Image sampler scaling issue

I am workin on a facade that shades itself.
My approach is to map an image of an shadestudy
onto a surface (same size). Using the image sampler
and setting its dims to size of the surface. But the
surface stays flat as long as the domain size of the
sampler is on the supposed values.

Am I missing some of the logic here?

20170104_curtain - (70.7 KB)


In the legacy forum I found this definition.
It solves my issues so I will use it instead.
I think the initial error I made lies within vector and point math.