I am trying to create an animation rig that uses nulls following track point data. Is there an expression I can use to make a null follow a track point like a parent? That is to say, the null point would stay at it's original position on the first frame and use the track point's position as a guide for animation. I'm able to somewhat do this by manually inputting x and y values for an offset, but since this rig will ultimately be used with a lots of different tracking data, I'm hoping there's a way to automatically calculate the offset without moving the null on the first frame at all.
I'd also like to have a slider control to be able to further tweak the null's position while still parented to the track point. Is this possible?