three.js/examples/jsm/geometries/DecalGeometry.js

import {
	BufferGeometry,
	Euler,
	Float32BufferAttribute,
	Matrix3,
	Matrix4,
	Mesh,
	Vector3
} from 'three';

/**
 * You can use this geometry to create a decal mesh, that serves different kinds of purposes.
 * e.g. adding unique details to models, performing dynamic visual environmental changes or covering seams.
 *
 * Constructor parameter:
 *
 * mesh — Any mesh object
 * position — Position of the decal projector
 * orientation — Orientation of the decal projector
 * size — Size of the decal projector
 *
 * reference: http://blog.wolfire.com/2009/06/how-to-project-decals/
 *
 */

class DecalGeometry extends BufferGeometry {

	constructor( mesh = new Mesh(), position = new Vector3(), orientation = new Euler(), size = new Vector3( 1, 1, 1 ) ) {

		super();

		// buffers

		const vertices = [];
		const normals = [];
		const uvs = [];

		// helpers

		const plane = new Vector3();

		const normalMatrix = new Matrix3().getNormalMatrix( mesh.matrixWorld );

		// this matrix represents the transformation of the decal projector

		const projectorMatrix = new Matrix4();
		projectorMatrix.makeRotationFromEuler( orientation );
		projectorMatrix.setPosition( position );

		const projectorMatrixInverse = new Matrix4();
		projectorMatrixInverse.copy( projectorMatrix ).invert();

		// generate buffers

		generate();

		// build geometry

		this.setAttribute( 'position', new Float32BufferAttribute( vertices, 3 ) );
		this.setAttribute( 'uv', new Float32BufferAttribute( uvs, 2 ) );

		if ( normals.length > 0 ) {

			this.setAttribute( 'normal', new Float32BufferAttribute( normals, 3 ) );

		}

		//

		function generate() {

			let decalVertices = [];

			const vertex = new Vector3();
			const normal = new Vector3();

			// handle different geometry types

			const geometry = mesh.geometry;

			const positionAttribute = geometry.attributes.position;
			const normalAttribute = geometry.attributes.normal;

			// first, create an array of 'DecalVertex' objects
			// three consecutive 'DecalVertex' objects represent a single face
			//
			// this data structure will be later used to perform the clipping

			if ( geometry.index !== null ) {

				// indexed BufferGeometry

				const index = geometry.index;

				for ( let i = 0; i < index.count; i ++ ) {

					vertex.fromBufferAttribute( positionAttribute, index.getX( i ) );

					if ( normalAttribute ) {

						normal.fromBufferAttribute( normalAttribute, index.getX( i ) );
						pushDecalVertex( decalVertices, vertex, normal );

					} else {

						pushDecalVertex( decalVertices, vertex );

					}

				}

			} else {

				if ( positionAttribute === undefined ) return; // empty geometry

				// non-indexed BufferGeometry

				for ( let i = 0; i < positionAttribute.count; i ++ ) {

					vertex.fromBufferAttribute( positionAttribute, i );

					if ( normalAttribute ) {

						normal.fromBufferAttribute( normalAttribute, i );
						pushDecalVertex( decalVertices, vertex, normal );

					} else {

						pushDecalVertex( decalVertices, vertex );

					}

				}

			}

			// second, clip the geometry so that it doesn't extend out from the projector

			decalVertices = clipGeometry( decalVertices, plane.set( 1, 0, 0 ) );
			decalVertices = clipGeometry( decalVertices, plane.set( - 1, 0, 0 ) );
			decalVertices = clipGeometry( decalVertices, plane.set( 0, 1, 0 ) );
			decalVertices = clipGeometry( decalVertices, plane.set( 0, - 1, 0 ) );
			decalVertices = clipGeometry( decalVertices, plane.set( 0, 0, 1 ) );
			decalVertices = clipGeometry( decalVertices, plane.set( 0, 0, - 1 ) );

			// third, generate final vertices, normals and uvs

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

				const decalVertex = decalVertices[ i ];

				// create texture coordinates (we are still in projector space)

				uvs.push(
					0.5 + ( decalVertex.position.x / size.x ),
					0.5 + ( decalVertex.position.y / size.y )
				);

				// transform the vertex back to world space

				decalVertex.position.applyMatrix4( projectorMatrix );

				// now create vertex and normal buffer data

				vertices.push( decalVertex.position.x, decalVertex.position.y, decalVertex.position.z );

				if ( decalVertex.normal !== null ) {

					normals.push( decalVertex.normal.x, decalVertex.normal.y, decalVertex.normal.z );

				}

			}

		}

		function pushDecalVertex( decalVertices, vertex, normal = null ) {

			// transform the vertex to world space, then to projector space

			vertex.applyMatrix4( mesh.matrixWorld );
			vertex.applyMatrix4( projectorMatrixInverse );

			if ( normal ) {

				normal.applyNormalMatrix( normalMatrix );
				decalVertices.push( new DecalVertex( vertex.clone(), normal.clone() ) );

			} else {

				decalVertices.push( new DecalVertex( vertex.clone() ) );

			}

		}

		function clipGeometry( inVertices, plane ) {

			const outVertices = [];

			const s = 0.5 * Math.abs( size.dot( plane ) );

