[yt-dev] Coordinate handling pull request

[Anthony Scopatz]

Hello Matt,

Thanks for putting this together!  After briefly looking this over, it
seems like some methods which have "cartesian" in the name are slightly
mis-named.  As implemented, cartesian_length() is really just a
path_length().

I agree with the sentiment of the convert_to/from_cartesian() methods.
 However, I think half of these could be redundant.  It basically depends
on whether you are choosing that Cartesian be the canonical form or not.
 So I see two strategies:

1) All other systems have to/from cartesian methods.  Only need two methods
per coord system.  All coord transformations go through this cartesian.
(What you have, cleaner structure).

2) All systems have either a to_<othersystem>() method.  One method per
conversion you wish to support.  Could be faster and have less floating
point error than #1.  This prevents the situation where to go from Polar ->
Spherical you actually have to do Polar -> Cartesian -> Spherical.


The disadvantage of #2 is that you have to write more methods.  I guess
nothing is stopping you from implementing #1 and then just tacking on these
extra methods from #2 for cases where it makes sense.

My initial thoughts are that there are only 4 or 5 of these coordinate
systems so the number of permutations is not so high as to be unmanageable.
I also doubt that this is the kind of thing that users will be subclassing
and creating their own versions of.  So this is basically a "write
once" piece of code (::crosses fingers::).

In any event, I think what you have now is great.  It would just be nice to
circumvent going through cartesian when that is clearly too much work.

Be Well
Anthony

PS Sorry if this was rambling.  Must. find. coffee.

On Wed, Aug 29, 2012 at 10:27 AM, Matthew Turk <matthewturk at gmail.com>wrote:

> Hi Casey,
>
> I've updated the PR:
>
>
> https://bitbucket.org/yt_analysis/yt-3.0/pull-request/5/initial-import-of-coordinate-handler-class
>
> I think for now I'm going to leave the ABC in as a bookkeeping
> exercise while implementing the other handlers, and remove it later.
>
> -Matt
>
> On Wed, Aug 29, 2012 at 11:18 AM, Casey W. Stark <caseywstark at gmail.com>
> wrote:
> > Hey Matt.
> >
> > Sounds good. I would guess the dimensionality issue comes down to
> > implementation, but I think subclasses are fine.
> >
> > I am -1 on using abstract classes in general. Completely a style thing,
> but
> > I think it ends up making things harder.
> >
> > - Casey
> >
> >
> > On Wed, Aug 29, 2012 at 7:52 AM, Matthew Turk <matthewturk at gmail.com>
> wrote:
> >>
> >> On Wed, Aug 29, 2012 at 10:43 AM, Casey W. Stark <caseywstark at gmail.com
> >
> >> wrote:
> >> > Hey Matt.
> >> >
> >> > I think this would be a big improvement, but I was wondering how it
> >> > interacts with other yt pieces. Does each output have geometry and
> >> > coordinate_handler objects as attributes?
> >>
> >> Yup, that is the plan.  The idea is that we move all of the IO and
> >> particle/fluid selection into the geometry handler, and the handling
> >> of spatial layout to the coordinate handler.  This would mean, for
> >> instance, that we could push periodicity as well as path length into
> >> the coordinate handler; this would remove some of the need to
> >> constantly do wraparound checks and the like.
> >>
> >> There is still somewhat the issue that *selection* of points to
> >> understand coordinate systems, which will require a bit more thought
> >> in the future but I think is still a tractable problem.
> >>
> >> >
> >> > Is it possible to replace axis_name, axis_id, x_axis, and y_axis with
> >> > only
> >> > axis_names = ['x', ...]?
> >>
> >> It is, but not with abc.abstractproperty.  (Initially I figured the
> >> cost of creating the dicts was low enough that we could do this to
> >> avoid worrying about mutable, class-level properties, but I think
> >> perhaps I like yours better.)  I'll remove some of the fancier
> >> ABC-stuff and slim it down.
> >>
> >> >
> >> > - Casey
> >> >
> >> >
> >> > On Wed, Aug 29, 2012 at 7:28 AM, Matthew Turk <matthewturk at gmail.com>
> >> > wrote:
> >> >>
> >> >> Hi all,
> >> >>
> >> >> I've issued a pull request to the 3.0 repository, as I think it
> >> >> warrants discussion.  It's here:
> >> >>
> >> >>
> >> >>
> >> >>
> https://bitbucket.org/yt_analysis/yt-3.0/pull-request/5/initial-import-of-coordinate-handler-class
> >> >>
> >> >> This includes a first pass at a coordinate handling system.  This is
> >> >> distinct from a geometry handling system; the coordinates here refer
> >> >> to how we handle coordinates and spatial locations internally,
> whereas
> >> >> geometry refers to how data is distributed throughout a domain and
> >> >> throughout places on disk.  For instance, coordinate handling would
> be
> >> >> cartesian, polar, spherical.
> >> >>
> >> >> The reason I'm bringing it up for discussion is that I believe we
> want
> >> >> to move as much coordinate handling and transformation into a
> >> >> separate, well-defined class as possible.  Periodicity, distances and
> >> >> so on are all currently scattered throughout the code, and I'd like
> to
> >> >> try to consolidate them.  Additionally, as new coordinate systems
> >> >> (polar, spherical) are added, we'll need clear ways to delegate
> >> >> responsibility for things like "How do I calculate path length as I
> >> >> integrate?" or "What's the way to turn this into an image?"  I
> believe
> >> >> the best way to do that is to attach a coordinate system to the
> >> >> dataset object itself.  (We now have a polar pixelizer
> >> >> http://i.imgur.com/a4UGg.png !)
> >> >>
> >> >> The interface is currently set such that you need to define these
> >> >> methods and properties in order to implement a coordinate system:
> >> >>
> >> >> coordinate_fields (this may go away, but it's for the analogs of 'x',
> >> >> 'y', 'z', as well as volume)
> >> >> pixelize
> >> >> convert_from_cartesian
> >> >> convert_to_cartesian
> >> >> axis_name
> >> >> axis_id
> >> >> x_axis
> >> >> y_axis
> >> >> period
> >> >>
> >> >> Some of these currently live in dictionaries in
> >> >> yt/utilities/definitions.py, which is pretty sub-optimal.  I'd like
> to
> >> >> ask for feedback:
> >> >>
> >> >> 1) Do these methods sufficiently cover everything we need to know in
> >> >> yt about a coordinate system?  Should any be added?
> >> >> 2) Do we need to directly address dimensionality as a separate
> >> >> subclass?
> >> >> 3) Should any of these be removed?
> >> >>
> >> >> This will also help address these issues:
> >> >>
> >> >>
> >> >>
> >> >>
> https://bitbucket.org/yt_analysis/yt/issue/418/use-a-right-handed-coordinate-system
> >> >>
> >> >>
> >> >>
> https://bitbucket.org/yt_analysis/yt/issue/422/ray-casting-in-cylindrical-coordinates
> >> >>
> >> >>
> >> >>
> https://bitbucket.org/yt_analysis/yt/issue/421/refactor-non-cartesian-geometry
> >> >>
> https://bitbucket.org/yt_analysis/yt/issue/345/non-cartesian-geometry
> >> >> https://bitbucket.org/yt_analysis/yt/issue/205/periodicity
> >> >>
> >> >> -Matt
> >> >> _______________________________________________
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