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webp-lossless-bitstream-spec: mv up prefix code group def

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Move this to the top-level of the "Details" section as prefix codes are
discussed in both "Decoding and Building the Prefix Codes" and "Decoding
of Meta Prefix Codes"

Bug: webp:611
Change-Id: Ide2dfedf081d6c94122ac16ecded968ed760f90c
This commit is contained in:
James Zern
2023-06-05 18:21:11 -07:00
parent e4f17a31a7
commit c437c7aace
1 changed files with 11 additions and 11 deletions
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22
doc/webp-lossless-bitstream-spec.txt
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@@ -816,6 +816,17 @@ The encoded image data consists of several parts:
1. Meta prefix codes
1. Entropy-coded image data
For any given pixel (x, y), there is a set of five prefix codes associated with
it. These codes are (in bitstream order):
* **Prefix code #1**: used for green channel, backward-reference length, and
color cache.
* **Prefix code #2, #3, and #4**: used for red, blue, and alpha channels,
respectively.
* **Prefix code #5**: used for backward-reference distance.
From here on, we refer to this set as a **prefix code group**.
#### 6.2.1 Decoding and Building the Prefix Codes
There are several steps in decoding the prefix codes.
@@ -975,17 +986,6 @@ The next bits contain an entropy image of width `prefix_xsize` and height
**Interpretation of Meta Prefix Codes:**
For any given pixel (x, y), there is a set of five prefix codes associated with
it. These codes are (in bitstream order):
* **Prefix code #1**: used for green channel, backward-reference length, and
color cache.
* **Prefix code #2, #3, and #4**: used for red, blue, and alpha channels,
respectively.
* **Prefix code #5**: used for backward-reference distance.
From here on, we refer to this set as a **prefix code group**.
The number of prefix code groups in the ARGB image can be obtained by finding
the _largest meta prefix code_ from the entropy image: