In December 2016, Mixamo decided to pull support for Unreal Engine 4 compatible animations. But what does this mean ? Does this make it impossible to get mixamo animations into UE4 ? In short, “No!”, it is still possible to get Mixamo animations into Unreal Engine 4. First I will explain why mixamo needed a separate option for unreal exports and afterwards I will describe a (at the time of writing atleast) working way to get mixamo animations into UE4.
The previous way of getting animations into UE4 would have been to export the mannequin from UE4 as an fbx file, upload this to mixamo, select the desired animations on the mannequin and download them from mixamo with the “unreal Engine 4 export” option. This was however made unavailable.
Mixamo had the need for this option because animations in mixamo work without an actual root bone. I don’t exactly know why, but I guess it isn’t necessary as the preview “scenes” only have the specific character and thus only 1 skeleton. At export time however, the skeleton gets a root bone. When selecting the UE4 option, they added a root bone at 0.0.0 via the way of a script. When you use the standard export, the pelvis gets transformed in a root bone, and this is where the problem lies. UE4 needs a root bone at 0.0.0 to correctly use root motion, and the root bone also describes the position of the mesh. Importing skeletons into UE4 which have the pelvis as root bone therefore results in meshes which “float” by means of the pelvis, resulting in the added effect that a pelvis will not move.
Now, for a way to actually import them correctly. First, we must modify the UE4 mannequin to have the pelvis as a root bone (I know , this sounds counter-intuitive). So, export the SK_Mannequin to an fbx. and open it in any modelling software you are familiar with (Maya is sure to work, you may also need to put the model into y-up). Then, remove the “root” bone and the IK bones for the left and right hand. This will leave you with a normal skeleton which has the pelvis as a root. We will call this skeleton_noroot to be clear.
Our skeleton_noroot is now ready to be uploaded to mixamo. You can now put any mixamo animations you want on it. Download the skeleton_noroot with the animations from mixamo in fbx and with a skin attached. Now, import this into UE4, this will give you a new mesh with it’s own animations, materials and skeleton, don’t worry, we will make these animations available on the standard mannequin.
The last thing we have to do is use the UE4 “retarget manager” to retarget our animations from skeleton_noroot to the standard mannequin. This is fairly trivial, as the bones have exactly the same name and the skeletons are actually identical except for the root and IK bones. More information on the retarget manager can be found on Epic’s site.