[cairo] Redoing SOURCE and CLEAR

Behdad Esfahbod behdad at cs.toronto.edu
Wed Aug 17 09:38:02 PDT 2005

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.

> (And it's tailoring, unless I'm doing it. :-)

Sorry about that.  Silly me. :)

> > Moreover, it reduces SOURCE to IN somehow, but a broken one.  The
> > fact that it's differentiating between a alpha of 0 in the
> > source, and a point ouside the mask is confusing.
> I don't see the relationship between the new SOURCE and IN.
> But what's *wrong* with doing:
>    cr = cairo_create (canvas);
>    cairo_set_operator (cr, CAIRO_OPERATOR_CLEAR);
>    cairo_fill();
>    cairo_set_operator (cr, CAIRO_OPERATOR_OVER);
>    cairo_set_source_rgba (cr, 0, 0, 0);
>    cairo_rectangle (cr, x, y, width, height);
>    cairo_fill (cr);
>    cairo_destroy (cr);
> Yes, it touches the pixels in the rectangle twice, but it's not
> going to be a big performance difference. And it's more apparent
> what it is doing.

Ok, then what's wrong with simply using OVER?  In other words,
why should people use SOURCE at all?

> (Note also that create_similar() clears the surface, for better
> or worse.)

I personally have been keeping my scratch buffers around and only
recreate on resize...

> Rgeards,
> 						Owen


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