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webp-lossless-bitstream-spec,cosmetics: reflow paragraphs
Change-Id: Ifd7472fe0678b45efc62faa66de8e8dc2e9931e3
This commit is contained in:
James Zern
parent
0ceeeab987
commit
e5fe2cfc1b
@ -1,8 +1,8 @@
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<!--
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Although you may be viewing an alternate representation, this document
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is sourced in Markdown, a light-duty markup scheme, and is optimized for
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the [kramdown](https://kramdown.gettalong.org/) transformer.
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Although you may be viewing an alternate representation, this document is
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sourced in Markdown, a light-duty markup scheme, and is optimized for the
|
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[kramdown](https://kramdown.gettalong.org/) transformer.
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See the accompanying specs_generation.md. External link targets are referenced
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at the end of this file.
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@ -27,10 +27,10 @@ WebP lossless is an image format for lossless compression of ARGB images. The
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lossless format stores and restores the pixel values exactly, including the
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color values for pixels whose alpha value is 0. The format uses subresolution
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images, recursively embedded into the format itself, for storing statistical
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data about the images, such as the used entropy codes, spatial predictors,
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color space conversion, and color table. LZ77, prefix coding, and a color cache
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are used for compression of the bulk data. Decoding speeds faster than PNG have
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been demonstrated, as well as 25% denser compression than can be achieved using
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data about the images, such as the used entropy codes, spatial predictors, color
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space conversion, and color table. LZ77, prefix coding, and a color cache are
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used for compression of the bulk data. Decoding speeds faster than PNG have been
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demonstrated, as well as 25% denser compression than can be achieved using
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today's PNG format.
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@ -41,18 +41,18 @@ today's PNG format.
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1 Introduction
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--------------
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This document describes the compressed data representation of a WebP
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lossless image. It is intended as a detailed reference for the WebP lossless
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encoder and decoder implementation.
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This document describes the compressed data representation of a WebP lossless
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image. It is intended as a detailed reference for the WebP lossless encoder and
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decoder implementation.
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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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In this document, we extensively use C programming language syntax to describe
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the bitstream, and assume the existence of a function for reading bits,
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`ReadBits(n)`. The bytes are read in the natural order of the stream containing
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them, and bits of each byte are read in least-significant-bit-first order. When
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multiple bits are read at the same time, the integer is constructed from the
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original data 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, the
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statement
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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b = ReadBits(2);
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@ -66,79 +66,76 @@ b |= ReadBits(1) << 1;
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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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
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uint8. A whole ARGB pixel is represented by a type called uint32, an
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unsigned integer consisting of 32 bits. In the code showing the behavior
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of the transformations, alpha value is codified in bits 31..24, red in
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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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represented using an 8-bit byte. We define the corresponding type as uint8. A
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whole ARGB pixel is represented by a type called uint32, an unsigned integer
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consisting of 32 bits. In the code showing the behavior of the transformations,
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alpha value is codified in bits 31..24, red in bits 23..16, green in bits 15..8
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and blue in bits 7..0, but implementations of the format are free to use another
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representation internally.
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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 four different types of
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transformation before being entropy encoded. The transform information
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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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Broadly, a WebP lossless image contains header data, transform information and
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actual image data. Headers contain width and height of the image. A WebP
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lossless image can go through four different types of transformation before
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being entropy encoded. The transform information in the bitstream contains the
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data required to apply the respective inverse transforms.
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2 Nomenclature
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--------------
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ARGB
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: A pixel value consisting of alpha, red, green, and blue values.
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: A pixel value consisting of alpha, red, green, and blue values.
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ARGB image
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: A two-dimensional array containing ARGB pixels.
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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, to be able
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to recall them with shorter codes.
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: A small hash-addressed array to store recently used colors, to be able to
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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 indexed using a small
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integer (up to 256 within WebP lossless).
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: A one-dimensional image of colors that can be indexed using a small integer
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(up to 256 within WebP lossless).
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color transform image
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: A two-dimensional subresolution image containing data about
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correlations of color components.
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: A two-dimensional subresolution image containing data about correlations of
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color components.
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distance mapping
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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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: 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 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 prefix code.
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: A two-dimensional subresolution image indicating which entropy coding should
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be used in a respective square in the image, i.e., each pixel is a meta
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prefix code.
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prefix code
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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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: A classic way to do entropy coding where a smaller number of bits are used
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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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: Dictionary-based sliding window compression algorithm that either emits
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symbols or describes them as sequences of past symbols.
