[cairo] Seams with CAIRO_ANTIALIAS_FAST

[Søren Sandmann]

The CAIRO_ANTIALIAS_FAST option uses a 2x2 sampling grid where each
sample has a weight of 64 when the depth of the mask is 8 bits.

This is problematic because the four samples then sum to 256, which
causes a seam a pixel is fully covered by a combination of independent
polygons.

The attached program demonstrates this. The blue rectangle is drawn as
four separate rectangles and the cross where they meet has a blue
channel of 0x74 where the rest of the rectangle is 0x73.

There are various fixes possible:

(a) Make ANTIALIAS_FAST use a 3x5 sampling grid. This is tempting
    because X Render already supports this antialiasing mode (rendering
    trapezoids to a 4 bit mask will use it).

(b) Make the weight of one of the samples 63 instead of 64. I don't know
    how difficult this will be in the tor22 rasterizer.

(c) Ignore it and document that cairo can't draw seamless polygons.

    Considering that the bug-seams.c test has been failing ever since
    the tor rasterizer was added, and that fixing that test will
    probably involve a performance hit from turning off analytical
    antialiasing, maybe this is the more honest way to go.


Soren


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