/**
 * Sobel Edge Detection (see https://youtu.be/uihBwtPIBxM)
 *
 * As mentioned in the video the Sobel operator expects a grayscale image as input.
 *
 */

THREE.SobelOperatorShader = {

	uniforms: {

		'tDiffuse': { value: null },
		'resolution': { value: new THREE.Vector2() }

	},

	vertexShader: [

		'varying vec2 vUv;',

		'void main() {',

		'	vUv = uv;',

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

		'}'

	].join( '\n' ),

	fragmentShader: [

		'uniform sampler2D tDiffuse;',
		'uniform vec2 resolution;',
		'varying vec2 vUv;',

		'void main() {',

		'	vec2 texel = vec2( 1.0 / resolution.x, 1.0 / resolution.y );',

		// kernel definition (in glsl matrices are filled in column-major order)

		'	const mat3 Gx = mat3( -1, -2, -1, 0, 0, 0, 1, 2, 1 );', // x direction kernel
		'	const mat3 Gy = mat3( -1, 0, 1, -2, 0, 2, -1, 0, 1 );', // y direction kernel

		// fetch the 3x3 neighbourhood of a fragment

		// first column

		'	float tx0y0 = texture2D( tDiffuse, vUv + texel * vec2( -1, -1 ) ).r;',
		'	float tx0y1 = texture2D( tDiffuse, vUv + texel * vec2( -1,  0 ) ).r;',
		'	float tx0y2 = texture2D( tDiffuse, vUv + texel * vec2( -1,  1 ) ).r;',

		// second column

		'	float tx1y0 = texture2D( tDiffuse, vUv + texel * vec2(  0, -1 ) ).r;',
		'	float tx1y1 = texture2D( tDiffuse, vUv + texel * vec2(  0,  0 ) ).r;',
		'	float tx1y2 = texture2D( tDiffuse, vUv + texel * vec2(  0,  1 ) ).r;',

		// third column

		'	float tx2y0 = texture2D( tDiffuse, vUv + texel * vec2(  1, -1 ) ).r;',
		'	float tx2y1 = texture2D( tDiffuse, vUv + texel * vec2(  1,  0 ) ).r;',
		'	float tx2y2 = texture2D( tDiffuse, vUv + texel * vec2(  1,  1 ) ).r;',

		// gradient value in x direction

		'	float valueGx = Gx[0][0] * tx0y0 + Gx[1][0] * tx1y0 + Gx[2][0] * tx2y0 + ',
		'		Gx[0][1] * tx0y1 + Gx[1][1] * tx1y1 + Gx[2][1] * tx2y1 + ',
		'		Gx[0][2] * tx0y2 + Gx[1][2] * tx1y2 + Gx[2][2] * tx2y2; ',

		// gradient value in y direction

		'	float valueGy = Gy[0][0] * tx0y0 + Gy[1][0] * tx1y0 + Gy[2][0] * tx2y0 + ',
		'		Gy[0][1] * tx0y1 + Gy[1][1] * tx1y1 + Gy[2][1] * tx2y1 + ',
		'		Gy[0][2] * tx0y2 + Gy[1][2] * tx1y2 + Gy[2][2] * tx2y2; ',

		// magnitute of the total gradient

		'	float G = sqrt( ( valueGx * valueGx ) + ( valueGy * valueGy ) );',

		'	gl_FragColor = vec4( vec3( G ), 1 );',

		'}'

	].join( '\n' )

};
