1. An image processing method for deriving text characteristic images from an input image, comprising: performing a plurality of edge detecting processes upon the input image to generate a plurality of edge images, respectively, comprising: performing a first edge detecting process of the edge detecting processes upon the input image to derive a first edge image of the edge images according to a first upper threshold and a first lower threshold, wherein the first upper threshold is larger than the first lower threshold; and performing a second edge detecting process of the edge detecting processes upon the input image to derive a second edge image of the edge images according to a second upper threshold and a second lower threshold, wherein the second upper threshold is larger than the second lower threshold, the second upper threshold is smaller than the first upper threshold and the second lower threshold is smaller than the first lower threshold; and deriving a first text characteristic image according to the edge images.