three.js/examples/js/shaders/PixelShader.js

/**
 * Pixelation shader
 */

THREE.PixelShader = {

	uniforms: {

		'tDiffuse': { value: null },
		'resolution': { value: null },
		'pixelSize': { value: 1. },

	},

	vertexShader: [

		'varying highp vec2 vUv;',

		'void main() {',

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

		'}'

	].join( '\n' ),

	fragmentShader: [

		'uniform sampler2D tDiffuse;',
		'uniform float pixelSize;',
		'uniform vec2 resolution;',

		'varying highp vec2 vUv;',

		'void main(){',

		'vec2 dxy = pixelSize / resolution;',
		'vec2 coord = dxy * floor( vUv / dxy );',
		'gl_FragColor = texture2D(tDiffuse, coord);',

		'}'

	].join( '\n' )
};