[yt-users] ray objects

[Sam Skillman]

Hi Elizabeth,

So the issue here, I think, is that while the rays are sorted, they traverse
varying sized cells.  When you query the field 'x', you are actually
querying the 'x' value of the cell that the ray intersects, not the position
of the ray itself.  This, in combination with the fact that fields like 'x'
are cell centered, means that you can see weird jumps when you go from a
highly refined cell to a root grid cell.  You could check this by looking at
what ray1['dx'] is, and if it is large for the second value, then this is
the issue.

If you'd like to recover the x,y, position of the ray at that point, I would
use the field 't', which is the integral of dt (distance) along the ray.
 You could then use the starting position and the direction to recover the
position along the ray.

That said, if all you are worried about is the Density, it should in fact be
giving you the ordered density values.  If you were to
matplotlib.pylab.plot(ray1['t'], ray1['Density']) it should give you a plot
of the density along the ray.

For reference, much of this is described here:
http://yt.enzotools.org/doc/analyzing/generating_processed_data.html#line-queries-and-planar-integrals

Best,
Sam

On Tue, Aug 16, 2011 at 12:49 PM, Elizabeth Tasker <taskere at mcmaster.ca>wrote:

> Hi,
>
> When you create a ray object with yt, e.g.
>
> ray1 = pf.h.ray([10,16,16], [10.3,16.1,16])
>
> I was expecting to get the values of the cells that lie inbetween those two
> sets of co-ordinates. Instead, it looks like the first two values back at
> the start and end points. .e.g.:
>
> -------> print(ray1["x"])
> Warning: divide by zero encountered in divide
> Warning: invalid value encountered in multiply
> Warning: divide by zero encountered in divide
> Warning: invalid value encountered in multiply
> yt : [INFO     ] 2011-08-16 14:47:27,962 Getting field t from 92
> yt : [INFO     ] 2011-08-16 14:47:28,972 Getting field x from 92
> [ 10.0078125   10.30078125  10.0234375   10.0390625   10.0390625
>  10.0546875   10.0703125   10.0859375   10.0859375   10.1015625
>  10.1171875   10.1328125   10.1328125   10.1484375   10.1640625
>  10.17578125  10.18359375  10.18359375  10.19140625  10.19921875
>  10.20703125  10.20703125  10.21484375  10.22265625  10.23046875
>  10.23046875  10.23828125  10.24609375  10.25390625  10.25390625
>  10.26171875  10.26953125  10.27734375  10.27734375  10.28515625
>  10.29296875]
>
> In [53]: print ray1["y"]
> -------> print(ray1["y"])
> yt : [INFO     ] 2011-08-16 14:48:23,698 Getting field y from 92
> [ 16.0078125   16.09765625  16.0078125   16.0078125   16.0234375
>  16.0234375   16.0234375   16.0234375   16.0390625   16.0390625
>  16.0390625   16.0390625   16.0546875   16.0546875   16.0546875
>  16.05859375  16.05859375  16.06640625  16.06640625  16.06640625
>  16.06640625  16.07421875  16.07421875  16.07421875  16.07421875
>  16.08203125  16.08203125  16.08203125  16.08203125  16.08984375
>  16.08984375  16.08984375  16.08984375  16.09765625  16.09765625
>  16.09765625]
>
>
> Is that right? Should I ignore the first two values of ray1 when I'm
> examining the data, e.g. ray1["Density"]?
>
> Thanks!
>
> Elizabeth
>
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