MeshVertex has been converted by component transform before being converted by instance transform.
It is ( Vertex * ComponentTransform ) * InstanceTransform.
I thought it has to be converted by (Vertex * InstanceTransform * ComponentTransform).
I found one quick solution.
// code placeholder void FMeshMergeHelpers::ExpandInstances(const UInstancedStaticMeshComponent* InInstancedStaticMeshComponent, FMeshDescription& InOutRawMesh, TArray<FSectionInfo>& InOutSections) { FMeshDescription CombinedRawMesh; bool bFirstMesh = true; for(const FInstancedStaticMeshInstanceData& InstanceData : InInstancedStaticMeshComponent->PerInstanceSMData) { FMeshDescription InstanceRawMesh = InOutRawMesh; #if 0 FMeshMergeHelpers::TransformRawMeshVertexData(FTransform(InstanceData.Transform), InstanceRawMesh); #else const FTransform ComponentTransform = InInstancedStaticMeshComponent->GetComponentTransform(); const FTransform InstanceTransform = ComponentTransform.Inverse() * FTransform(InstanceData.Transform) * ComponentTransform; FMeshMergeHelpers::TransformRawMeshVertexData(InstanceTransform, InstanceRawMesh); #endif
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There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-79140 in the post.