three.js/examples/js/shaders/LuminosityShader.js

( function () {

	/**
 * Luminosity
 * http://en.wikipedia.org/wiki/Luminosity
 */
	var LuminosityShader = {
		uniforms: {
			'tDiffuse': {
				value: null
			}
		},
		vertexShader: [ 'varying vec2 vUv;', 'void main() {', '	vUv = uv;', '	gl_Position = projectionMatrix * modelViewMatrix * vec4( position, 1.0 );', '}' ].join( '\n' ),
		fragmentShader: [ '#include <common>', 'uniform sampler2D tDiffuse;', 'varying vec2 vUv;', 'void main() {', '	vec4 texel = texture2D( tDiffuse, vUv );', '	float l = linearToRelativeLuminance( texel.rgb );', '	gl_FragColor = vec4( l, l, l, texel.w );', '}' ].join( '\n' )
	};

	THREE.LuminosityShader = LuminosityShader;

} )();