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meta prefix code
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: A small integer (up to 16 bits) that indexes an element in the meta
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prefix table.
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: A small integer (up to 16 bits) that indexes an element in the meta prefix
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table.
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predictor 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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: A two-dimensional subresolution image indicating which spatial predictor is
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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 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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: A way to entropy code larger integers that codes a few bits of the integer
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using an entropy code and codifies the remaining bits raw. This allows for
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the descriptions of the entropy codes to remain relatively small even when
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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-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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: A processing order of pixels, left-to-right, top-to-bottom, starting from
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the left-hand-top pixel, proceeding to the right. Once a row is completed,
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continue from the left-hand column of the next row.
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3 RIFF Header
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-------------
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@ -157,27 +154,26 @@ following 21 bytes:
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lossless stream.
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6. One byte signature 0x2f.
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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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The first 28 bits of the bitstream specify the width and height of the image.
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Width and height are decoded as 14-bit integers as follows:
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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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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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 14-bit dynamics for image size limit the maximum size of a WebP lossless
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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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The alpha_is_used bit is a hint only, and should not impact decoding. It should
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be set to 0 when all alpha values are 255 in the picture, and 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 set to 0. Any other value
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should be treated as an error. \[AMENDED\]
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The version_number is a 3 bit code that must be set to 0. Any other value should
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be treated as an error. \[AMENDED\]
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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int version_number = ReadBits(3);
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@ -187,19 +183,18 @@ int version_number = ReadBits(3);
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4 Transformations
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-----------------
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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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Transformations are reversible manipulations of the image data that can reduce
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the remaining symbolic entropy by modeling spatial and color correlations.
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Transformations can make the final compression more dense.
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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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An image can go through four types of transformation. A 1 bit indicates the
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presence of a transform. Each transform is allowed to be used only once. The
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transformations are used only for the main level ARGB image: the subresolution
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images have no transforms, not even the 0 bit indicating the end-of-transforms.
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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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Typically, an encoder would use these transforms to reduce the Shannon entropy
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in the residual image. Also, the transform data can be decided based on entropy
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minimization.
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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while (ReadBits(1)) { // Transform present.
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@ -212,8 +207,8 @@ while (ReadBits(1)) { // Transform present.
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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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type. There are four types of transforms.
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If a transform is present then the next two bits specify the transform type.
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There are four types of transforms.
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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enum TransformType {
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@ -224,25 +219,23 @@ enum TransformType {
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};
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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 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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The transform type is followed by the transform data. Transform data contains
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the information required to apply the inverse transform and depends on the
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transform type. Next we describe the transform data for different types.
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### 4.1 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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transform, the current pixel value is predicted from the pixels already
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decoded (in scan-line order) and only the residual value (actual -
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predicted) is encoded. The _prediction mode_ determines the type of
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prediction to use. We divide the image into squares and all the pixels
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in a square use the same prediction mode.
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The predictor transform can be used to reduce entropy by exploiting the fact
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that neighboring pixels are often correlated. In the predictor transform, the
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current pixel value is predicted from the pixels already decoded (in scan-line
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order) and only the residual value (actual - predicted) is encoded. The
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_prediction mode_ determines the type of prediction to use. We divide the image
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into squares and all the pixels in a square use the same prediction mode.
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The first 3 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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indexing two-dimensionally.
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The first 3 bits of prediction data define the block width and height in number
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of bits. The number of block columns, `block_xsize`, is used in indexing
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two-dimensionally.
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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int size_bits = ReadBits(3) + 2;
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@ -252,26 +245,24 @@ int block_height = (1 << size_bits);
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int block_xsize = DIV_ROUND_UP(image_width, 1 << size_bits);
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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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
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[Chapter 5](#image-data).
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The transform data contains the prediction mode for each block of the image. All
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the `block_width * block_height` pixels of a block use same prediction mode. The
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prediction modes are treated as pixels of an image and encoded using the same
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techniques described in [Chapter 5](#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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For a pixel _x, y_, one can compute the respective filter block address by:
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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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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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
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whose values are already known.
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There are 14 different prediction modes. In each prediction mode, the current
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pixel value is predicted from one or more neighboring pixels whose values are
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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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We choose the neighboring pixels (TL, T, TR, and L) of the current pixel (P) as
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follows:
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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O O O O O O O O O O O
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@ -282,12 +273,12 @@ X X X X X X X X X X X
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X X X X X X X X X X X
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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where TL means top-left, T top, TR top-right, L left pixel.
