void InverseTransform(uint8 red, uint8 green, uint8 blue,
|
||||
ColorTransfromElement *p,
|
||||
ColorTransformElement *p,
|
||||
uint8 *new_red, uint8 *new_blue) {
|
||||
// Applying inverse transform is just subtracting the
|
||||
// color transform deltas
|
||||
@@ -497,32 +490,31 @@ void AddGreenToBlueAndRed(uint8 green, uint8 *red, uint8 *blue) {
|
||||
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
|
||||
|
||||
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 of the color transform, and there is no additional data
|
||||
here, this transform can be coded using less bits than a full blown
|
||||
color transform.
|
||||
transform, but it is still often useful. Since it can extend the
|
||||
dynamics of the color transform and there is no additional data here,
|
||||
the subtract green transform can be coded using fewer bits than a
|
||||
full-blown color transform.
|
||||
|
||||
|
||||
### Color Indexing Transform
|
||||
|
||||
If there are not many unique values of the pixels then it may be more
|
||||
efficient to create a color index array and replace the pixel values by
|
||||
the indices to this color index array. Color indexing transform is used
|
||||
to achieve that. In the context of the WebP lossless, we specifically do
|
||||
not call this transform a palette transform, since another slightly
|
||||
similar, but more dynamic concept exists within WebP lossless encoding,
|
||||
called color cache.
|
||||
If there are not many unique pixel values, it may be more efficient to
|
||||
create a color index array and replace the pixel values by the array's
|
||||
indices. The color indexing transform achieves this. (In the context of
|
||||
WebP lossless, we specifically do not call this a palette transform
|
||||
because a similar but more dynamic concept exists in WebP lossless
|
||||
encoding: color cache.)
|
||||
|
||||
The color indexing transform checks for the number of unique ARGB values
|
||||
in the image. If that number is below a threshold (256), it creates an
|
||||
array of those ARGB values is created which replaces the pixel values
|
||||
with the corresponding index. The green channel of the pixels are
|
||||
replaced with the index, all alpha values are set to 255, all red and
|
||||
array of those ARGB values, which is then used to replace the pixel
|
||||
values with the corresponding index: the green channel of the pixels are
|
||||
replaced with the index; all alpha values are set to 255; all red and
|
||||
blue values to 0.
|
||||
|
||||
The transform data contains color table size and the entries in the
|
||||
color table. The decoder reads the color indexing transform data as
|
||||
follow:
|
||||
follows:
|
||||
|
||||
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
|
||||
// 8 bit value for color table size
|
||||
@@ -531,13 +523,13 @@ int color_table_size = ReadStream(8) + 1;
|
||||
|
||||
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 size, and transforms, assuming an height of one pixel, and
|
||||
a width of color_table_size. The color table is always subtraction coded
|
||||
for reducing the entropy of this image. The deltas of palette colors
|
||||
contain typically much less entropy than the colors themselves leading
|
||||
header, image size, and transforms, assuming a height of one pixel and
|
||||
a width of `color_table_size`. The color table is always
|
||||
subtraction-coded to reduce image entropy. The deltas of palette colors
|
||||
contain typically much less entropy than the colors themselves, leading
|
||||
to significant savings for smaller images. In decoding, every final
|
||||
color in the color table can be obtained by adding the previous color
|
||||
component values, by each ARGB-component separately and storing the
|
||||
component values by each ARGB component separately, and storing the
|
||||
least significant 8 bits of the result.
|
||||
|
||||
The inverse transform for the image is simply replacing the pixel values
|
||||
@@ -550,46 +542,48 @@ color.
|
||||
argb = color_table[GREEN(argb)];
|
||||
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
|
||||
|
||||
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 several (2, 4, or 8) pixels into a single pixel reducing
|
||||
the image width respectively. Pixel bundling allows for a more efficient
|
||||
joint distribution entropy coding of neighboring pixels, and gives some
|
||||
arithmetic coding like benefits to the entropy code, but it can only be
|
||||
used when there is a small amount of unique values.
