gdal vector buffer missing --geometry-type option

[geographika]

Feature description

Running the following command to buffer lines:

gdal vector buffer --distance=5 "/vsizip/TRONCON_VOIE_PARIS.zip" out.gpkg --overwrite

Returns a warning:

Warning 1: A geometry of type POLYGON is inserted into layer TRONCON_VOIE_PARIS of geometry type LINESTRING, which is not normally allowed by the GeoPackage specification, 
but the driver will however do it. To create a conformant GeoPackage, if using ogr2ogr, the -nlt option can be used to override the layer geometry type. This warning will 
no longer be emitted for this combination of layer and feature geometry type.

The output is created, but it is a line dataset with polygons, and so doesn't display in QGIS.

It would be very useful to be able to set --geometry-type Polygon similar to the -nlt option in ogr2ogr, or in gdal vector explode-collections.

Even better would be that the buffer tool automatically sets the output geometry to polygon when buffering lines.

A workaround is to use a pipeline as below, but this is quite verbose:

gdal vector pipeline `
    ! read "/vsizip/TRONCON_VOIE_PARIS.zip" `
    ! set-geom-type --geometry-type Polygon `
    ! buffer --distance=5 `
    ! write out.gpkg --overwrite

Additional context

No response

[jratike80]

I think the geometry type should be multipolygon to support buffering of multi(point/linestring/polygon).

[theroggy]

I think the geometry type should be multipolygon to support buffering of multi(point/linestring/polygon).

I think it is indeed not necessary to be able to specify it, always multipolygon is a safe choice.

Only exception is probably a buffer with distance 0, that should output the same type as the input