[cairo] Alpha masks

[Keith Packard]

Around 13 o'clock on Jan 6, Carl Worth wrote:

> Right. But I'm thinking about "display image at 50% opacity". Having
> to create a pattern and a mask for this operation would make it just
> as hard as the original complaint about drawing a solid color through
> a mask.

As cairo_set_rgb_color creates a source pattern, should cairo_set_alpha 
create a mask pattern?  Should we expose this 'mask pattern' as a part of 
the drawing pipeline?

	dst = (src IN (path IN mask IN CLIP)) OP dst

This has the minor advantage of clarifying the relationship between 
cairo_set_rgb_color and cairo_set_alpha as to whether the rgb values 
should be premultiplied.

We would then add:

	cairo_set_mask_pattern (cairo_t *cr, cairo_pattern_t *pattern)

and you could get at the Plan 9 operator with:

	cairo_set_pattern (cr, source);
	cairo_set_mask_pattern (cr, mask);
	cairo_rectangle (cr, x, y, w, h);
	cairo_fill (cr)

At this point, making common operations more convenient really is just a 
matter of convenience functions.

-keith


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