Bounding box is not properly calculated for the InstancedMesh objects in Box3.expandByObject

[Fang3s]

Description of the problem

Just as the issue title indicated, I create an InstancedMesh with more than one instances at different positions, but it's just the Box3 result of the Geometry attached to it. After reading the code, I have made a fix.

var _box = new THREE.Box3();
var _vector = new THREE.Vector3();
THREE.Box3.prototype.expandByObject = function (object) {

    // Computes the world-axis-aligned bounding box of an object (including its children),
    // accounting for both the object's, and children's, world transforms

    object.updateWorldMatrix(false, false);

    var geometry = object.geometry;

    if (geometry !== undefined) {

        if (geometry.boundingBox === null) {

            geometry.computeBoundingBox();

        }

        if (object.isInstancedMesh) {
            var matrixWorld = object.matrixWorld,
                matrix4Array = object.instanceMatrix.array,
                arrayLength = matrix4Array.length;
            for (var posOffset = 12; posOffset < arrayLength; posOffset += 16) {
                _vector.set(matrix4Array[posOffset], matrix4Array[1 + posOffset], matrix4Array[2 + posOffset]);
                _box.expandByPoint(_vector);
            }
        } else {
            _box.copy(geometry.boundingBox);
        }
        _box.applyMatrix4(object.matrixWorld);

        this.expandByPoint(_box.min);
        this.expandByPoint(_box.max);

    }

    var children = object.children;

    for (var i = 0, l = children.length; i < l; i++) {

        this.expandByObject(children[i]);

    }

    return this;
}

See also:

• Box3.expandByObject
• Box3.setFromObject
• InstancedMesh.instanceMatrix
• BufferAttribute.getX
• Matrix4.setPosition

Please also include a live example if possible. You can start from these templates:

• jsfiddle (dev branch 39)
• jsbin (latest release branch)
• codepen(latest release branch)

Three.js version

• Dev
• r113
• ...

Browser

• All of them
• Chrome
• Firefox
• Internet Explorer

OS

• All of them
• Windows
• macOS
• Linux
• Android
• iOS

Hardware Requirements (graphics card, VR Device, ...)

[Mugen87]

Duplicate of #18334

[Saltallica]

@Fang3s A huge thank you to providing this code. InstancedMeshes are cumbersome to work with and miss a lot of the practical methods and abilities of normal geometry - I hope that changes over time!