[vtkusers] vtkCameraInterpolator gives funny camera positions when camera position stays the same

[Kalle Pahajoki]

Hi

I've been using the vtkCameraInterpolator class as a basis for creating 
keyframe animations and noticed a "funny" behaviour with the camera 
position.
If the camera position stays completely the same for two consecutive 
timepoints, the camera still moves when interpolated between these 
timepoints. This only seems to happen, if there are timepoints where the 
camera position changes after the two timepoints, though.

Testing this with vtkKochanekSpline directly is easy (code from the top 
of my head, should give you the idea)

s=vtk.vtkKochanekSpline()
s.AddPoint(0,5.0)
s.AddPoint(1,5.0)
print s.Evaluate(0.5) # Result is 5.0
s.AddPoint(2,10.0)
print s.Evaluate(0.5)  # Result is 4.6875

The first result makes sense (when the value at the start and end point 
are the same) but if the spline "continues" after the two points, then 
the results are not
what one might intuitively expect.

This is probably just a feature of the spline functions, but from the 
camera movement point-of-view it is incorrect. Can anyone come up with 
any more elegant solution to this than checking the position and not 
using the interpolated results for the position if the camera doesn't move?
I guess (haven't tested) using linear interpolation would yield the 
correct result, but then the movement between keyframes would get jaggy.

Kalle Pahajoki

-- 
Debugging is twice as hard as writing the code in the first place.
Therefore, if you write the code as cleverly as possible, you are, by
definition not smart enough to debug it.        -- Brian W. Kerninghan