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At the time of predicting a value for P, all pixels O, TL, T, TR and L
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have been already processed, and pixel P and all pixels X are unknown.
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where TL means top-left, T top, TR top-right, L left pixel. At the time of
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predicting a value for P, all pixels O, TL, T, TR and L have already been
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processed, and pixel P and all pixels X are unknown.
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Given the above neighboring pixels, the different prediction modes are
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defined as follows.
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Given the above neighboring pixels, the different prediction modes are defined
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as follows.
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| Mode | Predicted value of each channel of the current pixel |
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| ------ | ------------------------------------------------------- |
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@ -342,8 +333,8 @@ uint32 Select(uint32 L, uint32 T, uint32 TL) {
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}
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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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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The functions `ClampAddSubtractFull` and `ClampAddSubtractHalf` are performed
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for each ARGB component as follows:
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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// Clamp the input value between 0 and 255.
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@ -365,30 +356,28 @@ int ClampAddSubtractHalf(int a, int b) {
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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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,
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the predicted value for the left-topmost pixel of the image is
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0xff000000, L-pixel for all pixels on the top row, and T-pixel for all
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pixels on the leftmost column.
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prediction transform, regardless of the mode \[0..13\] for these pixels, the
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predicted value for the left-topmost pixel of the image is 0xff000000, L-pixel
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for all pixels on the top row, and T-pixel for all pixels on the leftmost
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column.
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\[AMENDED2\]
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Addressing the TR-pixel for pixels on the rightmost column is
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exceptional. The pixels on the rightmost column are predicted by using
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the modes \[0..13\] just like pixels not on the border, but the leftmost pixel
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on the same row as the current pixel is instead used as the TR-pixel.
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\[AMENDED2\] Addressing the TR-pixel for pixels on the rightmost column is
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exceptional. The pixels on the rightmost column are predicted by using the modes
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\[0..13\] just like pixels not on the border, but the leftmost pixel on the same
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row as the current pixel is instead used as the TR-pixel.
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### 4.2 Color Transform
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\[AMENDED2\]
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The goal of the color transform is to decorrelate the R, G and B values
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of each pixel. The color transform keeps the green (G) value as it is,
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transforms red (R) based on green and transforms blue (B) based on green
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and then based on red.
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The goal of the color transform is to decorrelate the R, G and B values of each
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pixel. The color transform keeps the green (G) value as it is, transforms red
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(R) based on green and transforms blue (B) based on green and then based on red.
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As is the case for the predictor transform, first the image is divided
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into blocks and the same transform mode is used for all the pixels in a
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block. For each block there are three types of color transform elements.
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As is the case for the predictor transform, first the image is divided into
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blocks and the same transform mode is used for all the pixels in a block. For
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each block there are three types of color transform elements.
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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typedef struct {
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@ -398,11 +387,10 @@ typedef struct {
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} ColorTransformElement;
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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 the same for all the pixels in a particular block. The delta is
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subtracted during the color transform. The inverse color transform then is just
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adding those deltas.
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The actual color transformation is done by defining a color transform delta. The
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color transform delta depends on the `ColorTransformElement`, which is the same
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for all the pixels in a particular block. The delta is subtracted during the
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color transform. The inverse color transform then is just adding those deltas.
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The color transform function is defined as follows:
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@ -424,9 +412,9 @@ void ColorTransform(uint8 red, uint8 blue, uint8 green,
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}
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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`ColorTransformDelta` is computed using a signed 8-bit integer
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representing a 3.5-fixed-point number, and a signed 8-bit RGB color
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channel (c) \[-128..127\] and is defined as follows:
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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) \[-128..127\]
|
||||
and is defined as follows:
|
||||
|
||||
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
|
||||
int8 ColorTransformDelta(int8 t, int8 c) {
|
||||
@ -434,22 +422,20 @@ int8 ColorTransformDelta(int8 t, int8 c) {
|
||||
}
|
||||
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
|
||||
|
||||
A conversion from the 8-bit unsigned representation (uint8) to the 8-bit
|
||||
signed one (int8) is required before calling `ColorTransformDelta()`.
|
||||
It should be performed using 8-bit two's complement (that is: uint8 range
|
||||
\[128..255\] is mapped to the \[-128..-1\] range of its converted int8 value).
