STC-S to S-MOC implementation

[F.-X. Pineau]

What about using MOCPy to:
1. build a MOC from your Box at a HEALPix resolution near from the 
wanted image pixel resolution
2. then to compute the intersection of both MOCs
3. and finally to render the image using the WCS class (see 
https://cds-astro.github.io/mocpy/stubs/mocpy.WCS.html)?

See the MOCPy gallery:
https://cds-astro.github.io/mocpy/examples/user_documentation.html#gallery-of-notebooks-examples-using-smocs


Le 14/12/2023 à 19:43, Arnold Rots a écrit :
> On a different note, I have been looking for a tool that renders the 
> area covered by the intersection of a Box and a MOC.
> Does anyone have something that does that?
> Thanks,
>
>   - Arnold
>
> Arnold H Rots
>
> Research Associate
>
> SAO/HEAD
>
> Center for Astrophysics | Harvard & Smithsonian
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>
> Email: arots at cfa.harvard.edu
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> On Wed, Dec 13, 2023 at 6:49 AM F.-X. Pineau 
> <francois-xavier.pineau at astro.unistra.fr> wrote:
>
>     Dear all,
>
>     Whatever the future of the STC-S working draft is, having -- even
>     partial --
>     implementations may help the debate.
>
>     A few MOCPy users have asked for a S-MOCs from STC-String feature.
>     And it seems that S-MOCs are considered to replace (at least in
>     some places)STC.
>     I am curious to know where exactly since MOCs and STC-S are
>     complementary:
>     * STC-S is compact -- as far as it does not contains 
>     unions/intersection of hundreds
>       of shapes -- and precise, but not indexation oriented
>     * MOCs are approximations, not compact -- except at low resolutions --
>       but fast and indexation oriented
>     I believe that efficient STC-S queries must rely (internally) on MOCs
>     (or other similar mechanisms), but should not be replaced by MOC
>     queries
>     (even if MOC queries are interesting too and can help in STC-S
>     queries).
>
>
>     ## STC-S Parser
>
>     So, as previously announced, I implemented a STC-S parser
>     available on both
>      github and crates.io <http://crates.io>:
>     * https://github.com/cds-astro/cds-stc-rust
>     * https://crates.io/crates/stc-s
>     There is still room for improvement and feeback/comments from people
>     with STC experience would be much appreciated.
>
>
>     ## STC-S to S-MOC
>
>     This email to announced that I now have also implemented a *first
>     version of**
>     **the STC-S to S-MOC feature* in MOCLibRust and in:
>     * *moc-cli*: already available in pypi and github release
>         + https://pypi.org/project/moc-cli/
>         + https://github.com/cds-astro/cds-moc-rust/tree/main/crates/cli
>         + Example: echo "Circle ICRS 147.6 69.9 0.4" | moc from stcs
>     14 - fits stcs.moc.fits --force-u64
>     * moc-wasm: already available in github release
>         + https://github.com/cds-astro/cds-moc-rust/tree/main/crates/wasm
>     * *MOCPy*: implemented in the github source code, but not released yet
>         + https://github.com/cds-astro/mocpy
>     The feature has been put in place but now have to be tested more
>     thoroughly.
>     So far the limitations are:
>     * it *supports only frame=ICRS, flavor=SPHER2 and units=deg*
>     * it wrongly considers the*DIFFERENCE to be a SYMMETRIC
>     DIFFERENCE* (XOR)
>       instead of a "MINUS".
>       Note that for a single polygon whith a hole inside it, the
>     result is the same,
>       except that:
>     * it does not adopt the STC definition of a polygon:
>       + it *supports self-intersecting polygons*
>       + the polygon interior is (kind of) the part having the smallest
>     area, i.e.
>         the order of the vertices does not matter
>     We do support nested UNION/NOT/INTERSECTION/XOR operations.
>     To do so, we rely on BMOCs implemented in the CDS HEALPix Rust
>     library.
>     BMOCs are S-MOCs storing in each cell a boolean flag telling if
>     the cell is for
>     sure fully inside the geometrical shape or not.
>     For example, the NOT operation removes all cells having the
>     boolean set to 'true'
>     but preserves the cells having the boolean set to 'false' (and add
>     missing cells
>     setting their flag to 'true').
>     BMOCs have so far two drawback:
>     * operations on BMOCs has not yet been thoroughly tested
>     * operations on BMOCs are -- at least in the current
>     implementation -- much slower
>       than operations on MOCs
>     In the future, we may first detect the operations in a STC-String
>     to switch
>     between MOCs (neither NOT or DIFFERENCE operations) and BMOCs.
>     Except that BMOCs have also one big advantage: they can be spitted
>     into 2 sub-MOCs
>     * one fully inside the STC region (no additional test needed, e.g.
>     when retrieving sources)
>     * one "edges" MOC: additional tests needed to determine if a
>     source in this MOC
>       is in or out of the STC region.
>
>     *If you have example of STC-String, especially with operations
>     (Alberto?),
>     please send them to us so we can add them to our tests.*
>
>     Even if STC-S is dropped in the future, it should not be that
>     complex to
>     adapt to another standard like DALI "shape".
>     (Although STC-S is complex, it is quite complete and
>     self-consistent since it contains
>     information such as the frame: the two sides of a medal).
>
>
>     ## About polygons:
>
>     We support self-intersecting polygons.
>
>     Although it is useless when describing footprints, I do think it
>     makes sense
>     when you let a user create a polygon by clicking in interfaces such as
>     Aladin/Aladin Lite or TOPCAT.
>     In addition, the existing algorithm to deal with self-intersecting
>     polygons is
>     simple (at least in the plane, with some complications on the sphere)
>     and fast (see
>     https://wrfranklin.org/Research/Short_Notes/pnpoly.html).
>
>     The best option to define the interior/exterior of a
>     self-intersecting polygon
>      may be to provide a control point, provided it is not a vertex
>     and is not part
>     of an edge. (It's on of the several options that has been
>     implemented in the
>      CDS Healpix Lib).
>     The control point can be automatically computed from the current
>     standard for
>     non-intersecting polygons:
>     * for convex polygons: we can use the gravity center (or its
>     opposite) by testing it
>       with the current convention (if the gravity center is (0, 0, 0),
>       remove recursively one vertex, starting from the first one).
>     * concave polygons: by testing the gravity center of 3 successive
>     non-aligned vertices,
>       iterating till one lies inside the polygon by the current
>     standard (also ensuring
>       the point is not a vertex or on an edge).
>     * self-intersecting: the control point would have to be provided,
>     i.e. after the last
>       vertex (unless we all agree on a same algorithm, using the NOT
>     operation to define
>       the complement polygon).
>
>     Alberto:
>     If I understand correctly, a polygon with a hole is the intersection
>     of a CCW polygon with a (smaller) CW polygon, right?
>     Why not having used the difference between two CCW polygons?
>     Is it because the difference in STC-S is not a symmetric
>     difference and hence
>     you cannot use the logical XOR operation in a database expression?
>     (*That may advocate to replace the DIFFERENCE by/ or at least to add a
>      SYMMETRIC DIFFERENCE in STC :)* ).
>
>
>     fx
>
>
>
>
>
>
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