[vtkusers] Question on vtkPerspectiveTransform::SetupCamera()

[Müller Michael]

Good morning (at least for all those europeans reading this),

I have a question regarding the method vtkPerspectiveTransform::SetupCamera() which is used by the vtkCamera to recalculate the view transform matrix. Here is the complete code of the function since it will be easier to describe what irritates me:
--------------------
void vtkPerspectiveTransform::SetupCamera(const double position[3],
                                          const double focalPoint[3],
                                          const double viewUp[3])
{
  double matrix[4][4];
  vtkMatrix4x4::Identity(*matrix);

  // the view directions correspond to the rows of the rotation matrix,
  // so we'll make the connection explicit
  double *viewSideways =    matrix[0];
  double *orthoViewUp =     matrix[1];
  double *viewPlaneNormal = matrix[2];

  // set the view plane normal from the view vector
  viewPlaneNormal[0] = position[0] - focalPoint[0];
  viewPlaneNormal[1] = position[1] - focalPoint[1];
  viewPlaneNormal[2] = position[2] - focalPoint[2];
  vtkMath::Normalize(viewPlaneNormal);

  // orthogonalize viewUp and compute viewSideways
  vtkMath::Cross(viewUp,viewPlaneNormal,viewSideways);
  vtkMath::Normalize(viewSideways);
  vtkMath::Cross(viewPlaneNormal,viewSideways,orthoViewUp);

  // translate by the vector from the position to the origin
  double delta[4];
  delta[0] = -position[0];
  delta[1] = -position[1];
  delta[2] = -position[2];
  delta[3] = 0.0; // yes, this should be zero, not one

  vtkMatrix4x4::MultiplyPoint(*matrix,delta,delta);

  matrix[0][3] = delta[0];
  matrix[1][3] = delta[1];
  matrix[2][3] = delta[2];

  // apply the transformation
  this->Concatenate(*matrix);
}
-------------------
The calculation of the three rotation axes is straight forward but then the position is calculated by multiplying it with -1 and then with the previously defined rotation matrix. What is the reason for this?
I am working with pose estimation algorithms with the effect that I have to recalculate my estimated position in an inverted scheme (multiply with the inverted rotation matrix and then with -1) to adjust the vtk camera correctly.

Thank you for your answers,

Michael
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