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[Octave-bug-tracker] [bug #57471] streamtube.m should use surface object

From: Hg200
Subject: [Octave-bug-tracker] [bug #57471] streamtube.m should use surface object rather than patch object
Date: Sun, 2 Feb 2020 18:34:45 -0500 (EST)
User-agent: Mozilla/5.0 (X11; Fedora; Linux x86_64; rv:72.0) Gecko/20100101 Firefox/72.0

Follow-up Comment #6, bug #57471 (project octave):

Pros and Cons are:

When I call "streamtube()", I expect a "streamtube" as described for example
on Wikipedia, rather than a "divergence-plot". In addition, a classic
"streamline" can be plot for all types of vector fields, whereas a
"divergence-plot" will show numerical errors for those vector fields that are
divergence free. For example this is the case for incompressible fluid flows
which satisfy the continuity equation.

Examples for Matlab versus Octave Plots:


"Demo 1" shows a vector field with constant divergence everywhere. Octave
shows how the streamlines converge at the bottom. The "divergence-plot" only
shows numerical errors, since the tube radius is scaled to the min and max of
the divergence. "Demo 2" shows an electrostatic field caused by a point
charge. This field is free of divergence, except at the singularity. A classic
"streamtube" plot shows how the field converges. A "divergence-plot" is
useless because the divergence is zero everywhere. Consequence: When switching
the algorithm we have to remove all demos. I don't have any good examples to
plot normalized divergence.

On the other hand, after further research, I found that the "tube filter" in
Paraview allows plotting "tubes along a streamline" whereby the tube diameter
can be "controlled by a scalar value". So the Matlab solution can also be
found in some post-processing programs. I also feel compatibility is a primary
goal for Octave. 

In the end it is a niche function (I think nobody will use it at all ???) and
I don't want to extend the discussion too much. I need some time, but I can
upload here a patch that is Matlab compatible and that shows the normalized
divergence. However i would prefer to push only one of the two solutions and
not a mixed solution. It would be great if Philip can contribute with a third
opinion. We can then still decide which algorithm is Octave's final choice and
hopefully close the topic.


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