If I have a polygon with 5 vertices , I can just get the cross product of 3 and that would be the normal .... right.

If they all lie on the same plane, yes. If you are using 5 verts that might not always be the case though.

so four could also not also lie on the same plane so I am relgated to only using tirangles.

hmmmmmmm

Am I right in saying that 4 vertices can be on different planes.

(leading to the fact that I cant get the cross product of 3
vertices to get the normal of the quad) ?

You could take a look at the SIGGRAPH ''98 course notes, they have a tiny section that deals with finding normals for poly''s with more than 3 vertices.

Codito, ergo sum - Graphics Programming

Yes, four vertices can lie on different planes. For example (0, 0, 0) (1, 0, 0) (1, 1, 0) and (0, 1, 10) don''t all lie on the same plane. When you calculate the normal with points 1, 2 and 3, you get the plane that those three points lie on. But if you calculated the normal with points 2, 3 and 4 you would get an entirely different plane. If you are certain that the vertices of your five vertex face will always be on the same plane there is no problem, you can calculate the normal using just three of the points. But, if you cannot guarantee that the points will always lie on the same plane, then you cannot calculate a single normal for that face.

Nate Miller

Excellent , thanks guys.