1. A method for improving image quality, comprising steps of: inputting an image; computing a probability density function based on the image; generating a first luminance histogram; computing a weighing distribution function based on the first luminance histogram; generating a second luminance histogram through pixel redistribution of the first luminance histogram; computing a cumulative density function based on the second luminance histogram; computing a gamma conversion function based on the cumulative density function based on the second luminance histogram; and adjusting luminance distribution of the input image based on the gamma conversion function.