What are the parameters of UInstancedStaticMesh::UpdateInstances?

Development Programming & Scripting C++
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UInstancedStaticMesh has a member function UpdateInstances:

	/**
	* Lightweight interface to add, remove and update instances.
	*
	* @param AddInstanceTransforms:  The transforms of the new instances to add.
	* @param RemoveInstanceIds: The ids of the instances to remove.
	* @param UpdateInstanceIds: The ids of the new instances to update.
	* @param UpdateInstanceTransforms: The transforms of the new instances to update.
	* @param UpdateInstancePreviousTransforms:	The transforms of the new instances to update.
	* @return True on success
	*/
	virtual bool UpdateInstances(
		const TArray<int32>& UpdateInstanceIds, 
		const TArray<FTransform>& UpdateInstanceTransforms, 
		const TArray<FTransform>& UpdateInstancePreviousTransforms,
		int32 NumCustomFloats,
		const TArray<float>& CustomFloatData);

As you can see the documentation is incorrect:

  • There is no parameter AddInstanceTransforms

  • There is no parameter RemoveInstanceIds

  • The documentation of both UpdateInstanceTransforms and UpdateInstancePreviousTransforms says “The transforms of the new instances to update”. (Does it really want the same identical argument to both parameters? I doubt it.)

  • The parameters NumCustomFloats and CustomFloatData are undocumented.

Does anyone know what the actual semantics of this function and its parameters are? In particular:

  • What is UpdateInstancePreviousTransforms ? Previous to what? Previous to this call? So the caller needs to keep the old transforms?

  • What should the length of each vector be relative to each other and the other parameters?

  • How do you use the function to add instances? How do you use it to remove them? What are the instance ids?

1 Like
2

I solved this issue by looping ver all the instanced and setting the flag bMarkRenderStateDirty to true in all update calls:

// for tranforms
InstancedMeshCmp->UpdateInstanceTransform(ID, NewTransfrom, true, true, true);
// for custom data
InstancedMeshCmp->SetCustomDataValue(ID, 0, .5, true);

This makes the update instantaneous.