Making a local orient constraint

In my research project, at one stage I needed an orient constraint that works locally, i.e. it works only on the hierarchy below a particular transform. I believe we could do that by re-connecting orient constraint's targetMatrix. However, since I don't know how orient constraint works under the hood and it does not allow me to have dynamic offset, I had to come up with my own constraint using matrix calculations.

So I have been digging into how transforms work and how to visualize the matrix concatenation. I was successful at making such constraint. However, I am still working on adding an offset that can dynamically be changed without affecting the rotation of the controlled object. I have yet to use this on my research project (fingers crossed). But, knowing I had success with a sample hierarchy gives me hope.

Follow this thread for detailed discussion on this orient constraint. LINK

edit: (26.06.09)
Later in my research I realized that what I had found out was not the best solution for what I was doing.

Continue on Twist

The best way to understand twist is to think of the joint as a vector and twist is a spin around that vector. This leads to the topic of how quaternion transform works. More on that later.

When it comes to vector related implementation two tools mentioned below are a great help.

-> Single IK chain with pole vector can act as a vector. And spin is defined by pole vector or twist attribute.
-> Aim constraint comes very handy when vectors are involved. You can use aim vector and up vector to control the rotation of an object.

Keep in mind though that they do flip at one point or the other. The pose does not change generally, but if you are doing calculations based on their rotation then you are highly prone to flipping issues.

Twist Setup Notes

When doing a twist setup to distribute the rotation uniformly there are some important things to consider. Following are some notes from my experience while working on forearm and shoulder twist setups.

-> Twist cannot be associated with any single axis. Due to the nature of Euler angle, based on your rotation order axis that twist the joint could change.
For example,
A shoulder joint with zxy rotation order in rest position will twist around x-axis. But if you bring shoulders in front by rotating on y axis, now the z-axis is responsible for twist.

Hence, we cannot plug one axis from shoulder joint to the twist joints. Of course we can plug all three axis and it may work, but that's really not the elegant solution. Inspired by this tutorial, I think Aim constraint is the most elegant solution so far.


->Another issue is the source of twist and in which direction its propagating. For example, for forearm twist, rotation originates from the wrist joint and propagates towards the elbow joint. So the direction is from a child joint to its parent joint. However, in shoulder twist the direction is opposite, twist propagates from parent to the child joint. This changes the equation of how to make twist joints inherit motion from shoulder joint and dampen the rotation at the same time.

Scale Constraint on Joints

It does not work properly. As I had posted earlier, joints use a special scale connection with inverseScale attribute (only joints have this tranform property) when you create a chain hierarchy. This gives a special behavior when scaling a parent joint. And using scale constraint messes up this inverseScale calculation in joint transform and so the child joints scale weirdly when scaling the parent joint.

So use connection and a blend node(blendColors) on a joint chain to blend between scale values.

edit:
From reading on forums I get that the problem is with constraints not taking "segment scale compensation" into consideration. So orient constraint suffers from the same problem. I had encountered this problem when working on a stretchy setup on arms.