[yt-users] Plotting differences of two datasets

[David Collins]

Hi, Aaron--

What I typically do for such a task is to get the data form yt, then plot
the difference with matplotlib directly.  For instance,

>>> import matplotlib.pyplot as plt
>>> DifferenceCube = cube1['Density']-cube2['Density']
>>> plt.imshow( np.sum(DifferenceCube,axis=2), interpolation='nearest',
origin='lower')

To my knowlege, there's no way to feed a covering grid back into yt's
projection tools, so this is the best way that I've found.  (And you're
definitely right, a covering_grid is necessary for this task)

In general, if you want to make a new field you can't just assign it the
way you did, since there isn't a set method on the covering grid.  In
general, look at the add_field tool in the docs for making new fields.

d.




On Tue, Mar 11, 2014 at 5:27 PM, Aaron Lee <a.t.lee at berkeley.edu> wrote:

> All,
>
> Hopefully this is a simple question and is giving me trouble only because
> of my ignorance of how data is stored in YT structures.
>
> I have two data files of data tabulated over the same unigrid. Each side
> of the domain ranges from 0 to 1. I am interested in plotting a cell by
> cell difference of some quantity. For simplicity, let's just say it's the
> density.
>
> So what I have tried is this:
>
> (1) Load the files:
> pf1 = load("file1")
> pf2 = load("file2")
>
> (2) Following some YT cookbook pages, I extract the data :
> cube1 =
> pf1.h.covering_grid(1,left_edge=[0,0,0],dims=[128,128,128],fields=["Density"])
> cube2 =
> pf2.h.covering_grid(1,left_edge=[0,0,0],dims=[128,128,128],fields=["Density"])
>
> If I then type something like
>         cube1["Density"]
> the output is a 3d array of data. Similarly for cube2["Density"]. OK.
>
> (3) Now the part I'm more iffy on. To create a new data cube, I first
> instigate it as a copy of cube1 and then manipulate the data (at least, I
> thought I was... see below):
> test = cube1
> test["Density"] = cube1["Density"]-cube2["Density"]
>
> If I then type
>         test["Density"]
> the outputted array is an array of the differences, which is what I want
> to plot.
>
> However, when I then plot the data by doing:
>
> pc=PlotCollection(test,center=[0.5,0.5,0.5])
> p=pc.add_slice("Density",'y')
> p.save_to_pdf('Test.pdf')
>
> The Test.pdf is a plot of just the cube1 density. So the data does not
> appear to be manipulated at all. If I do not instigate 'test' first as a
> copy of cube1, yt complains because cube1["Density"]-cube2["Density"] is
> only an array of data.
>
> Another idea could be to create a new user-defined field that within the
> field definition loads another data file and does the difference for me. It
> would probably be clunky, but seems possible in theory. But you would know
> better than I.
>
> Thanks in advance for any notes and tips,
> Aaron
>
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>



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