( function () {

	/**
 * 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)
 */

	const FreiChenShader = {
		uniforms: {
			'tDiffuse': {
				value: null
			},
			'aspect': {
				value: new THREE.Vector2( 512, 512 )
			}
		},
		vertexShader:
  /* glsl */
  `

		varying vec2 vUv;

		void main() {

			vUv = uv;
			gl_Position = projectionMatrix * modelViewMatrix * vec4( position, 1.0 );

		}`,
		fragmentShader:
  /* glsl */
  `

		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);
		}`
	};

	THREE.FreiChenShader = FreiChenShader;

} )();