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

Cameron Hummels chummels at gmail.com
Wed Jul 16 16:22:37 PDT 2014


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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>> >>
>> >
>> >
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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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