[PROJ] Determine "cut line" for arbitrary projection

[Michael Sumner]

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
>> >>
>> >> _______________________________________________
>> >> PROJ mailing list
>> >> PROJ at lists.osgeo.org
>> >> https://lists.osgeo.org/mailman/listinfo/proj
>>
> _______________________________________________
> PROJ mailing list
> PROJ at lists.osgeo.org
> https://lists.osgeo.org/mailman/listinfo/proj
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.osgeo.org/pipermail/proj/attachments/20250309/250900be/attachment.htm>