The TriangleStripArray

A TriangleStripArray provides the most compact form of basic primitive when building geometry. Like the LineStripArray, the idea is to represent not just individual triangles, but a ribbon of triangles that all share common edges. To do this, you declare a starting triangle in the strip, and then for each additional triangle, you use the last two declared vertices as one edge of the triangle, and provide one more vertex to form the other two sides of your triangle. So, a strip of 2 triangles will declare a total of 4 vertices, a strip of 3 triangles uses 5 vertices and so on. This concept is illustrated in Figure 11 that shows three strips of length 3, 4 and 6.

Figure 11: A triangle strip array showing strips of length 3, 4 and 6

Construction

So, starting with a simple example that creates a strip with 2 triangles in it, you'll need the following code:

    int format = GeometryArray.COORDINATE |
                 GeometryArray.COLOR_3;

	int[] strip_counts = { 4 };

    TriangleStripArray tris = new TriangleStripArray(4, format, strip_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 strips as the renderer just starts from the first strip pulling coordinates from the array before moving onto the second strip etc.

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

    lines.setCoordinates(0, vertices);

Strips also follow the same rules for the individual triangles and quads when determining what is front facing and what is not. Since strips don't have any restriction on where you put vertices, it is possible for the strip to create a situation where a triangle folds backwards on the strip causing it to have the opposite winding as seen from the current viewpoint. When you look at this strip from the front, that folded back triangle would appear to be missing from the strip, as illustrated in Figure 12.

Figure 12: A triangle strip with a "missing" triangle due to winding rules to determine front facing for culling purposes