			// a single iteration clips one face,
			// which consists of three consecutive 'DecalVertex' objects

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

				let total = 0;
				let nV1;
				let nV2;
				let nV3;
				let nV4;

				const d1 = inVertices[ i + 0 ].position.dot( plane ) - s;
				const d2 = inVertices[ i + 1 ].position.dot( plane ) - s;
				const d3 = inVertices[ i + 2 ].position.dot( plane ) - s;

				const v1Out = d1 > 0;
				const v2Out = d2 > 0;
				const v3Out = d3 > 0;

				// calculate, how many vertices of the face lie outside of the clipping plane

				total = ( v1Out ? 1 : 0 ) + ( v2Out ? 1 : 0 ) + ( v3Out ? 1 : 0 );

				switch ( total ) {

					case 0: {

						// the entire face lies inside of the plane, no clipping needed

						outVertices.push( inVertices[ i ] );
						outVertices.push( inVertices[ i + 1 ] );
						outVertices.push( inVertices[ i + 2 ] );
						break;

					}

					case 1: {

						// one vertex lies outside of the plane, perform clipping

						if ( v1Out ) {

							nV1 = inVertices[ i + 1 ];
							nV2 = inVertices[ i + 2 ];
							nV3 = clip( inVertices[ i ], nV1, plane, s );
							nV4 = clip( inVertices[ i ], nV2, plane, s );

						}

						if ( v2Out ) {

							nV1 = inVertices[ i ];
							nV2 = inVertices[ i + 2 ];
							nV3 = clip( inVertices[ i + 1 ], nV1, plane, s );
							nV4 = clip( inVertices[ i + 1 ], nV2, plane, s );

							outVertices.push( nV3 );
							outVertices.push( nV2.clone() );
							outVertices.push( nV1.clone() );

							outVertices.push( nV2.clone() );
							outVertices.push( nV3.clone() );
							outVertices.push( nV4 );
							break;

						}

						if ( v3Out ) {

							nV1 = inVertices[ i ];
							nV2 = inVertices[ i + 1 ];
							nV3 = clip( inVertices[ i + 2 ], nV1, plane, s );
							nV4 = clip( inVertices[ i + 2 ], nV2, plane, s );

						}

						outVertices.push( nV1.clone() );
						outVertices.push( nV2.clone() );
						outVertices.push( nV3 );

						outVertices.push( nV4 );
						outVertices.push( nV3.clone() );
						outVertices.push( nV2.clone() );

						break;

					}

					case 2: {

						// two vertices lies outside of the plane, perform clipping

						if ( ! v1Out ) {

							nV1 = inVertices[ i ].clone();
							nV2 = clip( nV1, inVertices[ i + 1 ], plane, s );
							nV3 = clip( nV1, inVertices[ i + 2 ], plane, s );
							outVertices.push( nV1 );
							outVertices.push( nV2 );
							outVertices.push( nV3 );

						}

						if ( ! v2Out ) {

							nV1 = inVertices[ i + 1 ].clone();
							nV2 = clip( nV1, inVertices[ i + 2 ], plane, s );
							nV3 = clip( nV1, inVertices[ i ], plane, s );
							outVertices.push( nV1 );
							outVertices.push( nV2 );
							outVertices.push( nV3 );

						}

						if ( ! v3Out ) {

							nV1 = inVertices[ i + 2 ].clone();
							nV2 = clip( nV1, inVertices[ i ], plane, s );
							nV3 = clip( nV1, inVertices[ i + 1 ], plane, s );
							outVertices.push( nV1 );
							outVertices.push( nV2 );
							outVertices.push( nV3 );

						}

						break;

					}

					case 3: {

						// the entire face lies outside of the plane, so let's discard the corresponding vertices

						break;

					}

				}

			}

			return outVertices;

		}

		function clip( v0, v1, p, s ) {

			const d0 = v0.position.dot( p ) - s;
			const d1 = v1.position.dot( p ) - s;

			const s0 = d0 / ( d0 - d1 );

			const position = new Vector3(
				v0.position.x + s0 * ( v1.position.x - v0.position.x ),
				v0.position.y + s0 * ( v1.position.y - v0.position.y ),
				v0.position.z + s0 * ( v1.position.z - v0.position.z )
			);

			let normal = null;

			if ( v0.normal !== null && v1.normal !== null ) {

				normal = new Vector3(
					v0.normal.x + s0 * ( v1.normal.x - v0.normal.x ),
					v0.normal.y + s0 * ( v1.normal.y - v0.normal.y ),
					v0.normal.z + s0 * ( v1.normal.z - v0.normal.z )
				);

			}

			const v = new DecalVertex( position, normal );

			// need to clip more values (texture coordinates)? do it this way:
			// intersectpoint.value = a.value + s * ( b.value - a.value );

			return v;

		}

	}

}

// helper

class DecalVertex {

	constructor( position, normal = null ) {

		this.position = position;
		this.normal = normal;

	}

	clone() {

		const position = this.position.clone();
		const normal = ( this.normal !== null ) ? this.normal.clone() : null;

		return new this.constructor( position, normal );

	}

}

export { DecalGeometry, DecalVertex };