Hi. How difficult is skeleton retargeting? I want my mixamo character to use unreal engine’s skeleton and ues animations. I heard that it is a nightmare, a very difficult stuff that takes months to years to do for one skeleton. Is that true?
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Hello my friend. Althought I am still using UE3 (UDK), however I was able to develop a very good workflow for retargeting animations, from any skeleton to any other skeleton (biped, human).
It was hard for me to implement the workflow, however now in question of some minutes I am able to retargetting animations.
You will need Maya and Motion Builder.
1- Import any mixamo character to maya (it will act as a template). Open the human IK panel, then click on create skeleton definition, then you manually add each bone of the skeleton on the correct humanIK skeleton slot (to define which is the hand, spine1, spine2,…). Then you save the definition as mixamo (i.e)
2- Do the same to the Unreal Engine 4 Sample Robot (you need to import it to maya)
3- Now you need to export the mixamo character template from maya to FBX and import this on MotionBuilder.
4- Now in motion builder you need to manually set this character into T position by selecting the bones and rotating them. After set in T position, you need to open humanIK panel in MotionBuilder, and load the skeleton definition you saved from maya (maya and motion builder share the same skeleton definition)
5- Now you need to characterize the character in Motion Builder (create rig, FK/IK, Biped), name it as MIXAMO
6- Now save the file as something like mixamo_base_animations.FBX inside motion builder.
7- Do the same for the UE4 Robot in motion builder (set T position, load definition, characterization…), name the character as UE4
8- Now we have 2 motionbuilder files (ue4_base_animations and mixamo_base_animations), each one ready for retargetting.
9- Now you need to load any Unreal Engine 4 Animation on the ue4_base_animations motion builder file. I mean you need to extract the animations from the original UE4 animation set, you need to have each animation exported as a single FBX file.
10- Supposing you have the UE4 run forward animation in FBX format. So now open the ue4_base_animations motion builder file. Then import the fbx animation (run forward fbx) and select option to merge all. After import the animation, change the character source to none (humanIK panel), plot the character animation to control rig. Now select merge fbx no animation and select the mixamo_base_animations.fbx motion builder file.
11- Now the retarget magic, just change the character to MIXAMO in HumanIK panel, then on source, change from Control Rig to UE4 and VOILA!!! The animation is being retarget.
It should work almot 99% perfectly, just sometimes you need to edit some minor details, like finger rotation, because of difference between characters in size and form.
So this way I can retarget animations from any source to any skeleton. I work too much with game modding, so I can easily use animations from one game to another, specially character animations.
Cheers.
Wow man such a good method thank you very much!!! It’s very valuable
Re-targeting using Motion Builder is a snap depending on the animation set you have on hand. For example Kubold’s sets comes with the MB ready characterized source so if you have your rig characterized as well you can set your rig to character input and use a few tricks to match up the animations prior to plotting the takes down to the base rig.
The trick is if you can characterize the rig you can batch process 1…10…100 animation takes in about the same time as it takes to do just one so unlike doing the work in Maya, which could take a very long time to do one at a time, you can do all of the work in an afternoon over coffee and donuts. 
Re-targeting Mixamo animations is easy as the naming convention used is MB’s default so in most cases you can drag and drop a character template directly from the asset browser.
Sounding like an ad if one is serious about animation, video game or other wise, MotionBuilder is the tool of choice. 
The only negative is the cost but still cheaper than if your hobby was golf.
Yeah man, Motion Builder is not so cheap, however it is worth each cent 
I know there are many other options, however, MotionBuilder is the top. Be sure that almost 95% of pro game studios (Bethesda, Netherrealms, Ubisoft,…) use MotionBuilder in their workflow. Be sure that 95% of AAA games, like Battlefield 1, Call of Duty World War, and others have their animations captured, edited, created in MotionBuilder. Mortal Kombat X Fatalities and all animatons were created / captured in MotionBuilder.
About batch processing the retarget animation process, I sincerely prefeer not doing so, because you will indeed need to make some minor adjustments on the animations, some bone rotation, position. So I like to check each frame of my animations to ensure the retarget process is perfect
Cheers.
My…It’s 1000 dollars. My eyes almost escaped the orbits
So if you can`t afford MB, you can try this Animation Retargeting (Different Skeletons) | Unreal Engine Documentation