|
||||
A conversion from the 8-bit unsigned representation (uint8) to the 8-bit signed
|
||||
one (int8) is required before calling `ColorTransformDelta()`. It should be
|
||||
performed using 8-bit two's complement (that is: uint8 range \[128..255\] is
|
||||
mapped to the \[-128..-1\] range of its converted int8 value).
|
||||
|
||||
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 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.
|
||||
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 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.
|
||||
|
||||
Now we describe the contents of color transform data so that decoding
|
||||
can apply the inverse color transform and recover the original red and
|
||||
blue values. The first 3 bits of the color transform data contain the
|
||||
width and height of the image block in number of bits, just like the
|
||||
predictor transform:
|
||||
Now we describe the contents of color transform data so that decoding can apply
|
||||
the inverse color transform and recover the original red and blue values. The
|
||||
first 3 bits of the color transform data contain the width and height of the
|
||||
image block in number of bits, just like the predictor transform:
|
||||
|
||||
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
|
||||
int size_bits = ReadBits(3) + 2;
|
||||
@ -457,17 +443,15 @@ 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
|
||||
[Chapter 5](#image-data).
|
||||
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 [Chapter 5](#image-data).
|
||||
|
||||
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
|
||||
adding `ColorTransformElement` values to the red and blue
|
||||
channels. \[AMENDED3\]
|
||||
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 adding
|
||||
`ColorTransformElement` values to the red and blue channels. \[AMENDED3\]
|
||||
|
||||
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
|
||||
void InverseTransform(uint8 red, uint8 green, uint8 blue,
|
||||
@ -492,11 +476,10 @@ void InverseTransform(uint8 red, uint8 green, uint8 blue,
|
||||
|
||||
### 4.3 Subtract Green Transform
|
||||
|
||||
The subtract green transform subtracts green values from red and blue
|
||||
values of each pixel. When this transform is present, the decoder needs
|
||||
to add the green value to both red and blue. There is no data associated
|
||||
with this transform. The decoder applies the inverse transform as
|
||||
follows:
|
||||
The subtract green transform subtracts green values from red and blue values of
|
||||
each pixel. When this transform is present, the decoder needs to add the green
|
||||
value to both red and blue. There is no data associated with this transform. The
|
||||
decoder applies the inverse transform as follows:
|
||||
|
||||
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
|
||||
void AddGreenToBlueAndRed(uint8 green, uint8 *red, uint8 *blue) {
|
||||
@ -505,53 +488,47 @@ void AddGreenToBlueAndRed(uint8 green, uint8 *red, uint8 *blue) {
|
||||
}
|
||||
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
|
||||
|
||||
This transform is redundant as it can be modeled using the color
|
||||
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.
|
||||
This transform is redundant as it can be modeled using the color 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.
|
||||
|
||||
|
||||
### 4.4 Color Indexing Transform
|
||||
|
||||
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).
|
||||
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, 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 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, 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
|
||||
follows:
|
||||
The transform data contains color table size and the entries in the color table.
|
||||
The decoder reads the color indexing transform data as follows:
|
||||
|
||||
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
|
||||
// 8 bit value for color table size
|
||||
int color_table_size = ReadBits(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 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
|
||||
least significant 8 bits of the result.
|
||||
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 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 least
|
||||
significant 8 bits of the result.
|
||||
|
||||
The inverse transform for the image is simply replacing the pixel values
|
||||
(which are indices to the color table) with the actual color table
|
||||
values. The indexing is done based on the green component of the ARGB
|
||||
color.
|
||||
The inverse transform for the image is simply replacing the pixel values (which
|
||||
are indices to the color table) with the actual color table values. The indexing
|
||||
is done based on the green component of the ARGB color.
|
||||
|
||||
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
|
||||
// Inverse transform
|
||||
@ -559,15 +536,14 @@ argb = color_table[GREEN(argb)];
|
||||
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
|
||||
|
||||
If the index is equal or larger than `color_table_size`, the argb color value
|
||||
should be set to 0x00000000 (transparent black). \[AMENDED\]
|
||||
should be set to 0x00000000 (transparent black). \[AMENDED\]
|
||||
|
||||
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 16 or fewer 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 16 or fewer unique values.
|
||||
|
||||
`color_table_size` specifies how many pixels are combined:
|
||||
|
||||
@ -621,9 +597,9 @@ We use image data in five different roles:
|
||||
[meta prefix codes](#decoding-of-meta-prefix-codes). The red and green
|
||||
components of a pixel define the meta prefix code used in a particular
|
||||
block of the ARGB image.
