[yt-users] error in light_ray periodic ray creation

[Joshua Eric Moloney]

Hi all,

I've been trying to get a script (provided to me by Britton) working to
create a bunch of light rays from an Enzo simulation. Depending on the
initial seed value, it makes some of them correctly but eventually fails
with the following error:

Traceback (most recent call last):
  File "make_light_rays.py", line 32, in <module>
    get_los_velocity=True, get_nearest_galaxy=False)
  File "/home/student/jomo9722/Enzo/
yt-x86_64/src/yt-hg/yt/analysis_modules/light_ray/light_ray.py", line 275,
in make_light_ray
    (sub_ray[field])])
  File
"/home/student/jomo9722/Enzo/yt-x86_64/src/yt-hg/yt/data_objects/data_containers.py",
line 319, in __getitem__
    self.get_data(key)
  File
"/home/student/jomo9722/Enzo/yt-x86_64/src/yt-hg/yt/data_objects/data_containers.py",
line 508, in get_data
    if field not in ("dts", "t") and self._generate_field(field):
  File
"/home/student/jomo9722/Enzo/yt-x86_64/src/yt-hg/yt/data_objects/data_containers.py",
line 349, in _generate_field
    self[field] = self.pf.field_info[field](self)
  File
"/home/student/jomo9722/Enzo/yt-x86_64/src/yt-hg/yt/data_objects/field_info_container.py",
line 316, in __call__
    dd = self._function(self, data)
  File
"/data/moloney/cosmology_test/test_new/ion_balance/SD93IonBalance.py", line
178, in _ion_number_density
    return solarAbundance[atom] * data[fractionField] * data['Metallicity']
* \
  File
"/home/student/jomo9722/Enzo/yt-x86_64/src/yt-hg/yt/data_objects/data_containers.py",
line 319, in __getitem__
    self.get_data(key)
  File
"/home/student/jomo9722/Enzo/yt-x86_64/src/yt-hg/yt/data_objects/data_containers.py",
line 508, in get_data
    if field not in ("dts", "t") and self._generate_field(field):
  File
"/home/student/jomo9722/Enzo/yt-x86_64/src/yt-hg/yt/data_objects/data_containers.py",
line 349, in _generate_field
    self[field] = self.pf.field_info[field](self)
  File
"/home/student/jomo9722/Enzo/yt-x86_64/src/yt-hg/yt/data_objects/field_info_container.py",
line 316, in __call__
    dd = self._function(self, data)
  File
"/data/moloney/cosmology_test/test_new/ion_balance/SD93IonBalance.py", line
193, in _ion_fraction_field
    fraction = na.power(10, interp(data))
  File
"/home/student/jomo9722/Enzo/yt-x86_64/src/yt-hg/yt/utilities/linear_interpolators.py",
line 44, in __call__
    x_i = (na.digitize(x_vals, self.x_bins) - 1).astype('int32')
ValueError: Both x and bins must have non-zero length

Working through the debugger, I found that for the segment where it failed
the periodic_ray function had produced the following list of segments.

[[array([ 0.16103374,  0.78830872,  0.24846128]), array([ 0.14031834,
0.57367409,  1.        ])], [array([ 0.14031834,  0.57367409,  1.
]), array([ 0.14031834,  0.57367409,  1.        ])], [array([ 0.14031834,
0.57367409,  0.        ]), array([ 0.13268221,  0.49455532,  0.2770327 ])]]

The second segment here has identical start and end points, which is
leading yt to try to interpolate a field with nothing in it (and therefore
throwing the above error). It seems that what's happening is that due to
rounding errors the first segment being produced actually has a value < 1
for the third dimension so that when it checks to see if the segment hit
any of the box walls it thinks that it didn't. It then tries to start a new
segment that's already at the boundary of the box, producing a ray segment
containing 0 grid cells.

Has anyone run into this problem before, and is there a fix for it floating
around out there somewhere? If not I could probably work something up that
checks to see if the distance from the segment endpoint to the wall is <
SmallNumber and set it equal to the value at the wall if it is.

Thanks.
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