Question

Basically, I’m doing some sort of image processing using a screen-sized rectangle made of two triangles and a fragment shader, which is doing the whole processing stuff. The actual effect is something like an animation as it depends on a uniform variable, called current_frame.

I’m very much interested in measuring the performance in terms of “MPix/s”. What I do is something like that:

/* Setup all necessary stuff, including:                 */
/* - getting the location of the `current_frame` uniform */
/* - creating an FBO, adding a color attachment          */
/*   and setting it as the current one                   */

double current_frame = 0;
double step = 1.0f / NUMBER_OF_ITERATIONS;

tic(); /* Start counting the time */

for (i = 0; i < NUMBER_OF_ITERATIONS; i++)
{
    glUniform1f(current_frame_handle, current_frame);
    current_frame += step;
    glDrawArrays(GL_TRIANGLES, 0, NUMBER_OF_INDICES);
    glFinish();
}

double elapsed_time = tac(); /* Get elapsed time in seconds */

/* Calculate achieved pixels per second */
double pps = (OUT_WIDTH * OUT_HEIGHT * NUMBER_OF_ITERATIONS) / elapsed_time;

/* Sanity check by using reading the output into a buffer */
/* using glReadPixels and saving this buffer into a file  */

As far as theory goes, is there anything wrong with my concept?

Also, I’ve got the impression that glFinish() on mobile hardware doesn’t necessarily wait for previous render calls and may do some optimizations.

Of course, I can always force it by doing glReadPixels() after each draw, but that would be quite slow so that this wouldn’t really help.

Could you advise me as to whether my testing scenario is sensible and whether there is something more that can be done.

Answer 1

Concerning speed, using glDrawArrays() still duplicates the shared vertices.

glDrawElements() is the solution to reduce the number of vertices in
the array, so it allows transferring less data to OpenGL.

http://www.songho.ca/opengl/gl_vertexarray.html

Just throwing that in there to help speed up your results. As far as your timing concept, it looks fine to me. Are you getting results similar to what you had hoped?