A source driver providing an output image to a plurality of pixels and including a judgment unit, an image processing unit, and a digital-to-analog converter is disclosed. The judgment unit encodes a first input image to generate an encoded code and compares the encoded code with a preset code to generate a luminance controlling signal. The image processing unit processes an image signal by an algorithm and outputs the processed result when the judgment unit asserts the luminance controlling signal. The image processing unit directly outputs the image signal when the judgment unit un-asserts the luminance controlling signal. The digital-to-analog converter transforms the output of the image processing unit and outputs the transformed result to the pixels.