|
||||
When the color table is small (equal to or less than 16 colors), several
|
||||
pixels are bundled into a single pixel. The pixel bundling packs several
|
||||
(2, 4, or 8) pixels into a single pixel, reducing the image width
|
||||
respectively. Pixel bundling allows for a more efficient joint
|
||||
distribution entropy coding of neighboring pixels, and gives some
|
||||
arithmetic coding-like benefits to the entropy code, but it can only be
|
||||
used when there are a small number of unique values.
|
||||
|
||||
color_table_size specifies how many pixels are combined together:
|
||||
`color_table_size` specifies how many pixels are combined together:
|
||||
|
||||
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
|
||||
int width_bits = 0;
|
||||
int width_bits;
|
||||
if (color_table_size <= 2) {
|
||||
width_bits = 3;
|
||||
} else if (color_table_size <= 4) {
|
||||
width_bits = 2;
|
||||
} else if (color_table_size <= 16) {
|
||||
width_bits = 1;
|
||||
} else {
|
||||
width_bits = 0;
|
||||
}
|
||||
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
|
||||
|
||||
The _width_bits_ has a value of 0, 1, 2 or 3. A value of 0 indicates no
|
||||
`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 are combined together, and each pixel has a range of [0..15]. A
|
||||
value of 2 indicates that four pixels are combined together, and each
|
||||
pixel has a range of [0..3]. A value of 3 indicates that eight pixels
|
||||
are combined together and each pixels has a range of [0..1], i.e., a
|
||||
are combined together and each pixel has a range of [0..1], i.e., a
|
||||
binary value.
|
||||
|
||||
The values are packed into the green component as follows:
|
||||
|
||||
* _width_bits_ = 1: for every x value where x ≡ 0 (mod 2), a green
|
||||
* `width_bits` = 1: for every x value where x ≡ 0 (mod 2), a green
|
||||
value at x is positioned into the 4 least-significant bits of the
|
||||
green value at x / 2, a green value at x + 1 is positioned into the
|
||||
4 most-significant bits of the green value at x / 2.
|
||||
* _width_bits_ = 2: for every x value where x ≡ 0 (mod 4), a green
|
||||
* `width_bits` = 2: for every x value where x ≡ 0 (mod 4), a green
|
||||
value at x is positioned into the 2 least-significant bits of the
|
||||
green value at x / 4, green values at x + 1 to x + 3 in order to the
|
||||
more significant bits of the green value at x / 4.
|
||||
* _width_bits_ = 3: for every x value where x ≡ 0 (mod 8), a green
|
||||
* `width_bits` = 3: for every x value where x ≡ 0 (mod 8), a green
|
||||
value at x is positioned into the least-significant bit of the green
|
||||
value at x / 8, green values at x + 1 to x + 7 in order to the more
|
||||
significant bits of the green value at x / 8.
|
||||
@@ -607,12 +601,12 @@ to entropy coding, and three further roles related to transforms.
|
||||
code used in a particular area of the image.
|
||||
3. Predictor image. The green component defines which of the 14 values
|
||||
is used within a particular square of the image.
|
||||
4. Color indexing image. An array of up to 256 ARGB colors are used
|
||||
for transforming a green-only image, using the green value as an
|
||||
index to this one-dimensional array.
|
||||
4. Color indexing image. An array of up to 256 ARGB colors is used for
|
||||
transforming a green-only image, using the green value as an index
|
||||
to this one-dimensional array.
|
||||
5. Color transformation image. Defines signed 3.5 fixed-point
|
||||
multipliers that are used to predict the red, green, blue
|
||||
components to reduce entropy.
|
||||
multipliers that are used to predict the red, green, and blue
|
||||
components, to reduce entropy.
|
||||
|
||||
To divide the image into multiple regions, the image is first divided
|
||||
into a set of fixed-size blocks (typically 16x16 blocks). Each of these
|
||||
@@ -622,28 +616,29 @@ an entropy code, and in order to minimize this cost, statistically
|
||||
similar blocks can share an entropy code. The blocks sharing an entropy
|
||||
code can be found by clustering their statistical properties, or by
|
||||
repeatedly joining two randomly selected clusters when it reduces the
|
||||
overall amount of bits needed to encode the image. [See section
|
||||
_"Decoding of meta Huffman codes"_ in Chapter 5 for an explanation of
|
||||
how this _entropy image_ is stored.]
