Exact Halving of FPS when clipping an Alpha Blended Quad

I tested my engine using OGL on a P4 with a Geforce 4 MX. The FPS was constant 100 in 800x600, but on certain poins it dropped mysteriously to 50. When I turned VSync out, the problem remained, sometime the FPS was exactly halved for no good reason.
After performing a lot of test, I found out that the FPS drops enormously when OGL clippes an alpha blended polygon (4 vertices) against the FOV.
This is not because of low bandwith or to many triangles/pixels. I just rendered 1 alpha blended quad with no textures! When it clipped against the FOV, the FPS dropped from 100 to 20-40. This also happens when rendering a low-poly (100) scene in wireframe.
In window mode, this only happens when the viewport is large, like 800x600.
In fullscreen mode it happens at any resolution.

What’s going on? I use standard OGL 1.1 calls. The single alpha blended polygon was even rendered with the simplest OGL 1.0 calls, glPolygon and glVertex!
Only low poly primitives cause this behavior, large polygon batches (curved surfaces) don’t cause the FPS to be halved.

That sounds very strange.

  1. Which driver-version do you use?
  2. Is your viewport setup correct (maybe the viewport is bigger than the actual window?)
  3. Does it still happen, if you only have one single poly?
  4. Can you reproduce the bug in another very simple Glut-app?

If the answer to 3-4 is “yes”, than send that glut-app to nvidia, together with a very detailed description of your pc hardware and the bug.

Also you could give that app to us, and we try, if it happens on other people´s pc´s, too.

However, i never discovered anything like this, so i would think it has something to do with your app. But this error is very strange. Maybe your gfx card is damaged, so that clipping a blended poly is slower or not working correctly.