mirror of
https://salsa.debian.org/xorg-team/lib/pixman
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15aa37adec
Consider a DISJOINT_ATOP operation with the following pixels:
- source: 0xff (8 bits)
- source alpha: 0x01 (8 bits)
- mask alpha: 0x7b (8 bits)
- dest: 0x00 (8 bits)
- dest alpha: 0xff (8 bits)
When (src IN mask) is computed in 8 bits, the resulting alpha channel
is 0 due to rounding:
floor ((0x01 * 0x7b) / 255.0 + 0.5) = floor (0.9823) = 0
which means that since Render defines any division by zero as
infinity, the Fa and Fb for this operator end up as follows:
Fa = max (1 - (1 - 1) / 0, 0) = 0
Fb = min (1, (1 - 0) / 1) = 1
and so since dest is 0x00, the overall result is 0.
However, when computed in full precision, the alpha value no longer
rounds to 0, and so Fa ends up being
Fa = max (1 - (1 - 1) / 0.0001, 0) = 1
and so the result is now
s * ma * Fa + d * Fb
= (1.0 * (0x7b / 255.0) * 1) + d * 0
= 0x7b / 255.0
= 0.4823
so the error in this case ends up being 0.48235294, which is clearly
not something that can be considered acceptable.
In order to avoid this problem, we need to do all arithmetic in such a
way that a multiplication of two tiny numbers can never end up being
zero unless one of the input numbers is itself zero.
This patch makes all computations that involve divisions take place in
floating point, which is sufficient to fix the test cases
This brings the number of failures in pixel-test down to 14.
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|---|---|---|
| .. | ||
| a1-trap-test.c | ||
| affine-test.c | ||
| alpha-loop.c | ||
| alphamap.c | ||
| blitters-test.c | ||
| check-formats.c | ||
| combiner-test.c | ||
| composite-traps-test.c | ||
| composite.c | ||
| fetch-test.c | ||
| fuzzer-find-diff.pl | ||
| glyph-test.c | ||
| gradient-crash-test.c | ||
| infinite-loop.c | ||
| lowlevel-blt-bench.c | ||
| Makefile.am | ||
| Makefile.sources | ||
| Makefile.win32 | ||
| matrix-test.c | ||
| oob-test.c | ||
| pdf-op-test.c | ||
| pixel-test.c | ||
| prng-test.c | ||
| radial-perf-test.c | ||
| region-contains-test.c | ||
| region-test.c | ||
| region-translate-test.c | ||
| rotate-test.c | ||
| scaling-bench.c | ||
| scaling-crash-test.c | ||
| scaling-helpers-test.c | ||
| scaling-test.c | ||
| stress-test.c | ||
| thread-test.c | ||
| tolerance-test.c | ||
| trap-crasher.c | ||
| utils-prng.c | ||
| utils-prng.h | ||
| utils.c | ||
| utils.h | ||