toJSON( meta ) {

		// meta is a string when called from JSON.stringify
		const isRootObject = ( meta === undefined || typeof meta === 'string' );

		const output = {};

		// meta is a hash used to collect geometries, materials.
		// not providing it implies that this is the root object
		// being serialized.
		if ( isRootObject ) {

			// initialize meta obj
			meta = {
				geometries: {},
				materials: {},
				textures: {},
				images: {},
				shapes: {},
				skeletons: {},
				animations: {},
				nodes: {}
			};

			output.metadata = {
				version: 4.7,
				type: 'Object',
				generator: 'Object3D.toJSON'
			};

		}

		// standard Object3D serialization

		const object = {};

		object.uuid = this.uuid;
		object.type = this.type;

		if ( this.name !== '' ) object.name = this.name;
		if ( this.castShadow === true ) object.castShadow = true;
		if ( this.receiveShadow === true ) object.receiveShadow = true;
		if ( this.visible === false ) object.visible = false;
		if ( this.frustumCulled === false ) object.frustumCulled = false;
		if ( this.renderOrder !== 0 ) object.renderOrder = this.renderOrder;
		if ( Object.keys( this.userData ).length > 0 ) object.userData = this.userData;

		object.layers = this.layers.mask;
		object.matrix = this.matrix.toArray();
		object.up = this.up.toArray();

		if ( this.matrixAutoUpdate === false ) object.matrixAutoUpdate = false;

		// object specific properties

		if ( this.isInstancedMesh ) {

			object.type = 'InstancedMesh';
			object.count = this.count;
			object.instanceMatrix = this.instanceMatrix.toJSON();
			if ( this.instanceColor !== null ) object.instanceColor = this.instanceColor.toJSON();

		}

		if ( this.isBatchedMesh ) {

			object.type = 'BatchedMesh';
			object.perObjectFrustumCulled = this.perObjectFrustumCulled;
			object.sortObjects = this.sortObjects;

			object.drawRanges = this._drawRanges;
			object.reservedRanges = this._reservedRanges;

			object.geometryInfo = this._geometryInfo.map( info => ( {
				...info,
				boundingBox: info.boundingBox ? info.boundingBox.toJSON() : undefined,
				boundingSphere: info.boundingSphere ? info.boundingSphere.toJSON() : undefined
			} ) );
			object.instanceInfo = this._instanceInfo.map( info => ( { ...info } ) );

			object.availableInstanceIds = this._availableInstanceIds.slice();
			object.availableGeometryIds = this._availableGeometryIds.slice();

			object.nextIndexStart = this._nextIndexStart;
			object.nextVertexStart = this._nextVertexStart;
			object.geometryCount = this._geometryCount;

			object.maxInstanceCount = this._maxInstanceCount;
			object.maxVertexCount = this._maxVertexCount;
			object.maxIndexCount = this._maxIndexCount;

			object.geometryInitialized = this._geometryInitialized;

			object.matricesTexture = this._matricesTexture.toJSON( meta );

			object.indirectTexture = this._indirectTexture.toJSON( meta );

			if ( this._colorsTexture !== null ) {

				object.colorsTexture = this._colorsTexture.toJSON( meta );

			}

			if ( this.boundingSphere !== null ) {

				object.boundingSphere = this.boundingSphere.toJSON();

			}

			if ( this.boundingBox !== null ) {

				object.boundingBox = this.boundingBox.toJSON();

			}

		}

		//

		function serialize( library, element ) {

			if ( library[ element.uuid ] === undefined ) {

				library[ element.uuid ] = element.toJSON( meta );

			}

			return element.uuid;

		}

		if ( this.isScene ) {

			if ( this.background ) {

				if ( this.background.isColor ) {

					object.background = this.background.toJSON();

				} else if ( this.background.isTexture ) {

					object.background = this.background.toJSON( meta ).uuid;

				}

			}

			if ( this.environment && this.environment.isTexture && this.environment.isRenderTargetTexture !== true ) {

				object.environment = this.environment.toJSON( meta ).uuid;

			}

		} else if ( this.isMesh || this.isLine || this.isPoints ) {

			object.geometry = serialize( meta.geometries, this.geometry );

			const parameters = this.geometry.parameters;

			if ( parameters !== undefined && parameters.shapes !== undefined ) {

				const shapes = parameters.shapes;

				if ( Array.isArray( shapes ) ) {

					for ( let i = 0, l = shapes.length; i < l; i ++ ) {

						const shape = shapes[ i ];

						serialize( meta.shapes, shape );

					}

				} else {

					serialize( meta.shapes, shapes );

				}

			}

		}

		if ( this.isSkinnedMesh ) {

			object.bindMode = this.bindMode;
			object.bindMatrix = this.bindMatrix.toArray();

			if ( this.skeleton !== undefined ) {

				serialize( meta.skeletons, this.skeleton );

				object.skeleton = this.skeleton.uuid;

			}

		}

		if ( this.material !== undefined ) {

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

				const uuids = [];

				for ( let i = 0, l = this.material.length; i < l; i ++ ) {

					uuids.push( serialize( meta.materials, this.material[ i ] ) );

				}

				object.material = uuids;

			} else {

				object.material = serialize( meta.materials, this.material );

			}

		}

		//

		if ( this.children.length > 0 ) {

			object.children = [];

			for ( let i = 0; i < this.children.length; i ++ ) {

				object.children.push( this.children[ i ].toJSON( meta ).object );

			}

		}

		//

		if ( this.animations.length > 0 ) {

			object.animations = [];

			for ( let i = 0; i < this.animations.length; i ++ ) {

				const animation = this.animations[ i ];

				object.animations.push( serialize( meta.animations, animation ) );

			}

		}

		if ( isRootObject ) {

			const geometries = extractFromCache( meta.geometries );
			const materials = extractFromCache( meta.materials );
			const textures = extractFromCache( meta.textures );
			const images = extractFromCache( meta.images );
			const shapes = extractFromCache( meta.shapes );
			const skeletons = extractFromCache( meta.skeletons );
			const animations = extractFromCache( meta.animations );
			const nodes = extractFromCache( meta.nodes );

			if ( geometries.length > 0 ) output.geometries = geometries;
			if ( materials.length > 0 ) output.materials = materials;
			if ( textures.length > 0 ) output.textures = textures;
			if ( images.length > 0 ) output.images = images;
			if ( shapes.length > 0 ) output.shapes = shapes;
			if ( skeletons.length > 0 ) output.skeletons = skeletons;
			if ( animations.length > 0 ) output.animations = animations;
			if ( nodes.length > 0 ) output.nodes = nodes;

		}

		output.object = object;

		return output;

		// extract data from the cache hash
		// remove metadata on each item
		// and return as array
		function extractFromCache( cache ) {

			const values = [];
			for ( const key in cache ) {

				const data = cache[ key ];
				delete data.metadata;
				values.push( data );

			}

			return values;

		}

	}
Serializes the 3D object into JSON.

@param {?(Object|string)} meta - An optional value holding meta information about the serialization.
@return {Object} A JSON object representing the serialized 3D object.
@see {@link ObjectLoader#parse}