Scan documents as pdf with gamma correction

scan.bash

#!/bin/bash

#find out scanner model with scanimage -L
scanimage -d hp3900:libusb:003:005 --resolution=600 --mode=Gray > raw.pnm

#gamma correction (fiddle around with the value suitable for your printer)
convert raw.pnm -gamma 0.5 out.pnm

#convert to pdf
gm convert out.pnm out.pdf

#cleanup
rm *.pnm