Object3D: matrixNeedsUpdate and matrix/orientation optimizations

[CodyJasonBennett]

Related issue: #21387, #25115

Description

Implements matrixNeedsUpdate set from mutations to position/quaternion/rotation/scale properties in Object3D, reducing the update depth and frequency of local matrix updates and consequent world matrix updates to descendants (related: #14138, #28533). I've also removed the _onChangeCallback system in math classes and synchronized rotation and quaternion locally in Object3D once on read (versus unconditionally on write), where it's now possible to skip this conversion if users keep to one orientation property at a time. This is not something I've seen regarded anywhere than maybe #14044 (comment), but should be a nice benefit for both maintenance and performance outside of matrices. Most of the diff in this PR is from removing prior hacks.

World matrices are also no longer updated unconditionally, as we will only set matrixWorldNeedsUpdate when the local matrix (or parent world matrix) updates. This removes the primary use of matrixWorldAutoUpdate for those looking to squeeze out performance, as these optimizations are performed automatically. It's possible to further improve updateWorldMatrix in this regard so it can skip updates to ancestors -- #28533 (comment). The introduction of matrixNeedsUpdate is similarly useful for polling in user-land, which I think is a reasonable compromise for those hoping to detect updates in the scene-graph (e.g. updating bounding volumes or even using the scene as a virtual API for BatchedMesh).

Future work could include optimizing the cost of matrix composition itself (#14138 (comment)). Although position is a simple mutation, scale would be more involved by dividing/multiplying new/old scales, and quaternion the worst-case-scenario with a complete update to the upper-left 3x3 matrix. It's possible this can be performed in user-land, or we can reduce this check to matrix.setPosition or matrix.compose depending on whether orientation/scale change. This is not something I'll consider further, as I believe this is best handled in user-land where we can compile WASM etc., but I've outlined a solution below.

Partial Matrix Composition Example

// Partial Matrix4.compose https://github.com/mrdoob/three.js/blob/dev/src/math/Matrix4.js
const m = this.matrix.elements;

if ( this.quaternionChanged ) {

	const x = quaternion.x, y = quaternion.y, z = quaternion.z, w = quaternion.w;
	const x2 = x + x,	y2 = y + y, z2 = z + z;
	const xx = x * x2, xy = x * y2, xz = x * z2;
	const yy = y * y2, yz = y * z2, zz = z * z2;
	const wx = w * x2, wy = w * y2, wz = w * z2;

	const sx = scale.x, sy = scale.y, sz = scale.z;

	m[ 0 ] = ( 1 - ( yy + zz ) ) * sx;
	m[ 1 ] = ( xy + wz ) * sx;
	m[ 2 ] = ( xz - wy ) * sx;

	m[ 4 ] = ( xy - wz ) * sy;
	m[ 5 ] = ( 1 - ( xx + zz ) ) * sy;
	m[ 6 ] = ( yz + wx ) * sy;

	m[ 8 ] = ( xz + wy ) * sz;
	m[ 9 ] = ( yz - wx ) * sz;
	m[ 10 ] = ( 1 - ( xx + yy ) ) * sz;

} else if ( this.scaleChanged ) {

	// NOTE: this relies on either a cache or decomposing into two parts when orientation is changed:
	// - dividing old scale when orientation changes (either Quaternion or Euler)
	// - multiplying new scale when updating matrix
	const sx = ( 1 / this.oldScale.x ) * scale.x;
	const sy = ( 1 / this.oldScale.y ) * scale.y;
	const sz = ( 1 / this.oldScale.z ) * scale.z;

	m[ 0 ] *= sx;
	m[ 1 ] *= sx;
	m[ 2 ] *= sx;

	m[ 4 ] *= sy;
	m[ 5 ] *= sy;
	m[ 6 ] *= sy;

	m[ 8 ] *= sz;
	m[ 9 ] *= sz;
	m[ 10 ] *= sz;

}

// NOTE: might be best to do so unconditionally and leave tracking with orientation
if ( this.positionChanged ) {

	m[ 12 ] = this.position.x;
	m[ 13 ] = this.position.y;
	m[ 14 ] = this.position.z;

}

[github-actions]

📦 Bundle size

Full ESM build, minified and gzipped.

Filesize dev	Filesize PR	Diff
678.5 kB (168.2 kB)	678 kB (168.1 kB)	-495 B

🌳 Bundle size after tree-shaking

Minimal build including a renderer, camera, empty scene, and dependencies.

Filesize dev	Filesize PR	Diff
456.6 kB (110.3 kB)	456.1 kB (110.2 kB)	-568 B

[CodyJasonBennett]

listenToProperties adds an on-read and on-write callback to named keys (not a deopt), which we use here to set matrixNeedsUpdate when transform properties are modified. On-read is used to convert quaternion/rotation like before, but only if their counterpart is dirty when read, preventing many unneeded conversions on mutation (which can be many times). Notably, if users stick to quaternion, they will avoid this conversion entirely. This is nice so we don't have to modify math classes like before.

listenToMethods I have as a special case for the local matrix whose members would be deopt if I just listened to all elements in its array. This is because array accessors are not named and unstable, so v8 would need to create a layer of indirection to make them stable. We want matrixWorldNeedsUpdate to be set whenever the local matrix updates to preserve existing behavior, so I instead handle the cases where local matrix is modified by its methods (which are named keys), and don't handle the case where matrix.elements is mutated directly (matrixWorldNeedsUpdate has to then be set by the user). This keeps us on the fast path for common usage without breaking APIs who expected a 1-1 relationship between local and world matrix updates.

[CodyJasonBennett]

This relied on the fact that world matrices were always updated even transiently. Since #28533, we can do this instead.