Søren Sandmann wrote:
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
This example could be fixed by treating 0/0 as returning 0.
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