Raw laser data has already been scanmatched (open loop), yielding the
dark gray background.

The loop closer starts at the beginning and looks for loop
closures. Any place that it thinks *might* be visible from the current
location is indicated in cyan. 

After a bundle of possible loop closures is identified (usually after
the robot has left the area), the set of loop closures is shown in
yellow. SCGP runs, classifying those closures as correct (green) or
incorrect (blue). The correct loop closures are added to the graph and
a non-linear graph optimization algorithm runs.