doc: use two's complement explicitly for uint8->int8 conversion
BUG=webp:225
Change-Id: I6bad131e275dbd992484e95a1b834010121281b8
(cherry picked from commit 13ae011e4cd540088dfc80ee2f303811f5c51389)
diff --git a/doc/webp-lossless-bitstream-spec.txt b/doc/webp-lossless-bitstream-spec.txt
index 19fec9d..2d2dde1 100644
--- a/doc/webp-lossless-bitstream-spec.txt
+++ b/doc/webp-lossless-bitstream-spec.txt
@@ -429,6 +429,11 @@
}
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+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,
