1. A bit resolution-extension method, the method comprising: acquiring a shortest distance. from a central pixel to a positive contour and a shortest distance from the central pixel to a negative contour in a filtering region which scans the positive contour comprising large pixels of a fixed value and the negative contour comprising small pixels of a fixed value, based on a value of the central pixel; generating an extension bit of the central pixel by interpolating the shortest distance from the central pixel to the positive contour and the shortest distance from the central pixel to the negative contour; and adding the extension bit as a lower bit of the central pixel.