以數位半色調技術製作雙色圖像浮水印之研究

Abstract

現今印刷科技已相當成熟，相關複製設備亦愈趨普及，所造成的版權保護問題日益受到重視。有鑒於此，印刷防偽相關技術如數位浮水印等在近年有著非常廣泛的應用。但以往的防偽文件中的隱藏圖紋浮水印多為灰階之文字或圖紋，因為不具有顏色故缺乏色彩聯想，影響觀看者效果，因此本研究即以數位過網技術製作具有色彩之雙色文件底紋浮水印。本研究利用洋紅（Magenta）與青（Cyan）兩色網點，並透過複印機對調幅網點(Amplitude Modulation,AM) 與調頻網點(Frequency Modulation,FM)之取樣差異，進行混合網點的隱藏圖文設計。首先將選定的雙色圖文以色彩與複印後的效果分版，為達到在正常距離人眼觀察下，將鄰近網點濃度做積分之效果，將各版中的洋紅色網點與青色網點均佔該面積的二分之一並相互交錯，而後依隱藏訊息內容置入AM 網點及FM 網點。經實驗結果得知，本研究能達到於肉眼無法察覺之良好隱藏效果防偽圖文，經數位偵測、實品輸出後實際複印測試與光柵解密，其隱藏彩色圖文即浮現達到防偽效果，同時因具有色彩，故能增加圖像浮水印之色彩表現與應用範圍。

Nowadays, with the progress of printing technology, duplicating equipment also has been improved rapidly. It also causes the concern of copyright. For this reason, many researchers have been working on anti-counterfeiting technology such as digital watermarking. Nevertheless, most of former security documents use only monochrome for the hidden watermark pattern. This research is to produce a method of watermark with two colors by digital halftoning. The pattern consists inks of magenta and cyan, with the combination AM and FM halftone dots. Due to the difference of sampling requirements for AM and FM dots, it makes the hidden watermarks appear after duplication. At last, the watermark will be output on paper which will be copied to verify its feasibility. The results show that the proposed method successfully makes the two colors watermark pattern visible after duplicate. At the same time, it can enhance the valued-added applications of digital watermarking.