Painter's Algorithm
As the name suggests, the algorithm follows the standard practice of a painter, who would paint the background (such as a backdrop) first, then the major details (such as a face), and finally the finer details (such as eyes). This is also called Depth Sort or Priority algorithm.
To determine the specific nature of the relative positions of polygons, it is necessary to define the x-, y-, and z-extents, of a polygon in 3-D space as the respective coordinate ranges of the bounding (i.e. enclosing) box. For the quadrilateral ABCD, the x-extent is (xD - xB), y-extent is (yA - yC), and z-extent is (zB - zD). The x- and y-extents of two polygons will enable decisions to be made on their overlap, and the z-extent will define their relative positions with respect to the viewer.