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FBX Normals export issue (IndexToDirect interpreted wrong in Unity) #123088

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opened 2024-06-11 18:25:55 +02:00 by Lucas-Brown-SSTK · 8 comments
Lucas-Brown-SSTK commented 2024-06-11 18:25:55 +02:00
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System Information
Operating system: Windows 10

Blender Version
Broken: 4.1.1
Worked: 4.0.2

Short description of error
Normals are exported incorrectly to FBX files, artifacts show up when imported into Unity. It seems like normal smoothing information may be lost. I've confirmed this issue is in fact Blender, not Unity. Also confirmed it is still an issue on 4.3.0 Alpha.

Exact steps for others to reproduce the error

  1. See attached scene
  2. Open in Blender 4.1.1
  3. Export as FBX
  4. Import into Unity, broken normals (see attached images).

Tested attached FBX files in the following Unity version, all with the same results pictured.

  • 6000.0.4f1
  • 2022.3.20f1
  • 2021.3.24f1
**System Information** Operating system: Windows 10 **Blender Version** Broken: 4.1.1 Worked: 4.0.2 **Short description of error** Normals are exported incorrectly to FBX files, artifacts show up when imported into Unity. It seems like normal smoothing information may be lost. I've confirmed this issue is in fact Blender, not Unity. Also confirmed it is still an issue on 4.3.0 Alpha. **Exact steps for others to reproduce the error** 1. See attached scene 2. Open in Blender 4.1.1 3. Export as FBX 4. Import into Unity, broken normals (see attached images). Tested attached FBX files in the following Unity version, all with the same results pictured. - 6000.0.4f1 - 2022.3.20f1 - 2021.3.24f1
test_blender_4-0.png
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test_blender_4-1.png
67 KiB
test_4-1.fbx
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test_4-0.fbx
7.6 MiB
test.blend
28 MiB
test_blender_4-0.png test_blender_4-1.png
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YimingWu commented 2024-06-12 04:52:41 +02:00
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Recently there had been a few reports on possible FBX exporting errors indeed. @Mysteryem Could you take a look?

Recently there had been a few reports on possible FBX exporting errors indeed. @Mysteryem Could you take a look?
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Philipp Oeser commented 2024-06-12 15:27:39 +02:00
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To get this out of the way first:

  • test_4-0.fbx seems to have no texture associated with the normal map node
  • test_4-1.fbx seems to have a texture associated (HS_AK12_Forearm_Normal.png)

(at least this is what happens when you import them into blender)

When I open test.blend in Blender 4.1.1 and export as FBX, I dont get the texture associated when re-importing into Blender.

Not sure if this is responsible for the issue, just wanted to ask if both FBXes have really been exported from the exact same file?

To get this out of the way first: - `test_4-0.fbx` seems to have no texture associated with the normal map node - `test_4-1.fbx` seems to have a texture associated (`HS_AK12_Forearm_Normal.png`) (at least this is what happens when you import them into blender) When I open `test.blend` in Blender 4.1.1 and export as FBX, I dont get the texture associated when re-importing into Blender. Not sure if this is responsible for the issue, just wanted to ask if **both** FBXes have really been exported from the **exact** same file?
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Lucas-Brown-SSTK commented 2024-06-12 16:27:59 +02:00
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Whoops, I think I zipped up the wrong FBX files, there shouldn't be any reference to a normal map. I uploaded the correct FBXs now.
Originally I thought the normal map might be causing the problem so I created a flat normal map, same issue.
In the end I removed the normal map as it has no impact on this bug. I think it's something with the mesh data in the FBX.

Whoops, I think I zipped up the wrong FBX files, there shouldn't be any reference to a normal map. I uploaded the correct FBXs now. Originally I thought the normal map might be causing the problem so I created a flat normal map, same issue. In the end I removed the normal map as it has no impact on this bug. I think it's something with the mesh data in the FBX.
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Thomas Barlow commented 2024-06-12 17:02:09 +02:00
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From my investigation, this seems to be a Unity bug specifically with per-vertex normals exported with the IndexToDirect FBX reference mode, where Unity considers normals belonging to only a single face normals used by more than one vertex to be invalid and then throws out all the normals in the mesh and recalculates the normals itself. Unreal and FBX Review have no issues importing these normals (unreal will complain about no smoothing information in this case, but that can be remedied by exporting with FACE smoothing enabled).
image

I think this is a Unity bug because changing these normals to export with the Direct FBX reference mode works without issue and the only difference between Direct and IndexToDirect is how the data is stored in the FBX file: IndexToDirect can be used to remove duplicates and adds a second array of indices into the deduplicated array of normals that allows the full array, like would be exported with the Direct reference mode, to be reconstructed. Notably, the FBX SDK Documentation specifically states that normals should be exported as IndexToDirect.

Attached are two FBX models of a plane with per-vertex normals, one using the Direct reference mode and the other IndexToDirect.

Unfortunately, none of the models I tested with when testing the export of per-vertex normals had multiple vertices with exactly the same normals, so this wasn't noticed sooner.

