X/Render, rotation transform, Src PICTOP: can I ignore pixels in the bounding box but outside the polygon?

[Ilya Anfimov]

On Sun, May 22, 2016 at 03:07:17PM +1000, Nigel Tao wrote:
> On Wed, May 18, 2016 at 8:07 PM, Ilya Anfimov <ilan at astelecom.ru> wrote:
> >  But  anyway  you can specify clip-mask with ChangePicture to use
> > bitmap of enabled pixels. Whould be better IMO  than  restricting
> > output line-by-line.
> >  And that mask bitmap also could be calculated by RENDER.
> 
> I suppose that will 'work', clipping the dst rather than the src,
> although it seems inefficient to allocate and clean up an entire
> Pixmap for the mask every time I want to do this...

 Well, it's inefficiency needs investigation.

 And inefficiency of alpha operation needs investigation too.