Nikos Alexandris wrote: > > > So nodata=255 does not mean "loss of information"? > > > The red, green, blue channels are 8-bit (2 high 8 = 256, values counting > > > from 0 up to 255). > > > > > It means that NULL cells in the GRASS raster are replaced with 255 or > > the specified nodata value. > > And what happens to the 255 pixels if the NULL cells become 255 after > export?
Nothing happens to them. You just cannot distinguish between cells which are 255 and those which are null. > OK, let me figure this out: > > I have an RGB photo with values from 0 to 255. > Import in GRASS splits in red, green, blue with values from 0 to 255 > Why should I want to eliminate those 255 values after exporting? This is > what I mean loss of information. If you import data which doesn't contain any nulls, then re-export it, the nodata= option will typically have no effect, as there aren't any nulls which need to be converted. OTOH, if the GRASS raster which you are exporting contains values from 0 to 255 inclusive, and also contains nulls, that's 257 possible values. If the format to which you are exporting only has 256 possible values (i.e. 0 to 255), then you can't avoid losing information. -- Glynn Clements <[EMAIL PROTECTED]> _______________________________________________ grass-user mailing list [email protected] http://lists.osgeo.org/mailman/listinfo/grass-user
