Question Skewed Billboard

I've been trying to create a skewed billboard effect with the base of the quad (bottom two vertices) staying in place on the Y axis and the upper vertices get pushed back and forth looking down and up. Useful for trees (same method used in Mario 64).

I already have other billboard scripts, but I haven't figured out how to keep the bottom of the quad fixed like mario 64's trees.

Any suggestions?

 

For anyone who might come across this thread, using: transform.forward = cam.transform.forward method for billboarding you only need to set the pivot to the correct position (base of the tree) to get the Mario 64 result.