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Mixamo offers users the ability to upload characters to the website and re-target them to our skeleton. If you are not familiar with the term re-targeting, it is the process to transferring animation from a character skeleton to another one which may have a different size or number of bones. In most cases, Mixamo automatically maps the user’s skeleton, allowing for real-time retargeting. Once the mapping is taken care of, the retargeting process uses forward and inverse kinematics (FK and IK) to transfer the motion. By doing this, we can apply thousands of animations to your character instantly.
To understand how this works, it is very important to know a few terms. The first two are probably terms you have heard, Parent and Child. This is a bi-directional relationship, a child has a parent and a parent has a child. The child is simply grouped to the parent so that if the parent is moved, the child moves or rotates with it. However if the child moves, the parent is not affected. An example would be your knee and your foot. If your knee moves in space, your foot moves as well. However if you move your foot, your knee is not affected, unless your foot is pushing off of something.
If you understand parent and child relationships in a skeleton, then it is easy to see that a parent of a child can also be a child of another joint. The knee is the parent of the foot but child of the hip. If you want, you could say that the foot is the grandchild of the hip or the hip is the grandparent of the foot. What if something is a child of a child of a child of a child of a joint? This could get confusing! To simplify things, everything can be described as an Ancestor or a Descendant. A parent is an ancestor. A parent of a parent of a parent of a child is also an ancestor of that child in the same way that your father, grandfather and great grandfather would be all be your ancestors. Your children, grandchildren and great grandchildren would all be your descendants. To understand retargeting, ancestors and descendants are the two concepts you need to remember.
Let’s look at what happens when we go to map our character. Often when you upload a character to Mixamo, it will be automatically mapped by our system. If it is not automatically mapped, it is either because we don’t recognize the hierarchy or there is a fundamental problem. Here are some issues that you might run into.
The first thing to remember, and a common cause of mapping errors on Mixamo, is that the hierarchy can’t be broken. This means that every joint in the skeleton must be connected to the skeleton as a descendant of the root node. Conversely, the root node should be the ancestor of every other joint in the skeleton. This may be obvious but often, rigs will have broken hierarchies and use other methods such as expression to control movement. This is fine in a program like Maya or 3D Studio Max, but it won’t work if you upload it to Mixamo.
It is also good to upload only one skeleton, an FK rig. If you have multiple hierarchies for IK or other purposes, uploading them in the file will make your life harder. If you are driving your bind rig with FK and IK rigs through expressions or another method of control, upload only the FK rig. Once you have picked your animation, you can then download your FK rig and import the animation onto the FK rig inside the original file. If you uploaded a 3ds Max Biped or 3ds Max Cat character, you can download the animation and import it onto your control rig. See our 3ds Max and 3ds Max Cat workflows.
It is also important to upload a character in T-pose. This is the pose where the characters arms are out to the side parallel to the ground, and the legs are beneath the body. You can also upload in A-pose where the arms are at 45 degrees. Your character may still work in other poses, but the further away you get from the T-Pose, the harder it is to accurately transfer animation.
The other criterion for your skeleton is that it needs a minimum number of joints. If you look at the Mixamo skeleton, you will see joints with an inner circle. These are the joints that need to be mapped to a joint in your skeleton. There are fifteen required and they are:
Hips (or root)
Waist (a descendant of the Hips)
Left and Right Hips (descendants of the Hips)
Left and Right Knees (descendants of Left and Right Hips respectively)
Left and Right Feet (descendants of Left and Right Knees respectively)
Head (descendant of the waist or whatever the last spine joint is which would also be a descendant of the waist. You can map as many spine joints as you wish.)
Left and Right Shoulders (descendants of the waist or whatever the last spine joint is)
Left and Right Elbows (descendants of Left and Right Shoulders respectively)
Left and Right Hands (descendants of Left and Right Elbows respectively)
In total there are fifteen joints required with this Ancestor/Descendant relationship. If your skeleton does not have at least these fifteen joints with the same relationship, it cannot map to our system. To complete mapping, you need to select each of the required Mixamo joints and select the corresponding joint in your skeleton to assign it. Once you have completed the required joints, you can continue to map additional (optional) joints in the Mixamo skeleton to yours. This will in some cases improve the animation quality on your character.
One good example is the clavicle. Many rigs have clavicles and you may have one that you want to map. If you look at the clavicle joint in the Mixamo skeleton, you will see that it is a descendant of the last spine joint but an ancestor of the shoulder. Because of this, you will want to map it to one of your joints that is a descendant of your last spine joint but an ancestor to your shoulder. If you have more than one joint that fit that criterion, any of them can be mapped to our clavicle; it’s up to you to decide which is best.
You may notice that if your skeleton has hands in its hierarchy, they may automatically map when you assign the wrist. If not, you can map them manually. It is beneficial to map hands if you have them because many of Mixamo animations include finger motion. It is also important to map the ToeBase because it will keep your characters feet from penetrating the ground during animation. Remember, these additional joints are optional but it is a good idea to map them if you can.
With this understanding, you should be able to map your character in Mixamo and start applying animation.
