1. A method for classifying background regions of an image, comprising a plurality of steps: receiving an image, wherein the image comprises a plurality of pixels in a matrix, each pixel comprising one or more sub-pixels; analyzing the received image to obtain gray scale values for sub-pixels of pixels in a (2m+1)×(2n+1) sub-matrix, wherein a pixel in a sth row, tth column is chosen as a center of the (2m+1)×(2n+1) sub-matrix, and wherein s, m, t and n are positive integers; selecting, from the (2m+1)×(2n+1) sub-matrix, N sub-pixels all having greater gray scale values than any of the rest of the sub-pixels in the (2m+1)×(2n+1) sub-matrix; making comparisons between the gray scale values for the N sub-pixels and a given gray scale value as well as between a variance of the gray scale values for the N sub-pixels and a specified threshold; and classifying pixels in the (2m+1)×(2n+1) sub-matrix into a high-luminance background region, if the gray scale values for the N sub-pixels are all greater than the given gray scale value and the variance of the gray scale values for the N sub-pixels is less than or equal to the specified threshold; and otherwise, classifying pixels in the (2m+1)×(2n+1) sub-matrix into a non-high-luminance background region.