|
||||
1. Predictor image: Stores the metadata for [Predictor
|
||||
Transform](#predictor-transform). The green component of a pixel defines
|
||||
which of the 14 predictors is used within a particular block of the
|
||||
1. Predictor image: Stores the metadata for
|
||||
[Predictor Transform](#predictor-transform). The green component of a pixel
|
||||
defines which of the 14 predictors is used within a particular block of the
|
||||
ARGB image.
|
||||
1. Color transform image. It is created by `ColorTransformElement` values
|
||||
(defined in [Color Transform](#color-transform)) for different blocks of
|
||||
@ -683,8 +659,8 @@ while the extra bits are stored as they are (without an entropy code).
|
||||
|
||||
**Rationale**: This approach reduces the storage requirement for the entropy
|
||||
code. Also, large values are usually rare, and so extra bits would be used for
|
||||
very few values in the image. Thus, this approach results in better
|
||||
compression overall.
|
||||
very few values in the image. Thus, this approach results in better compression
|
||||
overall.
|
||||
|
||||
The following table denotes the prefix codes and extra bits used for storing
|
||||
different ranges of values.
|
||||
@ -709,8 +685,8 @@ values. For distance values, however, all the 40 prefix codes are valid.
|
||||
| 524289..786432 | 38 | 18 |
|
||||
| 786433..1048576 | 39 | 18 |
|
||||
|
||||
The pseudocode to obtain a (length or distance) value from the prefix code is
|
||||
as follows:
|
||||
The pseudocode to obtain a (length or distance) value from the prefix code is as
|
||||
follows:
|
||||
|
||||
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
|
||||
if (prefix_code < 4) {
|
||||
@ -729,8 +705,8 @@ previously seen pixel, from which the pixels are to be copied. This subsection
|
||||
defines the mapping between a distance code and the position of a previous
|
||||
pixel.
|
||||
|
||||
Distance codes larger than 120 denote the pixel-distance in scan-line
|
||||
order, offset by 120.
|
||||
Distance codes larger than 120 denote the pixel-distance in scan-line order,
|
||||
offset by 120.
|
||||
|
||||
The smallest distance codes \[1..120\] are special, and are reserved for a close
|
||||
neighborhood of the current pixel. This neighborhood consists of 120 pixels:
|
||||
@ -770,8 +746,8 @@ neighboring pixel, that is, the pixel above the current pixel (0 pixel
|
||||
difference in the X-direction and 1 pixel difference in the Y-direction).
|
||||
Similarly, the distance code `3` indicates the left-top pixel.
|
||||
|
||||
The decoder can convert a distance code `i` to a scan-line order distance
|
||||
`dist` as follows:
|
||||
The decoder can convert a distance code `i` to a scan-line order distance `dist`
|
||||
as follows:
|
||||
|
||||
\[AMENDED3\]
|
||||
|
||||
@ -812,16 +788,16 @@ int color_cache_size = 1 << color_cache_code_bits;
|
||||
`color_cache_code_bits` is \[1..11\]. Compliant decoders must indicate a
|
||||
corrupted bitstream for other values.
|
||||
|
||||
A color cache is an array of 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.
|
||||
A color cache is an array of 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.
|
||||
|
||||
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
|
||||
this color at decoding time. The state of the color cache is maintained
|
||||
by inserting every pixel, be it produced by backward referencing or as
|
||||
literals, into the cache in the order they appear in the stream.
|
||||
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 this color at
|
||||
decoding time. The state of the color cache is maintained by inserting every
|
||||
pixel, be it produced by backward referencing or as literals, into the cache in
|
||||
the order they appear in the stream.
|
||||
|
||||
|
||||
6 Entropy Code
|
||||
@ -871,9 +847,9 @@ stream. This may be inefficient, but it is allowed by the format.
|
||||
|
||||
\[AMENDED2\]
|
||||
|
||||
This variant is used in the special case when only 1 or 2 prefix symbols are
|
||||
in the range \[0..255\] with code length `1`. All other prefix code lengths
|
||||
are implicitly zeros.
|
||||
This variant is used in the special case when only 1 or 2 prefix symbols are in
|
||||
the range \[0..255\] with code length `1`. All other prefix code lengths are
|
||||
implicitly zeros.
|
||||
|
||||
The first bit indicates the number of symbols:
|
||||
|
||||
@ -882,10 +858,11 @@ int num_symbols = ReadBits(1) + 1;
|
||||
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
|
||||
|
||||
Following are the symbol values.
