Hi Sam, Great question. There have been in the past methods of casting rays that followed healpix-determined trajectories in yt. These could then be used as samples -- for instance, casting rays that traversed only an epsilon in delta r, at each of the values. Right now, because of licensing concerns, these are not currently *directly* exposed in yt. The stubs are still there and can be uncommented out and linked against, but I haven't tried it in a while. The alternative I would suggest would be to use healpy to generate the r, theta, phi values (or, better yet, directly compute the x', y', z' for them) for your N pixels at some level of healpix refinement. Use these to compute the values at whatever radius from the center you want so that you have an array of x, y, z points *in pixel indices*. Then use the find_field_values_at_points function to sample the values there. This will not conduct any interpolation, but with reasonable resolution this should not be a terrible impediment. There's some more info about this function at https://yt-project.org/doc/examining/low_level_inspection.html#finding-data-... Does that help? Additionally, two things -- it would *absolutely* be worth exploring if this set of analysis operations could be abstracted and upstreamed, and furthermore if we were able to utilize healpy instead of HEALpix inside our cython volume rendering routines we'd likely be able to utilize healpix-determined vectors in a standard installation of yt. -Matt On Wed, Mar 13, 2024 at 11:12 AM Samuel Patrick <spatrick@ed.ac.uk> wrote:

Hi all,

I would like to perform a spherical harmonic decomposition of the density within a spherical shell, essentially a Fourier transform of the density (or any scalar quantity) on the surface of a sphere object . I'm aware of python libraries like Healpy which have functions such as map2alm which take a healpix map and return the corresponding Alm coefficients; are there any utilities within yt that can either do this directly or return a healpix map from a sphere object?

Thanks and best, Sam Patrick

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