[yt-dev] ytdata frontend work

Britton Smith brittonsmith at gmail.com
Tue Oct 27 07:35:01 PDT 2015


Hi all,

I have spent a while looking at the issue involving the derived quantities
and I have come up with a solution for some of them, but not all of them.
I have pushed a couple changes to the PR that add a cell_volume and
cell_mass field specific to YTDataContainerDatasets that makes use of their
"dx", "dy", and "dz" fields.  As of now, the following derived quantities
work:
- min location
- max location
- extrema
- bulk velocity
- total mass
- weighted average quantity
- weighted variance
- total quantity

The ones that don't work are:
- spin parameter
- center of mass
- angular momentum vector

The final three all don't work for the same reason, which is that they
require access to the "index" position fields.  Unfortunately, these cannot
be overridden for reasons that have to do with the _determine_fields
function.  I have made a point during this development process to alter as
little as possible outside of the ytdata frontend.  However, at this point,
there is no more room to maneuver.  The only solutions I can come up with
are ugly and would make it very difficult to smoothly swap in a proper AMR
hierarchy when we get around to creating one and doing this properly.

The last thing I can think of would be to swap in special derived quantity
functions for this frontend only that use position fields appropriate for
the frontend.  The functions themselves would be simple to implement, but
because the derived quantity collection is added onto data container
objects and not dataset objects, I have no idea how one would do the swap.
For a similar reason, I don't know how to simply not allow these problem
cases to be called, or to fail in a clean way for them.

If anyone has any ideas, I would love to hear them.

Britton

On Mon, Oct 26, 2015 at 10:12 PM, John ZuHone <jzuhone at gmail.com> wrote:

> No worries—the main issues now are the tests (which don’t run under Python
> 2) and Python 3.
>
> Tomorrow I’ll point you at the head that I’ve been making changes on to
> address these.
>
> On Oct 26, 2015, at 6:09 PM, Britton Smith <brittonsmith at gmail.com> wrote:
>
> Hi John,
>
> At the PR hangout, we discussed the possibility of eventually creating a
> method to extract subsets of AMR hierarchies that would include only grid
> patches that are at least partially contained within the data container.
> With them, we would also include the mask arrays to be applied to each grid
> patch such that you would only get the cells within the container.
> However, this was deemed something that should not hold up this PR, but
> that could eventually subbed in to fill in any gaps in functionality
> created by treating the grid data like particles.  We decided that patching
> the existing machinery with a frontend-specific cell_volume field was the
> best thing as it would get this on front of users faster, especially since
> it contains a lot of other things that are fully working.  I may have
> missed some important parts of the discussion, but my memory faded a lot
> faster than I thought it would.
>
> I probably won't have much time to work on this until Friday, but I'll try
> to play around with the derived quantities when I can.
>
> Britton
>
> On Mon, Oct 26, 2015 at 9:45 PM, John ZuHone <jzuhone at gmail.com> wrote:
>
>> Hi all,
>>
>> Matt asked someone to send a quick note about the current issues
>> regarding the ytdata frontend PR:
>>
>>
>> https://bitbucket.org/yt_analysis/yt/pull-requests/1788/adding-ytdata-frontend
>>
>> I’m sending it because I’m working on it currently, but I was not able to
>> attend today’s PR hangout so anyone else who was more to add or to correct
>> should chime in.
>>
>> Currently, there are 3 issues, from most important to least important:
>>
>> 1. Tests don’t work
>> 2. Doesn’t support Python 3
>> 3. Derived quantities don’t work
>>
>> I’m currently working on 1 and 2, and there have been some ideas about 3.
>> The main issue with the latter is that for non-gridded data containers
>> (like sphere, disk, etc.) created from grid datasets, we treat the new
>> datasets as “particle data”. For that reason there is an issue with
>> calculating the cell_volume properly. Without that, we cannot use the
>> total_mass() quantity. Given that we have the cell deltas in the dataset,
>> we could have a special derived field for the cell_volume.
>>
>> There are other issues with some of the other derived quantities (e.g.,
>> weighted_average_quantity returning the wrong units), but I haven’t been
>> able to diagnose them yet.
>>
>> I’m working on some changes that will need to be merged into Britton’s
>> fork eventually to address at least 1 and 2.
>>
>> Best,
>>
>> John
>>
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