[Yt-dev] Stumped on off-axis projections

david collins antpuncher at gmail.com
Fri Sep 30 06:58:25 PDT 2011


Hey, Matt--

Here's my $0.02

I'd say that interpolating (field*weight) should be the primary
option, but I would leave both as an option.  As you say, they can
give different answers, but have different utility.

I think that this method (option 1) is more physical, since that's
what I would get if I were making an observation of some convolved
quantity, say rotation measure = \int{ n B dz }.  I then need a second
independent measurement of \int{n dz} to get B alone.

I'd also leave in the second option, since one might be weighting
purely for visual emphasis of a more "pure" quantity.

I'd call it "combined_interpolation" and default it to True.

d.


On Fri, Sep 30, 2011 at 7:30 AM, Matthew Turk <matthewturk at gmail.com> wrote:
> Hi all,
>
> I'm writing a helper function for off-axis projections.  I'm stuck on
> deciding what to do for weighting.  For a weighted projection, we
> integrate field * weight across the whole domain.  For on-axis, this
> is well-defined.  For off-axis, we have two choices of what to do,
> because we interpolate between vertices:
>
>  * Construct a field that is everywhere field * weight, and
> interpolate inside that to get our v's
>  * Interpolate field, interpolate weight, and multiply the two
> interpolated values to get our v's
>
> These are different operations, and can give very different results.
> (You can test this very easily in 1D; I was surprised at the magnitude
> of the differences.)  I am not sure which one is more 'correct' for
> our particular goal.  I can actually see arguments for both sides.
> The arguments in favor of #1 are that we are constructing a valid
> field everywhere in advance, and it will be more stable.  The
> arguments for #2 are that it may more accurately reflect the local
> value you would get compared to, say, a column density taken off axis
> (which is what we ultimately divide by at the end of the calculation.)
>
> Does anybody here have an opinion?
>
> 1D example code: http://paste.yt-project.org/show/1837/
>
> -Matt
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>



-- 
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