[PROJ] Determine "cut line" for arbitrary projection

[Javier Jimenez Shaw]

This question moved me to implement an easy idea I had on mind
https://github.com/OSGeo/PROJ/pull/4418

 The result is here:

https://osgeo-proj--4418.org.readthedocs.build/en/4418/operations/projections/all_images.html

(This is not solving your problem. It is just collecting a bunch of images)

On Sun, 9 Mar 2025, 00:13 Michael Sumner, <mdsumner at gmail.com> wrote:

> In the past I've had success filtering out bad segments that are "long"
> (in native CRS), for example with omerc. I guess we could generate spanning
> segments across the grid domain (maybe with a rectangle or triangle grid
> from longlat)and analytically find that boundary (endpoint pixels that have
> segments that are "too long).
>
> I think this would work for omerc and spilhaus, not sure about generally.
> Certainly repointing polygons back together is much harder and probably
> needs mesh decomposition.
>
> Will try, very interested in this discussion.
>
> Cheers, Mike
>
> On Sun, Mar 9, 2025, 08:27 Javier Jimenez Shaw via PROJ <
> proj at lists.osgeo.org> wrote:
>
>> Implementing the Spilhaus projection I learned how the images in the
>> documentation are generated, like the one in
>> https://proj.org/en/latest/operations/projections/spilhaus.html
>> if you have a look at docs/plot/plotdefs.json you will see that the
>> workaround is to not plot very long lines, that are usually crossing from
>> one border of the projection to another.
>>
>> On Sat, 8 Mar 2025 at 22:03, Nyall Dawson <nyall.dawson at gmail.com> wrote:
>>
>>> On Sun, 9 Mar 2025 at 04:06, Javier Jimenez Shaw <j1 at jimenezshaw.com>
>>> wrote:
>>> >
>>> > If you mean in a generic way, I think the answer is no.
>>> > Each projection has its own peculiarities and behaviours. Spilhaus (by
>>> the way, not yet in PROJ, but coming in 9.6.0) is a good example.
>>>
>>> Testing Spilhaus in QGIS was actually the motivation for this
>>> question. It works fine for rasters, but for vectors there's extreme
>>> artifacts caused by rendering features crossing the boundaries of this
>>> projection.
>>>
>>> >
>>> > "... latitude (or projected x coordinate) " Do you mean longitude? See
>>> that the longitude you are looking for may depend on the latitude.
>>>
>>> 🤦 of course! (*although for the above mentioned Spilhaus projection I
>>> guess we'd need something more complex than a single line longitude
>>> line anyway!)
>>>
>>> Nyall
>>>
>>> >
>>> >
>>> > On Sat, 8 Mar 2025 at 07:30, Nyall Dawson via PROJ <
>>> proj at lists.osgeo.org> wrote:
>>> >>
>>> >> Hi list,
>>> >>
>>> >> A question: given an arbitrary projection/CRS, is it possible to
>>> determine the latitude (or projected x coordinate) at which features would
>>> need to be "cut" in order to avoid artifacts when those features wrap
>>> around the projection extremes.
>>> >>
>>> >> Eg if it was EPSG:4326, we'd need to cut the features at +/- 180. But
>>> if it's another projection... say mercator or spilhaus or ... is there any
>>> way to reliably determine this cutting line?
>>> >>
>>> >> Nyall
>>> >>
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