// by Xaron: http://irrlicht.sourceforge.net/phpBB2/viewtopic.php?t=11333
/** 
 * @brief guided missile steering helper function 
 * 
 * @param missilePosition - current absolute position of the missile 
 * @param missileRotation - current absolute rotation of the missile 
 * @param targetPosition  - current absolute target position 
 * @param oversteer       - oversteer factor in degrees 
 * 
 * @return targetPitchRoll - delta pitch (x) and delta roll (z) values to the target 
 * 
 *******************************************************************************************************/core::vector3df getToTargetPitchRoll( core::vector3df& missilePosition, core::vector3df& missileRotation, core::vector3df& targetPosition, float oversteer = 0.0f ) 
{ 
  core::matrix4 rotMatrix, invRotMatrix; 
  rotMatrix.setRotationDegrees( missileRotation ); 
  core::vector3df targetPitchRoll( 0.0f, 0.0f, 0.0f ); 
  // get the current missile direction 
  core::vector3df currentMissileTargetVec( 0.0f, 0.0f, 1.0f ); 

  // get the vector to the target (from the missile position) 
  core::vector3df missileTargetVec = ( targetPosition - missilePosition ).normalize(); 
  // invert the rotation matrix to transform the target vector into the missile's local space 
  rotMatrix.getInverse( invRotMatrix ); 
  // transform the target vector into missile's local space 
  invRotMatrix.rotateVect( missileTargetVec ); 
  // compute the roll angle to the target 
  float deltaRoll = atan2f( missileTargetVec.Y, missileTargetVec.X ) * core::RADTODEG - 90.0f; 
  if( deltaRoll < 0.0f ) 
    deltaRoll += 360.0f; 
  if( deltaRoll >= 360.0f ) 
    deltaRoll -= 360.0f; 

  if( deltaRoll > 180.0f ) 
    deltaRoll -= 360.0f; 

  // compute the pitch angle to the target 
  float deltaPitch = acosf( currentMissileTargetVec.dotProduct( missileTargetVec ) ) 
    * core::RADTODEG + oversteer; 

  targetPitchRoll.X = -deltaPitch; 
  targetPitchRoll.Z = deltaRoll; 

  return targetPitchRoll; 
}