[PROJ] Migrating to proj API 6+ without forcing crs input

[Even Rouault]

I already answered (I admit I more and more terse with years) in 
https://github.com/OSGeo/PROJ/issues/3161#issuecomment-1094238124

So for your case where you don't have a CRS but a pipeline, and assuming 
it serves the role of the source, you'd need an alternate code path that 
would do

PJ* P = proj_create(ctxt, "+proj=pipeline " + 
your_pipeline_without_proj_equal_pipeline_and_with_steps_in_reverse_order_and_with_+inv_qualifier_added_when_absent_or_removed_when_present 
+ " +step " + proj_string_of_target )

proj_trans(P, PJ_FWD, ...);

so yes, there's a bunch of PROJ string manipulation to do.

Even

Le 14/04/2022 à 17:16, Johannes Schauer Marin Rodrigues a écrit :
> Dear proj list,
>
> in my application [1] I was using the ability of the Python mapnik library to
> render an arbitrary transformation by supplying my own custom proj string (a
> +proj=pipeline with +proj=tinshift). This stopped working when mapnik
> implemented support for the proj API 6+ [2] because mapnik followed the proj
> migration guide and turned code that looked like this before:
>
>      projPJ pj_longlat = pj_init_plus(source);
>      projPJ pj_merc = pj_init_plus(target);
>      pj_transform(pj_longlat, pj_merc, ...)
>
> Into code that looks like this:
>
>      PJ *P = proj_create_crs_to_crs(PJ_DEFAULT_CTX, source, target, NULL);
>      proj_trans(P, PJ_FWD, ...);
>
> This means that in contrast to before, the source and target transformation
> must now be a CRS. My own custom proj string (a pipeline) is not a CRS and thus
> my code is now non-functional with mapnik versions from git.
>
> I want to contribute an alternative codepath to mapnik which restores its
> previous ability to work with arbitrary proj strings (even those that are not a
> CRS). But how is this done? All projects that migrated to API 6+ that I was
> able to find follow the migration guide and call proj_create_crs_to_crs.
>
> So my question ultimately becomes: how do I implement proj API 6+ support
> without proj_create_crs_to_crs so that the software retains its ability to
> handle arbitrary user-supplied proj strings?
>
> I also asked the same question on stackoverflow [3] in case somebody wants to
> grab the bounty.
>
> Thanks!
>
> cheers, josch
>
>
> [1] https://blog.mister-muffin.de/2014/04/03/mapbender---maps-for-long-distance-travels/
> [2] https://github.com/mapnik/mapnik/commit/88241b32eeaccdc9eda7b92b7e7b6d4deac88e12
> [3] https://stackoverflow.com/questions/71823105/migrating-to-proj-api-6-without-forcing-crs-input
>
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