Game Programming using Qt 5 Beginner's Guide - Second Edition

Time for action – Creating a regular polygon item

Let's learn about the scene-graph by delivering an item class for rendering convex regular polygons. We will draw the polygon using the OpenGL drawing mode called "triangle fan". It draws a set of triangles that all have a common vertex. Subsequent triangles are defined by the shared vertex, the vertex from the previous triangle, and the next vertex specified. Take a look at the diagram to see how to draw a hexagon as a triangle fan using eight vertices as control points:

The same method applies for any regular polygon. The first vertex defined is always the shared vertex occupying the center ...