[cairo] Seams with the image backend
M Joonas Pihlaja
jpihlaja at cc.helsinki.fi
Sat May 2 04:33:31 PDT 2009
On Sat, 2 May 2009, Soeren Sandmann wrote:
> Actually, looking more closely at the application, there is actually
> also a seam even when the slider is at 1.0 and the shared edge is
> specified with the exact same coordinates in the two polygons.
Yow. I didn't expect that!
Turns out the problem is in the definition of the GRID_AREA_TO_ALPHA()
macro: It truncates rather than rounds when converting a span's area
to [0,255]. I'll commit a fix to that soon.
After that fix if we decide that the image backend's seaming is still
unacceptable in the collinear-but-not-exactly-equal-coordinates case,
we might try doing the edge walking DDA in 24.8 space rather than the
rasteriser's quantized space. That might mean needing to increase the
Y supersampling to 16x from 15x to keep the edge bucketing easy
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