[cairo] Avoiding seams

Eric Hughes eh-cairo at narthex.us
Mon Dec 12 20:01:02 PST 2005

I can't address everything yet, because I'm still investigating, but 
I can respond to one point.

>On Mon, 2005-12-12 at 18:19 -0700, Eric Hughes wrote:
> > Anti-aliasing can be done locally with path-as-boundary, with one
> > caveat.  You can draw either with luminosity (additive) or occlusion
> > (subtractive).  As long as one or the other is specified, you can get
> > a linear behavior at edges.

At 07:21 PM 12/12/2005, MenTaLguY wrote:
>There is a fundamental ambiguity:
>  http://lists.freedesktop.org/archives/cairo/2005-March/003481.html

The fundamental ambiguity of this example is because of the use of 
simple opaque occlusion, what is called there the "covered-by" 
operator.  Mathematically, this is the function max(), which isn't 
linear over the integers.  If you have a non-linear compositing 
operator, then you're in a situation where computing an alias filter 
independently per element doesn't work.  You have to have linearity 
to be able to compute the filter function independently per element.

If you need a non-linear compositing operator to deal with vector 
graphics, you still don't need to compute it over the whole 
screen.  Doing it at intersections (including collinear edges and 
coincident vertices) suffices.


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