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doc: use two's complement explicitly for uint8->int8 conversion

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BUG=webp:225

Change-Id: I6bad131e275dbd992484e95a1b834010121281b8
This commit is contained in:
Pascal Massimino 2016-09-05 10:57:56 -07:00
parent 9e478f808e
commit 13ae011e4c
1 changed files with 5 additions and 0 deletions
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doc/webp-lossless-bitstream-spec.txt
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@ -429,6 +429,11 @@ 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).
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,