three.js/examples/js/shaders/VignetteShader.js

( function () {

	/**
 * Vignette shader
 * based on PaintEffect postprocess from ro.me
 * http://code.google.com/p/3-dreams-of-black/source/browse/deploy/js/effects/PaintEffect.js
 */

	const VignetteShader = {
		uniforms: {
			'tDiffuse': {
				value: null
			},
			'offset': {
				value: 1.0
			},
			'darkness': {
				value: 1.0
			}
		},
		vertexShader: /* glsl */`

		varying vec2 vUv;

		void main() {

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

		}`,
		fragmentShader: /* glsl */`

		uniform float offset;
		uniform float darkness;

		uniform sampler2D tDiffuse;

		varying vec2 vUv;

		void main() {

			// Eskil's vignette

			vec4 texel = texture2D( tDiffuse, vUv );
			vec2 uv = ( vUv - vec2( 0.5 ) ) * vec2( offset );
			gl_FragColor = vec4( mix( texel.rgb, vec3( 1.0 - darkness ), dot( uv, uv ) ), texel.a );

		}`
	};

	THREE.VignetteShader = VignetteShader;

} )();