# How do I maintain the world pitch, roll or yaw of child objects?

I've got an application where I'm trying to display an object in 3D, with information provided by real-world accelerometers and inclinometers - sensors that pick up the angle of the equipment, relative to ground. The trouble comes when I want to show this 'world pitch' or 'world roll' on an object that is a child of another. I'd think that there is a trivial solution to the problem, but for the life of me, I can't figure out what it is.

Let's say I've got a robot arm with several pieces - "LowerArm", "Elbow", "UpperArm" and "Hand". "Hand" is the child of the "UpperArm", and "UpperArm" is the child of "Elbow". I'd like to place "Hand" at 0 degree pitch, relative to world-space, no matter how I move the parent objects. If I grab my "LowerArm", and rotate it 90 degrees, the hand should still lay 'flat', relative to the ground; in local coordinate space, it should moved 90 degrees, to compensate.

I want it to remain attached; so it should continue to follow the other parts, in terms of roll and yaw. If I twist the arm, the hand twists with everything else - it just maintains that 0 degree world pitch. Observe the yellow object, in the below example:

How can I do this? I would have thought that there was some kind of trick, with matrices or quaternions, that could resolve it; but I've had no luck, so far. It's hard to figure out what the new 'forward' vector should be.

• Anyone got any ideas? I'm tearing my hair out here, and I don't have all that much left! Jun 7 '17 at 12:43

Vector3 forward = Vector3.Cross(transform.parent.right, Vector3.up);