constructor( radius = 1, widthSegments = 32, heightSegments = 16, phiStart = 0, phiLength = Math.PI * 2, thetaStart = 0, thetaLength = Math.PI ) {

		super();

		this.type = 'SphereGeometry';

		/**
		 * Holds the constructor parameters that have been
		 * used to generate the geometry. Any modification
		 * after instantiation does not change the geometry.
		 *
		 * @type {Object}
		 */
		this.parameters = {
			radius: radius,
			widthSegments: widthSegments,
			heightSegments: heightSegments,
			phiStart: phiStart,
			phiLength: phiLength,
			thetaStart: thetaStart,
			thetaLength: thetaLength
		};

		widthSegments = Math.max( 3, Math.floor( widthSegments ) );
		heightSegments = Math.max( 2, Math.floor( heightSegments ) );

		const thetaEnd = Math.min( thetaStart + thetaLength, Math.PI );

		let index = 0;
		const grid = [];

		const vertex = new Vector3();
		const normal = new Vector3();

		// buffers

		const indices = [];
		const vertices = [];
		const normals = [];
		const uvs = [];

		// generate vertices, normals and uvs

		for ( let iy = 0; iy <= heightSegments; iy ++ ) {

			const verticesRow = [];

			const v = iy / heightSegments;

			// special case for the poles

			let uOffset = 0;

			if ( iy === 0 && thetaStart === 0 ) {

				uOffset = 0.5 / widthSegments;

			} else if ( iy === heightSegments && thetaEnd === Math.PI ) {

				uOffset = -0.5 / widthSegments;

			}

			for ( let ix = 0; ix <= widthSegments; ix ++ ) {

				const u = ix / widthSegments;

				// vertex

				vertex.x = - radius * Math.cos( phiStart + u * phiLength ) * Math.sin( thetaStart + v * thetaLength );
				vertex.y = radius * Math.cos( thetaStart + v * thetaLength );
				vertex.z = radius * Math.sin( phiStart + u * phiLength ) * Math.sin( thetaStart + v * thetaLength );

				vertices.push( vertex.x, vertex.y, vertex.z );

				// normal

				normal.copy( vertex ).normalize();
				normals.push( normal.x, normal.y, normal.z );

				// uv

				uvs.push( u + uOffset, 1 - v );

				verticesRow.push( index ++ );

			}

			grid.push( verticesRow );

		}

		// indices

		for ( let iy = 0; iy < heightSegments; iy ++ ) {

			for ( let ix = 0; ix < widthSegments; ix ++ ) {

				const a = grid[ iy ][ ix + 1 ];
				const b = grid[ iy ][ ix ];
				const c = grid[ iy + 1 ][ ix ];
				const d = grid[ iy + 1 ][ ix + 1 ];

				if ( iy !== 0 || thetaStart > 0 ) indices.push( a, b, d );
				if ( iy !== heightSegments - 1 || thetaEnd < Math.PI ) indices.push( b, c, d );

			}

		}

		// build geometry

		this.setIndex( indices );
		this.setAttribute( 'position', new Float32BufferAttribute( vertices, 3 ) );
		this.setAttribute( 'normal', new Float32BufferAttribute( normals, 3 ) );
		this.setAttribute( 'uv', new Float32BufferAttribute( uvs, 2 ) );

	}
Constructs a new sphere geometry.

@param {number} [radius=1] - The sphere radius.
@param {number} [widthSegments=32] - The number of horizontal segments. Minimum value is `3`.
@param {number} [heightSegments=16] - The number of vertical segments. Minimum value is `2`.
@param {number} [phiStart=0] - The horizontal starting angle in radians.
@param {number} [phiLength=Math.PI*2] - The horizontal sweep angle size.
@param {number} [thetaStart=0] - The vertical starting angle in radians.
@param {number} [thetaLength=Math.PI] - The vertical sweep angle size.