It is theoretically analyzed that the upper bound capacity of a gray image block with n pixels is not more than ⌊log2 n2+ 2 n+2⌋ binary bits, when not more than 2 pixels are modified and every pixel is modified not more than 1. It is proved that such a weighted matrix can’t be constructed, but a way to construct a suboptimum weighted matrix is proposed in this paper ， the matrix can get better balance between the hiding capacity and the imperceptibility compared with the optimum maxtrix, the suboptimum can hide one bit less than the optimum matrix at the worst condition when n is not more than 25 million pixels.
