1. An error diffusion processing method, comprising: quantizing an input grayscale value of a pixel (m, n) of an input image based on a determination of whether the pixel (m, n) belongs to one of areas divided by a plurality of thresholds, the areas including at least one shadow area which is darker than a median grayscale value, the input grayscale value being quantized to a predetermined grayscale value which is brighter than the median grayscale value to determine a binary grayscale value of the pixel (m, n); obtaining an error from the input grayscale value and the binary grayscale value of the pixel (m, n); and distributing the error to neighboring pixels of the pixel (m, n) according to a predetermined error diffusion coefficient corresponding to the error, and to modify input grayscale values of the neighboring pixels.