I've got a pretty old ATI HD 3400 video card , which have no support of OpenCL , so i'm wondering if i can actually play around with OpenGL libraries provided by ATI catalyst driver ?
If my algorithm is running in glutDisplayFunc ( displayFunc ) , that is in displayFun () , is it actually costing CPU power or GPU power ?
GLUT is just a library which manages platform-specific window and GL context creation. The function you pass to glutDisplayFunc is just called by GLUT at the appropriate time and context for the platform you're running on; it is not executed on the GPU.
It is not possible to have code that you've compiled in the normal fashion as part of a larger program run on the GPU.
However, the individual graphics operations run inside of your display func do of course perform the rendering on the GPU; the CPU is still computing which graphics operation to execute, but not actually rendering the results. Each gl function is a normal CPU function, but what it does is send a command through your system bus to your graphics card, which then does the actual rendering.
Furthermore, these operations are asynchronous; the gl functions don't wait for your GPU to finish the operation before letting your program continue. This is useful because your CPU and GPU can both be working simultaneously — the GPU draws graphics while the CPU figures out what graphics to draw. On the other hand, if you do need communication in the other direction — such as glReadPixels — then the CPU has to wait for the GPU to catch up. This is also the difference between glFlush and glFinish.
