precision mediump float;

#endif

​

// Pixel position

varying vec2 pos;

​

// Uniforms set by filterShader

uniform sampler2D filter_background; // contains the image being filtered

uniform vec2 filter_res; // contains the image resolution in pixels

​

void main() {

  // Read colour from image

  vec4 col = texture2D(filter_background, pos);

  // Average the rgb channels

  float avg = (col.r + col.g + col.b)/3.;

  // Output the average for rgb, with original alpha

  gl_FragColor = vec4(avg, avg, avg, col.a);

}

​

​

​