Fix Version/s: None
As part of
DM-31221, we need to be able to generate HIPS compatible HPX SkyWcs objects to warp coadds into the HPX projection used for HIPS viewing.
In the investigation I found no documentation for how to do this in https://www.ivoa.net/documents/HiPS/20170519/REC-HIPS-1.0-20170519.pdf or in https://www.atnf.csiro.au/people/mcalabre/WCS/hpx.pdf (which describes a full-sky mapping rather than the single-pixel mapping used by HIPS). However, this is done in the Aladin java source code available here: http://aladin.u-strasbg.fr/java/AladinSrc.jar (linked from http://aladin.u-strasbg.fr/java/nph-aladin.pl?frame=downloading).
This ticket ports the specific z-order mapping and related code used by the HPX code into afw.geom.makeHpxWcs to generate a SkyWcs suitable for use with the warping code.
I believe there are few cleanups and name differentiations (is it HEALPix or HPX) needed - then this is good to go. Plus the better description of tilepix as the number of projected pixels on the side of the HEALPIx pixel in the higher resolution projected space (or is it the inverse ..)
Wil O'Mullane Can you please take a look at the latest version? I've added more commentary and references, clarified the API (I hope!) and removed a couple of extraneous unused variables. I've tried to be clear about distinguishing between HEALPix the pixelization code and the HEALPix grid projection (HPX). Please let me know if there's anything missing.
PR is here: https://github.com/lsst/afw/pull/603