BUG: Lack of precision in ImageData finding procedures

[Odet Christophe]

Hi David,

Thanks for your help.

I agree with you about the floating overhead but I think that your solution does not solve 
completely the problem (may be I don't understand your solution !)

in vtkImageData::ComputeStructuredCoordinates the value -0.5 is converted to 0. Using a floor
statement (or vtkFloor) will correctly convert it to -1 and this effectively solves the problem for this
kind of values. Nevertheless, considering the case where your extent is (0,1), a value of 1.5 is converted to
1 (with or without the floor function) and the lines
 
else // if ( ijk[i] == this->Extent[i*2+1] )
      {
      if (this->Dimensions[i] == 1)
        {
        pcoords[i] = 0.0;
        }
      else
        {
        ijk[i] -= 1;
        pcoords[i] = 1.0;
        }
      }

will consider the 1.5 point to belong to the extent. This is false and the extent is artificially extended.
if the value x is exactly equal to 1 the fucntion is ok as it interpolates inside the 0,1 cell with pcoords=1.0.
But if x is greater than 1.0 (i.e 1.2) then answer is wrong giving a pcoords of 1.0 . 

So this is not only a problem of floor'ed int values.

The vtkImageData::ComputeStructuredCoordinates needs to be modified in a much more complicated
way. 

Can we really do something without some foat comparisons ?
May be with comparison based on some floor'ed and ceil'ed values....
Do you agree with that ?

Christophe

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