Is there a way to tell whether an OpenGL operation is finished?

Question

OpenGL functions are defined as though they operate synchronously. But rendering functions (and others) will often be executed asynchronously by the GPU. OpenGL will effectively hide this: if you do something that requires the results of an operation (such as reading from a framebuffer that has been rendered to), OpenGL will stop CPU operations until the GPU has reached that point.

That is functional, but hardly ideal for performance, since the CPU basically blocks until the GPU is done. Is there a way to determine if a particular operation has completed, so that you know that a dependent operation will be executed without so much CPU blocking?

Answer 1

Yes, there is. Assuming you have access to OpenGL 3.2+ or the ARB_sync extension.

If you want to know when a particular command has finished, you can insert a "fence" sync object into the command stream immediately after issuing that command. This is done with the glFenceSync function.

You must store the pointer returned by this function. With that pointer in hand, you can query whether the fence has completed by checking whether the fence is signaled with the glGetSync function. Like this:

GLint isSignaled = 0;
glGetSync(syncObj, GL_SYNC_STATUS, 1, NULL, &isSignaled);

if(isSignaled == GL_SIGNALED)
{
  //Prior commands have completed.
}
else
{
  //Not done yet.
}

If you run out of other stuff to do, you don't need to wait for the sync object to finish; you can just do whatever OpenGL process you wanted to do. That will induce the CPU block for you.

Once you're finished with the sync object, you must delete it. Use the glDeleteSync function to do so.