[vtkusers] vtkTransform problem: really roundoff error?

[Michele Conconi]

David Gobbi wrote:
> Hi Michele,
>
> You might be exhausting the 32-bit limit on the timestamps.  VTK uses
> timestamps to decide when things should be updated.  It uses an
> unsigned long to store the timestamp, and any time that a VTK object
> is "modified", the global VTK "timestamp" is incremented.
>
> Ordinarily, timestamp overflows do not occur in VTK, but you have a
> loop of 20000 followed by a loop of 1e7, inside of which you modifying
> VTK objects.  So you are essentially forcing an overflow to occur.
> (Note that loops of this size occur in VTK all the time, but usually
> only when iterating over data containers or doing other things that
> don't cause timestamp changes).
>
> It is easy for you to check whether timestamp overflow is the cause of
> your problems.  Just check to see if (transform->GetMTime() >
> 2147483647) and if it is, you are halfway towards an overflow.
>
> If timestamp overflow is the problem, the two ways to fix it are (1)
> re-write your code so that VTK objects aren't modified as often, e.g.
> by using non-VTK objects for computation inside the tightest loops, or
> (2) compile VTK as 64-bit, because even the loopiest program cannot
> overflow a 64-bit timestamp.
>
>    David
>
>
>   

Thank you very much David, that was exactly the problem.
I agree with everyone suggested me to modify my code and I will. Still, 
I could possible encounter again the same problem even with less 
inversion. Indeed, any modification affect the timestamp and I still 
have to modify many times my vtkTransform. Of course I can do something 
like delete and recreate the vtkTransform after a while, to reset the 
timestamp, but this should slow down my code. Do you know if there is a 
different way to obtain the same result, something like setMTime(zero)? 
What the purpose of timestamp? Why should I be limited in the number of 
modification to a vtk object?

Thank you once again, I feel much better now :)

michele
-- 
* Eng. Michele Conconi*
Ph.D. student
DIEM - Dept. of Mechanical Engineering
Alma Mater Studiorum - University of Bologna
Viale Risorgimento 2, 40136, Bologna, Italy
Email: michele.conconi at unibo.it
Website: http://www.diem.ing.unibo.it/grab
Office: (+39) 051 20 93451
Mobile: (+39) 329 0287996



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