A New Method for Discriminating Printers Based on Contours Qualities of Printed Characters Using Wavelet Decomposition

This article described a new method for discriminating models of laser printers by means of their printed characters, in particular details in contours of characters. A method we proposed was based on evaluating qualities in contours of printed characters using wavelet decomposition. Recently most of characters printed by laser printers were originally stored in printers or computers as vector outline such as Bezier or Spline. Raster Image Processor (RIP) implemented as hardware or software in printers or computers rasterized the outline into a pattern which was composed of subtle vast dots. There was a variety of types in contours of printed characters which were printed by each printer model. In Japan, stalkers typed their threatening letters using common used fonts such as MS Mincho in Japanese and Times or Century in English. Even though the kinds of fonts were known, there was not evidence since these fonts were equipped in almost all computers in Japan. Therefore a new method to discriminate models of printers was desired. Even though same too common fonts were used in threatening letters, subtle differences among contours of printed characters were observed since there was a variety of methods which each maker adopted in rendering and screening which converted from outline to pattern of dots. In order to detect the subtle differences among contours of printed documents, the article utilized wavelet decomposition and a high resolution i.e., 5400dpi flat bed image scanner. The article also used a simple method to analyze results of wavelet decomposition which was counting numbers of zero-crossing points at each scale of decomposition. The results of the experiment showed that the method we proposed was able to detect differences among models of laser printers even though using same too common fonts both Japanese and English.