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webp-lossless-bitstream-spec: fix dist mapping example

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The distance code read from the bitstream is reduced by 1 before doing
the lookup. The prose describing the lookup was correct, the pseudocode
failed to subtract 1 and used x/y instead of xi/yi from the lookup.

Bug: webp:448
Change-Id: I152477b888c26a0473a35373d3d331fddd14237f
This commit is contained in:
James Zern 2022-11-17 15:11:43 -08:00
parent fad0ece7ed
commit 44741f9c58
1 changed files with 3 additions and 3 deletions
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6
doc/webp-lossless-bitstream-spec.txt
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@ -764,14 +764,14 @@ The mapping between distance code `i` and the neighboring pixel offset
For example, distance code `1` indicates an offset of `(0, 1)` for the For example, distance code `1` indicates an offset of `(0, 1)` for the
neighboring pixel, that is, the pixel above the current pixel (0 pixel neighboring pixel, that is, the pixel above the current pixel (0 pixel
difference in X-direction and 1 pixel difference in Y-direction). Similarly, difference in X-direction and 1 pixel difference in Y-direction). Similarly,
distance code `3` indicates left-top pixel. distance code `3` indicates the left-top pixel.
The decoder can convert a distance code `i` to a scan-line order distance The decoder can convert a distance code `i` to a scan-line order distance
`dist` as follows: `dist` as follows:
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
(xi, yi) = distance_map[i] (xi, yi) = distance_map[i - 1]
dist = x + y * xsize dist = xi + yi * xsize
if (dist < 1) { if (dist < 1) {
dist = 1 dist = 1
} }