precision mediump float;
varying vec2 v_texcoord;
uniform sampler2D tex;

void main() {
  vec4 this_colour = texture2D( tex, v_texcoord ); 
  float new_colour = (this_colour.r+this_colour.g+this_colour.b)/3.0;
  gl_FragColor = vec4(new_colour,new_colour,new_colour,1.0);
}