three.js/examples/jsm/utils/SceneUtils.js

import {
	BufferAttribute,
	BufferGeometry,
	Color,
	Group,
	Matrix4,
	Mesh
} from 'three';

import { mergeGroups, deepCloneAttribute } from './BufferGeometryUtils.js';

const _color = /*@__PURE__*/new Color();
const _matrix = /*@__PURE__*/new Matrix4();

function createMeshesFromInstancedMesh( instancedMesh ) {

	const group = new Group();

	const count = instancedMesh.count;
	const geometry = instancedMesh.geometry;
	const material = instancedMesh.material;

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

		const mesh = new Mesh( geometry, material );

		instancedMesh.getMatrixAt( i, mesh.matrix );
		mesh.matrix.decompose( mesh.position, mesh.quaternion, mesh.scale );

		group.add( mesh );

	}

	group.copy( instancedMesh );
	group.updateMatrixWorld(); // ensure correct world matrices of meshes

	return group;

}

function createMeshesFromMultiMaterialMesh( mesh ) {

	if ( Array.isArray( mesh.material ) === false ) {

		console.warn( 'THREE.SceneUtils.createMeshesFromMultiMaterialMesh(): The given mesh has no multiple materials.' );
		return mesh;

	}

	const object = new Group();
	object.copy( mesh );

	// merge groups (which automatically sorts them)

	const geometry = mergeGroups( mesh.geometry );

	const index = geometry.index;
	const groups = geometry.groups;
	const attributeNames = Object.keys( geometry.attributes );

	// create a mesh for each group by extracting the buffer data into a new geometry

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

		const group = groups[ i ];

		const start = group.start;
		const end = start + group.count;

		const newGeometry = new BufferGeometry();
		const newMaterial = mesh.material[ group.materialIndex ];

		// process all buffer attributes

		for ( let j = 0; j < attributeNames.length; j ++ ) {

			const name = attributeNames[ j ];
			const attribute = geometry.attributes[ name ];
			const itemSize = attribute.itemSize;

			const newLength = group.count * itemSize;
			const type = attribute.array.constructor;

			const newArray = new type( newLength );
			const newAttribute = new BufferAttribute( newArray, itemSize );

			for ( let k = start, n = 0; k < end; k ++, n ++ ) {

				const ind = index.getX( k );

				if ( itemSize >= 1 ) newAttribute.setX( n, attribute.getX( ind ) );
				if ( itemSize >= 2 ) newAttribute.setY( n, attribute.getY( ind ) );
				if ( itemSize >= 3 ) newAttribute.setZ( n, attribute.getZ( ind ) );
				if ( itemSize >= 4 ) newAttribute.setW( n, attribute.getW( ind ) );

			}


			newGeometry.setAttribute( name, newAttribute );

		}

		const newMesh = new Mesh( newGeometry, newMaterial );
		object.add( newMesh );

	}

	return object;

}

function createMultiMaterialObject( geometry, materials ) {

	const group = new Group();

	for ( let i = 0, l = materials.length; i < l; i ++ ) {

		group.add( new Mesh( geometry, materials[ i ] ) );

	}

	return group;

}

/**
 * @param {InstancedMesh}
 * @param {function(int, int):int}
 */
function sortInstancedMesh( mesh, compareFn ) {

	// store copy of instanced attributes for lookups

	const instanceMatrixRef = deepCloneAttribute( mesh.instanceMatrix );
	const instanceColorRef = mesh.instanceColor ? deepCloneAttribute( mesh.instanceColor ) : null;

	const attributeRefs = new Map();

	for ( const name in mesh.geometry.attributes ) {

		const attribute = mesh.geometry.attributes[ name ];

		if ( attribute.isInstancedBufferAttribute ) {

			attributeRefs.set( attribute, deepCloneAttribute( attribute ) );

		}

	}


	// compute sort order

	const tokens = [];

	for ( let i = 0; i < mesh.count; i ++ ) tokens.push( i );

	tokens.sort( compareFn );


	// apply sort order

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

		const refIndex = tokens[ i ];

		_matrix.fromArray( instanceMatrixRef.array, refIndex * mesh.instanceMatrix.itemSize );
		_matrix.toArray( mesh.instanceMatrix.array, i * mesh.instanceMatrix.itemSize );

		if ( mesh.instanceColor ) {

			_color.fromArray( instanceColorRef.array, refIndex * mesh.instanceColor.itemSize );
			_color.toArray( mesh.instanceColor.array, i * mesh.instanceColor.itemSize );

		}

		for ( const name in mesh.geometry.attributes ) {

			const attribute = mesh.geometry.attributes[ name ];

			if ( attribute.isInstancedBufferAttribute ) {

				const attributeRef = attributeRefs.get( attribute );

				attribute.setX( i, attributeRef.getX( refIndex ) );
				if ( attribute.itemSize > 1 ) attribute.setY( i, attributeRef.getY( refIndex ) );
				if ( attribute.itemSize > 2 ) attribute.setZ( i, attributeRef.getZ( refIndex ) );
				if ( attribute.itemSize > 3 ) attribute.setW( i, attributeRef.getW( refIndex ) );

			}

		}

	}

}

export {
	createMeshesFromInstancedMesh,
	createMeshesFromMultiMaterialMesh,
	createMultiMaterialObject,
	sortInstancedMesh
};