[vtkusers] Visualize Vector Field Example Wrong?

[David Doria]

On Wed, Oct 14, 2015 at 5:40 PM, Werner Sembach <werner at sembach.de> wrote:
>
> Hello,
> i want to do exactly this: http://www.vtk.org/Wiki/VTK/Examples/Cxx/Visualization/VectorField
> but when you try the example code the 2nd coordinate of the vectors is completly ignored (even the picture on the wiki shows 2 parrallel vectors while in the code the vectors are (10, 10) and (-10, 5).
> hope someone can help me

You are right - something is wrong. As it is, I get this warning:

Warning: In /home/doriad/src/VTK-6.2.0/Common/DataModel/vtkDataSetAttributes.cxx,
line 1165
vtkPointData (0x23e62e0): Can not set attribute Vectors. Incorrect
number of components.

on this line:

image->GetPointData()->SetActiveVectors("ImageScalars");

The warning is resolved if I change

image->AllocateScalars(VTK_FLOAT,2);

to

image->AllocateScalars(VTK_FLOAT,3);

I'm not sure why 2D vectors are not allowed.

Something else is wrong as well, though. Once I add pixel[2] = 0.0; to
each of the pixels (to prevent referencing unallocated memory), the
vectors still don't behave as I'd expect. With them both set to
(10,10), they both point (up, right) as expected. If I change pixel
(20,20) to (10, 10), it now points (down, right), again as I'd expect.
However, when I change it to (-10, -10), it points (up, right) again
(where I'd expect (down, left)), and it turns red (where everything
previously had been blue.

Sorry I don't have an answer, but at least that should help define the
problem :)

David