An image processing method for deriving text characteristic images from an input image includes: performing a plurality of edge detecting processes upon the input image to generate a plurality of edge images, respectively, and deriving a first text characteristic image according to the edge images. The image detecting processes include: performing a first edge detecting process upon the input image to derive a first edge image according to a first upper threshold and a first lower threshold, and performing a second edge detecting process upon the input image to derive a second edge image according to a second upper threshold and a second lower threshold.