[cairo] Alpha masks

[Keith Packard]

Around 15 o'clock on Jan 6, Owen Taylor wrote:

> I think in terms of the original requested operation (basically, use
> an A8 surface in place of a polygon or set of glyphs), this doesn't
> fit very will.

In Twin (a tiny window system), I provide two levels in the API; the low 
level 'Composite' api which performs basic image compositing upon which 
the geometry is built, following the Plan 9 architecture which makes paths 
just an implicit mask to the path compositing operation.  In cairo, we 
would then have something like:

	cairo_composite (cairo_t	 dst, 
			 cairo_pattern_t src, 
			 cairo_pattern_t msk);

(this is not a formal proposal; I realize the confusion in having a 
pattern in dst).

We would then formally define cairo_fill and cairo_stroke in terms of this 
primitive -- the msk operand is build from the provided geometry.

And, your cairo_show_surface_mask becomes a simple wrapper around the 
above general compositing function.

I guess what I'm trying to avoid is having a collection of useful but not
obviously related rendering operations.

-keith


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