Polygons are typically drawn by filling in all the pixels enclosed within the boundary, but you can also draw them as outlined polygons, or simply as points at the vertices.
A filled polygon might be solidly filled, or stippled with a certain pattern.
A polygon has two sides - front and back - and might be rendered differently depending on which side is facing the viewer.
glPolygonMode() controls the drawing mode for a polygon:
void glPolygonMode( GLenum face, GLenum mode );
parameter face can be:
By default, both the front and back faces are drawn filled.
For example, you can have the front faces filled and the back faces outlined with two calls to this routine:
glPolygonMode(GL_FRONT, GL_FILL); glPolygonMode(GL_BACK, GL_LINE);
Polygons are drawn in such a way that if adjacent polygons share an edge or vertex, the pixels making up the edge or vertex are drawn exactly once - they're included in only one of the polygons. This is done so that partially transparent polygons don't have their edges drawn twice, which would make those edges appear darker (or brighter, depending on what color you're drawing with).