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 having a plurality of sets of pixels and a plurality of dynamic backlight sectors corresponding to the plurality of sets of pixels; 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, wherein a mapping ratio of a pixel of the set of pixels is generated by dividing a predetermined value by a maximum first RGB brightness level of first RGB brightness levels of the pixel when the saturation of the pixel is smaller than a threshold value; 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; converting the set of RGBW brightness levels to generate a set of RGBW gray levels of the set of pixels; measuring backlight diffusion effects of a dynamic backlight sector; forming a backlight diffusion coefficient matrix according to the measured backlight diffusion effects of the dynamic backlight sector and neighboring dynamic backlight sectors; generating a diffused backlight duty of the dynamic backlight sector involving the backlight diffusion effects of the neighboring dynamic backlight sectors according to an ideal backlight duty that is inversely proportional to the minimum mapping ratio and the backlight diffusion coefficient matrix; generating an interpolated backlight duty by interpolating among the neighboring dynamic backlight sectors according to the diffused backlight duty of the dynamic backlight sector; and generating RGBW signals according to recalculated mapping ratios derived by the interpolated backlight duty and brightness of pixels of the dynamic backlight sector.