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Using gdaldem hillshade, I get a one-band hillshade (download here, 2MB) :

 gdaldem hillshade crop_xl.tmp.tif Shadedrelief.tmp.tif -s 111120 -z 5 -az 315 -alt 60 -compute_edges

I currently can duplicate its band 1 (grey scale) into the band 2 (alpha) via :

 gdal_translate -b 1 -b 1 -co COMPRESS=LZW -co ALPHA=YES ./Shadedrelief.tmp.tif ./Shadedrelief.2bands.tmp.tif

But what I want to do is to get the inverted value of band 1 into band 2. The equation is :

 y = -x + 255; // where x is -b 1 's value, and y is -b 2 's value.

Given a tif hillshade as provided, how to get the inverted value of band 1 into band 2 ?

See also : How to conditionnally assign a new value to pixels of a raster image? , gdal_calc : fails with value 0?

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First, use gdal_calc.py to invert the band :

gdal_calc.py -A Shadedrelief.tif --outfile=InvertedShadedrelief.tif --calc="255-A"

gdalbuildvrt helps to merge 2 files into a two band image (or a 2 band vrt), it takes the first band of each input into band 1 and band 2:

gdalbuildvrt -separate final.vrt Shadedrelief.tif InvertedShadedrelief.tif

gdal_translate helps to convert to tif and compress the file:

gdal_translate -co COMPRESS=LZW -co ALPHA=YES ./final.vrt ./final.tif
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  • Thanks for the hint. I understand command 1, but It's my first contact with vrt and I don't understand the 2nd command. What are its input and output ? I would expect a kind of merge of input.tif -b 1 + result.tif -b 1 into a two bands final.tif -b 1 -b 2. Commented Apr 30, 2015 at 13:55
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    vrt is virtual raster file: you can manipulate it in most software as if it was a real 2 band image, but it is only a small xml file. Basically, if you want a single tif file, you can do : gdal_translate final.vrt final.tif Commented Apr 30, 2015 at 14:02
  • Tested, working, and happily validated ! Commented Apr 30, 2015 at 14:58

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