Unique vectors (Al Zimmermann)

Discuss about this problem

(a 2-dimensional extension to Golomb rulers) Consider an n-by-n grid of cells. Fill a few of the cells with X's. Draw a line between every pair of X's. (If there are k X's, then you will draw k(k-1)/2 lines) For a given n, the objective is to place as many X's as possible so that no pair of lines are the same length and at the same angle with respect to the horizontal. Please send us a simpler description.