Hi Matt,

Thanks for the pointer. 

I think the real difficulty here is that in theory the data object used to generate the photons could be literally anything, a sphere, box, disk, or some goofy shape defined by a region cut. 

The code in selection_routines.pyx that handles such periodic shifting appears (to me at least) to apply only for rectangular regions for which such an operation is relatively straightforward—but I’d need something that doesn’t necessarily know about “left edge” and “right edge”. You could special-case that for things like rectangular regions and spheres, perhaps, but for anything more general determining a “left edge” becomes a problem. For example, you simply couldn’t just use the minimum value of the coordinate because of course your region could straddle the domain boundary and your minimum coordinate could actually be on the “rightward” side of the object. 

Best,

John

On Sep 9, 2016, at 1:12 PM, Matthew Turk <matthewturk@gmail.com> wrote:

Hi John,


On Thu, Sep 8, 2016 at 10:22 AM, John ZuHone <jzuhone@gmail.com> wrote:
Hi all,

I’m working on some updates to my yt-based pyXSIM package and I’m stuck and I thought I would email this list for some guidance. 

I’m thinking of a situation where we have a data object (sphere, box, whatever) and it straddles a periodic boundary. I want to convert the coordinates such that the coordinates are translated with respect to some arbitrary origin (say, the center of a sphere but in theory it could be anywhere), but are also continuous, i.e, they do not wrap at the boundary. 

I’ve looked at the functions get_radius and get_periodic_rvec in yt/fields/field_functions.py, and based on that I have come up with the code below, but it doesn’t quite work for any arbitrary value of the “ctr” argument (the origin). I think it’s because the functions I mentioned only need to calculate absolute value differences in order to compute a radius. 

The x, y, and z arguments to the function below are the input coordinate arrays. Could anyone who is more familiar with this sort of thing point out what I should be doing? 

I think that Doug implemented something in the region selector that does exactly this; it computes whether or not something needs a coordinate shift, and then if so, it applies that.  If you look through selection_routines.pyx for anything with "shift" in the name, it should be there.  If that doesn't do what you're looking for, we can try to figure something else out.
 

Best,

John Z

def get_periodic_coords(ds, ctr, x, y, z):
    coords = ds.arr(np.zeros((3, x.size)), "kpc")
    coords[0, :] = (x - ctr[0]).to("kpc")
    coords[1, :] = (y - ctr[1]).to("kpc")
    coords[2, :] = (z - ctr[2]).to("kpc")
    if sum(ds.periodicity) == 0:
        return coords
    dw = ds.domain_width.to("kpc")
    for i in range(3):
        if ds.periodicity[i]: 
            c = coords[i, :]
            cdw = c - dw[i]
            mins = np.argmin([np.abs(c), np.abs(cdw)], axis=0)
            ii = np.where(mins == 1)[0]
            coords[i, ii] = coords[i, ii] - dw[i]
    return coords


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