Unable to create correct alpha band in GeoPackage using 'gdal raster mosaic'

[matsamentet]

What is the bug?

I have a GeoTIFF RGB raster which uses the color 255,255,252 around the edges (without defining it as nodata in the file). I want to translate this to a GeoPackage with this color transparent using an alpha band. I have been using gdalwarp with the combination of -srcnodata "255 255 252" -dstalpha to successfully accomplish this.

With the new CLI, as far as I can see, the equivalent is gdal raster mosaic. I am not able to accomplish the same result however. Instead of creating an alpha band where the specified color is transparent, the color is defined as "Nodata" in the gpkg_metadata table (which does not make sense as RGB GeoPackages does not support the concept on Nodata according to the format documentation )

Steps to reproduce the issue

Old gdalwarp command with correct result:
gdalwarp -f gpkg -dstalpha -srcnodata "255 255 252" rgb_raster.tif gdalwarp_output.gpkg

New CLI with incorrect output:
gdal raster mosaic --of gpkg --add-alpha --src-nodata "255,255,252" --dst-nodata 0,0,0 rgb_raster.tif gdalmosaic_output.gpkg

Versions and provenance

Windows 11
GDAL 3.11.0 from conda-forge

Additional context

No response

[rouault]

gdal raster mosaic isn't the way to go for your use case. You could achieve what you want (ab)using (abuse is similar to using galwarp) with gdal raster reproject --add-alpha --src-nodata "255,255,252" --dst-nodata 0,0,0 rgb_raster.tif gdalmosaic_output.gpkg

We perhaps need a dedicated "gdal raster nodata-to-alpha" command to avoid abusing other commands

[matsamentet]

gdal raster mosaic isn't the way to go for your use case. You could achieve what you want (ab)using (abuse is similar to using galwarp) with gdal raster reproject --add-alpha --src-nodata "255,255,252" --dst-nodata 0,0,0 rgb_raster.tif gdalmosaic_output.gpkg

We perhaps need a dedicated "gdal raster nodata-to-alpha" command to avoid abusing other commands

Understood. A similar usecase for me is having multiple input files, which was also handled by gdalwarp. As far as I can see from the docs, gdal raster reproject only supports single file input. It looks like multiple input files and nodata-to-alpha in the same operation is not currently possible with the new CLI? Perhaps something which could be achieved with piplines ?