billboardMode of sourceMesh changes instance rotation

kzhsw · May 16, 2025, 2:31am

Engine: WebGL2
Version: 8.8.1
Playground: Babylon.js Playground

Steps to reproduce:

Open the playground: Babylon.js Playground
Move camera
Watch the blue box

Expected:
The blue box kept exactly its rotation, and does not change on camera move, like it was cloned.

Actual:
It somehow “inherited” the billboardMode of sourceMesh, even if instance.billboardMode == 0.
Actual

Related code:

github.com/BabylonJS/Babylon.js

packages/dev/core/src/Meshes/instancedMesh.ts

98e6452ee


      
          this._currentLOD = this.sourceMesh;

github.com/BabylonJS/Babylon.js

packages/dev/core/src/Meshes/instancedMesh.ts

98e6452ee


      
                  // we are using the edge renderer of the source mesh

                  this._sourceMesh._renderingGroup._edgesRenderers.pushNoDuplicate(this._sourceMesh._edgesRenderer);

                  this._sourceMesh._edgesRenderer.customInstances.push(this.getWorldMatrix());

              } else if (this._edgesRenderer && this._edgesRenderer.isEnabled && this._sourceMesh._renderingGroup) {

                  // we are using the edge renderer defined for this instance

                  this._sourceMesh._renderingGroup._edgesRenderers.push(this._edgesRenderer);

              }

          }

          

          public override getWorldMatrix(): Matrix {

              if (this._currentLOD && this._currentLOD.billboardMode !== TransformNode.BILLBOARDMODE_NONE && this._currentLOD._masterMesh !== this) {

                  if (!this._billboardWorldMatrix) {

                      this._billboardWorldMatrix = new Matrix();

                  }

                  const tempMaster = this._currentLOD._masterMesh;

                  this._currentLOD._masterMesh = this;

                  TmpVectors.Vector3[7].copyFrom(this._currentLOD.position);

                  this._currentLOD.position.set(0, 0, 0);

                  this._billboardWorldMatrix.copyFrom(this._currentLOD.computeWorldMatrix(true));

                  this._currentLOD.position.copyFrom(TmpVectors.Vector3[7]);

                  this._currentLOD._masterMesh = tempMaster;

Deltakosh · May 16, 2025, 3:15pm

Nice one! wanna send a PR?

kzhsw · May 19, 2025, 1:26am

A quick fix would be adding this._currentLOD !== this._sourceMesh to line 441, like this:

But I’m not sure if this is the real fix.

Deltakosh · May 19, 2025, 2:59pm

I like it! This sounds right to me!

kzhsw · May 20, 2025, 12:44am

github.com/BabylonJS/Babylon.js

instancedMesh: fix getWorldMatrix in case billboardMode of source mesh enabled

master ← kzhsw:patch-1

opened 12:43AM - 20 May 25 UTC

kzhsw

+6 -1

The `_currentLOD` of InstancedMesh would be `_sourceMesh` if LOD of source mesh …not enabled, but if billboardMode of source mesh enabled, `getWorldMatrix` would incorrectly use the billboardMode of the `sourceMesh` to compute world matrix. This commit would add a check in `getWorldMatrix` to skip this if `_currentLOD` is `_sourceMesh`. Forum post: <https://forum.babylonjs.com/t/billboardmode-of-sourcemesh-changes-instance-rotation/58469> Playground: <https://playground.babylonjs.com/#1RQ8OR>

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billboardMode of sourceMesh changes instance rotation

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