When I use glPolygonMode(), I am confused about the GL_FRONT and GL_BACK. how to distinguish FRONT and BACK from a group of vertexes?
OpenGL uses the right-hand rule for deciding the front and back of polygons. take your right hand and curl your fingers in the direction of your vertices. your thumb is now representing the normal of the polygon. this is the front side. the other side is the back. glPolygonMode() allows you to specify front, back or both. as a general rule, you want to make sure all your polygons are “wound” the same way, be it clockwise or counter-clockwise.
The front of a polygon is the side that
has the normal faceing out of it. The
direction of the normal depends on the
winding of the vertices that make up the
polygon. (clockwise or counter-clockwise)