1. A method for adjusting an input image on the basis of characteristics of a display system, said input image comprising M horizontal lines, each of the M horizontal lines respectively comprising N pixels, M and N being positive integers, each pixel in the input image having an original gray level, a first look-up table being used for storing a plurality of compensation coefficients related to the characteristics of said display system, said method comprising the steps of: (a) processing the M horizontal lines, i being an integer index ranging from 1 to M, when processing the ith horizontal line among the M horizontal lines, performing the following sub-steps: (a1) according to the N original gray levels of the N pixels in the ith horizontal line, calculating an ith loading; (a2) based on the ith loading, selecting an ith compensation coefficient corresponding to the ith loading from the plurality of compensation coefficients in the first look-up table; and (a3) respectively multiplying the N original gray levels of the N pixels in the ith horizontal line by the ith compensation coefficient to generate N compensated gray levels for the N pixels in the ith horizontal line; whereby the input image is adjusted.