constructor( renderer, parameters = {} ) {

			this.enabled = true;
			const defaultThickness = parameters.defaultThickness !== undefined ? parameters.defaultThickness : 0.003;
			const defaultColor = new THREE.Color().fromArray( parameters.defaultColor !== undefined ? parameters.defaultColor : [ 0, 0, 0 ] );
			const defaultAlpha = parameters.defaultAlpha !== undefined ? parameters.defaultAlpha : 1.0;
			const defaultKeepAlive = parameters.defaultKeepAlive !== undefined ? parameters.defaultKeepAlive : false; // object.material.uuid -> outlineMaterial or
			// object.material[ n ].uuid -> outlineMaterial
			// save at the outline material creation and release
			// if it's unused removeThresholdCount frames
			// unless keepAlive is true.

			const cache = {};
			const removeThresholdCount = 60; // outlineMaterial.uuid -> object.material or
			// outlineMaterial.uuid -> object.material[ n ]
			// save before render and release after render.

			const originalMaterials = {}; // object.uuid -> originalOnBeforeRender
			// save before render and release after render.

			const originalOnBeforeRenders = {}; //this.cache = cache;  // for debug

			const uniformsOutline = {
				outlineThickness: {
					value: defaultThickness
				},
				outlineColor: {
					value: defaultColor
				},
				outlineAlpha: {
					value: defaultAlpha
				}
			};
			const vertexShader = [ '#include <common>', '#include <uv_pars_vertex>', '#include <displacementmap_pars_vertex>', '#include <fog_pars_vertex>', '#include <morphtarget_pars_vertex>', '#include <skinning_pars_vertex>', '#include <logdepthbuf_pars_vertex>', '#include <clipping_planes_pars_vertex>', 'uniform float outlineThickness;', 'vec4 calculateOutline( vec4 pos, vec3 normal, vec4 skinned ) {', '	float thickness = outlineThickness;', '	const float ratio = 1.0;', // TODO: support outline thickness ratio for each vertex
				'	vec4 pos2 = projectionMatrix * modelViewMatrix * vec4( skinned.xyz + normal, 1.0 );', // NOTE: subtract pos2 from pos because THREE.BackSide objectNormal is negative
				'	vec4 norm = normalize( pos - pos2 );', '	return pos + norm * thickness * pos.w * ratio;', '}', 'void main() {', '	#include <uv_vertex>', '	#include <beginnormal_vertex>', '	#include <morphnormal_vertex>', '	#include <skinbase_vertex>', '	#include <skinnormal_vertex>', '	#include <begin_vertex>', '	#include <morphtarget_vertex>', '	#include <skinning_vertex>', '	#include <displacementmap_vertex>', '	#include <project_vertex>', '	vec3 outlineNormal = - objectNormal;', // the outline material is always rendered with THREE.BackSide
				'	gl_Position = calculateOutline( gl_Position, outlineNormal, vec4( transformed, 1.0 ) );', '	#include <logdepthbuf_vertex>', '	#include <clipping_planes_vertex>', '	#include <fog_vertex>', '}' ].join( '\n' );
			const fragmentShader = [ '#include <common>', '#include <fog_pars_fragment>', '#include <logdepthbuf_pars_fragment>', '#include <clipping_planes_pars_fragment>', 'uniform vec3 outlineColor;', 'uniform float outlineAlpha;', 'void main() {', '	#include <clipping_planes_fragment>', '	#include <logdepthbuf_fragment>', '	gl_FragColor = vec4( outlineColor, outlineAlpha );', '	#include <tonemapping_fragment>', '	#include <encodings_fragment>', '	#include <fog_fragment>', '	#include <premultiplied_alpha_fragment>', '}' ].join( '\n' );

			function createMaterial() {

				return new THREE.ShaderMaterial( {
					type: 'OutlineEffect',
					uniforms: THREE.UniformsUtils.merge( [ THREE.UniformsLib[ 'fog' ], THREE.UniformsLib[ 'displacementmap' ], uniformsOutline ] ),
					vertexShader: vertexShader,
					fragmentShader: fragmentShader,
					side: THREE.BackSide
				} );

			}

			function getOutlineMaterialFromCache( originalMaterial ) {

				let data = cache[ originalMaterial.uuid ];

				if ( data === undefined ) {

					data = {
						material: createMaterial(),
						used: true,
						keepAlive: defaultKeepAlive,
						count: 0
					};
					cache[ originalMaterial.uuid ] = data;

				}

				data.used = true;
				return data.material;

			}

			function getOutlineMaterial( originalMaterial ) {

				const outlineMaterial = getOutlineMaterialFromCache( originalMaterial );
				originalMaterials[ outlineMaterial.uuid ] = originalMaterial;
				updateOutlineMaterial( outlineMaterial, originalMaterial );
				return outlineMaterial;

