Variations
•Sum a different distance
–f(d) = d2 
–or Manhattan distance.
–f(d) = 1 if d < threshold, 0 otherwise.
–This is called bounded error.
•Use maximum distance instead of sum.
–This is called: directed Hausdorff distance.
–Use median distance.
•Use other features
– Corners.
–Lines.  Then position and angles of lines must be similar.
•Model line may be subset of image line.