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

22 Nov 2011

      Problem solved everyone.  Britton and I tracked it down.

When you make a Homogenized Volume, you need to set the field explicitly 
that you are homogenizing.  By default it uses the "Density" field.  So 
effectively, I was comparing a projection of "Density" (from the 
homogenized volume method), and a projection of "CIV Number Density" 
(from the KDtree method)--of course these would differ.

This just goes to show you that one should always RTFM prior to emailing 
the list with problems.  Sorry everyone!

Cameron

On 11/22/11 3:31 PM, Cameron Hummels wrote:
...
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 mailto:chummels@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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