Painter's algorithm

Painter's algorithm is one of the simplest solution to visibility problem in 3D graphics. When projecting 3D scene into a 2D plane, at some point, you need to decide which polygons are visible and which are hidden.

The name "painter's algorithm" references to a silly painter who paints the distant parts of a scene at first and then covers them by those parts which are nearer. Similarly, when you want to use this algorithm in your rendering system, you have to sort all thy polygons by their depth and then paint them in this order. You will over-paint the parts that are normally not visible and thus solve the visibility problem.

Note that this approach has several problems. What happens, when polygon A partly covers B, B partly covers C and C partly covers A again? Painter's algorithm will fail in this case. Other problem is that it's very slow because the computer needs to calculate all the intensities at all points of all polygons even if they will be dropped out immediately, because the polygon is not visible.