Re: opengl and images

[Michael Goffioul]

On Mon, Oct 5, 2009 at 9:12 AM, Søren Hauberg <address@hidden> wrote:
> man, 05 10 2009 kl. 09:55 +0200, skrev Shai Ayal:
>> Maybe it's a driver issue? -- what graphics card do you use, and what
>> is the output of
>> glxinfo|grep render
>
> I'm not sure if you were asking me or Michael, but I have an Intel card,
> and I get the following form glxinfo
>
>        No kernel support for execution fencing, disabling texture
>        tiling
>        direct rendering: Yes
>        OpenGL renderer string: Mesa DRI Mobile Intel® GM45 Express
>        Chipset GEM 20090712 2009Q2 RC3 x86/MMX/SSE2
>
> If you look at the image I sent, you will see that the curve appears
> under the image, so both objects are rendered. My _guess_ is that the
> two objects have the same depth, and their order becomes somewhat
> arbitrary.

Drawing coplanar primitives in OpenGL is not reliable and you can't
rely on the draw order to put one on top of the other (especially as
computation is done in float, not double). When dealing with polygons,
you can use glPolygonOffset, but I'm not sure glDrawPixels is sensitive
to that setting.

Michael.