This process is part of creating light draft effect. And I am suggested to do the described process in order to enhance performance and I really want to do it.
However I am clueless on how I can apply "frustrum perspective matrix" from other camera to my camera perspective to chop off that square plane into shapes that fit into the "frustrum perspective matrix".
I am guessing I need to do this on CPU side first? But that's as far as my wild guess goes