1. An apparatus for processing an objective video frame in a video stream, the objective video frame comprising M image units and being adjacent to P adjacent video frames in the video stream, each of the M image units respectively comprising N original pixels P, 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 preliminary gain and a preliminary minimum input gray scale based on the maximum luminance (Y max ) and the minimum luminance (Y min ), the gain generating module generating a contrast gain (k) based on the preliminary gain with P adjacent gains relative to the P adjacent video frames, the gain generating module generating a minimum input gray scale (Min) based on the preliminary minimum input gray scale and P adjacent minimum gray scales relative to the P adjacent video frames; 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.