Model disappeared when same 3 models pathtraced

[dongho-shin]

Describe the bug

in three.js editor you can reproduce 100%

1. Create 1 cube(other models also reproduce)
2. PathTrace Render(set Realistic upper right)
3. Create 2 cubes
4. again PathTrace Render then you can see 2 cubes disappear in pathTraced Scene

2024-08-07.8.13.15-1.mov

Only this scenario reproduce this bug

add 1 model -> pathTrace render -> add same model of two -> PathTrace render

Expected behavior

all 3 cubes rendered in realistic

Platform:

• Device: [Desktop]
• OS: [MacOS]
• GPU: [NVidia ??, Radeon ??, ...]
• Browser: [Chrome]
• Three.js version: [r167(editor)]
• Library version: [current library version]

[gkjohnson]

Can you reproduce in a jsfiddle or smaller example? Something like initializing / rendering the initial scene with 1 cube and then 1 second later (ore pressing a button) adding two more cubes?

[dongho-shin]

@gkjohnson I found a reason but not perfectly digged
this line is a problem
PathTracingSceneGenerator.js in line.217

if ( result.changeType === GEOMETRY_REBUILT ) {

				const bvhOptions = {
					strategy: SAH,
					maxLeafTris: 1,
					indirect: true, //<-- this options causes a problem
					onProgress,
					...this.bvhOptions,
				};

because most(react three fiber, etc...) of indirect option is false as default, but PathTracingSceneGenerator this.generateBVH true as a default

so MeshBVH instantiates indirectBuffer at PathTracingSceneGenerator constructor, and MeshBVH's indirect check like below code (MeshBVH line.111)

	get indirect() {

		return ! ! this._indirectBuffer;

	}

so it causes a bug. it affects many codes ex. MeshBVHUniformStruct updateFrom // indirect false but true

[dongho-shin]

Open a pr for check a bug @gkjohnson

[dongho-shin]

I found a reason and why it's 3 same model causes a bug
if indirect set true in bvhOptions it causes an error because bvhToTextures not supported Multi root Nodes

function bvhToTextures( bvh, boundsTexture, contentsTexture ) {

	const roots = bvh._roots;

	if ( roots.length !== 1 ) {
               //if indirect false this error occurred
		throw new Error( 'MeshBVHUniformStruct: Multi-root BVHs not supported.' );

	}

https://github.com/gkjohnson/three-mesh-bvh/blob/9718501eee2619f1015fa332d7bddafaf6cf562a/src/gpu/MeshBVHUniformStruct.js#L52
and in three-mesh-bvh

// dereference a new index attribute if we're using indirect storage
		if ( bvh.indirect ) {

			const indirectBuffer = bvh._indirectBuffer;
			if (
				this._cachedIndexAttr === null ||
				this._cachedIndexAttr.count !== indirectBuffer.length
			) {

				if ( geometry.index ) {

					this._cachedIndexAttr = geometry.index.clone();

				} else {

					const array = getIndexArray( getVertexCount( geometry ) );
					this._cachedIndexAttr = new BufferAttribute( array, 1, false );

				}

			}

			dereferenceIndex( geometry, indirectBuffer, this._cachedIndexAttr );
			this.index.updateFrom( this._cachedIndexAttr );

		}

MeshUniformBVHStruct is a struct that in PhysicalPathTracingMaterial

three-gpu-pathtracer/src/materials/pathtracing/PhysicalPathTracingMaterial.js

Line 87 in 46b395e

bvh: { value: new MeshBVHUniformStruct() },

focus on here

if ( bvh.indirect ) {

			const indirectBuffer = bvh._indirectBuffer;
			if (
				this._cachedIndexAttr === null ||
				this._cachedIndexAttr.count !== indirectBuffer.length
			) {

				if ( geometry.index ) {

					this._cachedIndexAttr = geometry.index.clone();

				}

in BoxGeometry case

1. at first PathTrace Rendering

• this._cachedIndexAttr.count(null) !== indirectBuffer.length(12)
• this._cachedIndexAttr = BufferAttribute(length 36)

2. add two BoxGeometry

• this._cachedIndexAttr.count(36) !== indirectBuffer.length(36) <-- it's same so it Pass update _cachedIndexAttr
  -> there's no update at _cachedIndexAttr it causes a bug