Transformation offsets of Copy Rotation, Copy Location, Copy Scale constraints

Right now we have Copy Rotation , Copy Location and Copy Scale constraints that copy corresponding transforms from target bone/object. But there is no way of adding any offsets to copied transforms through bone transforms or constraints options (there is a workaround with empties that I'll describe later). To simplify everything I'll describe only Copy Rotation behavior but Copy Location and Copy Scale constraints behavior is pretty much the same.

Default Copy Rotation constraint behavior

Both rigs have an arm bone chain with different bone rolls of upper arm bone. If we simply copy rotation of that bone we'll end up with different arm orientations on bone's local Y axis (one can notice that - wrists are not aligned). And there is no way of fixing that. Child Of instead of Copy Rotation also will not help here because rest pose of both rigs is different.

Workaround with Empties

Here instead of Copy Rotation directly from target bone we:

1. Create an Empty and add Copy Rotation and Copy Location constraints with bone from which we want to copy rotation as target;
2. Create Empty.001 and make it child of Empty ;
3. Add Copy Rotation constraint to the bone to which we want to copy rotation with Empty.001 as target;
4. Add all the rotation offsets you want to Empty.001

That's the most common way of doing retargeting from one rig to another in Blender and used in such addons like UEtoRigify. As one can see that's a bit overcomplicated way of handling offsets and also overloads a scene with lot of empties.

There are few ways of how we could solve that to be more user friendly and to unlock more options to riggers and animators in use cases different from mocap retargeting:

Proposed №1: Add Rotation constraint

The best way IMO is to add additional constraint that adds Rotation , Location or Scale offset to bone/object. Basically this constraint will just add defined values to rotation, location or scale. That also will open a whole new world to riggers.

Proposed №2: Add Offset option to existing Copy Rotation , Copy Location , Copy Scale constraints

We could also have additional option in Copy Rotation , Copy Location , Copy Scale constraints where one could define offsets of constraint.

Proposed №3: Delta Transforms

Empties and other objects have this option by default, one can specify delta transforms. These are the transforms added to original transforms. But there is no such feature for bones! So implementing bone delta transforms also is an option here. But that implementation has a limit of just one delta transform. With Proposed №1: Add Rotation constraint approach theoretically we could have as much transformation additions as we want.

Copy Rotation constraint example .blend file