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

Sam Skillman samskillman at gmail.com
Tue Nov 22 12:26:29 PST 2011


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> 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<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
> ______________________________**_________________
> yt-dev mailing list
> yt-dev at lists.spacepope.org
> http://lists.spacepope.org/**listinfo.cgi/yt-dev-spacepope.**org<http://lists.spacepope.org/listinfo.cgi/yt-dev-spacepope.org>
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.spacepope.org/pipermail/yt-dev-spacepope.org/attachments/20111122/a6ff69ad/attachment.htm>


More information about the yt-dev mailing list