[vtk-developers] issue 7887 revisited

[Andreas Buykx]

Hi all,

Last few days I've been struggling to find the cause of a strange contouring effect caused, I think, by vtkMapper::MapScalarsToTexture()

    // Get the texture map from the lookup table.
    // Create a dummy ramp of scalars.
    // In the future, we could extend vtkScalarsToColors.
    double k = (range[1]-range[0]) / (ColorTextureMapSize-1);
    vtkFloatArray* tmp = vtkFloatArray::New();
    tmp->SetNumberOfTuples(ColorTextureMapSize);
    float* ptr = tmp->GetPointer(0);
    for (int i = 0; i < ColorTextureMapSize; ++i)
      {
      *ptr = range[0] + i * k;
      ++ptr;
      }

I set the color scale to have 3 colors, and, as you can see, when using color interpolation before mapping, I get 2 almost identical greens, neither of which matches the green of the color scale. This happens only when I run our application under  Windows remote desktop. When I run a local installation on Windows, or a VNC session on Linux, the colors are fine.

I accidentally came across issue 7887 while studying vtkContourValues::GenerateValues()

    for (i= 0; i < numContours; ++i)
      {
      // no we cannot factorize the ratio
      // (range[1] - range[0])/(numContours - 1) out of the loop.
      // we want the whole expression to be evaluated on the FPU.
      // see bug discussion: http://www.vtk.org/Bug/view.php?id=7887
      this->SetValue(i, range[0] + i*(range[1] - range[0])/(numContours - 1));
      }

The same loop occurs in said method of vtkMapper but here the multiplier is calculated outside the loop.  I suspect that this may cause the difference in behavior. I have no way for testing it though. Could someone please try to reproduce? Should I file this as an issue in mantis?
Thanks a lot,

Andreas Buykx
Senior Software Engineer

TNO DIANA BV
Software Developers and Analysis Consultants for Civil and Geotechnical Engineering

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