i need to measure the time the GPU needs to calculate an image on the shader.
For this test I upload a texture compile, load ... the shader and display the calculated image. My end time stamp is the return of glFinish but what can i use for my start timestamp? I want to make my measure so independent as possible from the used cpu.
Is there any OpenGL specific command to start the pipeline?
Or is there any function i anyway use in a standard implementation?

I hope to hear from you