the visibility ordering of polyhedral cells is efficiently determined by building an ordering graph , comprising oriented edges between two cells . each edge corresponds to the fact that cell a has to be projected , or rendered , before b . a set of ordering relations and rules that can be shown to generate , if one exists , a global ordering of the polyhedral cell complex . three different types of edges are used to accomplish this : mpvo , bsp and ppc edges . mpvo edges exist between two cells that share a face . to define the bsp edges , a bsp - tree of the boundary faces of the cell complex is constructed . during this construction , some of the boundary faces of the cells will be ‘ cut ’ by the bsp - tree ‘ extended ’ faces , into multiple pieces . if c is the boundary cell , and c ′, c ″, and so on , are the pieces of its boundary faces , the bsp_edge is defined to mean that cell c can only be projected after c ′ has been projected by the bsp . a cell c is in the ppc if one of the pieces that compose it , say c ′, has been projected by the bsp , but there exist other pieces of cell c that have not been projected . in this case , additional checks are done to ensure cells are not being projected out of order . this is accomplished by performing ‘ ray shooting ’.