Hi John,

I'm not averse to the changes in the GDF standard you propose.  I think now that yt-3.0 is incorporating units as cleanly as it is, this makes sense to allow the format to specify an arbitary unittype instead of requiring cgs, since it can be so easily modified within current/future versions of yt.  

I honestly don't know how many people are using GDF, but it seems like these changes would break backwards-compatibility.  Is it possible for us to retain that compatibility by automatically running "field_to_cgs" on GDF files which do not possess the HDF5 attributes specifying the units?  Or perhaps since yt is the main source of GDF files, we can just move forward without having to worry about too much backwards-compatibility issues?

Anyway, this seems like a positive change. +1 to continue on this.

Cameron


On Sat, Jul 12, 2014 at 11:48 AM, John Zuhone <jzuhone@gmail.com> wrote:
Hi all,

I have a work-in-progress PR in the hopper regarding GDF reading and writing:

https://bitbucket.org/yt_analysis/yt/pull-request/1001/wip-some-gdf-writing-work/diff

What it does is implement direct writing of covering grids to GDF
files (something I've been using in my research, to work with the
resulting files directly in yt) and clobbering existing files with the
same path if the user allows for it.

However, when fiddling around with this I determined that it was
necessary to make GDF more unit-aware. Ultimately, I determined that
changes were needed that would break the standard that we had
determined for the files (http://yt-project.org/gdf.txt).

The two main changes are:

1) Remove "field_to_cgs", and allow the fields to be in whatever units
we wish them to be in the file (which are specified as HDF5
attributes).

2) Add a new top-level group containing the information for
"length","mass", and "time" units. These will be used when the file is
opened up in yt to determine the units.

Since (for now) we do automatic conversion to cgs, that is something
that is still left unimplemented, but otherwise I think this is all to
do for now.

I'm writing the email in case anyone has any suggestions or objections
to the format changes--particularly Jeff or Kacper. We'll obviously
need to document them if they are accepted.

Best,

John

--
John ZuHone

Postdoctoral Researcher
NASA/Goddard Space Flight Center

jzuhone@gmail.com
john.zuhone@nasa.gov
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--
Cameron Hummels
Postdoctoral Researcher
Steward Observatory
University of Arizona
http://chummels.org