Comment 21 for bug 1631859

What the SANE driver is telling us (from the log attached to this report):

...
[+15,55s] DEBUG: scanner.vala:885: sane_get_option_descriptor (24)
[+15,55s] DEBUG: scanner.vala:735: Option 24: name='brightness' title='Brightness' type=int size=4 min=-100, max=100, quant=1 cap=soft-select,soft-detect
[+15,55s] DEBUG: scanner.vala:738: Description: Controls the brightness of the acquired image.
[+15,55s] DEBUG: scanner.vala:885: sane_get_option_descriptor (25)
[+15,55s] DEBUG: scanner.vala:735: Option 25: name='contrast' title='Contrast' type=int size=4 min=-100, max=100, quant=1 cap=soft-select,soft-detect
[+15,55s] DEBUG: scanner.vala:738: Description: Controls the contrast of the acquired image.
...

i.e. there are two settings that go from -100 to 100. Simple scan is assuming -100 is darkest/lowest contrast and 100 is brightest/highest contrast.