加密數位浮水印圖像應用於數位印刷之品質分析研究

The Study on the Quality Analysis of Encryption Digital Watermarking Images Applied to Digital Printing

陳雅莉(Ya-Li Chen) ； 陳昌郎(Chang-Lang Chen)

《中華印刷科技年報》 2008 (2008/03) Pp. 182-207

https://doi.org/10.30153/JCAGST.200803.0014

摘要

隨著全球仿冒案件的激增，防偽技術也正以每年30%的成長率持續成長當中，數位浮水印技術以其能達成防止非經授權的印刷複製又不需耗費巨額成本特點成為防偽技術新寵，然相關研究中，皆尚未針對數位浮水印實際應用於正快速蓬勃成長之數位印刷領域的品質進行探討。
本研究係應用數位過網原理，以數位隱藏圖像訊息技術設計製作具有雙重色版加密之數位浮水印影像，並瞭解柱狀透鏡陣列板之最佳透鏡數、數位印刷機加密檔案的最佳網點線數範圍、加密圖像與原圖像之色彩差異程度，及其經掃描設備及複印設備後，是否能保留版權宣告之功能。研究採實驗研究法，針對數位印刷機印刷之網點線數、柱狀透鏡陣列板上之透鏡數及雙重色版加密角度之組合，進行最佳化加密條件之選定、正式實驗圖片加密、數位印刷、色彩差異量測之驗證、掃描及彩色複印複製浮水印之強韌性檢驗等研究過程，以獲致結論。
實驗結果顯示，以HP Indigo Press 5000數位印刷機印製的加密數位浮水印圖像確實可達到加密防偽及達到版權宣告目的，其柱狀透鏡陣列板之最佳加密透鏡數大約在100 lpi~150 lpi之間，以175線印刷網點線數、C15°+M75°之加密網點角度製作之加密圖像經數位印刷後之效果最佳。色彩差異分析部份，加密對原稿顏色加深10%-25%不等，若於加密前將濃度曲線下修可減少加密圖像與原稿圖像的色彩差異。浮水印強韌性之驗證部份，以EPSON Stylus Pro 9800輸出掃描之加密圖像，124.8 lpi以下之加密圖像可被成功解密；而彩色複印部分，以Xerox Docucolor 1290彩色複印，僅101 lpi可被成功解密，若以Xerox Document Centre C250彩色複印，最高可解密至141 lpi，顯示當複印設備解析度及網點複製能力足夠時，可以成功以柱狀透鏡陣列板還原加密之訊息。

並列摘要

With the explosive increase of counterfeiting cases in the world, forgery proof technology has also been continuously growing at rate of 30% every year. Digital watermark technology with the characteristics of preventing the illegal printing duplication and consuming less cost becomes a newly favor in forgery proof technology. However, there are not any discussions about the quality of digital watermark technology applied to the rapid growth of digital printing field in the related studies.
In this study, the digital halftone principle is used with Digital Hidden Image Information Technology to design and make dual color prints with encrypted digital watermark images, as well as to understand the best number of lens per inch on the columned lens array board, the best encrypted screen ruling range in digital printing press, the color differences between encrypted image and original copy, and whether or not the function of copyright announcement is retained after copied by scanner and reproduction equipment.
This study adopts the experiment methodology, including research processes of screen rulings printed by digital printing press, number of lens per inch on the columned lens array board, the combination of dual color prints encrypted screen angles, and the choice of the best encrypted condition, official experiment of picture encryption, digital printing, confirmation of color differences measurement, the obdurability examination of encrypted digital watermark after copying by means of scanning and color reproduction, etc. Then the conclusion is obtained.
The experimental result shows that encrypted digital watermark images printed with HP Indigo Digital Press 5000 can prevent the illegal printing duplication and can get copyright announcement lens array board is between 100 lpi to 150 lpi. The best quality is achieved in the digital encrypted watermark images on the prints with the screen of 175 lpi, and the encrypted angle of Cyan 15°, Magenta 75°. In the part of color differences, the original color will darken about 10%-25% after encryption. If reducing the density curve can decrease the color differences between encrypted image and original. In the part of watermark obdurability, encrypted image scanned under 124.8lpi printed with EPSON Stylus Pro 9800 can be decrypted successfully, 101 lpi encrypted image copied with Xerox Docucolor 1290 can be decrypted. If the encrypted image copied with Xerox Document Centre C250 can be decrypted up to 141 lpi. It shows encrypted image can be decrypted with columned lens array board, if the resolution of reproduction equipment and dot duplication ability are good enough.