（转）Ogre中计算Tangent, 感觉很不错！

Vector3 Math::calculateTangentSpaceVector(
 const Vector3& position1, const Vector3& position2, const Vector3& position3,
 Real u1, Real v1, Real u2, Real v2, Real u3, Real v3)
 {
   //side0 is the vector along one side of the triangle of vertices passed in, 
   //and side1 is the vector along another side. Taking the cross product of these returns the normal.
   Vector3 side0 = position1 - position2;
   Vector3 side1 = position3 - position1;
   //Calculate face normal
   Vector3 normal = side1.crossProduct(side0);
   normal.normalise();
   //Now we use a formula to calculate the tangent. 
   Real deltaV0 = v1 - v2;
   Real deltaV1 = v3 - v1;
   Vector3 tangent = deltaV1 * side0 - deltaV0 * side1;
   tangent.normalise();
   //Calculate binormal
   Real deltaU0 = u1 - u2;
   Real deltaU1 = u3 - u1;
   Vector3 binormal = deltaU1 * side0 - deltaU0 * side1;
   binormal.normalise();
   //Now, we take the cross product of the tangents to get a vector which 
   //should point in the same direction as our normal calculated above. 
   //If it points in the opposite direction (the dot product between the normals is less than zero), 
   //then we need to reverse the s and t tangents. 
   //This is because the triangle has been mirrored when going from tangent space to object space.
   //reverse tangents if necessary
   Vector3 tangentCross = tangent.crossProduct(binormal);
   if (tangentCross.dotProduct(normal) < 0.0f)
   {
     tangent = -tangent;
     binormal = -binormal;
   }

 return tangent;

 }