I’ve seen tutorials in which the guy draws a bounding box in the UV/Image Editor and then it renders only that part of the image, which’d be quite handy so that you can see the effects of little changes on some particular part of the scene without waiting for the full render. How is it done?
I don’t know how it would work in the UV/Image editor, but in the 3D viewport you can Shift-B and drag in Camera view to set a render border. To remove the border selection, either Shift-B and select the entire camera view, or turn off “Border” in the Render properties-> Dimensions panel.
That’s the ticket.