[yt-users] Point Pairs in the Two Point Function

[Piyanat Kittiwisit]

Hi Stephen,

Rephrasing my questions:

If the pixel coordinates of my data volume is on a 3D rectangular grid (assuming pixel coordinate here), i.e.

0 0 0
0 0 1
0 0 2
...
0 1 0
0 1 1
0 1 2
...

Would the random points be chosen out of these coordinates or from arbitrary coordinates, i.e. (0.2, 3.1, 5.8) instead of (0, 3, 6)?

Now, if the random point coordinates are arbitrary, this implies an infinite number of points within a spherical volume for a given max separation. How does the TPF avoid overlapping voxels here, i.e. does it put a constraint on the lower limit of separation to avoid overlapping voxels? 

If the random point coordinates are not arbitrary but chosen from the fixed rectangular coordinates, this implies a limited number of points for a given maximum separation, so how does total_values work here? Do I have to pre-estimate the number of pairs within a sphere of a given max separation and set it total_values to this number to avoid over counting the same pairs?

I am not sure if my use here make sense. I have a temperature field in a big 3D numpy array that I would like to calculate an absolute difference in temperature between pixel pairs, so I load the data into the yt and run it with the tpf function. It seems to work although I have not gotten mpirun to work with the script., and I am not sure if this is actually the right thing to do.

Any comment would be appreciated. Thanks!
-Piyanat

On Sep 15, 2014, at 3:29 PM, Stephen Skory <s at skory.us> wrote:

> Hi Piyanat,
> 
> If memory serves (and it might not!),
> 
> 1. The first point in each pair is chosen randomly within the volume.
> The second point is chosen by picking a random-ish distance away
> (within limits) from the first point, and the 3D angle is randomly
> chosen on a sphere (so all angles are equally likely).
> 
> 2. I'm not sure I understand this question. The algorithm should not
> chose point pairs with distances greater than the size of your volume
> (periodicity considered). Can you rephrase your question?
> 
> Sorry I can't be more specific, but yt isn't part of my job anymore! :(
> 
> 
> 
> On Mon, Sep 15, 2014 at 3:02 PM, Piyanat Kittiwisit
> <piyanat.kittiwisit at asu.edu> wrote:
>> Hello,
>> 
>> I have some questions regarding the two point function in yt-2.6.
>> 
>> 1) If my volume is a generic data on a rectangular grid, would the random point pairs be on a regular grid, or would they be truly random on a spherical grid an interpolated/averaged from the nearest neighborhoods on a rectangular grid?
>> 
>> 2) If the former is true in 1), does it meant that you have to pick total_values that will not exceed the number of point pairs within your distance?
>> 
>> -Piyanat
>> 
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> 
> 
> 
> -- 
> Stephen Skory
> s at skory.us
> http://stephenskory.com/
> 510.621.3687 (google voice)
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