Environment
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Unreal Engine: 5.8.3
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Platform: Windows 11
Description
I encountered an issue where a Static Mesh can produce incorrect normals after being converted to a Packaged Level Actor (BPP) when the actor has a negative scale.
The issue appears to be related to FastGeo / ISMTransformer, and is particularly noticeable when the material is Two-Sided.
Steps to Reproduce
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Download a Static Mesh asset from Megascans.
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Create eight copies of the Static Mesh Actor and assign different scale combinations to them, covering the permutations of positive and negative scale signs.
For example:
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(+, +, +) -
(-, +, +) -
(+, -, +) -
(+, +, -) -
(-, -, +) -
(-, +, -) -
(+, -, -) -
(-, -, -)
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![Simple Material with two-sided option turned on|566x500]
3. Create a simple material using the Static Mesh.
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Enable Two Sided on the material.
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Assign the material to the Static Mesh.
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Select the actors and convert/package them into a Packaged Level Actor (BPP).
7 Observe the resulting rendering and visualize the world-space normals.
Actual Result
After the actors are converted into a Packaged Level Actor, some instances with negative scale produce incorrect normals and therefore incorrect lighting/shading.
The issue can be observed using World Normal Visualization.
The same meshes render correctly before being converted into the Packaged Level Actor.
Incorrect Result
Comparison
When Two Sided is disabled, the problem appears to disappear or behave differently.
Expected Result
The generated Packaged Level Actor should preserve the original mesh orientation, winding/handedness, and normal behavior regardless of whether the source actor uses positive or negative scale.
In particular, the combination of:
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Static Mesh
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Negative Actor Scale
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Two-Sided Material
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Packaged Level Actor
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FastGeo / ISMTransformer
should not result in incorrect normals.
Affected Features
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Packaged Level Actor
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FastGeo
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ISMTransformer
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Static Meshes using negative scale
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Two-Sided materials
Additional Information
The issue seems to be specifically related to how the negative scale is handled when the actor is transformed/instanced during the Packaged Level Actor conversion.
I would like to know:
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Is this a known issue in UE 5.8.3?
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Is negative scale currently supported by FastGeo / ISMTransformer?
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Is there a known workaround for preserving the correct normal orientation when converting these actors into Packaged Level Actors?
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Is there a relevant source-code path or fix that should be investigated?
5. Saw this note in UnrealEngine Code, but could not find that issue descripti on Unreal Engine Issues and Bug Tracker.