12. The image processing method according to claim 8 , further comprising identifying a text direction where characters printed in the document are aligned, wherein the type of the document is identified further based on the text direction.
8. An image processing method comprising: acquiring an input image generated by reading a document; generating a histogram of valid pixels in the image; extracting continuous regions in the histogram and calculating a character string variance in the input image, based on a distance between a maximum value of classes of the histogram and a center of a continuous region closest to the maximum value, and distances between centers of the continuous regions adjacent to each other; and identifying, using a computer, a type of the document based on the character string variance.