[Paraview] ParticleTracer particle lifetime

[Ryan Abernathey]

Hello,

I am continuing my ongoing quest to do something like this
http://earth.nullschool.net/#current/wind/isobaric/1000hPa/orthographic=267.73,5.54,350
or this
http://www.youtube.com/watch?v=xusdWPuWAoU
in Paraview using ***time dependent velocity vectors***.

While the LIC plugin is very cool, it does something different. I followed
the previous suggestion and tried to use the streamline filter:
http://paraview.org/Wiki/ParaView/Custom_Filters
Unfortunately that is not quite right either.

The problem with the streamline filter is that it treats each timestep as
completely independent and regenerates the streamlines whenever the
velocity field changes. (This is the correct behavior: streamlines are
defined by the *instantaneous* flow.)

What we see in those videos are truly particle trajectories. In particular:
- particles are seeded randomly (in space and time)
- they leave a decaying trail (sometimes called a streaklet)
- the particles disappear after a short lifetime
This is the combination of ingredients I need to reproduce in paraview.

The best candidate is clearly the ParticleTracer filter. However, I have
hit a serious problem: it doesn't appear that this filter is able to make
the particles "die" after a temporal lifetime. Compare the v 3.3
documentation
http://paraview.org/OnlineHelpCurrent/ParticleTracer.html
with the current documentation
http://paraview.org/Wiki/ParaView/Users_Guide/List_of_filters#ParticleTracer
In the old version, there was an option called "Termination Time" that is
missing from the new version.

Without such an option, the particles will never disappear, the domain will
get more and more crowded, and the computational expense will grow with
time.

Let me know if you have any suggestions or if you know how to re-enable
this Termination Time option.

Thanks a lot,
Ryan
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