What is the post process effect that you are trying to limit to certain space. You could just simply make view space AABB and test if the pixel position is inside it and process that pixel and if not then just pass through.
Did you read through the thread?
There is not one single but various post process effects I'm using, one of them is (fake) light reflections on the floor. And I have to select which shader to activate in the host application which is not capable of doing any directX API / raw 3d stuff - so, nothing here to "process" or "pass through". It's a question of dealing with a set of coordinates and a handful of camera properties (world pos, view direction, FOV). Mainly a math / geometrical / trigo question.
DvDmanDT's idea is the best so far, but I'm sort of stuck on making an algorithm that works well (in terms of creating the closest possible match, as it's an approximative approach).
I mean, the box to compare the camera pos and dir against consists of 8 corner points. So if I would be able to check how many / which of those corner points are located inside the current screen boundaries, or outside of the screen - what would be the best way to check whether the inside of the box actually "fills" most of the screen?