[vtkusers] what changes the coordinates?

[Petru Pau]

I can give you a practical example. The two attached poly data files contain
two faces each: a quadrilateral and a triangle. The points in 
testPolydata07.vtk
have integer coordinates, and those from testPolydata08.vtk were obtained
by multiplying each point with 3/25.

The coordinates of the points in the vtkPolyData created by reading these
files differ from those on files by more than 10^(-8). Due to this fact the
volume of the tetrahedron is of order 10^(-9). The following code snippet
allows you to see this, if you run it in debug mode and insert a 
breakpoint at
the last sentence:

    vtkPolyDataReader* reader = vtkPolyDataReader::New();
    polyData = vtkPolyData::New();

    reader->SetFileName("testPolydata08.vtk");
    vtkPolyData* readData = reader->GetOutput();
    readData->Update();

    vtkCell* cell = readData->GetCell(0);
    bool planar = true;
    if (cell->GetCellType()==VTK_QUAD)
    {
        vtkPoints *cellPoints = cell->GetPoints();
        double vertices[4][3];
        for (int i=0; i<4; i++)
        {
            double *aVert = cellPoints->GetPoint(i);
            memcpy(vertices[i], aVert, 3*sizeof(double));
        }
        double vol = vtkTetra::ComputeVolume(vertices[0], vertices[1], 
vertices[2], vertices[3]);
        if (abs(vol)>1.0e-10)
        {
            planar = false;
        }
    }

Of course, I am not urging you to try this out.

My theory is that reading is done in float, and the vtkPolyData arithmetic
is in double; when storing the read points to the vtkPolyData points, 
the latter
are not initialized with 0 and some residual non-zero digits of small order
are preserved.

For some real-life examples, the errors are even bigger (10^(-6)); you 
can imagine
the influence of these errors to precision-sensible computations...

For the moment, I see no way to eliminate this effect.

Petru
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