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

[Suoqing JI]

Hi Cameron,

That’s true, or we can multiply a sign function to get the correct sign even. But for a 2-D plot, maybe the symlog scaling is a suitable solution...

Thanks!

Best wishes,
--
Suoqing JI
Ph.D Student
Department of Physics
University of California, Santa Barbara
CA 93106, USA

On Jul 16, 2014, at 4:22 PM, Cameron Hummels <chummels at gmail.com> wrote:

> What I've done to address this issue in the past is plot the absolute value in logspace, and then have the symbol plotted be an up arrow if it is positive or a down arrow if it is negative.  This way you can keep it all on a single plot.  Example:
> 
> http://i.imgur.com/Y77UokD.png
> 
> Cameron
> 
> 
> On Wed, Jul 16, 2014 at 4:13 PM, Suoqing JI <suoqing at physics.ucsb.edu> wrote:
> Hi Nathan, John,
> 
> The example I gave does be on a linear scale and not appropriate… but yes, John’s information is exactly I want.
> 
> Although it might be possible to superpose two plots (one positive and one negative) with two color bars, but it appears a little bit redundant.
> 
> Thanks a lot!
> 
> Best wishes,
> Suoqing
> 
> On Jul 16, 2014, at 4:09 PM, Nathan Goldbaum <nathan12343 at gmail.com> wrote:
> 
>> 
>> 
>> 
>> 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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>> 
>> 
>> --
>> John ZuHone
>> 
>> Postdoctoral Researcher
>> NASA/Goddard Space Flight Center
>> 
>> jzuhone at gmail.com
>> john.zuhone at nasa.gov
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> 
> -- 
> Cameron Hummels
> Postdoctoral Researcher
> Steward Observatory
> University of Arizona
> http://chummels.org
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