Hey! and thanks for taking the time to stopping by 
So im making a window for a project, and it got a pattern that the light shines thru.
I was intending to just use opacity and make the pattern on a plane to save geometry however i would still like my static light that shines thru the window to display the pattern in its shadow.
here is the reference for the window:
How would you guys suggest achieving it? 
I wonder if an opacity mask couldn’t do the trick. One plane, one material, and opacity handling only in the window slot.
You could make a high res window with full pattern geo and a low res plane with material only pattern. Place them both in the same spot and then turn off visibility on the high res and shadows on the plane. Like this:
Not entirely sure this would work but if you try it out do let us know.
Thank you both
Tried out both metods but they got too blurry thru opacity so i went with what waves said.
Well if the light never moves maybe you can just create a mesh that matches your pattern exactly and then bake the shadow in. Afterwards just delete the mesh pattern so you can still create the illusion that the pattern is casting shadows. That might be one possibility, it wouldn’t work if you’re using dynamic lights though because if the light ever moves the shadows won’t follow along.