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

/**
 * 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" )

};

export { LuminosityShader };