[vtkusers] vtkTransform problem: really roundoff error?

[David Gobbi]

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


On Mon, Feb 8, 2010 at 8:45 AM, Michele Conconi
<michele.conconi at unibo.it> wrote:
> Hi again.
> I'm sorry, I probably did not explain my problem in the best way.
> I do not think that it is a round off error. Indeed, what I'm doing is not
> reading the output after a bing number of inversion. Instead, I set the
> transformation matrix to some values, invert it many times, than I reset the
> matrix into my check configuration and see if the output still corresponds
> with what I impose. So the problem seems to be that by inverting the
> vktTransofrm enough times, it "obeys" no more, i.e. it goes no more where it
> should.
>
> As both David and Murat suggested, I also thought at the beginning that the
> problem was caused by some roundoff error. So, to compensateit, I  reset the
> matrix of my transformation to the identity before any change. I also
> performed my checks introducing some tolerance as you may see in the new
> code below. Unfortunately, with epsilon up to 1.5 I  obtain the same
> problem. The filter change its behavior always after the same number of
> cycle (214 for your knowledge). Also, the error I obtain is not of the
> dimension of a roundoff error. Indeed, at the failure the center of the
> polydata is
>
> x = -0.0438237 , y =0.0876474 , z = -4.98359  instead of  x = 9.95618,  y =
> -23.462, z = 26.4141
>
> and the matrix is the identity instead of
>
> 1   0   0   10
> 0    0.707107   0.707107   -20
> 0   -0.707107   0.707107   30
> 0   0   0   1
>
> Where could be the problem?
> Michele
>
> #include "vtkTransformPolyDataFilter.h"
> #include "vtkSTLReader.h"
> #include "vtkTransform.h"
> #include <iostream>
>
>
> using namespace std;
>
>
> int main(int argc, char *argv[])
> {
>   double center[]={0,0,0};
>   double c[]={0,0,0};
>
>   vtkSTLReader *volumeReader = vtkSTLReader::New();
>   vtkTransformPolyDataFilter *TF = vtkTransformPolyDataFilter::New();
>   vtkTransform *ActualPosition = vtkTransform::New();
>   vtkPolyData *poly = vtkPolyData::New();
>
>   volumeReader->SetFileName("cube.stl");
>   TF->SetInput(volumeReader->GetOutput());
>   volumeReader->Delete();
>
>   TF->SetTransform(ActualPosition);
>   poly=TF->GetOutput();
>
>   poly->GetCenter(center);
>   cout<<center[0]<<" "<<center[1]<<" "<<center[2]<<endl;//the center of the
> cube containing the polydata in its ZERO configuration, i.e. no rotation or
> translation
>
>   ActualPosition->Identity();
>   ActualPosition->PostMultiply();
>   ActualPosition->RotateX(-45);
>   ActualPosition->Translate(10,-20,30);
>   poly->Update();
>
>   poly->GetCenter(center);
>   cout<<center[0]<<" "<<center[1]<<" "<<center[2]<<endl;//the center of the
> cube containing the polydata in the control configuration  configuration
>   ActualPosition->Print(cout);
>     int i=0;
>   while(i<200000)
>   {
>       //modify the vtkTransform
>       ActualPosition->Identity();
>       ActualPosition->PostMultiply();
>       ActualPosition->RotateX(0);
>       ActualPosition->Translate(0,0,0);
>       poly->Update();
>
>       for (int j=0; j<1e7; j++)
>       {
>           ActualPosition->Inverse();
>           //my operations...
>           ActualPosition->Inverse();
>       }
>
>
>       if(i%10000==0)
>           cout<<i<<endl;
>
>       //bring it back to the control configuration
>       ActualPosition->Identity();
>       ActualPosition->PostMultiply();
>       ActualPosition->RotateX(-45);
>       ActualPosition->Translate(10,-20,30);
>       poly->Update();
>
>
>       poly->GetCenter(c);
>       //check if the center of the cube containing the polydata is still in
> the right position
>       double epsilon=1.5;
>       if(abs(c[0]-center[0])<epsilon && abs(c[1]-center[1])<epsilon &&
> abs(c[2]-center[2])<epsilon)
>           i++;
>       else
>       {
>           cout<<c[0]<<" "<<c[1]<<" "<<c[2]<<endl;
>           cout<<i<<endl;
>           ActualPosition->Print(cout);
>           i=2000000;
>       }
>   }
>
>   ActualPosition->Delete();
>   TF->Delete();
> }
> --
> * 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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