FBX Export: Duplicate materials cause material indices to be offset #103976

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Mysteryem · 2023-01-18T18:36:03+01:00

Mysteryem commented

2023-01-18 18:36:03 +01:00

System Information
Operating system: Windows-10-10.0.19045-SP0 64 Bits
Graphics card: NVIDIA GeForce GTX 1070 Ti/PCIe/SSE2 NVIDIA Corporation 4.5.0 NVIDIA 516.94

Blender Version
Broken: version: 3.4.1, branch: blender-v3.4-release, commit date: 2022-12-19 17:00, hash: blender/blender@55485cb379, version: 2.80.75
Worked:

Short description of error
When meshes have duplicate materials in their slots, the material is added to the fbx only once, but the material index for that material is incremented each time the material is found. This causes the material index to increase beyond the number of materials, offsetting subsequent material indices and causing material indices to go out of bounds.

The polygons assigned to either slot containing Duplicate Material were given a material index of 2, which now corresponds to Other Material in the exported FBX. (green has become red)
The polygons assigned to Other Material were given a material index of 3, which is now out of bounds, so the FBX importer has reassigned them to the first slot. (red has become purple)

Exact steps for others to reproduce the error

Create a mesh with the same material in at least two material slots
Assign some polygons to one of the material slots containing a duplicate material or a subsequent material slot
Export as FBX (default settings)
Open the system console
Import the exported .fbx
Observe the warning in the system console and that the polygons have been assigned to the wrong materials
Attached is the .blend file shown in the description (authored in 3.4.1) and the .fbx exported from it
FBXDuplicateMaterialTest.blend
FBXDuplicateMaterialTest.fbx

**System Information** Operating system: Windows-10-10.0.19045-SP0 64 Bits Graphics card: NVIDIA GeForce GTX 1070 Ti/PCIe/SSE2 NVIDIA Corporation 4.5.0 NVIDIA 516.94 **Blender Version** Broken: version: 3.4.1, branch: blender-v3.4-release, commit date: 2022-12-19 17:00, hash: `blender/blender@55485cb379`, version: 2.80.75 Worked: **Short description of error** When meshes have duplicate materials in their slots, the material is added to the fbx only once, but the material index for that material is incremented each time the material is found. This causes the material index to increase beyond the number of materials, offsetting subsequent material indices and causing material indices to go out of bounds. ![image.png](https://archive.blender.org/developer/F14172352/image.png) ![image.png](https://archive.blender.org/developer/F14172262/image.png) The polygons assigned to either slot containing `Duplicate Material` were given a material index of 2, which now corresponds to `Other Material` in the exported FBX. (green has become red) The polygons assigned to `Other Material` were given a material index of 3, which is now out of bounds, so the FBX importer has reassigned them to the first slot. (red has become purple) ![image.png](https://archive.blender.org/developer/F14172354/image.png) **Exact steps for others to reproduce the error** - Create a mesh with the same material in at least two material slots - Assign some polygons to one of the material slots containing a duplicate material or a subsequent material slot - Export as FBX (default settings) - Open the system console - Import the exported .fbx - Observe the warning in the system console and that the polygons have been assigned to the wrong materials Attached is the .blend file shown in the description (authored in 3.4.1) and the .fbx exported from it [FBXDuplicateMaterialTest.blend](https://archive.blender.org/developer/F14172357/FBXDuplicateMaterialTest.blend) [FBXDuplicateMaterialTest.fbx](https://archive.blender.org/developer/F14172358/FBXDuplicateMaterialTest.fbx)

Mysteryem commented

2023-01-18 18:36:03 +01:00

Added subscriber: @Mysteryem-2