Which are you looking for–rendering a region of the camera image or building a region of the level? Those are completely different things. There’s little use for the ability to render a camera region(like Maya) even with a low-end graphics card it renders the full image very quickly since the engine is optimized for that purpose. The thing that takes a long time is baking the lighting which you can’t do by a level region as you were illustrating in the first post.