Oyvind, The best abstraction to deal with a nodata triplet would be the NODATA_VALUES metadata item at the dataset level described in http://www.gdal.org/gdal_datamodel.html
You can do : gdal_translate your.sid your.tif -mo NODATA_VALUES="255 255 255" Since a few GDAL releases this will results in an implicit mask band that will only mark pixels where R=G=B=255 as transparent. However mask bands are not honoured very much in GDAL. Unless you use latest trunk version where you could do : gdal_translate your.tif rgba.tif -b 1 -b 2 -b 3 -b mask -mo NODATA_VALUES="" to make a RGBA image by translating the implicit mask band into a explicit alpha channel. > Thanks for fast reply ! > > I assume the IMAGE__NO_DATA_VALUE=255,255,255 comes from the MrSid SDK > library. > > These values could easily be mapped to the GDAL-bands NoData, as they > practically means > the same ? (255 for each band) > > > -- Oyvind > > > On Thu, Oct 28, 2010 at 1:09 PM, Chaitanya kumar CH > <chaitanya...@gmail.com>wrote: > > > Oyvind, > > > > GDAL handles nodata values band by band, whereas in MrSID all the bands of > > the particular pixel location must match the nodata values for that pixel > to > > be considered nodata. > > Currently nodata values are ignored in this driver. I don't know if > > anything is planned for future releases. > > > > On Thu, Oct 28, 2010 at 4:21 PM, Oyvind Idland > <oyvind.idl...@gmail.com>wrote: > > > >> Hello, > >> > >> stumbled across a problem when reading MrSid: > >> > >> The MrSid dataset contains 3 bands, R/G/B. None of the 3 bands gives me a > >> GetNoDataValue(), > >> however, the metadata contains IMAGE__NO_DATA_VALUE=255,255,255. > >> > >> Does this mean, that I have to check for both cases ? > >> > >> The version is GDAL 1.7.2. > >> > >> > >> Regards, > >> > >> > >> Oyvind Idland > >> > >> _______________________________________________ > >> gdal-dev mailing list > >> gdal-dev@lists.osgeo.org > >> http://lists.osgeo.org/mailman/listinfo/gdal-dev > >> > > > > > > > > -- > > Best regards, > > Chaitanya kumar CH. > > /tÊaɪθÉnjÉ/ /kÊmÉr/ > > +91-9494447584 > > 17.2416N 80.1426E > > > _______________________________________________ gdal-dev mailing list gdal-dev@lists.osgeo.org http://lists.osgeo.org/mailman/listinfo/gdal-dev