1. A method for discriminating luminance backgrounds for images, comprising the steps of: receiving image information that is to be discriminated, the image information comprising gray scale values for respective sub-pixels in each pixel; forming the gray scale values for specific sub-pixels of pixels within the s±mth row and the t±nth column in the image information, having a pixel of the sth row, tth column as the center, into a digit group, and arranging the digit group in order, wherein s, m, t and n are natural numbers; if the gray scale values for N greater specific sub-pixels in the digit group are all greater than a given gray scale value, and a variance is less than or equal to a specified threshold, it is determined that the specific sub-pixels within the s±mth row and the t±nth column are a high-luminance background region; otherwise, it is determined that the specific sub-pixels within the s±mth row and the t±nth column are a non-high-luminance background region.