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!
On 11/22/11 3:31 PM, Cameron Hummels wrote:
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.
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...
> On Tue, Nov 22, 2011 at 1:07 PM, Cameron Hummels <chummels(a)gmail.com
> <mailto:firstname.lastname@example.org>> 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
> 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
> Any ideas on what could be making this difference? Any ideas on
> which is the value to trust?
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