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SciVisFall2007/Assignment 2/FAQ

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Here are some answers to FAQs on assignment 2.

The most important issue we have seen is that it seems people are getting confused when it comes to divergence and curl. Divergence and curl are scalar fields, that are computed from a vector field. To visualize them, you need to remember what type of data they are, and create appropriate visualizations. There are several pipelines in the example vistrail that create a scalar field from a vector field, and visualizes both of them at the same time.

Q: When running vtkStreamTracer, I get an error "Coincident points in polyline...can't compute normals". What happened?

Short A: You can ignore this error - everything else still works fine

Long A: The stream tracer hit a point in the vector field where speed is exactly zero. The result is that the next point in the streamline will exactly the same as the original one. VTK needs two distinct points to compute normals correctly, and complains when it can't find it.

Q: When executing a module, I get an error "NoneType has no attribute vtkInstance". What happened?

A: This is a bug in VisTrails that is easy to work around by reseting the cache and reexecuting. Click on Run -> Clear Execution Cache, and try again.

Q: Specifying a path as "c = '\documents and settings\file'" doesn't work.. What's wrong?

A: Python uses backslashes inside strings for escape characters:

Q: When executing a module, I get an error: "NameError: global name 'width' is not defined".

A: Leah Wouters found this issue and fixed it, in an email sent about a week ago:

"I fixed it by changing the make_ix function to the following:

def make_ix(x, y, width1 = width):
  return x + width1 * y"

(grads only)

Q: On problem 4b, should I create simply a colormap of divergence/curl?

A: No - you have to build on your previous vector field visualization.

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