[GRASS-dev] Re: [GRASS GIS] #73: r.out.gdal tiff output does not work

[Glynn Clements]

Markus Metz wrote:

> > so the user will have to understand the conversion issues even if they
> > never use an out-of-range value. Ultimately, its either usability or
> > flexibility.
> 
> Without the -f flag I would opt for usability, with the -f flag for 
> flexibility.

Having -f affect the interpretation of nodata= also harms usability.

> >> The GDAL API documentation says "To clear the nodata value, just set it 
> >> with an "out of range" value." This is currently possible with the -f 
> >> flag, but r.out.gdal then does not give any information on what value is 
> >> assigned to NULL cells, only what nodata value is written to the metadata.
> >
> > Right. There's only one nodata= option, and it's performing a dual
> > function: what to store for null cells and what to set GDAL's no-data
> > value to. If you want to allow a mismatch, you really need two
> > options.
> 
> I thought in this case (allow mismatch) it would be enough to write it 
> as double to metadata without prior cast to the selected GDAL datatype.

Yeah, but what are you going to write for nulls in the input data?

Suppose the user specifies a CELL input map (which contains at least
0-255 and null), byte (GDT_UInt8) output, -f, and nodata=9999. So you
use 9999.0 as GDAL's no-data value, but how will you treat null values
in the input data?

-- 
Glynn Clements <glynn at gclements.plus.com>