I have encountered the same problem yesterday. My solution is this one, it is vertex type independent and works quite good.
protected BoundingBox UpdateBoundingBox(Model model, Matrix worldTransform)
{
// Initialize minimum and maximum corners of the bounding box to max and min values
Vector3 min = new Vector3(float.MaxValue, float.MaxValue, float.MaxValue);
Vector3 max = new Vector3(float.MinValue, float.MinValue, float.MinValue);
// For each mesh of the model
foreach (ModelMesh mesh in model.Meshes)
{
foreach (ModelMeshPart meshPart in mesh.MeshParts)
{
// Vertex buffer parameters
int vertexStride = meshPart.VertexBuffer.VertexDeclaration.VertexStride;
int vertexBufferSize = meshPart.NumVertices * vertexStride;
// Get vertex data as float
float[] vertexData = new float[vertexBufferSize / sizeof(float)];
meshPart.VertexBuffer.GetData<float>(vertexData);
// Iterate through vertices (possibly) growing bounding box, all calculations are done in world space
for (int i = 0; i < vertexBufferSize / sizeof(float); i += vertexStride / sizeof(float))
{
Vector3 transformedPosition = Vector3.Transform(new Vector3(vertexData[i], vertexData[i + 1], vertexData[i + 2]), worldTransform);
min = Vector3.Min(min, transformedPosition);
max = Vector3.Max(max, transformedPosition);
}
}
}
// Create and return bounding box
return new BoundingBox(min, max);
}
All calculations are done in world space, this gives you world space bounding boxes (which is quite common for AABBs). Since it is quite expensive (but still performs good in real-time) just update it on dynamic objects and only when a rotation is performed (you can just translate AABBs with objects if that is the only transform).
