Hi - bit of a story, and I’m not sure this is a Maya or a UE question, but the tl;dr is that importing alembics from maya has flipped tangent space Y vectors and I don’t know how to fix it (maya export or ue import issue?).
Longer story: I’m trying to extract metahuman eyes for use in characters modeled and animated in maya and exported as alembic, then brought back into UE.
I was able to export a metahuman mesh from UE, import to maya, remove everything but the eyes, then reexport that and bring into UE. My FBX export looks correct (So: UE metahuman eye looks identical to UE->FBX->maya->FBX->UE metahuman eye). The alembic export looks almost correct, except the pupil and iris refraction direction is reversed in the up-down direction. After some investigation, it looks like the tangent space Y vector (which I guess is the bi-tangent vector) is inverted in the ABC but not the FBX. So, UE->FBX->maya->ABC->UE metahuman eye has flipped tangent space Y vector compared to UE metahuman eye).
For normal maps this isn’t a big deal as you can just flip the green, but the metahuman eye material is using tangent space to fake the iris/pupil refraction. I can add an option to make the metahuman eye material work with the inverted vector, but was just wondering if anybody knows what’s going on.
Is this a Maya export issue or a UE import issue? I don’t know enough about Maya to thoroughly debug, but I have found the tangent space right-handed / left-handed coordinate system toggle and winding order options and have tried all combinations and it doesn’t seem to fix anything.