Source PostScript (sortp3d.pps)

/sortp3d {
6 dict begin
   /M1 defpoint3d
   /M0 defpoint3d
   M1
   /z1 exch def
   /y1 exch def
   /x1 exch def
   M0
   /z0 exch def
   /y0 exch def
   /x0 exch def
   x0 x1 lt {
      M0 M1
   } {
      x0 x1 gt {
         M1 M0
      } {
         y0 y1 lt {
            M0 M1
         } {
            y0 y1 gt {
               M1 M0
            } {
               z0 z1 lt {
                  M0 M1
               } {
                  M1 M0
               } ifelse
            } ifelse
         } ifelse
      } ifelse
   } ifelse
end
} def