](https://blenderartists.org/forum/member.php?13572-motorsep)“That’s not the topic of this thread.”
I’m sorry if you are offended by my answer or don’t see it as helpful, sorry. I answered because you obviously had issue understanding what the root of the problem is. It’s not so much to do with any specific software, but the fact that you have to do a round trip. Authoring skeleton in Blender will naturally produce “working” skeleton+animation.
“If you import UE4’s Mannequin FBX into Blender, you’ll see bones are oriented entirely differently from how you’d usually have bones for characters in Blender.”
“It has to do with bones’ axes, which are different from software to software.”
I mentioned in my first post: “First of all, AFAIK Blender only has a very “rigid” understanding of bones - it doesn’t even have a separate bone and joint, it always makes joint align with bone, and bones only can have one specific length axis (was it Y?)”
I’m not sure which part you didn’t understand, I specifically stated it’s related to bone axis, which are different but in Blender’s case limited.
“bones are oriented entirely differently” because of this: AFAIK Blender limits itself to Y-only as main axis of bone. You can see this by enabling bone axis. World up is Z-axis… This is not so much true if you come from Maya. First of all, default world up is Y, not Z. There is already first issue. Also, you can define main axis of joints anyway you like in Maya. Just go to orient joints panel. You can define main axis as any axis. In Blender it’s always Y, and Maya, default is X-axis along length. And FBX has more flexibility than Blender.
Like mentioned already this is the same situation in Blender, as you are only going to have bone along Y-axis. Try this. Change default main axis of 2-bone chain in Maya from default X to Y, export it to Blender, you’ll see bones pointing up in Blender world space instead of along what should be joint to joint span.
You can compensate this using FBX import settings, swapping “Primary bone axis” from X to Y. But this doesn’t seem to work with full rigs. Default FBX importer does bad job, it either gets the skeleton looking good with messed up animation or vice versa. I haven’t been able to find proper settings to do proper axis swapping, if that is even possible.
Anyway, what I mentioned is IMHO very much related to your question, these limitations are the core reason why this happens with any other software… in your case of UE Marketplace and Unity/Maya/Max and so on, and yes, it can’t be that different, if they author skeleton hierarchy in Maya style for example.
But maybe I’ll continue this discussion elsewhere, I have asked it couple of times here in Blender Artists, but no-one seems to be interested, or gives similar low-level solution that is not acceptable in real world use.