Gentoo linux install usb scanner

problems description

Scanning is failing because the scanner software can't find the device.

lsusb shows scanner (so its not a driver problem)

$  lsusb                                                                                                                                             
Bus 001 Device 007: ID 04a9:190a Canon, Inc. CanoScan LiDE 210

sane-find-scanner list the scanner

# sane-find-scanner
...
found USB scanner (vendor=0x04a9 [Canon], product=0x190a [CanoScan], chip=GL124) at libusb:001:006

running xsane as root still reports "no devices available"... so it is not a user permission problem

Errors

xsane: no devices available

simple-scan : similar to above

root cause

Xsane backend not installed

solution

1. lookup the required xsane backend for your scanner hardware here:  http://www.sane-project.org/cgi-bin/driver.pl
2. add sane backend from above to make.conf (optional)
    SANE_BACKENDS="the_name_of_your_backend_goes_here"
3. add "usb" & "scanner" use flag to make.conf if needed
4. recompile sane-backends
    # emerge -uavND --quiet sane-backends 

TEST

# scanimage -L                                                                                                                                    

device `genesys:libusb:001:007' is a Canon LiDE 210 flatbed scanner

success! scanning now works