/// <summary>
/// The matricies can only be calculated if the mesh has all positions, indices and uv coordinates initialized.
/// Thus call this after initializing the mesh.
/// </summary>
private void CalcTangentsAndBitangents()
{
float maxId = IDs.Max();
if (maxId > TexCoord.Count)
{
throw new ArgumentException("Not all TexCoords are set, tangents and bitangents can not be calculated.");
}
//Calculate TBN for every triangle in the mesh
List <Vector3> tangentPerTriangle = new List <Vector3>();
List <Vector3> bitangentPerTriangle = new List <Vector3>();
for (int i = 0; i < IDs.Count; i += 3)
{
int id0 = (int)IDs[i + 0];
int id1 = (int)IDs[i + 1];
int id2 = (int)IDs[i + 2];
Vector3 p0 = Position[id0];
Vector3 p1 = Position[id1];
Vector3 p2 = Position[id2];
Vector3 q1 = p1 - p0;
Vector3 q2 = p2 - p0;
float u0 = TexCoord[id0].X;
float u1 = TexCoord[id1].X;
float u2 = TexCoord[id2].X;
float v0 = TexCoord[id0].Y;
float v1 = TexCoord[id1].Y;
float v2 = TexCoord[id2].Y;
Vector2 s = new Vector2(u1 - u0, u2 - u0);
Vector2 t = new Vector2(v1 - v0, v2 - v0);
var leftSide = Mat2x2(t.Y, -t.X, -s.Y, s.X);
var rightSide = VecToMat(q1, q2);
var det = (1.0f / (s.X * t.Y - s.Y * t.X));
Matrix4x4 tb = Matrix4x4.Multiply(Matrix4x4.Multiply(leftSide, det), rightSide);
Vector3 tan = new Vector3(tb.M11, tb.M12, tb.M13);
Vector3 bitan = new Vector3(tb.M21, tb.M22, tb.M23);
tangentPerTriangle.Add(Vector3.Normalize(tan));
bitangentPerTriangle.Add(Vector3.Normalize(bitan));
}
//Average TBN on every vertex
for (int i = 0; i < Position.Count; i++)
{
int countOfPointMatches = 0;
Vector3 resultingTangent = Vector3.Zero;
Vector3 resultingBitangent = Vector3.Zero;
for (int k = 0; k < IDs.Count; k += 3)
{
int id0 = (int)IDs[k + 0];
int id1 = (int)IDs[k + 1];
int id2 = (int)IDs[k + 2];
if (id0 == i || id1 == i || id2 == i)
{
countOfPointMatches++;
resultingTangent += tangentPerTriangle[(k - k % 3) / 3];
resultingBitangent += bitangentPerTriangle[(k - k % 3) / 3];
}
}
if (countOfPointMatches != 0)
{
resultingTangent = resultingTangent / countOfPointMatches;
resultingBitangent = resultingBitangent / countOfPointMatches;
}
Tangent.Add(resultingTangent);
Bitangent.Add(resultingBitangent);
}
}