In this case, it looks like Unity is recalculating of the normals is using an Area Weighted normal calculation which does not work well for this model. As a workaround for this issue, changing the normal mode in the FBX's import settings in Unity to Angle Weighted gives a much better result: image

From my investigation, this seems to be a Unity bug specifically with per-vertex normals exported with the `IndexToDirect` FBX reference mode, where Unity considers ~~normals belonging to only a single face~~ normals used by more than one vertex to be invalid and then throws out all the normals in the mesh and recalculates the normals itself. Unreal and FBX Review have no issues importing these normals (unreal will complain about no smoothing information in this case, but that can be remedied by exporting with FACE smoothing enabled). ![image](/attachments/57ddabb1-c563-436e-8c03-81ef38fc0b01) I think this is a Unity bug because changing these normals to export with the `Direct` FBX reference mode works without issue and the only difference between `Direct` and `IndexToDirect` is how the data is stored in the FBX file: `IndexToDirect` can be used to remove duplicates and adds a second array of indices into the deduplicated array of normals that allows the full array, like would be exported with the `Direct` reference mode, to be reconstructed. Notably, the FBX SDK Documentation specifically states that normals should be exported as `IndexToDirect`. Attached are two FBX models of a plane with per-vertex normals, one using the `Direct` reference mode and the other `IndexToDirect`. Unfortunately, none of the models I tested with when testing the export of per-vertex normals had multiple vertices with exactly the same normals, so this wasn't noticed sooner. In this case, it looks like Unity is recalculating of the normals is using an `Area Weighted` normal calculation which does not work well for this model. As a workaround for this issue, changing the normal mode in the FBX's import settings in Unity to `Angle Weighted` gives a much better result: ![image](/attachments/3b8c72a8-4b58-4175-a431-555214c43c85)
unityVertexNormalsBugPlaneIndexToDirect.fbx
12 KiB
unityVertexNormalsBugPlaneDirect.fbx
12 KiB
image.png
73 KiB
image.png
108 KiB
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Philipp Oeser commented 2024-06-13 08:58:46 +02:00
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Thx a lot for the info @Mysteryem !

How did you change it from IndexToDirect to Direct? Change the exporter code here?

# FBX SDK documentation says that normals should use IndexToDirect.
 elem_data_single_string(lay_nor, b"ReferenceInformationType", b"IndexToDirect")
Thx a lot for the info @Mysteryem ! How did you change it from `IndexToDirect` to `Direct`? Change the exporter code here? ```python # FBX SDK documentation says that normals should use IndexToDirect. elem_data_single_string(lay_nor, b"ReferenceInformationType", b"IndexToDirect") ```
Thomas Barlow commented 2024-06-13 14:28:44 +02:00
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I've updated my findings because it seems that what Unity doesn't like is any normals being used by more than one vertex, which somewhat defeats the purpose of the IndexToDirect reference mode. Unity does not complain if I use IndexToDirect without deduplicating the normals, so the indices array is just [0, 1, 2, 3, 4, ...].

Thx a lot for the info @Mysteryem !

How did you change it from IndexToDirect to Direct? Change the exporter code here?

# FBX SDK documentation says that normals should use IndexToDirect.
 elem_data_single_string(lay_nor, b"ReferenceInformationType", b"IndexToDirect")

In this case I modified the file manually by converting it to json with the fbx2json.py script, adjusted the normals in a text editor and then converted back with the json2fbx.py script.

Here's a commit with changes to make the exporter use Direct instead of IndexToDirect: dc7c1add1d

I've updated my findings because it seems that what Unity doesn't like is any normals being used by more than one vertex, which somewhat defeats the purpose of the `IndexToDirect` reference mode. Unity does not complain if I use `IndexToDirect` without deduplicating the normals, so the indices array is just `[0, 1, 2, 3, 4, ...]`. > Thx a lot for the info @Mysteryem ! > > How did you change it from `IndexToDirect` to `Direct`? Change the exporter code here? > > ```python > # FBX SDK documentation says that normals should use IndexToDirect. > elem_data_single_string(lay_nor, b"ReferenceInformationType", b"IndexToDirect") > ``` In this case I modified the file manually by converting it to json with the fbx2json.py script, adjusted the normals in a text editor and then converted back with the json2fbx.py script. Here's a commit with changes to make the exporter use `Direct` instead of `IndexToDirect`: https://projects.blender.org/Mysteryem/blender/commit/dc7c1add1d2734e3f62120d5d46fbf7593e7bbee
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Philipp Oeser commented 2024-06-14 08:57:25 +02:00
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@Mysteryem : do we have an explanation why this worked in 4.0.2? (that might be interesting to mention in !123186)

@Mysteryem : do we have an explanation why this worked in 4.0.2? (that might be interesting to mention in !123186)
Philipp Oeser changed title from Normals export incorrectly to FBX to FBX Normals export issue (IndexToDirect interpreted wrong in Unity) 2024-06-14 08:59:04 +02:00
Philipp Oeser commented 2024-06-14 08:59:16 +02:00
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Will confirm for now

Will confirm for now
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