|
||||
overall amount of bits needed to encode the image. See the section
|
||||
[Decoding of Meta Huffman Codes](#decoding-of-meta-huffman-codes) in
|
||||
[Chapter 5](#entropy-code) for an explanation of how this entropy image
|
||||
is stored.
|
||||
|
||||
Each pixel is encoded using one of three possible methods:
|
||||
|
||||
1. Huffman coded literals, where each channel (green, alpha, red,
|
||||
blue) is entropy-coded independently,
|
||||
blue) is entropy-coded independently;
|
||||
2. LZ77, a sequence of pixels in scan-line order copied from elsewhere
|
||||
in the image, or,
|
||||
in the image; or
|
||||
3. Color cache, using a short multiplicative hash code (color cache
|
||||
index) of a recently seen color.
|
||||
|
||||
In the following sections we introduce the main concepts in LZ77 prefix
|
||||
coding, LZ77 entropy coding, LZ77 distance mapping, and color cache
|
||||
codes. The actual details of the entropy code are described in more
|
||||
detail in chapter 5.
|
||||
detail in [Chapter 5](#entropy-code).
|
||||
|
||||
|
||||
### LZ77 prefix coding
|
||||
### LZ77 Prefix Coding
|
||||
|
||||
Prefix coding divides large integer values into two parts, the prefix
|
||||
Prefix coding divides large integer values into two parts: the prefix
|
||||
code and the extra bits. The benefit of this approach is that entropy
|
||||
coding is later used only for the prefix code, reducing the resources
|
||||
needed by the entropy code. The extra bits are stored as they are,
|
||||
@@ -652,9 +647,9 @@ without an entropy code.
|
||||
This prefix code is used for coding backward reference lengths and
|
||||
distances. The extra bits form an integer that is added to the lower
|
||||
value of the range. Hence the LZ77 lengths and distances are divided
|
||||
into prefix codes and extra bits performing the Huffman coding only on
|
||||
into prefix codes and extra bits. Performing the Huffman coding only on
|
||||
the prefixes reduces the size of the Huffman codes to tens of values
|
||||
instead of otherwise a million (distance) or several thousands (length).
|
||||
instead of a million (distance) or several thousands (length).
|
||||
|
||||
| Prefix code | Value range | Extra bits |
|
||||
| ----------- | --------------- | ---------- |
|
||||
@@ -676,13 +671,13 @@ The code to obtain a value from the prefix code is as follows:
|
||||
if (prefix_code < 4) {
|
||||
return prefix_code;
|
||||
}
|
||||
uint32 extra_bits = (prefix_code - 2) >> 1;
|
||||
uint32 offset = (2 + (prefix_code & 1)) << extra_bits;
|
||||
int extra_bits = (prefix_code - 2) >> 1;
|
||||
int offset = (2 + (prefix_code & 1)) << extra_bits;
|
||||
return offset + ReadBits(extra_bits) + 1;
|
||||
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
|
||||
|
||||
|
||||
### LZ77 backward reference entropy coding
|
||||
### LZ77 Backward Reference Entropy Coding
|
||||
|
||||
Backward references are tuples of length and distance. Length indicates
|
||||
how many pixels in scan-line order are to be copied. The length is
|
||||
@@ -692,13 +687,13 @@ limiting the maximum length to 4096. For distances, all 40 prefix codes
|
||||
are used.
|
||||
|
||||
|
||||
### LZ77 distance mapping
|
||||
### LZ77 Distance Mapping
|
||||
|
||||
120 smallest distance codes [1..120] are reserved for a close
|
||||
neighborhood within the current pixel. The rest are pure distance codes
|
||||
in scan-line order, just offset by 120. The smallest codes are coded
|
||||
into x and y offsets by the following table. Each tuple shows the x and
|
||||
the y coordinates in 2d offsets -- for example the first tuple (0, 1)
|
||||
the y coordinates in 2D offsets -- for example the first tuple (0, 1)
|
||||
means 0 for no difference in x, and 1 pixel difference in y (indicating
|
||||
previous row).
