[GRASS-user] GRASS export GeoTiff adventure... continued :-)
glynn at gclements.plus.com
Sun Aug 3 14:32:44 EDT 2008
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
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 <glynn at gclements.plus.com>
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