when viewed from a particular reference point r accordingly the orientation decision variable indicates that the actual orientation of vertices 16 of the polygon 14 is inverted from the apparent orientation of vertices 16 as indicated by the sign of the cpt as evidenced in the exemplary set of equations provided above the orientation decision variable may be a three bit variable that is determined substantially concurrently with sorting of the positional data for each vertes 16 of a polygon 14 by way of example the following table illustrates for each sort indication variable the order in which vertices 16 of polygon 14 have been sorted from sort step 2 to sort step 0 or 2 0 relative to the presorting orientation as well as the corresponding one bit orientation decision variable t sort 2 0 relative vertex order t 000 v0 v2 v1 1 001 n a n a 010 v0 vl v2 0 011 v1 v0 v2 1 100 v2 v0 v1 0 101 v2 v0 v1 1 110 n a n a 111 v1 v2 v0 0 based on the sorted vertices a cpt may be calculated e g by the processor as indicated at reference character 116 of drawing fig2 the sign of the cpt which is determined e g by the processor as known in the art at reference character 118 of drawing fig2 along with the orientation decision variable may then be.