[yt-users] cut_region bafflement

Wed Oct 7 16:33:32 PDT 2015

Hi Nathan,

Thanks for the quick response!  I will see if I can reproduce this on a
test dataset.

Different is the confusing part to me.  These all should be selecting
exactly the same region.  If P3 and P4 are trying to select a region from
ds.all_data() instead of ds.disk() then all three should be different.
Another strange thing, if I select x > 65 kpc (only the top half of the
cylinder, then the volume is 3.7384e-4, half of the values that don't seem
right to me!

If I do
r4c = ds.cut_region(["obj['Metal_Density']/obj['density'] > 0.33"])
then the volume doesnt change, is 7.4768e-4

If I do:
r4 = ds.all_data()
r4c = r4.cut_region(["obj['Metal_Density']/obj['density'] > 0.33"])
then the volume is, as it should be, 0.3

*I think that something going wrong when I have more than one cut_region
command.*  If I do:

r3 = ds.disk([0.5,0.5,0.5],[-1,0,0],(4,"kpc"),(2,"kpc"))
r3c = r3.cut_region(["(obj['x'].in_units('kpc') > 60.0)"])
 or
r2 = ds.disk([0.5,0.5,0.5],[-1,0,0],(4,"kpc"),(2,"kpc"))
r2c = r2.cut_region(["obj['Metal_Density']/obj['density'] > 0.33"])

then the volume is 9.346e-5, as it should be.

But when I put those together:

r3 = ds.disk([0.5,0.5,0.5],[-1,0,0],(4,"kpc"),(2,"kpc"))
r3r = r3.cut_region(["(obj['x'].in_units('kpc') > 60.0)"])
r3c = r3r.cut_region(["obj['Metal_Density']/obj['density'] > 0.33"])

the volume is 7.4768e-4 again!

And when I do this:
r4 = ds.all_data()
r4r = r4.cut_region(["obj['Metal_Density']/obj['density'] > 0.33"])
r4c = r4r.cut_region(["(obj['z'].in_units('kpc') > 0.0)"])

I get 2.4467, which is more than my box size.

--
Dr. Stephanie Tonnesen
Alvin E. Nashman Postdoctoral Fellow
Carnegie Observatories, Pasadena, CA
stonnes at gmail.com

On Wed, Oct 7, 2015 at 3:30 PM, Nathan Goldbaum <nathan12343 at gmail.com>
wrote:

> Hi Stephanie,
>
> I'm not sure off-hand why this is happening.
>
> It may help one of us to be able to reproduce and understand what you're
> seeing if you can phrase your question in the form of an example that one
> of can run locally. This would make use of one of the test datasets on
> yt-project.org/data, and a full script that we can run locally pasted to
> paste.yt-project.org/data
>
> Finally, just to clarify, the confusion is that the volumes of P3 and P4
> are *bigger* than P2 and identical?
>
> It might be the case that the derived quantity infrastructure is somehow
> selecting the data as if it were selecting from ds.all_data() instead of
> your disk selector.  Can you try computing what the total volume of cells
> in a data object constructed by applying cut_regions on ds.all_data()
> instead of ds.disk()?
>
> -Nathan
>
> On Wed, Oct 7, 2015 at 5:21 PM, Stephanie Tonnesen <stonnes at gmail.com>
> wrote:
>
>> Dear yt-users,
>>
>> I am running
>>
>> Version = 3.2-dev
>> Changeset = dc2467c4eae7 (yt) tip
>>
>> I have been fiddling around with cut_region in a disk region and have
>> gotten really confused.  I was initially trying to see whether defining a
>> disk region such that the cylinder height was on the x-axis changed how I
>> need to make a cut in the cylinder height.  Anyway, as I was fiddling with
>> these cuts I found that if I made a "spatial" cut (rather than a different
>> variable cut) I was getting different volumes.  Let me show you:
>>
>> small bit of background:  the whole box is 130 kpc across, so 0.5 0.5 0.5
>> is 65 65 65 kpc.  Also, I am cutting and pasting bits here so the
>> indentation is off.
>>
>> ds =
>> yt.load("/data/stonnes/dwarf_nowind_lowmassgal_bigtracer/DD"+loop[i]+"/DD44sMBhn25now"+loop[i])
>>
>> r2 = ds.disk([0.5,0.5,0.5],[-1,0,0],(4,"kpc"),(2,"kpc"))
>>     r2c = r2.cut_region(["obj['Metal_Density']/obj['density'] > 0.33"])
>>
>>     r3 = ds.disk([0.5,0.5,0.5],[-1,0,0],(4,"kpc"),(2,"kpc"))
>>     r3r = r3.cut_region(["(obj['x'].in_units('kpc') > 63.0)"])
>>     r3r = r3r.cut_region(["(obj['x'].in_units('kpc') < 67.0)"])
>>     r3c = r3r.cut_region(["obj['Metal_Density']/obj['density'] > 0.33"])
>>
>>     r4 = ds.disk([0.5,0.5,0.5],[-1,0,0],(4,"kpc"),(2,"kpc"))
>>     r4r = r4.cut_region(["(obj['x'].in_units('kpc') > 61.)"])
>>     r4c = r4r.cut_region(["obj['Metal_Density']/obj['density'] > 0.33"])
>>
>>  P2 =
>> np.append(P2,r2c.quantities.total_quantity([("index","cell_volume")]))
>>  P3 =
>> np.append(P3,r3c.quantities.total_quantity([("index","cell_volume")]))
>>  P4 =
>> np.append(P4,r4c.quantities.total_quantity([("index","cell_volume")]))
>>
>> P2 volume is 9.346e-5 : within 2% of my calculation for the volume I
>> should be getting
>> P3 volume is 7.4768e-4
>> P4 volume is 7.4768e-4
>>
>> When I print out the x- y- and z- maxes and mins, they are the same for
>> each of the selected regions.
>> (By doing this:
>> m2 = np.append(m2,r2c.quantities.extrema([("index","z")])[0])
>> rho2 = np.append(rho2,r2c.quantities.extrema([("index","z")])[1])
>>
>> )
>>
>> Can anyone help me figure out/tell me what is going on here?
>>
>> Thanks!
>> Stephanie
>>
>> --
>> Dr. Stephanie Tonnesen
>> Alvin E. Nashman Postdoctoral Fellow
>> Carnegie Observatories, Pasadena, CA
>> stonnes at gmail.com
>>
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>>
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