three.js/examples/js/shaders/UnpackDepthRGBAShader.js

/**
 * @author alteredq / http://alteredqualia.com/
 *
 * Unpack RGBA depth shader
 * - show RGBA encoded depth as monochrome color
 */

THREE.UnpackDepthRGBAShader = {

	uniforms: {

		"tDiffuse": { type: "t", value: null },
		"opacity":  { type: "f", value: 1.0 }

	},

	vertexShader: [

		"varying vec2 vUv;",

		"void main() {",

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

		"}"

	].join("\n"),

	fragmentShader: [

		"uniform float opacity;",

		"uniform sampler2D tDiffuse;",

		"varying vec2 vUv;",

		// RGBA depth

		"float unpackDepth( const in vec4 rgba_depth ) {",

			"const vec4 bit_shift = vec4( 1.0 / ( 256.0 * 256.0 * 256.0 ), 1.0 / ( 256.0 * 256.0 ), 1.0 / 256.0, 1.0 );",
			"float depth = dot( rgba_depth, bit_shift );",
			"return depth;",

		"}",

		"void main() {",

			"float depth = 1.0 - unpackDepth( texture2D( tDiffuse, vUv ) );",
			"gl_FragColor = opacity * vec4( vec3( depth ), 1.0 );",

		"}"

	].join("\n")

};