We give the structure of discrete two-dimensional finite sets A, B ⊆ R2 which are extremal for the recently obtained inequality |A + B| ≥ ( |A| m + |B| n − 1)(m + n − 1), where m and n are the minimum number of parallel lines covering A and B respectively. Via compression techniques, the above bound also holds when m is the maximal number of points of A contained in one of the parallel lines co...