there are a few methods out there creating an upper arm roll, however, none of those methods work in all possible situations, so it’s quite normal you will have to switch to one of 2 solutions (like backwards and forward arm or something like this). if you actually animating and come to this area you have a problem…
so for my diploma thesis i came up with the following method i like to share with you.
- You will need an upper arm with at least 2 joints, better 3 or 4 to give a nice volume preservation. Then you will need also a driver arm setup for the ik with just 2 bones. The chains must have the same LRA-Orientation for this setup to work.
- Create a rotate-plane ik fort he ik-joints. This will be a simple arm-setup to later drive the deform joints.
- Now comes the funky part…
Create another rotate-plane ik for the first deform joint, set the polevector of the rotate-plane ik to 0-0-0. By doing this the ik will „remember“ it’s orientation and will ony flip if the ik-handle gets to the exact opposite. This trick will ensure the the first deform joint will not self-rotate, it will always keep it’s original orientation. This ik-setup acts a little bit like a aim-constraint.
- Now parent the rp-ik to the first upper-arm joint from the driver ik-chain. By doing the deform-chain will follow the driver-chain without doing the self-rotation. And thats exactly what we wanted: NO SELF ROTATION !!!
- The next step is, you guessed it, to apply the exact self-rotation or roll-difference gradually back to next 3 joints of the deform chain. To measure this difference we create a locator, which we parent under the non-self-rotating first deform-joint. Then we create an orient constraint from the first driver-ik-joint to the locator. This way the locator rotates with driver-chain, but since its parented to the deform-chain, which also rotates with the driver, except for the roll-component, you get the exact rotation difference.
- The last step is take the rotate value of the locator and divide it through the number of deform-joint – 1 and connect this value to the defrom-joints self-rotation.
- The Setup is finished. You will see it produces very stable results. It will never flip unless the arm is positioned to the opposite, which is for more or less every character impossible.
Hope you liked the Tutorial!