distance(oA,oB,vD);
vD is set to the distance between objects oA and oB. The formula used for calculating the distance is abs(x1-x2) + abs(y1-y2) where x1,y1 and x2,y2 are the co-ordinates of the center of the baselines of oA and oB.
If one or more of the objects is not on screen, the distance is 255.