protected double criterion(int i,
Given a point at x,y in an image of size a,b
in the rectangle 0,a 0,b
find the minimum distance to the edge. We
assume that x,y is contained in the rectangle.
If x,y is outside the rectangle, then this
should return a negative number.
In this version the distance is scaled according
to the size of the image. Unfortunately we cannot
assume that this is constant. This ImageFinder is
called when we have images with substantially different
sizes in the survey.