Twin Skeleton is a tool to quickly, simply and consistently build a skeleton and attach it to some predefined points. This can be used if you happen to have consistently named control objects (position them and then let the tool build the skeleton), but it is most useful in games.

Sometimes (especially with game engines) you cannot use complicated autorigging systems. But those systems have great tools that come with them, GUI pickers etc. The compromise is to create a lesser Twin Skeleton attached to the more complicated one, sometimes with less joints than the complicated one too (such as omitting twist joints). Then skin that Twin Skeleton to the model instead.

This script facilitates that.

To install, download the "install.mel" file and drag/drop it into Maya. Or else download this folder. Unzip and name it "twinSkeleton". Throw it into your scripts directory.

To run, add a shelf icon with the text:

import twinSkeleton as twin
twin.Main()

Creating the skeleton is a three step process, of which all steps are reusable.

Manually create the Skeleton you want to reuse across all characters. The size and joint position don't matter, but make sure the hierarchy and joint names are what you want. Take the time to get it right here!

Click "Build Skeleton" to create a skeleton file and save it somewhere.

Open a file with the advanced rig that you want to attach your Twin Skeleton to. Run the tool and click "Retarget Skeleton". Work your way around the advanced rig selecting and clicking the appropriate buttons to target the rig. You can click Position, Rotation, Scale to set them individually or simply click the joint name to attach all three. TIP: The rotation order determines the axis that aims down the joint. Use this if you need something other than "x" to aim down the joint.

Save the file. The file contains all the skeletons info as well as attachment information too, so you can save over the previous file if you like. If you have a new rig to attach your Twin Skeleton to, reload this file, retarget and save a new copy.

With your scene containing the advanced rig open, load the tool a third and final time. Click "Attach Skeleton" to place the skeleton on the rig.

TIP: The joints are oriented at each other, but also in relation to the bend in the existing rig upon creation. If you bend all the joints that need to bend (ie closed hand, bent elbows, knees, toes) when creating the Twin Skeleton, you'll give the tool the most accurate information you can to ensure rotations are correct. After building the Twin Skeleton in this case, be sure to revert the advanced rig back to its previous zeroed out pose before skinning.

The following options are important.

When picking joint rotation angles, limbs are straight forward. Point one joint at another. But when it comes to multi-joint junctions such as the hips or the hand it's not as clear.

With the option turned on, the root joint will be pointed at the furthest joint in the group. This works in most cases, but can cause problems if your character has certain body shapes (such as wide hips and a short torso) and you want it to point down the (shortest distance) spine. With the option off, the joint will be angled to match world space.

It is recommended to keep this on, and to pay special attention to the joint angles in potentially troubled areas (using the display option below).

With this option on it will keep rotations around an entire limb the same. ie: on an S shaped spine, + and - rotations will move in the same direction on all joints. This makes it easier to directly manipulate joints. Use this if you are planning on animating the joints directly or you need consistent rotation information.

With this off, joints will orient consistently to the angles presented on an individual bases.

Generally you would want to keep this option off if you're not manually touching the Twin Skeleton.

With this option on, all rotational axis will be displayed upon creation. You may have noticed all of these options are to do with rotation. It is vital you get your rotations consistent between rigs. So it's highly recommended that you turn this on to get a sense of what the joints are doing and to manually correct anything if need be.

So you've probably noticed by now that joint orientation is KEY. This goes double for reusing a skeletons animations in a game or copy / pasting to another rig. So to prepare the rig, there is a utility tool.

Select the joints that you need to reorient, and click the "Attach Marker" button.

Manually rotate the markers using whatever method you like into the correct rotation, then press "Orient Joints" to update all joint orientations to match their corresponding markers.