|
||||
|
||||
@@ -723,15 +718,16 @@ previous row).
|
||||
The distances codes that map into these tuples are changes into
|
||||
scan-line order distances using the following formula:
|
||||
_dist = x + y * xsize_, where _xsize_ is the width of the image in
|
||||
pixels.
|
||||
pixels. If a decoder detects a computed _dist_ value smaller than 1,
|
||||
the value of 1 is used instead.
|
||||
|
||||
|
||||
### Color Cache Code
|
||||
|
||||
Color cache stores a set of colors that have been recently used in the
|
||||
image. Using the color cache code, the color cache colors can be
|
||||
referred more efficiently than emitting the respective ARGB values
|
||||
independently or by sending them as backward references with a length of
|
||||
referred to more efficiently than emitting the respective ARGB values
|
||||
independently or sending them as backward references with a length of
|
||||
one pixel.
|
||||
|
||||
Color cache codes are coded as follows. First, there is a bit that
|
||||
@@ -745,15 +741,15 @@ int color_cache_code_bits = ReadBits(br, 4);
|
||||
int color_cache_size = 1 << color_cache_code_bits;
|
||||
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
|
||||
|
||||
_color_cache_code_bits_ defines the size of the color_cache by (1 <<
|
||||
_color_cache_code_bits_). The range of allowed values for
|
||||
_color_cache_code_bits_ is [1..11]. Compliant decoders must indicate a
|
||||
`color_cache_code_bits` defines the size of the color_cache by (1 <<
|
||||
`color_cache_code_bits`). The range of allowed values for
|
||||
`color_cache_code_bits` is [1..11]. Compliant decoders must indicate a
|
||||
corrupted bitstream for other values.
|
||||
|
||||
A color cache is an array of the size _color_cache_size_. Each entry
|
||||
A color cache is an array of the size `color_cache_size`. Each entry
|
||||
stores one ARGB color. Colors are looked up by indexing them by
|
||||
(0x1e35a7bd * _color_) >> (32 - _color_cache_code_bits_). Only one
|
||||
lookup is done in a color cache, there is no conflict resolution.
|
||||
(0x1e35a7bd * `color`) >> (32 - `color_cache_code_bits`). Only one
|
||||
lookup is done in a color cache; there is no conflict resolution.
|
||||
|
||||
In the beginning of decoding or encoding of an image, all entries in all
|
||||
color cache values are set to zero. The color cache code is converted to
|
||||
@@ -765,33 +761,34 @@ literals, into the cache in the order they appear in the stream.
|
||||
5 Entropy Code
|
||||
--------------
|
||||
|
||||
### Huffman coding
|
||||
### Huffman Coding
|
||||
|
||||
Most of the data is coded using a canonical Huffman code. This includes
|
||||
the following:
|
||||
|
||||
* A combined code that defines either the value of the green
|
||||
component, a color cache code, or a prefix of the length codes,
|
||||
* the data for alpha, red and blue components, and
|
||||
* a combined code that defines either the value of the green
|
||||
component, a color cache code, or a prefix of the length codes;
|
||||
* the data for alpha, red and blue components; and
|
||||
* prefixes of the distance codes.
|
||||
|
||||
The Huffman codes are transmitted by sending the code lengths, the
|
||||
The Huffman codes are transmitted by sending the code lengths; the
|
||||
actual symbols are implicit and done in order for each length. The
|
||||
Huffman code lengths are run-length-encoded using three different
|
||||
prefixes, and the result of this coding is further Huffman coded.
