uniform sampler2D uTexSceneDepth;
uniform vec3 uInverseRatioDepth;
uniform vec3 uInverseRatioMain;
uniform int uDepthMode;
uniform float uDitherCutoff;

float linear_depth( float depth, float near, float far ) {
   float z = depth * 2.0 - 1.0;
   return (2.0 * near * far) / (far + near - z * (far - near));	
}

void depth_compare_dither()
{
   if( uDepthMode == 1 )
   {
      vec2 back_coord = gl_FragCoord.xy * uInverseRatioMain.xy 
                                        * uInverseRatioDepth.xy;
      float back_depth = texture( uTexSceneDepth, back_coord ).r;
      float front_depth = gl_FragCoord.z/gl_FragCoord.w;

      back_depth = linear_depth( back_depth, 0.1, 2100.0 );
      float diff = back_depth - front_depth;

      vec2 ssuv = gl_FragCoord.xy;
      vec3 vDither = vec3( dot( vec2( 171.0, 231.0 ), ssuv) );
      float dither = fract( vDither.g / 71.0 ) - 0.5;

      if( step(0.0,diff)+dither<0.3 )
         discard;
   }

   if( uDepthMode == 2 )
   {
      vec2 ssuv = gl_FragCoord.xy;
      vec3 vDither = vec3( dot( vec2( 171.0, 231.0 ), ssuv) );
      float dither = fract( vDither.g / 71.0 );
      if( dither<uDitherCutoff ) discard;
   }
}