|
||||
|
||||
This first symbol is coded using 1 or 8 bits depending on the value of
|
||||
`is_first_8bits`. The range is \[0..1\] or \[0..255\], respectively.
|
||||
The second symbol, if present, is always assumed to be in the range \[0..255\]
|
||||
and coded using 8 bits.
|
||||
`is_first_8bits`. The range is \[0..1\] or \[0..255\], respectively. The second
|
||||
symbol, if present, is always assumed to be in the range \[0..255\] and coded
|
||||
using 8 bits.
|
||||
|
||||
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
|
||||
int is_first_8bits = ReadBits(1);
|
||||
@ -897,13 +874,12 @@ if (num_symbols == 2) {
|
||||
}
|
||||
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
|
||||
|
||||
**Note:** Another special case is when _all_ prefix code lengths are _zeros_
|
||||
(an empty prefix code). For example, a prefix code for distance can be empty
|
||||
if there are no backward references. Similarly, prefix codes for alpha, red,
|
||||
and blue can be empty if all pixels within the same meta prefix code are
|
||||
produced using the color cache. However, this case doesn't need special
|
||||
handling, as empty prefix codes can be coded as those containing a single
|
||||
symbol `0`.
|
||||
**Note:** Another special case is when _all_ prefix code lengths are _zeros_ (an
|
||||
empty prefix code). For example, a prefix code for distance can be empty if
|
||||
there are no backward references. Similarly, prefix codes for alpha, red, and
|
||||
blue can be empty if all pixels within the same meta prefix code are produced
|
||||
using the color cache. However, this case doesn't need special handling, as
|
||||
empty prefix codes can be coded as those containing a single symbol `0`.
|
||||
|
||||
**(ii) Normal Code Length Code:**
|
||||
|
||||
@ -940,8 +916,8 @@ int length_nbits = 2 + 2 * ReadBits(3);
|
||||
int max_symbol = 2 + ReadBits(length_nbits);
|
||||
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
|
||||
|
||||
A prefix table is then built from `code_length_code_lengths` and used to read
|
||||
up to `max_symbol` code lengths.
|
||||
A prefix table is then built from `code_length_code_lengths` and used to read up
|
||||
to `max_symbol` code lengths.
|
||||
|
||||
* Code \[0..15\] indicates literal code lengths.
|
||||
* Value 0 means no symbols have been coded.
|
||||
@ -964,8 +940,8 @@ distance) is formed using their respective alphabet sizes:
|
||||
#### 6.2.2 Decoding of Meta Prefix Codes
|
||||
|
||||
As noted earlier, the format allows the use of different prefix codes for
|
||||
different blocks of the image. _Meta prefix codes_ are indexes identifying
|
||||
which prefix codes to use in different parts of the image.
|
||||
different blocks of the image. _Meta prefix codes_ are indexes identifying which
|
||||
prefix codes to use in different parts of the image.
|
||||
|
||||
Meta prefix codes may be used _only_ when the image is being used in the
|
||||
[role](#roles-of-image-data) of an _ARGB image_.
|
||||
@ -1019,8 +995,8 @@ int num_prefix_groups = max(entropy image) + 1;
|
||||
where `max(entropy image)` indicates the largest prefix code stored in the
|
||||
entropy image.
|
||||
|
||||
As each prefix code group contains five prefix codes, the total number of
|
||||
prefix codes is:
|
||||
As each prefix code group contains five prefix codes, the total number of prefix
|
||||
codes is:
|
||||
|
||||
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
|
||||
int num_prefix_codes = 5 * num_prefix_groups;
|
||||
@ -1037,8 +1013,8 @@ PrefixCodeGroup prefix_group = prefix_code_groups[meta_prefix_code];
|
||||
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
|
||||
|
||||
where, we have assumed the existence of `PrefixCodeGroup` structure, which
|
||||
represents a set of five prefix codes. Also, `prefix_code_groups` is an array
|
||||
of `PrefixCodeGroup` (of size `num_prefix_groups`).
|
||||
represents a set of five prefix codes. Also, `prefix_code_groups` is an array of
|
||||
`PrefixCodeGroup` (of size `num_prefix_groups`).
|
||||
|
||||
The decoder then uses prefix code group `prefix_group` to decode the pixel
|
||||
(x, y) as explained in the [next section](#decoding-entropy-coded-image-data).
|
||||
|
||||
Loading…
Reference in New Issue
Block a user