Around 7 o'clock on May 16, Keith Packard wrote:
> I'm going to run some numbers and see whether I can fix this by changing
> the precision of various alpha computations
I think this particular problem is occuring because we're losing half a
bit of alpha in the conversion from area to alpha. Remember that the
area to alpha computation is currently specified by:
int AreaAlpha (Fixed_1_31 area, int depth)
{
((area >> depth) * ((1 << depth) - 1)) >> (31 - depth)
}
Rounding instead of truncating in that computation fixes this particular
problem:
int AreaAlpha (Fixed_1_31 area, int depth)
{
((((area + (1 << (depth - 1)) >> depth) * ((1 << depth) - 1) +
(1 << (30 - depth))) >> (31 - depth);
}
I'm going to give that a try.
Keith Packard XFree86 Core Team HP Cambridge Research Lab
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