Merge Actors fails when no mesh has UV channel 1 AND Lightmap Index set to 1

In 4.14 p3, Merge Actors creates an empty illegal static mesh asset when none of the selected actors has LightMap channel set to 1 and a corresponding Channel to begin with.

Reproduction Steps

  1. Import Box Mesh from 3dsMax (or any other DCC-app) without Lightmap-Generation on (and without collision in my case) but of course proper UV0. For example box mapping done in 3dsmax for the first channel.
  2. Add actors of that Box to the scene
  3. Open Merge-Actors Dialog and Merge Actors. No matter the settings, the merge fails and an error is thrown in the output log that the raw mesh is corrupted. (The static mesh is empty). my settings: No Material Merge activated. Rest as default.
  4. Open the Box_Asset and create a 2nd (lightmap) channel. Save the asset.
  5. Try to merge again. The merged asset is still corrupted.
  6. Now Open the Box-Asset again and set the Lightmap Coordinate Index to 1. The merge will now succeed.

From what I understand this is a bug. Especially because the Merge-Dialog includes an option to generate a Lightmap Channel for the merged mesh. So this should always succeed and there shouldn’t be an error when the Actor assets dont have a lightmap.
Also interesting observation: If you have multiple different assets (actors) and ONLY 1 has a proper second UV channel + LM-index set to 1, the merge works, too. So at least 1 actors needs a “proper” setup.

If this is no bug and by design, there should be at least a helping output log error message. Atm. only a “raw mesh is corrupted” message from RawMesh.cpp is generated for the merged static mesh, which does not help at all.

Hi LeFxGuy,

Thanks for the detailed steps. I’ve submitted a ticket for this to be tracked: Unreal Engine Issues and Bug Tracker (UE-38520)

I didn’t get the exact issue you had where it would fail to create the mesh, but when mine was created it would have no geometry assigned to the merged mesh that was created. This lead to a number of crashes when clicking parts of the toolbar that would need that geometry to work properly.



That’s what i meant with “The static mesh is empty”. We got the same results. Corrupted Static mesh asset created in the Content Browser with Error Output log that the mesh is corrupted but no error log why it would generate the empty static mesh asset in the first place. :slight_smile: Glad i could help :slight_smile:

oh. dur. Misread that part about the emptiness. Glad we’ve got it all sorted and reported. Hopefully this can get fixed for 4.14’s final release in time. :slight_smile:

I’ve got stuck with the same issue.
However, I couldn’t reproduce the workaround mentioned in LeFxGuy’s step 6.

Did you try to generate LightMap UVSs on import or by using the static mesh built settings? Maybe you got another problem that is not related to UVs and a second UV channel when using Merge Actors? Which Engine Version did you try?