[cairo] Re: Using ADD operator to prevent seams

[Bill Spitzak]

I'm pretty convinced that there is a need to draw adjacent 
non-overlapping filled polygons so that seams are prevented.

But I don't like the idea of "turn off antialiasing" however. I would 
like to be able to take advantage of whatever the solution is without 
losing the ability to have antialiasing on other parts of the drawing, 
including the outer edge of the same shape! "turn off antialiasing" is 
thus the wrong approach, and supporting it will be misleading and 
discourage fixing this for real.

I think marking some edges of a path as being "adjoining" would solve 
this. This will only affect fill and using the path as a clip. Initially 
it will produce an aliased edge, but it could enable supersampling or 
whole new compositing algorithims instead, so it should be named to make 
it clear that it does not necessarily disable antialiasing. As I see it 
only two new calls are needed: adjoining-lineto and adjoining-curveto.

I would rather not see any way to turn off antialiasing of fonts, lines, 
and patterns (though I would like 0-width lines like other backends, as 
this gets rid of the other reason aliasing is often requested). Fonts in 
particular may be *slowed* by aliasing, as it disallows algorithms like 
scaling larger images which are otherwise the fastest method on modern 
hardware.

A special "aliased" backend can also be provided. This would be totally 
unrelated to the normal device backend, it would not be hardware 
accelerated, and would draw into an image buffer, and then copy that to 
the device. If you can set the scale from the image to the device this 
will also produce supersampled antialiasing.