float3 main(in float4 col : COL0,
	    in float3 pos : WPOS,
	    uniform float stepSize,
	    uniform samplerRect depthTex) : COLOR
{
   float zold = texRect(depthTex, pos.xy);

   if(pos.z < (zold + stepSize))
      discard;
   else;

   return col;
   
}