Since gl_FragCoord does not work properly on AMD hardware I need to fake it. I am having a little trouble visualizing this.

Let's say texcoord is the vec2 I want to write out for gl_FragCoord. The upper-left corner of the screen would be (0,0,z), and the bottom-right corner would be (1,1,z).

Here is my light volume vertex shader:
Code :
varying vec3 texcoord;
void main(void) {
	gl_Position = ftransform();
	gl_FrontColor = gl_Color;

Now how do I turn the screen position into the gl_FragCoord value?