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

Cameron Hummels chummels at astro.columbia.edu
Tue Nov 22 12:31:14 PST 2011


Hey Sam,

When I run each method over the whole volume, the kd-tree and the HV 
take the same duration to process, however, the ratio of the two is 
still the same factor of 2e-8.

Cameron

On 11/22/11 3:26 PM, Sam Skillman 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...
>
> Sam
>
> On Tue, Nov 22, 2011 at 1:07 PM, Cameron Hummels <chummels at gmail.com 
> <mailto: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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