/*!
SLMesh::calcTangents computes the tangent and bi-tangent per vertex used for
GLSL normal map bumb mapping. The code and mathematical derivation is in detail
explained in: http://www.terathon.com/code/tangent.html
*/
void SLMesh::calcTangents()
{
if (P && N && Tc)
{
// allocat tangents
delete[] T;
T = new SLVec4f[numV];
// allocate temp arrays for tangents
SLVec3f* T1 = new SLVec3f[numV * 2];
SLVec3f* T2 = T1 + numV;
memset(T1, 0, numV * sizeof(SLVec3f) * 2);
for (SLuint m = 0; m < numM; ++m)
{ for (SLuint f = 0; f < M[m].numF; ++f)
{
// Get the 3 vertex indexes
SLushort iVA = F[M[m].startF + f].iA;
SLushort iVB = F[M[m].startF + f].iB;
SLushort iVC = F[M[m].startF + f].iC;
float x1 = P[iVB].x - P[iVA].x;
float x2 = P[iVC].x - P[iVA].x;
float y1 = P[iVB].y - P[iVA].y;
float y2 = P[iVC].y - P[iVA].y;
float z1 = P[iVB].z - P[iVA].z;
float z2 = P[iVC].z - P[iVA].z;
float s1 = Tc[iVB].x - Tc[iVA].x;
float s2 = Tc[iVC].x - Tc[iVA].x;
float t1 = Tc[iVB].y - Tc[iVA].y;
float t2 = Tc[iVC].y - Tc[iVA].y;
float r = 1.0F / (s1*t2 - s2*t1);
SLVec3f sdir((t2*x1 - t1*x2) * r, (t2*y1 - t1*y2) * r, (t2*z1 - t1*z2) * r);
SLVec3f tdir((s1*x2 - s2*x1) * r, (s1*y2 - s2*y1) * r, (s1*z2 - s2*z1) * r);
T1[iVA] += sdir;
T1[iVB] += sdir;
T1[iVC] += sdir;
T2[iVA] += tdir;
T2[iVB] += tdir;
T2[iVC] += tdir;
}
}
for (SLuint i=0; i < numV; ++i)
{
// Gram-Schmidt orthogonalize
T[i] = T1[i] - N[i] * N[i].dot(T1[i]);
T[i].normalize();
// Calculate temp. bitangent and store its handedness in T.w
SLVec3f bitangent;
bitangent.cross(N[i], T1[i]);
T[i].w = (bitangent.dot(T2[i]) < 0.0f) ? -1.0f : 1.0f;
}
delete[] T1;
}
}
