[Pixman] [PATCH 1/3] Add CLEAR and SRC linear interpolation operators

Søren Sandmann sandmann at cs.au.dk
Thu Sep 15 02:36:57 PDT 2011

Chris Wilson <chris at chris-wilson.co.uk> writes:

> Cairo, for instance, has a subtly different interpretation of how to use
> the mask in combination with the Porter-Duff operators. In particular,
> it has the notion of a clip mask, for which pixman has no parallel.
> Quoting Soeren:
>   Another aspect is that cairo uses a different rendering equation in some
>   cases. The two equations used are:
> 	unbounded:   [(src IN shape) OP dest          ] LERP_clip dest
> 	bounded:     [(src OP dest) LERP_shape dest   ] LERP_clip dest
>   With shaped clips, the LERP_clip part is taken care of, and the first
>   equation is already directly supported by pixman. The second one could
>   be supported by adding new <op>_LERP operators that would use the
> 	(src OP dest) LERP_shape dest
>   equation. For cairo's purposes, all we need is CLEAR_LERP and SRC_LERP,
>   but I think it could be useful to have the full set of LERP operators.

Did you see Taekyun Kim's reply to this? He proposes this API:

     pixman_image_composite_xrender(op, src, shape, clip, dst)
     pixman_image_composite_bounded(op, src, shape, clip, dst)
     pixman_image_composite_simple(op, src, shape, clip, dst)

instead, which would add direct one-pass support for all of cairo's
compositing schemes, including clip masks. See:



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