OpenGL: Drawing a 1920 * 1080 quad = -5000 FPS?

Question

Am I doing something wrong here?

I am drawing a quad over the whole screen with glRectf( -1, -1, 1, 1 )

Per rect the FPS of my program go down by roughly 5000.

• What is happening?

This is my OpenGL initialization:

  glMatrixMode( GL_PROJECTION );
  glLoadIdentity();
  glViewport( 0, 0, width, height );
  glOrtho( 1, -1, 1, -1, 1, -1 );
  glMatrixMode( GL_MODELVIEW );
  glDisable( GL_DEPTH_TEST );

Answer 1

If glRectf(-1, -1, 1, 1) is the only thing you are doing, then you are experiencing a bad case of premature framerate freakout. Shawn Hargreaves covers this very nicely on his blog.

The fact is that it takes quite some time to spin up the graphics pipeline and draw a frame. As compared to not drawing anything (as I suspect you are doing), the framerate is going to drop by a huge amount, since doing anything is much more expensive than doing nothing.

To test my hypothesis, try:

for (int i = 0; i < 1000; ++i)
    glRectf(-1, -1, 1, 1);

Then see if your framerate changes at all. It probably won't.

Answer 2

1920x1080 is a lot of pixels. 7-11 fps seems pretty fast for the code snippet suggested by Travis G:

for (int i = 0; i < 1000; ++i)
    glRectf(-1, -1, 1, 1);

Let's do the math:

1920 * 1080 * 1000 * 11 pixels / 1 second = 22.8 Gpix/s

Wikipedia has tables of headline figures for ATI cards here: http://en.wikipedia.org/wiki/Comparison_of_AMD_graphics_processing_units and for nVidia cards here: http://en.wikipedia.org/wiki/Comparison_of_Nvidia_graphics_processing_units

I'm going to guess you have a Radeon HD 5850 just because they're popular and the numbers fit (the 5850 apparently has 23 Gpix/s fillrate)