Incompatible tangents are generated in models using a single UV "shell" about mikktspace HOT 9 CLOSED

Thanks for reaching out. Before I dig in just a quick thing to confirm. If you triangulate the mesh in blender first (you can use the modifier stack so it’s not destructive), then do the bake, then export this version of the mesh so you bring the same triangulation of the mesh into your engine. Does the problem go away? Similar to Gouraud shading how you tringulate your polygons has an impact on your interpolated vertex attributes.

Morten

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The two copies of mikktspace should definitely generate identical results. Ill take a look at that

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Thanks for the lighting fast reply! I'm pretty sure that they were triangulated before baking/export (I'll double-check with our artist).

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Actually, the warping is a bit hard to see in my earlier screenshots, here's a short video we did at some point to compare the results:

The face without any kind of warping corresponds to the case where tangents between Blender and our converter were matching:

• No Tangents Exported: tangent vertex attributes were generated through our converter (which was using this implementation).
• Tangents Exported: we use the tangent data directly from the FBX we get from Blender (i.e. using Tangent Space export option).

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Yeah, they should've been per-triangulated in Blender according to our artist. I opened them in Blender and they do indeed seem to be proper. The normal map was baked in Substance. Contained within mikkt_tests.zip are our test assets. Here's a short description:

• mikkt_tests_01_blender_high.fbx: high-polygon ground truth made in Blender (pretty much a cube with some rounded edges).
• mikkt_tests_01_blender_n.tga: baked per-pixel MikkTSpace normal map for a single UV shell (i.e. for the two FBX files below).
• mikkt_tests_01_blender_tangents_exported.fbx: low-poly single UV shell version of the asset, tangents exported from Blender.
• mikkt_tests_01_blender_no_tangents_exported.fbx: low-poly single UV shell, tangents need to be generated through our code.
• mikkt_tests_01_blender_separate_shells_n.tga: baked per-pixel MikkTSpace normal map with multiple UV islands (see below).
• mikkt_tests_01_blender_tangents_exported_separate_shells.fbx: low-poly multiple UV islands, tangents exported from Blender.
• mikkt_tests_01_blender_no_tangents_exported_separate_shells.fbx: low-poly multi-UV islands, tangents need to be generated.

Artists have been trying to follow this guide: Generating Perfect Normal Maps for Unity, but maybe we're still missing something. I'll also try to do a bit more digging later this week and see if I can pin down what's wrong, since I still have both versions of the code.

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bakes.zip

Could you try these two? It's a bake from xnormal and one from knald. Neither one uses Blender's version of the .c file but both are working as intended. What happens when you use these in your engine?

Btw this was baked to mikkt_tests_01_blender_no_tangents_exported.fbx

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Hey Morten, thanks for checking that out for us! I downloaded those bakes and proceeded to try them in our engine with that asset:

I think the left one was the one from Knald and the one on the right is the one from xNormal, notice the warping in that closest one.

Since it sounded like it worked on your side that got me thinking what could be different between our implementation and the one in this repository (and why it worked when we pulled the one from Blender). Well, it turns out that it was something embarrassing... Our compiler is setup to treat warnings as errors, even in middleware code (which is kind of stupid to begin with), so when I ported over mikktspace.c we got quite a few warnings that I needed to fix. I quickly hacked that together and didn't think too much of it.

Well, it turns out when doing vimdiff between those changes and the code here reveals a pretty stupid change I didn't notice I did:

I didn't actually notice that was a union before double checking now, so that explains the weird edge connections I was getting 😅. Instead of fixing those compilation warnings, I just pulled this repository again and disabled warnings with a few pragma directives. And yeah, after that, the results are completely perfect xD (not so surprisingly). So there is absolutely nothing wrong with the code here, it was just me being too carefree when patching out the warnings and not thinking too much about it afterwards... **sigh**

Thank you very much for your patience and help Morten! I think the mystery has been solved on our side at least 😅. Closing issue!

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No problem. I'm glad it was an easy fix for you :)

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After reading the tech deep dive of Northlight / Alan Wake 2 today, I've been feeling that impostor syndrome. You've really done an amazing job on that engine, twisting the GPU to do your bidding for you in ways I couldn't have imagined. Very impressive.

Then I found this issue while integrating mikktspace in my engine, and realize it's one of you guys. Instantly felt much better lol. What are the odds? Great work on Northlight, you are an inspiration.

And sorry for off-topic.

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