According to one embodiment of the invention, a smoothing module is configured to smooth and reduce grayscale differences in a plain area of an input digital image signal according to a collection parameter. A histogram acquisition module is configured to acquire a histogram value of the input digital image signal for one frame. A histogram value buffer module is configured to buffer histogram values for a plurality of frames. A luminance level transition detection module is configured to detect an interval during which an image gradually changes from light to dark or from dark to light, based on the histogram values for a plurality of frames, and to output the correction parameter.