s_region problem

[Markus Demleitner]

Felix,

On Wed, Nov 29, 2017 at 10:27:32AM +0100, Felix Stoehr wrote:
> > Question 2: Assuming some tool support (i.e., "STC-S-to-MOC
> > converter", and MOC support in their backen databases), how would
> > current STC-S providers feel about migrating to MOCs on a time frame
> > of a few years?
> > 
[...]
> ALMA can certainly compute MOCs and has actually been planning to do so.
> My personal impression is that however the MOCs serve a different
> purpose than our STC-S footprints. MOCs: fast x-match, high-order
> display in AladinLite. STC-S footprints: exact outline and precise
> display in AladinLite.

I don't think precision is a major issue here -- at level 29, the
typcial extent of a healpix cell is of the order 0.5 mas.  Even in
the Gaia age, that, I believe, should go a long way.

It is true, though, that MOCs hide some physics; it is, I suspect,
virtually impossible to reconstruct that a certain coverage, for
instance, has been produced as a union of n distinct circles, and...

> This seems also to be what ESA sky or ESO are doing with the new
> interface. We also want that users can select individual footprints by
> clicking on the boundary, which is a use-case that I am not sure how it
> could be supported by MOC.

...finding the outline of a MOC is probably non-trivial (though it
sounds like something that's algorithmically feasible).  

Then again, I'm not convinced if letting people go for outlines
actually is a good solution for the UI problem of letting people
select one of several overlapping shapes; after all, it's perfectly
possible that several of these shapes have the same outline, at least
at a given resolution, and then you're back at having to be explicit
about the stack again.

So, for this particular problem I'd say going for, for instance,
popups that let people select which of the set of shapes below the
pixel in question they'd like to use would be preferable, and that's
at least as straightforward with MOCs as with explicit geometry
(whether written in STC-S on in some sanitation of it).

That doesn't mean there aren't (other) use cases for explicit
geometries.  But it'd be great to understand them before going for a
sanitation/standardisation of the TAP 1.0 appendix.

         -- Markus