[cairo] Scrolling/Copying

[Bill Spitzak]

Carl Worth wrote:

> So, I think I just talked myself into the fact that Owen had the right
> idea all along that cairo_set_source should lock the pattern matrix to
> refer to the user space at the time of cairo_set_source, (rather than
> the user space in effect at the time of subsequent drawing
> operations).
> 
> This will allow usage like the following to directly copy from a
> source surface:
> 
> 	cairo_identity_matrix (cr);
> 	cairo_set_source_surface (cr, surface, x, y);
> 	cairo_scale (cr, sx, sy);
> 	cairo_rectangle (cr, dx, dy, width, height);

I agree this will do what is wanted. Also it will allow the pattern to 
align in multiple fills even if you change the transformation between 
the fills.

So I guess this means that copying areas should be detected. I don't 
think there is any need to work correctly unless opaque copy is being 
done, ie any compositing operation other than src, and any repeating or 
alpha, will produce undefined results.