node_modules/three/examples/js/shaders/ParallaxShader.js

// Parallax Occlusion shaders from
//    http://sunandblackcat.com/tipFullView.php?topicid=28
// No tangent-space transforms logic based on
//   http://mmikkelsen3d.blogspot.sk/2012/02/parallaxpoc-mapping-and-no-tangent.html

THREE.ParallaxShader = {
	// Ordered from fastest to best quality.
	modes: {
		none:  'NO_PARALLAX',
		basic: 'USE_BASIC_PARALLAX',
		steep: 'USE_STEEP_PARALLAX',
		occlusion: 'USE_OCLUSION_PARALLAX', // a.k.a. POM
		relief: 'USE_RELIEF_PARALLAX'
	},

	uniforms: {
		"bumpMap": { value: null },
		"map": { value: null },
		"parallaxScale": { value: null },
		"parallaxMinLayers": { value: null },
		"parallaxMaxLayers": { value: null }
	},

	vertexShader: [
		"varying vec2 vUv;",
		"varying vec3 vViewPosition;",
		"varying vec3 vNormal;",

		"void main() {",

			"vUv = uv;",
			"vec4 mvPosition = modelViewMatrix * vec4( position, 1.0 );",
			"vViewPosition = -mvPosition.xyz;",
			"vNormal = normalize( normalMatrix * normal );",
			"gl_Position = projectionMatrix * mvPosition;",

		"}"

  ].join( "\n" ),

	fragmentShader: [
		"uniform sampler2D bumpMap;",
		"uniform sampler2D map;",

		"uniform float parallaxScale;",
		"uniform float parallaxMinLayers;",
		"uniform float parallaxMaxLayers;",

		"varying vec2 vUv;",
		"varying vec3 vViewPosition;",
		"varying vec3 vNormal;",

		"#ifdef USE_BASIC_PARALLAX",

			"vec2 parallaxMap( in vec3 V ) {",

				"float initialHeight = texture2D( bumpMap, vUv ).r;",

				// No Offset Limitting: messy, floating output at grazing angles.
				//"vec2 texCoordOffset = parallaxScale * V.xy / V.z * initialHeight;",

				// Offset Limiting
				"vec2 texCoordOffset = parallaxScale * V.xy * initialHeight;",
				"return vUv - texCoordOffset;",

			"}",

		"#else",

			"vec2 parallaxMap( in vec3 V ) {",

				// Determine number of layers from angle between V and N
				"float numLayers = mix( parallaxMaxLayers, parallaxMinLayers, abs( dot( vec3( 0.0, 0.0, 1.0 ), V ) ) );",

				"float layerHeight = 1.0 / numLayers;",
				"float currentLayerHeight = 0.0;",
				// Shift of texture coordinates for each iteration
				"vec2 dtex = parallaxScale * V.xy / V.z / numLayers;",

				"vec2 currentTextureCoords = vUv;",

				"float heightFromTexture = texture2D( bumpMap, currentTextureCoords ).r;",

				// while ( heightFromTexture > currentLayerHeight )
				// Infinite loops are not well supported. Do a "large" finite
				// loop, but not too large, as it slows down some compilers.
				"for ( int i = 0; i < 30; i += 1 ) {",
					"if ( heightFromTexture <= currentLayerHeight ) {",
						"break;",
					"}",
					"currentLayerHeight += layerHeight;",
					// Shift texture coordinates along vector V
					"currentTextureCoords -= dtex;",
					"heightFromTexture = texture2D( bumpMap, currentTextureCoords ).r;",
				"}",

				"#ifdef USE_STEEP_PARALLAX",

					"return currentTextureCoords;",

				"#elif defined( USE_RELIEF_PARALLAX )",

					"vec2 deltaTexCoord = dtex / 2.0;",
					"float deltaHeight = layerHeight / 2.0;",

					// Return to the mid point of previous layer
					"currentTextureCoords += deltaTexCoord;",
					"currentLayerHeight -= deltaHeight;",

					// Binary search to increase precision of Steep Parallax Mapping
					"const int numSearches = 5;",
					"for ( int i = 0; i < numSearches; i += 1 ) {",

						"deltaTexCoord /= 2.0;",
						"deltaHeight /= 2.0;",
						"heightFromTexture = texture2D( bumpMap, currentTextureCoords ).r;",
						// Shift along or against vector V
						"if( heightFromTexture > currentLayerHeight ) {", // Below the surface

							"currentTextureCoords -= deltaTexCoord;",
							"currentLayerHeight += deltaHeight;",

						"} else {", // above the surface

							"currentTextureCoords += deltaTexCoord;",
							"currentLayerHeight -= deltaHeight;",

						"}",

					"}",
					"return currentTextureCoords;",

				"#elif defined( USE_OCLUSION_PARALLAX )",

					"vec2 prevTCoords = currentTextureCoords + dtex;",

					// Heights for linear interpolation
					"float nextH = heightFromTexture - currentLayerHeight;",
					"float prevH = texture2D( bumpMap, prevTCoords ).r - currentLayerHeight + layerHeight;",

					// Proportions for linear interpolation
					"float weight = nextH / ( nextH - prevH );",

					// Interpolation of texture coordinates
					"return prevTCoords * weight + currentTextureCoords * ( 1.0 - weight );",

				"#else", // NO_PARALLAX

					"return vUv;",

				"#endif",

			"}",
		"#endif",

		"vec2 perturbUv( vec3 surfPosition, vec3 surfNormal, vec3 viewPosition ) {",

 			"vec2 texDx = dFdx( vUv );",
			"vec2 texDy = dFdy( vUv );",

			"vec3 vSigmaX = dFdx( surfPosition );",
			"vec3 vSigmaY = dFdy( surfPosition );",
			"vec3 vR1 = cross( vSigmaY, surfNormal );",
			"vec3 vR2 = cross( surfNormal, vSigmaX );",
			"float fDet = dot( vSigmaX, vR1 );",

			"vec2 vProjVscr = ( 1.0 / fDet ) * vec2( dot( vR1, viewPosition ), dot( vR2, viewPosition ) );",
			"vec3 vProjVtex;",
			"vProjVtex.xy = texDx * vProjVscr.x + texDy * vProjVscr.y;",
			"vProjVtex.z = dot( surfNormal, viewPosition );",

			"return parallaxMap( vProjVtex );",
		"}",

		"void main() {",

			"vec2 mapUv = perturbUv( -vViewPosition, normalize( vNormal ), normalize( vViewPosition ) );",
			"gl_FragColor = texture2D( map, mapUv );",

		"}"

  ].join( "\n" )

};