XNA Xtended:
        Creating a BoundingSphere that encapsulates the entire model

BoundingSpheres are really useful, they allow you to check collision with only one addition (their position) and one distance calculation (which is three squares and additions, and one square root). The problem is, you get one BoundingSphere for each mesh, what do you do if you have a multi-mesh model? You've gotta calculate your own BoundingSphere. Thankfully, it's not that hard, let's have a look:

First, let's figure out the math of how and why. If a mesh is inside the sphere of the main mesh (which is safe to assume it's mesh[0], as you'll probably make the hull of a ship before the superstructure, engines, bridge or whatever), then you don't need to do anything. If it's outside... well, you could try and calculate a sphere that contains both spheres, but that would take some more complex operations, you would have to find the two farthest points of the spheres and create a sphere centered midway between them with a radius half their distance, but we can spare the time for now, as well as a calculation or two. You can try this however yourself.

So when a sphere is outside the original sphere, we'll expand the original sphere to include it. One step at a time though:

Step one, determine if the original sphere includes the secondary sphere. To be within the original sphere, the distance between the centers has to be less than the radius of the original, and it has to be less by at least the radius of the secondary. You can just pass the radius of the second sphere to the right-hand side of the equation. So the evaluation reads as follows:

Vector3.Distance(mm.BoundingSphere.Center, sphereCenter) > sphereRadius - mm.BoundingSphere.Radius

Now, throw that into an if() statement, and you know if the original is sufficient. Next, you need to expand the original sphere. One, perhaps naive thought, is to just add the radius of the second sphere, the problem with this is that, in rare occasions, the second mesh may be far enough from the original mesh that it lays entirely outside the original mesh's sphere. The solution is for the original sphere to increase by the distance to the farthest point of the second sphere from the first one's center. The center remains the same, but the radius now becomes the distance between the two spheres plus the radius of the second sphere:

Vector3.Distance(mm.BoundingSphere.Center, sphereCenter) + mm.BoundingSphere.Radius

Now let's fill in the gaps, essentially we just have to store the data into a new BoundingSphere structure:

    this.sphereCenter = model.Meshes[0].BoundingSphere.Center;
    this.sphereRadius = model.Meshes[0].BoundingSphere.Radius;
    foreach (ModelMesh mm in model.Meshes)
    {
        if (Vector3.Distance(mm.BoundingSphere.Center, sphereCenter) > sphereRadius - mm.BoundingSphere.Radius)
        {
            this.sphereRadius = Vector3.Distance(mm.BoundingSphere.Center, sphereCenter) + mm.BoundingSphere.Radius;
        }
    }
    this.boundingSphere = new BoundingSphere(sphereCenter, sphereRadius);

Where mm is the ModelMesh inside the model. This concludes the tutorial, check out my other tutorials if you didn't come here through my tutorial listing, just click on the big fancy picture of my name at the top of the page!