three.js/examples/js/csm/CSMFrustum.js

( function () {

	const inverseProjectionMatrix = new THREE.Matrix4();
	class CSMFrustum {

		constructor( data ) {

			data = data || {};
			this.vertices = {
				near: [ new THREE.Vector3(), new THREE.Vector3(), new THREE.Vector3(), new THREE.Vector3() ],
				far: [ new THREE.Vector3(), new THREE.Vector3(), new THREE.Vector3(), new THREE.Vector3() ]
			};
			if ( data.projectionMatrix !== undefined ) {

				this.setFromProjectionMatrix( data.projectionMatrix, data.maxFar || 10000 );

			}

		}
		setFromProjectionMatrix( projectionMatrix, maxFar ) {

			const isOrthographic = projectionMatrix.elements[ 2 * 4 + 3 ] === 0;
			inverseProjectionMatrix.copy( projectionMatrix ).invert();

			// 3 --- 0  vertices.near/far order
			// |     |
			// 2 --- 1
			// clip space spans from [-1, 1]

			this.vertices.near[ 0 ].set( 1, 1, - 1 );
			this.vertices.near[ 1 ].set( 1, - 1, - 1 );
			this.vertices.near[ 2 ].set( - 1, - 1, - 1 );
			this.vertices.near[ 3 ].set( - 1, 1, - 1 );
			this.vertices.near.forEach( function ( v ) {

				v.applyMatrix4( inverseProjectionMatrix );

			} );
			this.vertices.far[ 0 ].set( 1, 1, 1 );
			this.vertices.far[ 1 ].set( 1, - 1, 1 );
			this.vertices.far[ 2 ].set( - 1, - 1, 1 );
			this.vertices.far[ 3 ].set( - 1, 1, 1 );
			this.vertices.far.forEach( function ( v ) {

				v.applyMatrix4( inverseProjectionMatrix );
				const absZ = Math.abs( v.z );
				if ( isOrthographic ) {

					v.z *= Math.min( maxFar / absZ, 1.0 );

				} else {

					v.multiplyScalar( Math.min( maxFar / absZ, 1.0 ) );

				}

			} );
			return this.vertices;

		}
		split( breaks, target ) {

			while ( breaks.length > target.length ) {

				target.push( new CSMFrustum() );

			}

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

				const cascade = target[ i ];
				if ( i === 0 ) {

					for ( let j = 0; j < 4; j ++ ) {

						cascade.vertices.near[ j ].copy( this.vertices.near[ j ] );

					}

				} else {

					for ( let j = 0; j < 4; j ++ ) {

						cascade.vertices.near[ j ].lerpVectors( this.vertices.near[ j ], this.vertices.far[ j ], breaks[ i - 1 ] );

					}

				}

				if ( i === breaks.length - 1 ) {

					for ( let j = 0; j < 4; j ++ ) {

						cascade.vertices.far[ j ].copy( this.vertices.far[ j ] );

					}

				} else {

					for ( let j = 0; j < 4; j ++ ) {

						cascade.vertices.far[ j ].lerpVectors( this.vertices.near[ j ], this.vertices.far[ j ], breaks[ i ] );

					}

				}

			}

		}
		toSpace( cameraMatrix, target ) {

			for ( let i = 0; i < 4; i ++ ) {

				target.vertices.near[ i ].copy( this.vertices.near[ i ] ).applyMatrix4( cameraMatrix );
				target.vertices.far[ i ].copy( this.vertices.far[ i ] ).applyMatrix4( cameraMatrix );

			}

		}

	}

	THREE.CSMFrustum = CSMFrustum;

} )();