[cairo] Redoing SOURCE and CLEAR

[Owen Taylor]

On Wed, 2005-08-17 at 12:38 -0400, Behdad Esfahbod wrote:
> On Wed, 17 Aug 2005, Owen Taylor wrote:
> 
> > On Wed, 2005-08-17 at 11:55 -0400, Behdad Esfahbod wrote:
> >
> >  - What is useful for people
> >  - What makes sense to people
> 
> So far I've been escaping from understanding what's going on by
> not touching cairo_set_operator at all, and that's what people
> need most of the time!  The question here is what do SOURCE with
> your proposed definition would offer that OVER doesn't offer.

It copies the alpha channel of the source. So, if you say,
want to set just a rectangle of your destination image to
50% transparent red, then you need to use SOURCE, rather than
OVER.

If you look at the images in my first mail, you can see the
difference between the second and fourth column. (It's a little
more obvious what is going on if you load the images into a
program (EOG say) that uses a checker background instead of white.)

Regards,
					Owen


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