uniform sampler2D ParticlesShot;
uniform sampler2D FinalRender;
uniform vec4      parameters;
 
float getGrey(vec4 color)
{
  return dot(color, vec4(0.30, 0.59, 0.11, 0.0));
}

void main(void)
{
  float yComponents[4];
  vec3  normal;
 
  vec2  offsets[4],
  sampler;

  offsets[0] = vec2(-parameters.x, 0.0);
  offsets[1] = vec2( parameters.x, 0.0);
  offsets[2] = vec2(         0.0,-parameters.y);
  offsets[3] = vec2(         0.0, parameters.y);
 
  for(int i = 0; i < 4; i++)
    yComponents[i] = getGrey(texture2D(ParticlesShot, gl_TexCoord[0].xy + offsets[i]));

  normal.x     = yComponents[0] - yComponents[1];
  normal.z     = yComponents[2] - yComponents[3];
  normal.y     = 0.05;
 
  normal       = normalize(normal);
        
  sampler      = vec2(normal.x*parameters.z +  gl_TexCoord[0].x,
                        normal.z*parameters.w +  gl_TexCoord[0].y);

  gl_FragColor = texture2D(FinalRender, sampler);
}