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

Cameron Hummels chummels at gmail.com
Tue Nov 22 12:07:07 PST 2011


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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