1. A method for reducing discontinuity of gray level in an image buffer memory, which reduces N bits to (N−k) bits for each pixel on an input image stored in said image buffer memory (N and k are positive integers and N>k), so as to save required memory and reduce discontinuity of gray level, the input image including a plurality of pixels in a matrix, the method comprising the steps of: (A) selecting a plurality of interleaved pixels from the input image stored in said image buffer memory, wherein the pixels selected have coordinate (X, 3Y+0) in the matrix, where X and Y are non-negative integers and X=3 M for M as a non-negative integer; or coordinate (X, 3Y+2), where X and Y are non-negative integers and X=3 M+1 for M as a non-negative integer; or coordinate (X, 3Y+1), where X and Y are non-negative integers and X=3 M+2 for M as a non-negative integer; (B) adding a specific value of 2 k−1 in the pixels selected from the input image stored in said image buffer memory; and (C) deleting lower k bits for each pixel of the input image stored in said image buffer memory to reduce a size of said image buffer memory required by the image and reduce said discontinuity of gray levels in said image.