Provides a floor gradient. It is computed by the directions to the targets of the respective pedestrian, regardless of obstacles (i.e. the direct way). The point x with d(x,target)=0 would give problems when calculating the direction: x/norm(x-target). The mollified version (Koester et al. 2012) is this: x/sqrt((x-target).^2+epsilon^2) with a small epsilon.