An apparatus for processing an input image including M image units is provided. Each of the M image units respectively includes N pixels. Each of the pixels respectively has an original gray scale. The apparatus includes a converting module, a gain generating module, and a contrast enhancing module. The converting module is used for generating M luminances. Based on the original gray scales of the N pixels in the ith image unit among the M image units, the converting module generates the ith luminance among the M luminances. A maximum luminance and a minimum luminance are generated based on the M luminances. The gain generating module generates a contrast gain and a minimum gray scale based on the maximum luminance and the minimum luminance. Based on the contrast gain and the minimum gray scale, the contrast enhancing module adjusts the original gray scales of the pixels in the input image.