text: /**
   * Edge Detection Shader using Frei-Chen filter
   * Based on http://rastergrid.com/blog/2011/01/frei-chen-edge-detector
   *
   * aspect: vec2 of (1/width, 1/height)
   */
  
  THREE.FreiChenShader = {
  
  	uniforms: {
  
  		'tDiffuse': { value: null },
  		'aspect': { value: new THREE.Vector2( 512, 512 ) }
  	},
  
  	vertexShader: [
  
  		'varying vec2 vUv;',
  
  		'void main() {',
  
  		'	vUv = uv;',
  		'	gl_Position = projectionMatrix * modelViewMatrix * vec4( position, 1.0 );',
  
  		'}'
  
  	].join( '\n' ),
  
  	fragmentShader: [
  
  		'uniform sampler2D tDiffuse;',
  		'varying vec2 vUv;',
  
  		'uniform vec2 aspect;',
  
  		'vec2 texel = vec2(1.0 / aspect.x, 1.0 / aspect.y);',
  
  
  		'mat3 G[9];',
  
  		// hard coded matrix values!!!! as suggested in https://github.com/neilmendoza/ofxPostProcessing/blob/master/src/EdgePass.cpp#L45
  
  		'const mat3 g0 = mat3( 0.3535533845424652, 0, -0.3535533845424652, 0.5, 0, -0.5, 0.3535533845424652, 0, -0.3535533845424652 );',
  		'const mat3 g1 = mat3( 0.3535533845424652, 0.5, 0.3535533845424652, 0, 0, 0, -0.3535533845424652, -0.5, -0.3535533845424652 );',
  		'const mat3 g2 = mat3( 0, 0.3535533845424652, -0.5, -0.3535533845424652, 0, 0.3535533845424652, 0.5, -0.3535533845424652, 0 );',
  		'const mat3 g3 = mat3( 0.5, -0.3535533845424652, 0, -0.3535533845424652, 0, 0.3535533845424652, 0, 0.3535533845424652, -0.5 );',
  		'const mat3 g4 = mat3( 0, -0.5, 0, 0.5, 0, 0.5, 0, -0.5, 0 );',
  		'const mat3 g5 = mat3( -0.5, 0, 0.5, 0, 0, 0, 0.5, 0, -0.5 );',
  		'const mat3 g6 = mat3( 0.1666666716337204, -0.3333333432674408, 0.1666666716337204, -0.3333333432674408, 0.6666666865348816, -0.3333333432674408, 0.1666666716337204, -0.3333333432674408, 0.1666666716337204 );',
  		'const mat3 g7 = mat3( -0.3333333432674408, 0.1666666716337204, -0.3333333432674408, 0.1666666716337204, 0.6666666865348816, 0.1666666716337204, -0.3333333432674408, 0.1666666716337204, -0.3333333432674408 );',
  		'const mat3 g8 = mat3( 0.3333333432674408, 0.3333333432674408, 0.3333333432674408, 0.3333333432674408, 0.3333333432674408, 0.3333333432674408, 0.3333333432674408, 0.3333333432674408, 0.3333333432674408 );',
  
  		'void main(void)',
  		'{',
  
  		'	G[0] = g0,',
  		'	G[1] = g1,',
  		'	G[2] = g2,',
  		'	G[3] = g3,',
  		'	G[4] = g4,',
  		'	G[5] = g5,',
  		'	G[6] = g6,',
  		'	G[7] = g7,',
  		'	G[8] = g8;',
  
  		'	mat3 I;',
  		'	float cnv[9];',
  		'	vec3 sample;',
  
  		/* fetch the 3x3 neighbourhood and use the RGB vector's length as intensity value */
  		'	for (float i=0.0; i<3.0; i++) {',
  		'		for (float j=0.0; j<3.0; j++) {',
  		'			sample = texture2D(tDiffuse, vUv + texel * vec2(i-1.0,j-1.0) ).rgb;',
  		'			I[int(i)][int(j)] = length(sample);',
  		'		}',
  		'	}',
  
  		/* calculate the convolution values for all the masks */
  		'	for (int i=0; i<9; i++) {',
  		'		float dp3 = dot(G[i][0], I[0]) + dot(G[i][1], I[1]) + dot(G[i][2], I[2]);',
  		'		cnv[i] = dp3 * dp3;',
  		'	}',
  
  		'	float M = (cnv[0] + cnv[1]) + (cnv[2] + cnv[3]);',
  		'	float S = (cnv[4] + cnv[5]) + (cnv[6] + cnv[7]) + (cnv[8] + M);',
  
  		'	gl_FragColor = vec4(vec3(sqrt(M/S)), 1.0);',
  		'}'
  
  	].join( '\n' )
  };
  