glReadPixels problem

OpenGL OpenGL: Basic Coding
1

hello,

i have a little problem with glReadPixels. Maybe someone can help me.

i have an int variable could color.
then i render a pixel by setting the color with
glColor3ub(color,color,color).
when i want to read out the color with
glReadPixels(x,y,1,1,GL_RGB,GL_UNSIGNED_BYTE,&select)
(select is defined as GLubyte) other values for red,green and blue then rendered are appearing.

thanks,
Poons

2

Hi,

The colors that get to the framebuffer are not necessarily the same colors you set with glColor. If you’re for example running in 16-bit mode, the color values get rounded to some nearby value. If you want to guarantee similar results, like for picking, use stencil buffer instead of colors.

-Ilkka

3

And note that glDrawBuffer and glReadBuffer can be set to different buffers.
Default for double buffering is GL_BACK for both, so if you readback after SwapBuffers, you don’t read the image you see but some (possibly undefined) stuff from the backbuffer.

4

Thanks a lot.

i´m now using the stencil buffer it works just fine.