1. An image processing method comprising: providing a set of first RGB (red, green, blue) brightness levels of a set of pixels in a display panel; generating a set of saturation levels according to the set of first RGB brightness levels; generating a set of mapping ratios according to the set of saturation levels and the set of first RGB brightness levels; generating a set of second RGB brightness levels according to the set of first RGB brightness levels and a minimum mapping ratio of the set of mapping ratios; generating a set of brightness levels of white sub-pixels, each brightness level of white sub-pixel being generated according to a minimum second RGB brightness level of second RGB brightness levels of each pixel; generating a set of RGBW (red, green, blue, white) brightness levels according to the set of second RGB brightness levels and the set of brightness levels of white sub-pixels; and converting the set of RGBW brightness levels to generate a set of RGBW gray levels of the set of pixels.