[task #15713] Make quad structure to make unique hashes of 4 objects

[Sachin Kumar Singh]

Follow-up Comment #6, task #15713 (project gnuastro):

The resolution part is where I had a doubt about. In astrometry.net, they have
written in their paper <https://arxiv.org/pdf/0910.2233.pdf> that:
"The grid size is chosen so that grid cells are a small factor smaller than
the query
images. Typically we choose the grid cells to be about a third of the size of
the query
images, and select 10 stars from each grid cell so that most query images will
contain about
a hundred query stars."


But I think using a HEALPix size to find quad within the circle is more
efficient. And then we can use the kd-tree to find the nth nearest neighbour.
This I think will be a more reliable method than randomly finding the points.

IN astrometry.net, they use brightness as stated below in their paper:
"We search for quads starting with the brightest stars, but for each star, we
track the number of times it has already been used
to build a quad, and we skip stars that have been used too many times already.
We repeat
this process, sweeping through the grid cells and attempting to build a quad
in each one, a
the number of times."

But your suggestion really seems efficient. Also, we can use their approach of
not using a star more than some n number of time.

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