I have a spot light that's keyframed to move around, XYZ axes, and keyframed to pointofinterest around, again 3 axes.
I would like to parent another spot to the "point of interest" and X, Y, Z position (I have them split up), but have everything reversed. I assume expressions are the way to go, but I have very little knowledge of what they are capable of.
If you imagine a physical spot light, I basically want to stick another one on the back of it and point it in the other direction, no matter where the origina spotlight goes.
I haven't found anyone online who has done this, but if there are any resources or plugins that might help me, please point me in the right direction.
I have a few questioins of my own.
You say you want to parent another spot to the point of interest but have everything reversed. This makes no sense to me. The point of interest is where the spot is pointed and that has no inverse. There is a line between the spotlight position and the point of interest. That's how you would do it with an expression, but there's a far easier way.
Here's the steps. Create spot 1. Set the point of interest to the comp center for X and Y then set Z to something like 500. Set the X and Y position of the spot 1 to the comp center and Z to 0. What you want to do is end up with the X and Y values of the position and point of interest to be the same and the Z value offset to a reasonable distance.
Now duplicate spot 1, add a 3D null. Copy the position value for spot 1 to the null. parent Spot 1 to the null. Rotate the null 180º in y. Parent spot 2 to the null. Animate the null.