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webp-lossless-bitstream-spec: update transformations text

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follow up to:
29c9f2d4 webp-lossless-bitstream-spec: minor wording updates

Universally use 'transform' over 'transformation'. The doc generally
uses 'transform' to talk about the individual transforms themselves as
well as their related data.

Bug: webp:611
Change-Id: I097701873bbc440bd399ab5440a914a14f7c62f6
This commit is contained in:
James Zern
2023-06-22 16:35:32 -07:00
parent 08d60d6006
commit 6d6d4915ce
1 changed files with 12 additions and 12 deletions
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24
doc/webp-lossless-bitstream-spec.txt
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@@ -63,15 +63,15 @@ We assume that each color component, that is, alpha, red, blue and green, is
represented using an 8-bit byte. We define the corresponding type as uint8. A represented using an 8-bit byte. We define the corresponding type as uint8. A
whole ARGB pixel is represented by a type called uint32, which is an unsigned whole ARGB pixel is represented by a type called uint32, which is an unsigned
integer consisting of 32 bits. In the code showing the behavior of the integer consisting of 32 bits. In the code showing the behavior of the
transformations, these values are codified in the following bits: alpha in bits transforms, these values are codified in the following bits: alpha in bits
31..24, red in bits 23..16, green in bits 15..8 and blue in bits 7..0; however, 31..24, red in bits 23..16, green in bits 15..8 and blue in bits 7..0; however,
implementations of the format are free to use another representation internally. implementations of the format are free to use another representation internally.
Broadly, a WebP lossless image contains header data, transform information, and Broadly, a WebP lossless image contains header data, transform information, and
actual image data. Headers contain the width and height of the image. A WebP actual image data. Headers contain the width and height of the image. A WebP
lossless image can go through four different types of transformation before lossless image can go through four different types of transforms before being
being entropy encoded. The transform information in the bitstream contains the entropy encoded. The transform information in the bitstream contains the data
data required to apply the respective inverse transforms. required to apply the respective inverse transforms.
2 Nomenclature 2 Nomenclature
@@ -174,17 +174,17 @@ int version_number = ReadBits(3);
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
4 Transformations 4 Transforms
----------------- ------------
Transformations are reversible manipulations of the image data that can reduce The transforms are reversible manipulations of the image data that can reduce
the remaining symbolic entropy by modeling spatial and color correlations. the remaining symbolic entropy by modeling spatial and color correlations. They
Transformations can make the final compression more dense. can make the final compression more dense.
An image can go through four types of transformations. A 1 bit indicates the An image can go through four types of transforms. A 1 bit indicates the
presence of a transform. Each transform is allowed to be used only once. The presence of a transform. Each transform is allowed to be used only once. The
transformations are used only for the main-level ARGB image; the subresolution transforms are used only for the main-level ARGB image; the subresolution images
images have no transforms, not even the 0 bit indicating the end of transforms. have no transforms, not even the 0 bit indicating the end of transforms.
Typically, an encoder would use these transforms to reduce the Shannon entropy Typically, an encoder would use these transforms to reduce the Shannon entropy
in the residual image. Also, the transform data can be decided based on entropy in the residual image. Also, the transform data can be decided based on entropy