Normal for surface of a cube in XNA - xna

Question

Can anyone please help.
I have a cube which I have made in 3DS Max. I don't know the dimensions of the cube. Is there a way to get the vertices of each of the triangles of the faces of the cube? I am trying to get the normal to one of the faces of the cube to determine which way its pointing. So if I can determine the vertices I can get the normal for the face if I have 3 vertices, V1, V2 and V3, ordered in counterclockwise order, I can obtain the direction of the normal by computing (V2 - V1) x (V3 - V1), where x is the cross product of the two vectors.
I have looked in my models .fbx file and I can see a number of values there:
Vertices: *24 {
a: -15,-12.5,0,15,-12.5,0,-15,12.5,0,15,12.5,0,-15,-12.5,0.5,15,-12.5,0.5,-15,12.5,0.5,15,12.5,0.5}
PolygonVertexIndex: *36 {
a: 0,2,-4,3,1,-1,4,5,-8,7,6,-5,0,1,-6,5,4,-1,1,3,-8,7,5,-2,3,2,-7,6,7,-4,2,0,-5,4,6,-3}
Are these my models vertices?
Also, I would assume that Vertices: * 24 would be my list of vertices, but why is there only 24? Should a cube not have 36 vertices? And finally, if the coordinates for my vertices are PolygonVertexIndex: * 36 these values just seem off to me when I imagine the cube in my head with those dimensions?
Or alternatively, is there a automatic way to get the vertices of a cube without having to manually enter all the values for each vertex? I might have a couple of models to
Any help would be greatly appreciated

Answer 1

I can't figure why you need that... because when you load a model it is calculated , internally each vertex will have the normal,...
Anyway it is easy to calc...
The three first indexes define the first triangle of a face, the next three, the other triangle of a face.
You need only one triangle to calculate the normal...
So with the three indexes access to the veretex array and get three points... A, B and C
Now your normal is the result of the cross product between two vectors formed with that vertex.
Vector3 Normal = Vector3.Cross(B-A, C-B);
If the normal go back or forward will depend on the A,B,C order, can be CounterClockWise or ClockWise, but every triangle of the model will be ordered in one way. So you will have try it and fix it

Answer 2

You can write an XNA program which reads your normals without much hassle.
If you still want to calculate them, however, use this C# code, taken from FFWD, as a guide. Check the URL for a more detailed discussion on pros and cons. Personally, I'm not too happy with the result, but for the time being it works. Of course, since this code is FFWD related (implementation of Unity's API for XNA), it does not match XNA exactly, but the mathematics remain the same.
/// <summary>
/// Recalculates the normals.
/// Implementation adapted from http://devmaster.net/forums/topic/1065-calculating-normals-of-a-mesh/
/// </summary>
public void RecalculateNormals()
{
Vector3[] newNormals = new Vector3[_vertices.Length];
// _triangles is a list of vertex indices,
// with each triplet referencing the three vertices of the corresponding triangle
for (int i = 0; i < _triangles.Length; i = i + 3)
{
Vector3[] v = new Vector3[]
{
_vertices[_triangles[i]],
_vertices[_triangles[i + 1]],
_vertices[_triangles[i + 2]]
};
Vector3 normal = Vector3.Cross(v[1] - v[0], v[2] - v[0]);
for (int j = 0; j < 3; ++j)
{
Vector3 a = v[(j+1) % 3] - v[j];
Vector3 b = v[(j+2) % 3] - v[j];
float weight = (float)Math.Acos(Vector3.Dot(a, b) / (a.magnitude * b.magnitude));
newNormals[_triangles[i + j]] += weight * normal;
}
}
foreach (Vector3 normal in newNormals)
{
normal.Normalize();
}
normals = newNormals;
}