The TriangleFanArray

The TriangleFanArray provides a different way of providing efficient primitive description. In the previous section the TriangleStripArray used a base triangle and then added each new triangle by using the last two vertices of the previous triangle and adding a new vertex to complete the shape. For this representation, all of the triangles in a single fan use one common vertex and then build the structure around that, much like taking the top of a cone, slicing down one side and unfurling it into a flat shape (though flatness is not a requirement for fans).

To describe the geometry for a single fan, you start with the common vertex. This is the first vertex of the fan. Next declare two more vertices to create your first triangle. To add another triangle to the fan, take the last vertex declared, the initial vertex and the new vertex triangle and that is now the new triangle. As with the strip version, each fan must consist of a minimum of 3 vertices and then an additional vertex for each triangle added after the first. Figure 13 illustrates 3 fans with 3, 4 and 6 vertices.

Figure 13: A triangle fan array showing strips of length 3, 4 and 6

Construction

    int format = GeometryArray.COORDINATE |
                 GeometryArray.COLOR_3;

	int[] fan_counts = { 4 };

    TriangleFanArray tris = new TriangleFanArray(4, format, fan_counts);

After creating the basic primitive, all you need to do is fill in the vertex values through the usual setCoordinate() method. The coordinates must be placed in, in the order that they will be used in the fans as the renderer just starts from the first fan pulling coordinates from the array before moving onto the second fan etc.

    float[] vertices =
    {
        0,   0,  0,   // v1
        1,   5, -3,   // v2
        0,   2,  0,   // v3
        -1,  3,  1,   // v4
    };

    lines.setCoordinates(0, vertices);

Figure 14: A triangle fan with a "missing" triangle due to winding rules to determine front facing for culling purposes