[Yt-dev] Density Column calculation

[Stephen Skory]

Hi Matt,

thanks for the stupid-check in the last email. I probably could have
figured that out but I had a mental block.

Now to my next questions, but first the set-up. I think I essentially
have this working in my test case. I've vectorized most of the Python
calculations so it doesn't feel excessively slow. Using the 'Ones'
field and centering at [0.5,0.5,0.5], I calculate four column density
spheres with radii between pf.h.get_smallest_dx() and 0.5 (spheres
with radii outside the box are... troublesome, I'm avoiding that for
now) using HEALPix. The 'Ones' field means that the values on these
spheres are the radii of the spheres themselves. Handy. Then I
interpolate the values on those spheres to all the points on each grid
that are on or inside the largest sphere. Points outside are given
values of 0 for now.

For now I'm just attaching the new fields manually to the grids like this:

pf.h.grid[grid_num]['NewColumnDensityField'] = result_for_this_grid

which works fine in the respect that I can then compare this field to
a manual cell-to-[0.5,0.5,0.5] calculation and the answers make sense.

My feeling is that this might be the way to go because it will allow
people to use this new field in as many ways as possible. I don't see
how this can be a derived field. Do you agree, Matt? If not, what else
could work?

If this is a good approach, then there are more questions (that can be
skipped if this is not a good approach). How do I add this field in
all the right places? I know it should be added to pf.h.field_list,
and that there are other stuff that field_info_container.py is related
to. Can you point me to a place in the code where fields are added?
I've never worked with that part of yt before.

Also, there's the question of parallelism, and if attaching to the
grids like this works. I'm thinking probably.

Thanks for your help, Matt.

-- 
Stephen Skory
s at skory.us
http://stephenskory.com/
510.621.3687 (google voice)