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. Have a look at the diagram to see how to draw a hexagon as a triangle fan using 8 vertices as control points:

The same method applies for any regular polygon. The first vertex defined is always the shared vertex occupying the center of the shape. The remaining points are positioned on the circumference of a bounding circle of the shape at equal angular distances. The angle is easily calculated by dividing the full angle by the number of sides. For a hexagon, this yields 60 degrees.

Let's get down to business and the subclass QQuickItem. We will give it a very...