I would like to render a part of my whole scene. The purpose is following: I have an Ocean in my scene, where a fish is comeing out of the ocean and back into the ocean. I want to render out only the part of the fish that is shown in the scene without the ocean. I wanna do it that way, cause I want to composite it in an external prog which applies different filters to the different parts of the scene (fish, ocean, background).
Any ideas how I can render out the seeable part of the fish only?