[yt-dev] Issue #775: Cannot generate isosurfaces of fields that depend on field parameters (yt_analysis/yt)

[Nathan Goldbaum]

New issue 775: Cannot generate isosurfaces of fields that depend on field parameters
https://bitbucket.org/yt_analysis/yt/issue/775/cannot-generate-isosurfaces-of-fields-that

Nathan Goldbaum:

Currently in 3.0 isosurfaces that depend on field parameters cannot be generated.

As an example:


```
#!python

from yt.mods import *
pf = load('HiResIsolatedgalaxy/DD0044/DD0044')

dd = pf.h.disk('c', [0,0,1], (20, 'kpc'), (3, 'kpc'))

radii = (np.arange(100)+1)/100.*20
fluxes = []

for r in radii:
    iso = pf.h.surface(dd, 'radius', r)
    print iso['density']
    flux = iso.calculate_flux('x-velocity', 'y-velocity', 'z-velocity', 'Density')
    fluxes.append(flux/1.9891e33*3.15569e13) # solar masses per megayear
    print r, fluxes[-1]

```

On both unitrefactor and the current 3.0 tip, this fails with the following traceback:


```
#!python

Traceback (most recent call last):
  File "radial_mass_flux.py", line 21, in <module>
    print iso['density']
  File "/Users/goldbaum/Documents/yt-hg/yt/data_objects/data_containers.py", line 205, in __getitem__
    self.get_data(f)
  File "/Users/goldbaum/Documents/yt-hg/yt/data_objects/construction_data_containers.py", line 826, in get_data
    mask, fields, sample_type)
  File "/Users/goldbaum/Documents/yt-hg/yt/data_objects/construction_data_containers.py", line 846, in _extract_isocontours_from_grid
    vals = grid.get_vertex_centered_data(field, no_ghost = False)
  File "/Users/goldbaum/Documents/yt-hg/yt/data_objects/grid_patch.py", line 293, in get_vertex_centered_data
    np.add(new_field, cg[field][1: ,1: ,1: ], new_field)
  File "/Users/goldbaum/Documents/yt-hg/yt/data_objects/data_containers.py", line 205, in __getitem__
    self.get_data(f)
  File "/Users/goldbaum/Documents/yt-hg/yt/data_objects/construction_data_containers.py", line 491, in get_data
    if len(gen) > 0: self._generate_fields(gen)
  File "/Users/goldbaum/Documents/yt-hg/yt/data_objects/data_containers.py", line 626, in _generate_fields
    fd = self._generate_field(field)
  File "/Users/goldbaum/Documents/yt-hg/yt/data_objects/data_containers.py", line 242, in _generate_field
    tr = self._generate_fluid_field(field)
  File "/Users/goldbaum/Documents/yt-hg/yt/data_objects/data_containers.py", line 262, in _generate_fluid_field
    rv = finfo(gen_obj)
  File "/Users/goldbaum/Documents/yt-hg/yt/fields/derived_field.py", line 181, in __call__
    dd = self._function(self, data)
  File "/Users/goldbaum/Documents/yt-hg/yt/fields/geometric_fields.py", line 49, in _radius
    return get_radius(data, "")
  File "/Users/goldbaum/Documents/yt-hg/yt/fields/field_functions.py", line 19, in get_radius
    center = data.get_field_parameter("center").in_units("cm")
AttributeError: 'NoneType' object has no attribute 'in_units'
```

I ran this script in unitrefactor, but the same issue is present in the current 3.0 tip.

In this context, 'data' is a covering grid that gets generated by `grid_patch` objects, which have no field parameters.

A quick fix might be to somehow pass the field parameters associated with the disk object to the grid patches.

When I chatted with Matt a little while ago about this issue, he mentioned that we would need to use the chunking system to generate the covering grid, rather than the old grid-based method. I'm unsure how to do that, so I'm reporting the issue here in hopes of getting some advice or guidance on how to fix this issue.