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

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

class LineGeometry extends LineSegmentsGeometry {

	constructor() {

		super();

		this.isLineGeometry = true;

		this.type = 'LineGeometry';

	}

	setPositions( array ) {

		// converts [ x1, y1, z1,  x2, y2, z2, ... ] to pairs format

		const length = array.length - 3;
		const points = new Float32Array( 2 * length );

		for ( let 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 ];

		}

		super.setPositions( points );

		return this;

	}

	setColors( array ) {

		// converts [ r1, g1, b1,  r2, g2, b2, ... ] to pairs format

		const length = array.length - 3;
		const colors = new Float32Array( 2 * length );

		for ( let 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 ];

		}

		super.setColors( colors );

		return this;

	}

	setFromPoints( points ) {

		// converts a vector3 or vector2 array to pairs format

		const length = points.length - 1;
		const positions = new Float32Array( 6 * length );

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

			positions[ 6 * i ] = points[ i ].x;
			positions[ 6 * i + 1 ] = points[ i ].y;
			positions[ 6 * i + 2 ] = points[ i ].z || 0;

			positions[ 6 * i + 3 ] = points[ i + 1 ].x;
			positions[ 6 * i + 4 ] = points[ i + 1 ].y;
			positions[ 6 * i + 5 ] = points[ i + 1 ].z || 0;

		}

		super.setPositions( positions );

		return this;

	}

	fromLine( line ) {

		const geometry = line.geometry;

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

		// set colors, maybe

		return this;

	}

}

export { LineGeometry };