[cairo] cairo anti aliasing

[Tim Janik]

hi carl et al.

i am seriously considering to use cairo as a rendering backend in an
anti-aliased canvas i'm currently working on.

there is one major essential feature i currently see cairo
lacking for that though.
adjusting the amount of anti-aliasing:
   currently, i can render rectangles with varying amounts of smoothness at
   the edges, similar to the "Feather Edge" feature of the gimp "rectangle
   select" command. this is essentially adjusting the radius of a 2D gaussian
   curve in pixels, used to smooth the figure edges.
   as an example, i have attached an image, containing 4 times the same
   rectangle, rendered with feather edges of 0, 1, 2 and 3 from left to right.

to adress this issues, i'd like to know:
where in cairo or pixman is the current anti-aliasing amount encoded
(approximately 1.5 pixels feather edge if comapared to my example picture)
and what is required to make that configurable?

any answer would be greatly apprechiated, especially source pointers to places
i can hack for getting better anti-aliasing results ;)

---
ciaoTJ
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