1. An apparatus for processing an objective video frame in a video stream, the objective video frame comprising M image units, each of the M image units respectively comprising N original pixels, M and N being positive integers, each of the original pixels respectively having an original gray scale, said apparatus comprising: a converting module for generating M luminances, based on the original gray scales of the N original pixels in the ith image unit among the M image units, the converting module generating the ith luminance among the M luminances, i being an integer index ranging from 1 to M, the converting module generating a maximum luminance (Y max ) and a minimum luminance (Y min ) based on the M luminances; a gain generating module for generating a contrast gain (k) and a minimum input gray scale (Min) based on the maximum luminance (Y max ) and the minimum luminance (Y min ); and a contrast enhancing module, based on the contrast gain (k) and the minimum input gray scale (Min), the contrast enhancing module adjusting the original gray scales of the original pixels in the objective video frame to generate an output video frame.