potential mdim support via cli Alg

[mdsumner]

(as suggested in #759)

I'm not sure we can do much more than generate a new dataset (VRT or MEM or file), because there's no cli version of AsClassicDataset().

From here we can create a new GDALRaster in C++, which is convenient but doesn't use AsClassicDataset, and can't support VRT afaics.

dsn <- normalizePath("~/gdal/autotest/gdrivers/data/netcdf/MODIS_ARRAY.nc")
alg <- new(GDALAlg, "gdal mdim convert", c("--input", dsn, "/vsimem/out.tif"))
alg$run()
new(GDALRaster, alg$output())  ## we could do this in mdim part of gdalalg.cpp
alg$release()

gdal mdim does not support GDALG streaming, and gdal mdim convert only returns a "classic dataset" when only one array is present and the format implied or specified is 2D-only. (for example if we used /vsimem/out.vrt above that would be a MDIM dataset, even though it's only one array - that might be a valid feature request for GDAL itself, --classic mode on gdal mdim convert )

Unless I'm missing something, I think we have to have dedicated C++ code for this, I'll keep exploring. I tried a "factory" approach for GDALRaster here and that works (no support for View yet) - I hadn't understood that stuff before.

main...mdsumner:gdalraster:as-classic-mdim-factory

[ctoney]

Yes this is doable in C++ and wouldn't need a cli version of AsClassicDataset(). The factory functions are a feature of Rcpp Modules that I used in a few cases, but they aren't necessary to return objects of class GDALRaster or GDALVector from C++. RCPP_EXPOSED_CLASS automatically provides as<>() and wrap() functions for conversion. We can allocate a GDALRaster or GDALVector on the heap with new() and Rcpp::wrap() a const ref to it. Rcpp makes that a SEXP for R (as an object making use of the S4 system, but not a true S4 class, and automatically managing the R external pointers to the underlying C++ objects for us).

Mdim is analogous to the case when cli output is a vector dataset. GDALVector represents one OGRLayer with a pointer to the GDALDataset that owns it. So gdalraster::GDALAlg has a setting for vector output, exposed as the writable string field outputLayerNameForOpen. It defaults to "" in which case the first layer by index will be opened on the output vector dataset. Or, the user can set the name of a specific layer that will be opened in the returned GDALVector object.

Mdim is similar but it's possibly more complicated to specify the MDArray to return as a classic (when cli output is a GDALDataset of type GDAL_OF_MULTIDIM_RASTER). From a multidim GDALDatasetH that the cli alg returns, we'll need C++ code to get to the MDArray that we want to return as classic. A specific one can be requested in one or more settings exposed as fields of class GDALAlg, starting with a boolean returnMdimAsClassicRaster. If false, only the DSN string will be returned. If true, a GDALRaster object will be returned and this will be an mdim version analogous to the vector output code starting at https://github.com/ctoney/gdalraster/blob/9bf5dd9793061d64d36ac9d5e95dcc1e8056b083/src/gdalalg.cpp#L1108 (i.e., instantiating on a specific MDArray, possibly specified with additional field settings).