for a school project I am suppose make a simulation. I want to do a day and night cycle and at night time there is the option to put down a light source the make things brighter. I'm an trying to do this by having a picture at the back and have a black  rectangle that I change the opacity on to make it darker or brighter. darker = transparency low and bright = high transparency. I don't know how to make a light source in the dark, my idea is to have a circle/circles with high transparency but the problem is that the shapes stack on top of each other so I can't put a circle on top of a dark rectangle and still decrease the transparency. Is there a way to erase the rectangle and replace it with a circle that has high transparency. Is there a better approach to this? please share.