1. A video-based handwriting input method, comprising: while a user writes on a writing surface with a stylus having thereon a special mark, capturing images including raw strokes and receiving writing sounds generated through contact of the stylus with the writing surface; filtering the captured images to extract, from the captured images, images of raw strokes that contain the special mark on the stylus and to filter out captured images that do not contain the special mark; and retrieving effective strokes from the raw strokes by utilizing the received writing sounds.