[yt-users] Coexistence of positive and negative values on a SlicePlot in log scale

[Nathan Goldbaum]

On Wed, Jul 16, 2014 at 4:06 PM, John Zuhone <jzuhone at gmail.com> wrote:

> Hi Suoqing,
>
> Probably cannot do what you're looking for with PlotWindow, but you
> might want to try Matplotlib's symlog:
>
> http://matplotlib.org/examples/pylab_examples/symlog_demo.html


I didn't know this was a thing.  We could probably add support for it to
PlotWindow.


>
>
> This is what the S-Z projection analysis module does, see the images
> of the S-Z decrement/increment here:
>
> http://yt-project.org/doc/analyzing/analysis_modules/sunyaev_zeldovich.html
>
> If you look in the code in
> yt.analysis_modules.sunyaev_zeldovich.projection.py, the SZProjection
> class has a write_png method that uses the symlog in a color plot.
>
> So my suggestion would be to write your image to a fixed resolution
> buffer and plot it directly with Matplotlib's imshow using the symlog
> scaling on the colorbar.
>
> Best,
>
> John
>
> On Wed, Jul 16, 2014 at 6:53 PM, Nathan Goldbaum <nathan12343 at gmail.com>
> wrote:
> >
> >
> >
> > On Wed, Jul 16, 2014 at 3:50 PM, Suoqing JI <suoqing at physics.ucsb.edu>
> > wrote:
> >>
> >> Hi,
> >>
> >> I understand that typically the negative values would appear as white
> >> color on a log slice plot. However, if I directly plot the fields that
> are
> >> generically defined in FLASH, the negative values DO appear correctly.
> For
> >> example, the velx field: http://pbrd.co/1jz3iqR.
> >
> >
> > Yup - that plot is on a linear scale.  I believe the FLASH frontend has
> it
> > explicitly defined *not* to log-scale velx.
> >
> >>
> >>
> >> But for any derived fields, for example, defining velx1 = 1.*velx, the
> >> negative values appear white again: http://pbrd.co/1jz4oD8.
> >
> >
> > That happens because the default for a derived field is to log scale the
> > field in plots.
> >
> >>
> >>
> >> So, is there anyway to show both positive and negative values on a log
> >> slice plot for derived fields (I’m using YT2)?
> >>
> >
> > Nope.  In general you can't show negative values on a log-scaled plot.
> >
> > You could plot the absolute value of a field but then you aren't really
> > plotting the field as such.
> >
> > What is the visualization goal you are trying to accomplish here?
> >
> >>
> >> Thanks!
> >>
> >> Best wishes,
> >> --
> >> Suoqing JI
> >> Ph.D Student
> >> Department of Physics
> >> University of California, Santa Barbara
> >> CA 93106, USA
> >>
> >>
> >> _______________________________________________
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> >>
> >
> >
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> >
>
>
>
> --
> John ZuHone
>
> Postdoctoral Researcher
> NASA/Goddard Space Flight Center
>
> jzuhone at gmail.com
> john.zuhone at nasa.gov
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