modify( geometry, cutOffAngle, tryKeepNormals = true ) {

		function computeNormals() {

			normals = new Float32Array( indexes.length * 3 );

			for ( let i = 0; i < indexes.length; i += 3 ) {

				let index = indexes[ i ];

				_A.set(
					positions[ 3 * index ],
					positions[ 3 * index + 1 ],
					positions[ 3 * index + 2 ] );

				index = indexes[ i + 1 ];
				_B.set(
					positions[ 3 * index ],
					positions[ 3 * index + 1 ],
					positions[ 3 * index + 2 ] );

				index = indexes[ i + 2 ];
				_C.set(
					positions[ 3 * index ],
					positions[ 3 * index + 1 ],
					positions[ 3 * index + 2 ] );

				_C.sub( _B );
				_A.sub( _B );

				const normal = _C.cross( _A ).normalize();

				for ( let j = 0; j < 3; j ++ ) {

					normals[ 3 * ( i + j ) ] = normal.x;
					normals[ 3 * ( i + j ) + 1 ] = normal.y;
					normals[ 3 * ( i + j ) + 2 ] = normal.z;

				}

			}

		}


		function mapPositionsToIndexes() {

			pointToIndexMap = Array( positions.length / 3 );

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

				const index = indexes[ i ];

				if ( pointToIndexMap[ index ] == null ) {

					pointToIndexMap[ index ] = [];

				}

				pointToIndexMap[ index ].push( i );

			}

		}


		function edgeSplitToGroups( indexes, cutOff, firstIndex ) {

			_A.set( normals[ 3 * firstIndex ], normals[ 3 * firstIndex + 1 ], normals[ 3 * firstIndex + 2 ] ).normalize();

			const result = {
				splitGroup: [],
				currentGroup: [ firstIndex ]
			};

			for ( const j of indexes ) {

				if ( j !== firstIndex ) {

					_B.set( normals[ 3 * j ], normals[ 3 * j + 1 ], normals[ 3 * j + 2 ] ).normalize();

					if ( _B.dot( _A ) < cutOff ) {

						result.splitGroup.push( j );

					} else {

						result.currentGroup.push( j );

					}

				}

			}

			return result;

		}


		function edgeSplit( indexes, cutOff, original = null ) {

			if ( indexes.length === 0 ) return;

			const groupResults = [];

			for ( const index of indexes ) {

				groupResults.push( edgeSplitToGroups( indexes, cutOff, index ) );

			}

			let result = groupResults[ 0 ];

			for ( const groupResult of groupResults ) {

				if ( groupResult.currentGroup.length > result.currentGroup.length ) {

					result = groupResult;

				}

			}


			if ( original != null ) {

				splitIndexes.push( {
					original: original,
					indexes: result.currentGroup
				} );

			}

			if ( result.splitGroup.length ) {

				edgeSplit( result.splitGroup, cutOff, original || result.currentGroup[ 0 ] );

			}

		}

		let hadNormals = false;
		let oldNormals = null;

		if ( geometry.attributes.normal ) {

			hadNormals = true;

			geometry = geometry.clone();

			if ( tryKeepNormals === true && geometry.index !== null ) {

				oldNormals = geometry.attributes.normal.array;

			}

			geometry.deleteAttribute( 'normal' );

		}

		if ( geometry.index == null ) {

			geometry = BufferGeometryUtils.mergeVertices( geometry );

		}

		const indexes = geometry.index.array;
		const positions = geometry.getAttribute( 'position' ).array;

		let normals;
		let pointToIndexMap;

		computeNormals();
		mapPositionsToIndexes();

		const splitIndexes = [];

		for ( const vertexIndexes of pointToIndexMap ) {

			edgeSplit( vertexIndexes, Math.cos( cutOffAngle ) - 0.001 );

		}

		const newAttributes = {};
		for ( const name of Object.keys( geometry.attributes ) ) {

			const oldAttribute = geometry.attributes[ name ];
			const newArray = new oldAttribute.array.constructor( ( indexes.length + splitIndexes.length ) * oldAttribute.itemSize );
			newArray.set( oldAttribute.array );
			newAttributes[ name ] = new BufferAttribute( newArray, oldAttribute.itemSize, oldAttribute.normalized );

		}

		const newIndexes = new Uint32Array( indexes.length );
		newIndexes.set( indexes );

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

			const split = splitIndexes[ i ];
			const index = indexes[ split.original ];

			for ( const attribute of Object.values( newAttributes ) ) {

				for ( let j = 0; j < attribute.itemSize; j ++ ) {

					attribute.array[ ( indexes.length + i ) * attribute.itemSize + j ] =
						attribute.array[ index * attribute.itemSize + j ];

				}

			}

			for ( const j of split.indexes ) {

				newIndexes[ j ] = indexes.length + i;

			}

		}

		geometry = new BufferGeometry();
		geometry.setIndex( new BufferAttribute( newIndexes, 1 ) );

		for ( const name of Object.keys( newAttributes ) ) {

			geometry.setAttribute( name, newAttributes[ name ] );

		}

		if ( hadNormals ) {

			geometry.computeVertexNormals();

			if ( oldNormals !== null ) {

				const changedNormals = new Array( oldNormals.length / 3 ).fill( false );

				for ( const splitData of splitIndexes )
					changedNormals[ splitData.original ] = true;

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

					if ( changedNormals[ i ] === false ) {

						for ( let j = 0; j < 3; j ++ )
							geometry.attributes.normal.array[ 3 * i + j ] = oldNormals[ 3 * i + j ];

					}

				}


			}

		}

		return geometry;

	}
Returns a new, modified version of the given geometry by applying an edge-split operation.
Please note that the resulting geometry is always indexed.

@param {BufferGeometry} geometry - The geometry to modify.
@param {number} cutOffAngle - The cut off angle in radians.
@param {boolean} [tryKeepNormals=true] - Whether to try to keep normals or not.
@return {BufferGeometry} A new, modified geometry.