[yt-dev] Off-axis projections -- Discrepancies between homogenized volume vs KDtree methods

Matthew Turk matthewturk at gmail.com
Tue Nov 22 12:28:06 PST 2011


Hi Sam,

On Tue, Nov 22, 2011 at 3:26 PM, Sam Skillman <samskillman at gmail.com> wrote:
> Hey Cameron,
> Are the answers similar if you do the entire volume?  The kd-tree can not
> accept things like spheres to homogenize over, so maybe it is because it is
> projecting the entire box?  I'll keep thinking...

I think he's using a sphere that fully-encloses the ray box, so it
should give the same answer.

-Matt

> Sam
>
> On Tue, Nov 22, 2011 at 1:07 PM, Cameron Hummels <chummels at gmail.com> wrote:
>>
>> Hello peeps (mostly Britton, Matt and Sam),
>>
>> I have recently been doing some off-axis projections in my cosmological
>> runs (using the supercool new off_axis_projection helper function Matt
>> wrote), and I've encountered some problems.  I find different results when I
>> do the off-axis projection using a homogenized volume versus when I do not
>> use a homogenized volume (when it uses the default behavior for camera
>> objects -- ie a KDtree).
>>
>> Of course, these two results should be identical, and they are when I use
>> a normal field like "Density".  However, I'm trying to use a derived field
>> from some code Britton wrote, part of a package called ion_balance, which
>> creates derived fields for different atomic ions.  So when I compare the CIV
>> Number Density from these two methods, I get very different results.  Even
>> when I do this on a normal vanilla yt field, like "Density", the KDtree
>> method takes exceptionally longer than the homogenized volume method (I
>> think this is because I'm only doing the HV for a small subsample of the
>> overall volume).  On the other hand, they both take about the same amount of
>> time when my sample volume is the entire box volume.
>>
>> I've pastebinned a demonstration script which shows this discrepancy at:
>> http://paste.yt-project.org/show/1953.  If you don't have ion_balance, you
>> can comment that import out, and comment the line for defining the field as
>> "CIV_Cloudy_eq_NumberDensity", and run it to see the time discrepancy
>> between the two methods.  It should work on any sort of parameter file, not
>> just the specific one I'm using.  What I do is take an off-axis projection
>> using each method, then divide the two images against each other to form a
>> ratio image, and then output the average and stddev for this ratio.  The
>> average of the ratio is: 2e-8.
>>
>> I've changed the width of the off-axis projection and it has a minimal
>> (but nonzero) change on the overall ratio between the two.
>>
>> So I'm not sure what to do.  It appears that the CIV field is initiated in
>> the same way that a normal field is, with the projection_conversion set to
>> 'cm', just as it is for "Density".  Any ideas on what could be making this
>> difference?  Any ideas on which is the value to trust?
>>
>> Cameron
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