Feature distance
How to define the difference between two features f1, f2?
Better approach:  ratio distance = SSD(f1, f2) / SSD(f1, f2’)
f2 is best SSD match to f1 in I2
f2  is  2nd best SSD match to f1 in I2
gives large values (~1) for ambiguous matches
f1
f2'
f2
I1
I2