BatchedMesh: setInstanceCount can't shrink available instances

[andreas-hilti]

Description

setInstanceCount doesn't correctly remove availableInstanceIds and thus, setInstanceCount can't be used to shrink the maxInstanceCount if more instances were used at some point.

For instance, create a BatchedMesh with 4 Instances, remove the last two instances, and then try to do setInstanceCount(2). This will fail with:
Instance ids outside the range 2 are being used. Cannot shrink instance count.

Root cause seems to be here:

three.js/src/objects/BatchedMesh.js

Line 1254 in e9fc133

while ( availableInstanceIds[ availableInstanceIds.length - 1 ] === instanceInfo.length ) {

it should instead be:

while ( availableInstanceIds[ availableInstanceIds.length - 1 ] === instanceInfo.length - 1) {

(the largest id is instanceInfo.length - 1)

Reproduction steps

1. Add the unit test as provided below
2. Run it

This results in

message: "Died on test #1: BatchedMesh: Instance ids outside the range 2 are being used. Cannot shrink instance count.
    at Object.<anonymous> (file:///[...]/three.js/test/unit/src/objects/BatchedMesh.tests.js

Code

/* global QUnit */

import { BatchedMesh } from '../../../../src/objects/BatchedMesh.js';
import { BoxGeometry } from '../../../../src/geometries/BoxGeometry.js';
import { MeshBasicMaterial } from '../../../../src/materials/MeshBasicMaterial.js';

export default QUnit.module( 'Objects', () => {

	QUnit.module( 'BatchedMesh', () => {

		// PUBLIC

		QUnit.test( 'setInstanceCount', ( assert ) => {

			const box = new BoxGeometry( 1, 1, 1 );
			const material = new MeshBasicMaterial( { color: 0x00ff00 } );
			
			// initialize and add geometries into the batched mesh
			const batchedMesh = new BatchedMesh( 4, 5000, 10000, material );
			const boxGeometryId = batchedMesh.addGeometry( box );
			
			// create instances of those geometries
			let boxInstanceIds = []
			for (let i = 0; i < 4; i++){
				boxInstanceIds.push( batchedMesh.addInstance( boxGeometryId ) );
			}
			
			batchedMesh.deleteInstance( boxInstanceIds[2] );
			batchedMesh.deleteInstance( boxInstanceIds[3] );

			// shrink the instance count
			batchedMesh.setInstanceCount(2);

			assert.ok( batchedMesh.instanceCount === 2, 'instance count unequal 2' );

		} );

	} );

} );

Live example

• jsfiddle-latest-release WebGLRenderer

Screenshots

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dev

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