Ya i sort of side stepped the initial problem with a dual pass vertex shader.
but…
Unfortunately one final problem associated to the same thing appeared at the very end anyways. This time however i could see no way around it.
Below depth cones that don’t interpolate vertice to vertice throw off the distance, as im purposely throwing either the start or the end vertex out of bounds for a special case.
The shown artifact here can be seen as a arc from a point light in the grey box extending two arc’ed cones out above the box. Unfortunately while i can mitigate it i can’t properly fix it.
To make it work properly i would have to be able to create a vertice at a specific position between two vertices on the shader on the fly, to fix it which would allow me to rework the original double pass solution, into a single pass as well.
The only other option i can think of is probably not worth trying. Which is to use setdata to create a depth map per frame, while set data has decent speed and this would in other ways simplify things. Doing this computationally cpu side kinda defeats the whole objective.
So on this idea im going to admit defeat.