emulate NV_depth_clamp on ATI

OpenGL OpenGL: Advanced Coding
1

I’m looking for a way to clamp depth values to near plane without clipping. I tried small fragment shader:


void main()
{
	gl_FragDepth = max( gl_FragCoord.z, 0. );
}

but polygons are still clipped.

2

the clipping runs before the fragment shader. So the fragments are already clipped away.

3

I’ve finally found solution. This is for ortho projection, for perspective it will need additional check on Zeye - to clip everything behind the eye.


// Vertex shader:
varying float Zw;
void main()
{
	vec4 clip = ftransform();
	float Zd = clip.z/clip.w;
	clip.z = max( Zd, -1. )*clip.w;
	Zw = Zd*0.5 + 0.5;
	gl_Position = clip;
}

// Fragment shader:
varying float Zw;
void main()
{
	gl_FragDepth = max( Zw, 0. );
}