Camera obscura
At its most basic, the camera obscura is a simple box (which may be room-sized) with a small hole in one side, (see pinhole camera for details on how to build one). Light from only one part of a scene will pass through the hole and strike a specific part of the back wall. (The projection can be made on paper on which an artist can copy the image.) As the pinhole is made smaller, the image gets sharper, but the light-sensitivity decreases. With this simple apparatus, the image is always upside-down. By using mirrors, as in the 18th century overhead version, it is also possible to project a "right-side-up" image.