[maemo-developers] Problem with Framebuffer Objects and glReadPixels in N900

[From: Torste Aikio]

I'm currently developing a software that involves drawing 3d scene
with OpenGL (ES) 2.0. I want users to be able to pick objects from the
scene by clicking (or touching) on the screen. My current approach is
to render the scene to an off-screen buffer and deduce the clicked
object by reading color values from that buffer. While this worked on
PC with regular OpenGL, it fails on N900. Most times glReadPixels
segfaults when reading a single pixel, and when it doesn't, it returns
invalid values. And if I read the whole buffer, I get strange, striped
results, where each row of pixels is accompanied with three empty
rows.

If anyone has any idea why the above is happening, or has suggestions
how to implement better picking, then any help would be appreciated. I
have attached an example code to this email which demonstrates my
problem. The example code uses Qt and QGLFramebufferObject, but I had
similar results with 'handmade' FBO too. The code is also available
from <http://zokier.net/stuff/fail.cpp> Also image of erroneous FBO is
available at <http://zokier.net/stuff/fbo.png>, it should have a
pinkish square in the middle, but it doesn't.

/Torste Aikio

ps. I am not very experienced with mailing lists, I hope this is the
correct place to discuss this.
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