[cairo] Stange alpha channel problem.

[Paul Giblock]

I just tried to run this source on my desktop.  I forgot to mention a
few things:  you must add -lX11 to your compile line.  You may have to
add #include <stdlib.h> to the source.

Finally, on my desktop there is no darkening problem. The darkening
still occurs on my laptop.  Both systems are running cairo-1.1.6.


On 6/13/06, Paul Giblock <drfaygo at gmail.com> wrote:
> Oops, forgot to "RE All".  Thanks for the quick response. Here is a
> recreation of the problem.  The code isn't the best since it was just
> stitched together from several other files in my real project.
>
> -Paul
>
>
> On 6/13/06, Carl Worth <cworth at cworth.org> wrote:
> > On Tue, 13 Jun 2006 15:53:22 -0400, "Paul Giblock" wrote:
> > >
> > > First, I would like to thank the Cairo developers for providing such a
> > > nice library.
> >
> > On behalf of all of us, you are most welcome.
> >
> > > This works, in that the image is displayed in the right place and is
> > > properly clipped.  Unfortunately, the areas of the surface underneith
> > > the transparent areas get darker each time I draw the image.  For
> > > example: My surface I am rendering to may be 255,0,0 (red). Each time
> > > I draw the image over the surface the visible red areas get darker.
> > > These areas will only get as dark as (128,0,0).
> >
> > I've tried to replicate this, but I haven't succeeded yet.
> >
> > I've attached the test program I wrote which starts by filling a 3x3
> > surface with red, then hits the center pixel with several different
> > forms of zero alpha, (including drawing an image surface through a
> > clip). In every case I've tried so far, that center pixel never
> > changes.
> >
> > If you can identify what distinguishes this test from your code, and
> > then augment this test to demonstrate the bug, that would be most
> > appreciated.
> >
> > I've attached the test in two forms.
> >
> > The first (zero-alpha.c) is a self-contained program, whose first line
> > indicates how it should be compiled. It targets the image backend
> > only.
> >
> > The second (test-zero-alpha.c) is ready to be dropped into the cairo
> > test suite. Just copy it into cairo/test and edit test/Makefile.am
> > adding the name of the file (without the .c extension) to the TESTS
> > list. This will target all backends available to the test suite,
> > (which includes the xlib backend that you are using).
> >
> > Good luck. And have fun with cairo!
> >
> > -Carl
> >
> >
> >
> >
>
>
>