			}

			function isCompatible( object ) {

				const geometry = object.geometry;
				let hasNormals = false;

				if ( object.geometry !== undefined ) {

					if ( geometry.isBufferGeometry ) {

						hasNormals = geometry.attributes.normal !== undefined;

					} else {

						hasNormals = true; // the renderer always produces a normal attribute for Geometry

					}

				}

				return object.isMesh === true && object.material !== undefined && hasNormals === true;

			}

			function setOutlineMaterial( object ) {

				if ( isCompatible( object ) === false ) return;

				if ( Array.isArray( object.material ) ) {

					for ( let i = 0, il = object.material.length; i < il; i ++ ) {

						object.material[ i ] = getOutlineMaterial( object.material[ i ] );

					}

				} else {

					object.material = getOutlineMaterial( object.material );

				}

				originalOnBeforeRenders[ object.uuid ] = object.onBeforeRender;
				object.onBeforeRender = onBeforeRender;

			}

			function restoreOriginalMaterial( object ) {

				if ( isCompatible( object ) === false ) return;

				if ( Array.isArray( object.material ) ) {

					for ( let i = 0, il = object.material.length; i < il; i ++ ) {

						object.material[ i ] = originalMaterials[ object.material[ i ].uuid ];

					}

				} else {

					object.material = originalMaterials[ object.material.uuid ];

				}

				object.onBeforeRender = originalOnBeforeRenders[ object.uuid ];

			}

			function onBeforeRender( renderer, scene, camera, geometry, material ) {

				const originalMaterial = originalMaterials[ material.uuid ]; // just in case

				if ( originalMaterial === undefined ) return;
				updateUniforms( material, originalMaterial );

			}

			function updateUniforms( material, originalMaterial ) {

				const outlineParameters = originalMaterial.userData.outlineParameters;
				material.uniforms.outlineAlpha.value = originalMaterial.opacity;

				if ( outlineParameters !== undefined ) {

					if ( outlineParameters.thickness !== undefined ) material.uniforms.outlineThickness.value = outlineParameters.thickness;
					if ( outlineParameters.color !== undefined ) material.uniforms.outlineColor.value.fromArray( outlineParameters.color );
					if ( outlineParameters.alpha !== undefined ) material.uniforms.outlineAlpha.value = outlineParameters.alpha;

				}

				if ( originalMaterial.displacementMap ) {

					material.uniforms.displacementMap.value = originalMaterial.displacementMap;
					material.uniforms.displacementScale.value = originalMaterial.displacementScale;
					material.uniforms.displacementBias.value = originalMaterial.displacementBias;

				}

			}

			function updateOutlineMaterial( material, originalMaterial ) {

				if ( material.name === 'invisible' ) return;
				const outlineParameters = originalMaterial.userData.outlineParameters;
				material.morphTargets = originalMaterial.morphTargets;
				material.morphNormals = originalMaterial.morphNormals;
				material.fog = originalMaterial.fog;
				material.toneMapped = originalMaterial.toneMapped;
				material.premultipliedAlpha = originalMaterial.premultipliedAlpha;
				material.displacementMap = originalMaterial.displacementMap;

				if ( outlineParameters !== undefined ) {

					if ( originalMaterial.visible === false ) {

						material.visible = false;

					} else {

						material.visible = outlineParameters.visible !== undefined ? outlineParameters.visible : true;

					}

					material.transparent = outlineParameters.alpha !== undefined && outlineParameters.alpha < 1.0 ? true : originalMaterial.transparent;
					if ( outlineParameters.keepAlive !== undefined ) cache[ originalMaterial.uuid ].keepAlive = outlineParameters.keepAlive;

				} else {

					material.transparent = originalMaterial.transparent;
					material.visible = originalMaterial.visible;

				}

				if ( originalMaterial.wireframe === true || originalMaterial.depthTest === false ) material.visible = false;

				if ( originalMaterial.clippingPlanes ) {

					material.clipping = true;
					material.clippingPlanes = originalMaterial.clippingPlanes;
					material.clipIntersection = originalMaterial.clipIntersection;
					material.clipShadows = originalMaterial.clipShadows;

				}

				material.version = originalMaterial.version; // update outline material if necessary

			}

			function cleanupCache() {

				let keys; // clear originialMaterials

				keys = Object.keys( originalMaterials );

				for ( let i = 0, il = keys.length; i < il; i ++ ) {

					originalMaterials[ keys[ i ] ] = undefined;

				} // clear originalOnBeforeRenders


				keys = Object.keys( originalOnBeforeRenders );

				for ( let i = 0, il = keys.length; i < il; i ++ ) {

					originalOnBeforeRenders[ keys[ i ] ] = undefined;