|
||||
|
||||
|
||||
### Spatially-variant Huffman coding
|
||||
### Spatially-variant Huffman Coding
|
||||
|
||||
For every pixel (x, y) in the image, there is a definition of which
|
||||
entropy code to use. First, there is an integer called 'meta Huffman
|
||||
code' that can be obtained from a subresolution 2d image. This
|
||||
code' that can be obtained from a subresolution 2D image. This
|
||||
meta Huffman code identifies a set of five Huffman codes, one for green
|
||||
(along with length codes and color cache codes), one for each of red,
|
||||
blue and alpha, and one for distance. The Huffman codes are identified
|
||||
by their position in a table by an integer.
|
||||
|
||||
### Decoding flow of image data
|
||||
|
||||
### Decoding Flow of Image Data
|
||||
|
||||
Read next symbol S
|
||||
|
||||
@@ -809,14 +806,14 @@ Read next symbol S
|
||||
1. Use ARGB color from the color cache, at index S - 256 + 24
|
||||
|
||||
|
||||
### Decoding the code lengths
|
||||
### Decoding the Code Lengths
|
||||
|
||||
There are two different ways to encode the code lengths of a Huffman
|
||||
code, indicated by the first bit of the code: _simple code length code_
|
||||
(1), and _normal code length code_ (0).
|
||||
|
||||
|
||||
#### Simple code length code
|
||||
#### Simple Code Length Code
|
||||
|
||||
This variant can codify 1 or 2 non-zero length codes in the range of [0,
|
||||
255]. All other code lengths are implicitly zeros.
|
||||
@@ -846,11 +843,11 @@ can be empty if all pixels within the same meta Huffman code are
|
||||
produced using the color cache.
|
||||
|
||||
|
||||
#### Normal code length code
|
||||
#### Normal Code Length Code
|
||||
|
||||
The code lengths of a Huffman code are read as follows. _num_codes_
|
||||
specifies the number of code lengths, the rest of the codes lengths
|
||||
(according to the order in _kCodeLengthCodeOrder_) are zeros.
|
||||
The code lengths of a Huffman code are read as follows: `num_codes`
|
||||
specifies the number of code lengths; the rest of the code lengths
|
||||
(according to the order in `kCodeLengthCodeOrder`) are zeros.
|
||||
|
||||
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
|
||||
int kCodeLengthCodes = 19;
|
||||
@@ -863,20 +860,20 @@ for (i = 0; i < num_codes; ++i) {
|
||||
}
|
||||
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
|
||||
|
||||
* Code length code [0..15] indicate literal code lengths.
|
||||
* Value 0 means no symbols have been coded,
|
||||
* Code length code [0..15] indicates literal code lengths.
|
||||
* Value 0 means no symbols have been coded.
|
||||
* Values [1..15] indicate the bit length of the respective code.
|
||||
* Code 16 repeats the previous non-zero value [3..6] times, i.e.,
|
||||
3 + ReadStream(2) times. If code 16 is used before a non-zero value
|
||||
has been emitted, a value of 8 is repeated.
|
||||
* Code 17 emits a streak of zeros [3..10], i.e., 3 + ReadStream(3)
|
||||
3 + `ReadStream(2)` times. If code 16 is used before a non-zero
|
||||
value has been emitted, a value of 8 is repeated.
|
||||
* Code 17 emits a streak of zeros [3..10], i.e., 3 + `ReadStream(3)`
|
||||
times.
|
||||
* Code 18 emits a streak of zeros of length [11..138], i.e.,
|
||||
11 + ReadStream(7) times.
|
||||
11 + `ReadStream(7)` times.
|
||||
|
||||
The entropy codes for alpha, red and blue have a total of 256 symbols.
|
||||
The entropy code for distance prefix codes has 40 symbols. The entropy
|
||||
code for green has 256 + 24 + _color_cache_size_, 256 symbols for
|
||||
code for green has 256 + 24 + `color_cache_size`, 256 symbols for
|
||||
different green symbols, 24 length code prefix symbols, and symbols for
|
||||
the color cache.
|
||||
|
||||
@@ -885,7 +882,7 @@ Huffman codes there are. There are always 5 times the number of Huffman
|
||||
codes to the number of meta Huffman codes.
