Le vendredi 17 juin 2016 09:01:43, Mark Johnson a écrit : > During the process georeferencing a Grayscale image, the image must be > converted to a RGB image (-expand rgb -ot Byte) before calling > gdal_translate setting the GCP's.
You can run gdal_translate and gdalwarp directly on your greyscale image. No need for the RGB intermediate. > > After gdalwarp, the image is of course then RGB (3 Bands) > - and is rendered looking as a Grayscale image in QGIS > > My question is: how to convert this back to the original Grayscale? gdal_translate rgb.tif grey.tif -b 1 > > I assume, during the '-expand', the gray values have been separated into > the 3 bands according to some default logic so that it renders as a > Grayscale image when viewed. The logic of -expand rgb , from a greyscale image, is just to replicate each grey value on the R,G,B channels :-) > > I have seen samples of converting a (truly) coloured image to Grayscale: > gdal_calc.py -A input.tif --A_band=1 -B input.tif --B_band=2 -C input.tif > --C_band=3 --outfile=result.tif --calc="A*0.2989+B*0.5870+C*0.1140" > > Would it be something in this form? You can do this but as A=B=C, gdal_translate -b 1 will do the same more efficiently :-) > > Mark Johnson, Berlin, Germany -- Spatialys - Geospatial professional services http://www.spatialys.com _______________________________________________ gdal-dev mailing list [email protected] http://lists.osgeo.org/mailman/listinfo/gdal-dev