				} // remove unused outlineMaterial from cache


				keys = Object.keys( cache );

				for ( let i = 0, il = keys.length; i < il; i ++ ) {

					const key = keys[ i ];

					if ( cache[ key ].used === false ) {

						cache[ key ].count ++;

						if ( cache[ key ].keepAlive === false && cache[ key ].count > removeThresholdCount ) {

							delete cache[ key ];

						}

					} else {

						cache[ key ].used = false;
						cache[ key ].count = 0;

					}

				}

			}

			this.render = function ( scene, camera ) {

				let renderTarget;
				let forceClear = false;

				if ( arguments[ 2 ] !== undefined ) {

					console.warn( 'THREE.OutlineEffect.render(): the renderTarget argument has been removed. Use .setRenderTarget() instead.' );
					renderTarget = arguments[ 2 ];

				}

				if ( arguments[ 3 ] !== undefined ) {

					console.warn( 'THREE.OutlineEffect.render(): the forceClear argument has been removed. Use .clear() instead.' );
					forceClear = arguments[ 3 ];

				}

				if ( renderTarget !== undefined ) renderer.setRenderTarget( renderTarget );
				if ( forceClear ) renderer.clear();

				if ( this.enabled === false ) {

					renderer.render( scene, camera );
					return;

				}

				const currentAutoClear = renderer.autoClear;
				renderer.autoClear = this.autoClear;
				renderer.render( scene, camera );
				renderer.autoClear = currentAutoClear;
				this.renderOutline( scene, camera );

			};

			this.renderOutline = function ( scene, camera ) {

				const currentAutoClear = renderer.autoClear;
				const currentSceneAutoUpdate = scene.autoUpdate;
				const currentSceneBackground = scene.background;
				const currentShadowMapEnabled = renderer.shadowMap.enabled;
				scene.autoUpdate = false;
				scene.background = null;
				renderer.autoClear = false;
				renderer.shadowMap.enabled = false;
				scene.traverse( setOutlineMaterial );
				renderer.render( scene, camera );
				scene.traverse( restoreOriginalMaterial );
				cleanupCache();
				scene.autoUpdate = currentSceneAutoUpdate;
				scene.background = currentSceneBackground;
				renderer.autoClear = currentAutoClear;
				renderer.shadowMap.enabled = currentShadowMapEnabled;

			};
			/*
     * See #9918
     *
     * The following property copies and wrapper methods enable
     * OutlineEffect to be called from other *Effect, like
     *
     * effect = new StereoEffect( new OutlineEffect( renderer ) );
     *
     * function render () {
     *
    	 * 	effect.render( scene, camera );
     *
     * }
     */


			this.autoClear = renderer.autoClear;
			this.domElement = renderer.domElement;
			this.shadowMap = renderer.shadowMap;

			this.clear = function ( color, depth, stencil ) {

				renderer.clear( color, depth, stencil );

			};

			this.getPixelRatio = function () {

				return renderer.getPixelRatio();

			};

			this.setPixelRatio = function ( value ) {

				renderer.setPixelRatio( value );

			};

			this.getSize = function ( target ) {

				return renderer.getSize( target );

			};

			this.setSize = function ( width, height, updateStyle ) {

				renderer.setSize( width, height, updateStyle );

			};

			this.setViewport = function ( x, y, width, height ) {

				renderer.setViewport( x, y, width, height );

			};

			this.setScissor = function ( x, y, width, height ) {

				renderer.setScissor( x, y, width, height );

			};

			this.setScissorTest = function ( boolean ) {

				renderer.setScissorTest( boolean );

			};

			this.setRenderTarget = function ( renderTarget ) {

				renderer.setRenderTarget( renderTarget );

			};

		}
Reference: https://en.wikipedia.org/wiki/Cel_shading

API

1. Traditional

const effect = new OutlineEffect( renderer );

function render() {

	effect.render( scene, camera );

}

2. VR compatible

const effect = new OutlineEffect( renderer );
let renderingOutline = false;

scene.onAfterRender = function () {

	if ( renderingOutline ) return;

	renderingOutline = true;

	effect.renderOutline( scene, camera );

	renderingOutline = false;

};

function render() {

	renderer.render( scene, camera );

}

// How to set default outline parameters
new OutlineEffect( renderer, {
	defaultThickness: 0.01,
	defaultColor: [ 0, 0, 0 ],
	defaultAlpha: 0.8,
	defaultKeepAlive: true // keeps outline material in cache even if material is removed from scene
} );

// How to set outline parameters for each material
material.userData.outlineParameters = {
	thickness: 0.01,
	color: [ 0, 0, 0 ]
	alpha: 0.8,
	visible: true,
	keepAlive: true
};