[yt-users] Projection questions

[Nathan Goldbaum]

On Tue, Oct 4, 2016 at 3:26 PM, Scott Feister <sfeister at gmail.com> wrote:

> Hi Nathan,
>
> Thanks a lot for the clarifications. I got confused on just one of your
> sentences:
>
> *It will average all the cells along the line of sight at that x,y value.
> Those cells might have different spatial resolutions.*
>
> So, a projection doesn't factor in every cell in the simulation, but it
> takes some subset? So, if there is a single cell in the grid that happens
> to have an absolutely enormous field value, it's quite possible that this
> will be under-factored or over-factored in a projection (depending on if
> the projection rays "misses" or "hits" it)?
>

For each pixel, it takes into account all cells that contribute along the
line of sight for that pixel.

If a single cell in the grid has an enormous value, it will be factored in
at the same level of weighting as other cells at the same resolution level.


>
> When you say the cells might have different resolution, do you mean they
> would have *sub-cells *or that they would have different
> lengths/widths/heights?
>

I'm not quite sure what you mean when you refer to "sub-cells" here. Let me
see if I can explain in a little bit more detail.

For a single pixel in a projection, yt figures out a "stack" of cells in
your simulation that are at different Z values but also enclose the X and Y
values of your pixel.

To figure out the projection at that pixel, yt computes the sum of cell
values times dZ in the "stack" of cells at that X,Y location. Optionally,
there might be a weight field here as well, so for a weighted projection it
would be the sum of the cell values multiplied by the weight field values
multiplied by dZ.

There's a little bit of text in the docs describing how different
projection types are formulated mathematically:

http://yt-project.org/docs/dev/visualizing/plots.html#types-of-projections

This is also covered in the yt method paper (section 3.2):

http://adsabs.harvard.edu/abs/2011ApJS..192....9T

Hope that explains it,

Nathan


>
> Thanks a lot!
>
> Best,
>
> Scott
>
>
>
>
> Scott Feister, Ph.D.
> Postdoctoral Researcher, Flash Center for Computational Science
> University of Chicago, Department of Astronomy and Astrophysics
>
> On Tue, Oct 4, 2016 at 2:17 PM, Nathan Goldbaum <nathan12343 at gmail.com>
> wrote:
>
>>
>>
>> On Tue, Oct 4, 2016 at 11:26 AM, Scott Feister <sfeister at gmail.com>
>> wrote:
>>
>>> Hi all,
>>>
>>> 1) Is there a way to do a "mean" ProjectionPlot? As far as I can tell,
>>> the options are "integrate", "mip", and "sum", so I am guessing the answer
>>> is no. I know you can do this with regions by reg.mean(), and make your own
>>> plot in matplotlib.
>>>
>>
>> If you use a weight field you'll get back a mean value (weighted by that
>> field). If you want an unweighted mean, you can weight by the "ones" field
>> (although this will likely not be what you want for AMR data). If you want
>> a mass-weighted mean, you can weight by the cell_mass field. If you want a
>> volume weighted mean, use cell_volume.
>>
>>
>>> 2) If I do a Z projection on a domain with high X,Y spatial resolution,
>>> will it average cells along X and Y (within the pixel limits) at each Z
>>> depth as it projects? Or, just pick a single cell at X,Y for each pixel?
>>>
>>
>> It will average all the cells along the line of sight at that x,y value.
>> Those cells might have different spatial resolutions.
>>
>>
>>>
>>> Also, does anyone know if the reg.mean() function weights the cells
>>> equally in the mean if there is varied spatial resolution? For example, I
>>> know the reg.integrate() function multiplies by path length, but reg.sum()
>>> simply adds up cell values.
>>>
>>
>> The mean() function takes a field to weight by:
>>
>> http://yt-project.org/docs/dev/reference/api/generated/yt.da
>> ta_objects.selection_data_containers.YTRegion.mean.html#yt.
>> data_objects.selection_data_containers.YTRegion.mean
>>
>> By default it uses "ones" or "particle_ones", depending on whether you're
>> averaging a particle or mesh field.
>>
>>
>>>
>>> Thanks!
>>>
>>> Scott
>>>
>>> Scott Feister, Ph.D.
>>> Postdoctoral Researcher, Flash Center for Computational Science
>>> University of Chicago, Department of Astronomy and Astrophysics
>>>
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>>> yt-users mailing list
>>> yt-users at lists.spacepope.org
>>> http://lists.spacepope.org/listinfo.cgi/yt-users-spacepope.org
>>>
>>>
>>
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