three.js/examples/js/lines/LineGeometry.js

( function () {

	var LineGeometry = function () {

		THREE.LineSegmentsGeometry.call( this );
		this.type = 'LineGeometry';

	};

	LineGeometry.prototype = Object.assign( Object.create( THREE.LineSegmentsGeometry.prototype ), {
		constructor: LineGeometry,
		isLineGeometry: true,
		setPositions: function ( array ) {

			// converts [ x1, y1, z1,	x2, y2, z2, ... ] to pairs format
			var length = array.length - 3;
			var points = new Float32Array( 2 * length );

			for ( var i = 0; i < length; i += 3 ) {

				points[ 2 * i ] = array[ i ];
				points[ 2 * i + 1 ] = array[ i + 1 ];
				points[ 2 * i + 2 ] = array[ i + 2 ];
				points[ 2 * i + 3 ] = array[ i + 3 ];
				points[ 2 * i + 4 ] = array[ i + 4 ];
				points[ 2 * i + 5 ] = array[ i + 5 ];

			}

			THREE.LineSegmentsGeometry.prototype.setPositions.call( this, points );
			return this;

		},
		setColors: function ( array ) {

			// converts [ r1, g1, b1,	r2, g2, b2, ... ] to pairs format
			var length = array.length - 3;
			var colors = new Float32Array( 2 * length );

			for ( var i = 0; i < length; i += 3 ) {

				colors[ 2 * i ] = array[ i ];
				colors[ 2 * i + 1 ] = array[ i + 1 ];
				colors[ 2 * i + 2 ] = array[ i + 2 ];
				colors[ 2 * i + 3 ] = array[ i + 3 ];
				colors[ 2 * i + 4 ] = array[ i + 4 ];
				colors[ 2 * i + 5 ] = array[ i + 5 ];

			}

			THREE.LineSegmentsGeometry.prototype.setColors.call( this, colors );
			return this;

		},
		fromLine: function ( line ) {

			var geometry = line.geometry;

			if ( geometry.isGeometry ) {

				console.error( 'THREE.LineGeometry no longer supports Geometry. Use THREE.BufferGeometry instead.' );
				return;

			} else if ( geometry.isBufferGeometry ) {

				this.setPositions( geometry.attributes.position.array ); // assumes non-indexed

			} // set colors, maybe


			return this;

		},
		copy: function ( ) {

			// todo
			return this;

		}
	} );

	THREE.LineGeometry = LineGeometry;

} )();