I am drawing a projected shadow onto a floor, by drawing the floor in a second pass, with a shadow texture, and a darken blend mode. I am clipping away the unneeded parts of the floor with glClipPlane(), so that I don't have to blend a lot of pixels that would be invisible anyways. On this card, the max clip planes if 6.

If I use 0 to 2 clipping planes, everything appears fine. If I use more than two clipping planes, the shadow z-fights with the floor. I am using a depth function of GL_LEQUAL, and the shadow looks fine if no clipping planes are used. I am sure the plane equations are correct, and there is not a clipping plane lying along the floor. It does not matter which clipping planes I use, it always z-fights if 3 or more clipping planes are enabled.

The card is a GE Force 440 MX, which is probably the problem, but I still need to try to solve this.