|
||||
|
||||
|
||||
### Decoding of meta Huffman codes
|
||||
### Decoding of Meta Huffman Codes
|
||||
|
||||
There are two ways to code the meta Huffman codes, indicated by one bit
|
||||
for the ARGB image and is an implicit zero, i.e., not present in the
|
||||
@@ -906,15 +903,15 @@ Huffman code, i.e., the entropy image is of subresolution to the real
|
||||
image.
|
||||
|
||||
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
|
||||
int huffman_bits = ReadBits(4);
|
||||
int huffman_bits = ReadBits(3) + 2;
|
||||
int huffman_xsize = DIV_ROUND_UP(xsize, 1 << huffman_bits);
|
||||
int huffman_ysize = DIV_ROUND_UP(ysize, 1 << huffman_bits);
|
||||
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
|
||||
|
||||
_huffman_bits_ gives the amount of subsampling in the entropy image.
|
||||
`huffman_bits` gives the amount of subsampling in the entropy image.
|
||||
|
||||
After reading the _huffman_bits_, an entropy image stream of size
|
||||
_huffman_xsize_, _huffman_ysize_ is read.
|
||||
After reading the `huffman_bits`, an entropy image stream of size
|
||||
`huffman_xsize`, `huffman_ysize` is read.
|
||||
|
||||
The meta Huffman code, identifying the five Huffman codes per meta
|
||||
Huffman code, is coded only by the number of codes:
|
||||
@@ -931,12 +928,12 @@ meta_codes[(entropy_image[(y >> huffman_bits) * huffman_xsize +
|
||||
(x >> huffman_bits)] >> 8) & 0xffff]
|
||||
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
|
||||
|
||||
The _huffman_code[5 * meta_code + k]_, codes with _k_ == 0 are for the
|
||||
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 3, are for codes of length 256 for red, blue and alpha,
|
||||
respectively.
|
||||
|
||||
The value of k for the reference position in _meta_code_ determines the
|
||||
The value of _k_ for the reference position in `meta_code` determines the
|
||||
length of the Huffman code:
|
||||
|
||||
* k = 0; length = 256 + 24 + cache_size
|
||||
@@ -947,12 +944,12 @@ length of the Huffman code:
|
||||
6 Overall Structure of the Format
|
||||
---------------------------------
|
||||
|
||||
Below there is a eagles-eye-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 Backus-Naur form. It does not cover
|
||||
all details. End-of-image (EOI) is only implicitly coded into the number
|
||||
of pixels (xsize * ysize).
|
||||
|
||||
|
||||
#### Basic structure
|
||||
#### Basic Structure
|
||||
|
||||
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
|
||||
<format> ::= <RIFF header><image size><image stream>
|
||||
@@ -961,7 +958,7 @@ into the number of pixels (xsize * ysize).
|
||||
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
|
||||
|
||||
|
||||
#### Structure of transforms
|
||||
#### Structure of Transforms
|
||||
|
||||
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
|
||||
<optional-transform> ::= 1-bit <transform> <optional-transform> | 0-bit
|
||||
@@ -974,11 +971,11 @@ into the number of pixels (xsize * ysize).
|
||||
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
|
||||
|
||||
|
||||
#### Structure of the image data
|
||||
#### Structure of the Image Data
|
||||
|
||||
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
|
||||
<entropy-coded image> ::= <color cache info><optional meta huffman><huffman codes>
|
||||
<lz77-coded image>
|
||||
<entropy-coded image> ::= <color cache info><optional meta huffman>
|
||||
<huffman codes><lz77-coded image>
|
||||
<optional meta huffman> ::= 1-bit value 0 |
|
||||
(1-bit value 1;
|
||||
<huffman image><meta Huffman size>)
|
||||
@@ -995,7 +992,7 @@ into the number of pixels (xsize * ysize).
|
||||
(<lz77-coded image> | "")
|
||||
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
|
||||
|
||||
A possible example sequence
|
||||
A possible example sequence:
|
||||
|
||||
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
|
||||
<RIFF header><image size>1-bit value 1<subtract-green-tx>
|
||||
|
||||
Reference in New Issue
Block a user