[yt-dev] Issue #941: CutRegionSelector doesn't work with some data types (yt_analysis/yt)

Nathan Goldbaum nathan12343 at gmail.com
Fri Nov 21 09:54:34 PST 2014


On Fri, Nov 21, 2014 at 8:20 AM, Douglas Harvey Rudd <drudd at uchicago.edu>
wrote:

> Instead use an integer index field?  Most frontends probably already have
> them, and those that don't should be encouraged...
>
>
That's the trouble - there isn't an integer index field.


> Douglas Rudd
> Scientific Computing Consultant
> Research Computing Center
> drudd at uchicago.edu
>
>

> > On Nov 21, 2014, at 3:09 AM, Nathan Goldbaum <issues-reply at bitbucket.org>
> wrote:
> >
> > New issue 941: CutRegionSelector doesn't work with some data types
> >
> https://bitbucket.org/yt_analysis/yt/issue/941/cutregionselector-doesnt-work-with-some
> >
> > Nathan Goldbaum:
> >
> > It turns out the `CutRegionSelector` I recently added doesn't work so
> well for FLASH data:
> >
> >
> > ```
> > #!python
> >
> > import yt
> >
> > ds = yt.load("GasSloshing/sloshing_nomag2_hdf5_plt_cnt_0150")
> >
> > dd = ds.all_data()
> > dd = dd.cut_region("obj['density'] > 1e-27")
> >
> > slc = yt.SlicePlot(ds, 2, 'density', data_source=dd)
> > slc.set_zlim('density', 1e-30, 1e-25)
> > slc.set_figure_size(5)
> > slc.save()
> >
> > ```
> >
> > ![sloshing_nomag2_hdf5_plt_cnt_0150_Slice_z_density.png](
> https://bitbucket.org/repo/BXbAb/images/3572484347-sloshing_nomag2_hdf5_plt_cnt_0150_Slice_z_density.png
> )
> >
> > This is happening because `CutRegionSelector` is using a fragile exact
> comparison between floats. It does this to take advantage of an efficient
> hashing algorithm, but it looks like this comes at the cost of maintaining
> bitwise identicality between two different ways of calculating cell
> positions.
> >
> > The fix is either:
> >
> > 1. Figure out why the FLASH cell position fields are not bitwise
> identical to the cell positions calculated inside
> `SelectorObject.fill_mask`.  It seems Enzo data is unaffected by the
> round-off artifacts we see with FLASH data, so we know that means it is
> possible to rely on the cell positions matching exactly at some level.
> >
> > 2. Use a more forgiving data structure to retrieve matching cell
> positions.  A KD-tree seems like a natural choice, but I'm unsure if
> there's a ready-to-use KD-tree library available.
> >
> > Responsible: ngoldbaum
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