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

import { LineSegmentsGeometry } from '../lines/LineSegmentsGeometry.js';

var LineGeometry = function () {

	LineSegmentsGeometry.call( this );

	this.type = 'LineGeometry';

};

LineGeometry.prototype = Object.assign( Object.create( 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 ];

		}

		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 ];

		}

		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 ( /* source */ ) {

		// todo

		return this;

	}

} );

export { LineGeometry };