[GRASS-dev] [gdal-dev] Setting NODATA to -nan

[Even Rouault]

Hi,

(sorry to top post, just subscribed to the list)

A few notes w.r.t this thread:

- there's no universally "good" nodata value for GDAL. In the GDAL API, it is 
a double value, but the way it is stored is highly dependant of formats. Might 
be binary, or text (like in GeoTIFF with the GDAL_NODATA tag : 
http://www.awaresystems.be/imaging/tiff/tifftags/gdal_nodata.html). In the case 
of GeoTIFF, here's how it is serialized :

void GTiffDataset::WriteNoDataValue( TIFF *hTIFF, double dfNoData )

{
    char szVal[400];
    if (CPLIsNan(dfNoData))
        strcpy(szVal, "nan");
    else
        CPLsnprintf(szVal, sizeof(szVal), "%.18g", dfNoData);
    TIFFSetField( hTIFF, TIFFTAG_GDAL_NODATA, szVal );
}

The nan special case was done for cross OS compatibility, since there's no 
standardized way among OS to serialize NaN. AFAIK, Linux must output "nan", 
Windows "1#.QNAN" etc... Messy. On the reading side, same thing, "nan" is then 
converted to the "most common" binary representation of a quiet NaN. GDAL code 
that compare pixel values to nodata has a special case for nodata.

- the nodata value assigned to a GDAL band should theoretically be compatible 
with the range of values compatible with the raster band datatype (e.g. in 
[0,255] for byte), but this is not checked universally in the code 
(gdal_translate -a_nodata checks it for example, drivers not as you can see in 
the GeoTIFF case for example).

- for non-NaN floating point values, GDAL should normally compare the pixel 
value with the nodata value with an epsilon test (due to potential 
serialization as text).

- I've had a look at https://trac.osgeo.org/grass/changeset/65602 and my 
understanding is that now a pixel value at NaN will be considered as null, 
even if null_val is not NaN (but this is perhaps intended, if so, ignore 
this). 

- This is just for the sake of my curiosity: do you know how -nan pixel values 
can be generated by GRASS in practice ? (This was what triggered all this: the 
GDAL reporter was surprised to see pixels at -nan and nodata at nan, and 
thought that something was wrong)

Anyway, too much time spent discussing about numbers that are not numbers ;-) 
Let's move on.


Even


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