Blender to UE4 physics issues

There’s Blender to UE4 workflow issues if you’d rather watch that instead.

tl;dr summary (I ramble but there’re details in the long version that are needed):

Blender to UE4 export/import procedure results in UE4 physics model exploding. Ensure you’ve applied scale and rotation (Ctrl-A->”Scale & Rotation”) to the objects before exporting, then set the FBX export scale to the correct value. Just because the model imports at what visually is a suitable size, doesn’t mean it’s the right scale. It may work fine in every respect (animations, appearance etc.) but still be wrong when you get to the UE4 PHAT editor. Details for how in the long version.

Long version:

I’ve had plenty of problems importing from Blender to UE4 previously but thought I’d got past those issues. Working with the PHAT editor to modify the physics body on a model I’m working on I discovered another. Happily I’ve also worked out what it is and how to fix it.

No matter how the physics body appears to match the model, it may be that it doesn’t really. I was happy with how the generated physics model looked with only some minor modifications needed, a few sphyls needing collision disabling manually, some rescaling etc.

After that though I couldn’t get the simulation to run without the model exploding like a cartoon clockwork toy.

The first thing to check is that in Blender you’ve applied the Scale and Rotation on all of the objects to be exported, but most especially the skeleton/armature. To do this go into Object Mode, select all of the objects you’ll be exporting and in the 3D view press Ctrl-A to bring up the ‘Apply’ menu, then select “Scale & Rotation“.

At this point your armature may mangle horribly, I’m afraid the only solution I’ve found so far is to start from scratch and apply the scale and rotation as soon as you’ve performed any scales or rotates, either that or manually move everything back into place. In a couple of my cases they were so badly mangled it was quicker to start over. I do like to learn the hard way 🙂

Applying scale and rotation will fix most of the issues.

Eventually I noticed that if I transformed a sphyl it would move faster than the transform arrows I was dragging. On a hunch I went back to Blender and halved the FBX export scale – deleting the previous import in UE4 and importing it from scratch made PHAT behave much better, then making some adjustments like turning off a few overlapping sphyl collisions it worked perfectly!

The following should work for you as well as it now does for me:

  1. Follow the Unit Scale in Blender” instructions on the Unreal Engine wiki.
  2. Remember to Ctrl-A->”Scale & Rotation” at suitable points in the Blender modelling process.
  3. When you Export->FBX, set the following options: Scale=0.005; Armature/Mesh (unset Empty, Camera, Lamp and Other); Smoothing=Face; Check “Only Deform Bones“. Everything else should be okay as default.
  4. In UE4 you should be able to import with the default settings.

You should now have a valid physics model, though you may need to turn off collisions (hover over the “Collision Off” button and hold Ctrl+Alt for an explanation of what it does and how to use it).

I hope this is of some use!


EDIT: I’ve put up Blender to UE4 workflow issues.