不同網點面積百分比之浮水印底紋於個人化郵票應用
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2017
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隨著數位科技的蓬勃發展，使得影像的精準複製更為方便，但也造成非法複製與偽造現象日益猖獗，增進各類有價證券之防偽功能已成為重要議題。個人化郵票是極少數之使用者可以參與設計的有價證券，故有學者進行個人化郵票浮水印加密及可變內藏訊息之研究，但對於個人化郵票中可變訊息及固定防偽機制結合的研究則相對較少。本研究主要目的在個人化郵票可變印紋中，加入固定防偽網點位移浮水印，藉由設計不同網點面積百分比之浮水印底紋於個人化郵票，探討浮水印之解密效果及經複印後之解密效果。本研究透過數位半色調過網演算技術，利用12x12臨界值矩陣中網點微結構內點數的不同，設計不同網點面積百分比之浮水印底紋，先以間隔取樣數位模擬柱狀透鏡解密效果，再以調制轉換函數（Modulation Transfer Function, MTF）評估其解密效果，找出最佳網點面積百分比之浮水印底紋，加密於個人化郵票附籤之黑色印版，經由平版印刷以2400dpi輸出形成網點位移浮水印底紋於個人化郵票。結果顯示，網點位移浮水印底紋網點面積5.5%解密效果最佳且經複印後無法解密，達到防偽效果。目前我國個人化郵票並無固定浮水印，本研究之網點位移浮水印底紋，將在可變印紋的個人化郵票附籤上形成固定的防偽功能，成為我國首枚兼具有可變印紋及固定防偽浮水印的郵票。
With the rapid development of digital technology, it is convenient to duplicate images that resulting in illegal copies. As a consequence it has become an important issue to enhance the security features of various types of security documents. Personalized stamps is one of the rare case which users participating in the design of security documents. Therefore, some studies embed watermark encryption and variable hidden data into personalized stamps. Nevertheless, there are only few discussions about variable images and digital watermark technology with the fixed anti-forgery mechanism of personalized stamps. In this study, the variable images of personalized stamps is used with fixed anti-forgery encrypted digital watermark images, as well as to discuss the decryption effect of the watermark and the decryption effect after copying . This study adopts the digital halftoning by different 12x12 threshold matrix to generate the various microstructural dots with various percentages dot area. Sampling interval is used to simulate the decrypted watermark by grating lens. After that, the study works with Modulation Transfer Function(MTF) for estimation of the decrypted results to finds out the best parameter of dot area percentage in black color plate of the personalized stamps output at 2400dpi by offset printing with dot shifting of encrypted digital watermark. The results shows that dot shifting for 5.5% dot area has the best effect on decoding and anti-forgery effect after coping. Due to there is no fixed watermark in personalized stamps, this study result can be used on the variable images for personalized stamp products. This personalized stamp will be the first and unique one with not only variable images but also fixed anti-counterfeiting watermark design.
With the rapid development of digital technology, it is convenient to duplicate images that resulting in illegal copies. As a consequence it has become an important issue to enhance the security features of various types of security documents. Personalized stamps is one of the rare case which users participating in the design of security documents. Therefore, some studies embed watermark encryption and variable hidden data into personalized stamps. Nevertheless, there are only few discussions about variable images and digital watermark technology with the fixed anti-forgery mechanism of personalized stamps. In this study, the variable images of personalized stamps is used with fixed anti-forgery encrypted digital watermark images, as well as to discuss the decryption effect of the watermark and the decryption effect after copying . This study adopts the digital halftoning by different 12x12 threshold matrix to generate the various microstructural dots with various percentages dot area. Sampling interval is used to simulate the decrypted watermark by grating lens. After that, the study works with Modulation Transfer Function(MTF) for estimation of the decrypted results to finds out the best parameter of dot area percentage in black color plate of the personalized stamps output at 2400dpi by offset printing with dot shifting of encrypted digital watermark. The results shows that dot shifting for 5.5% dot area has the best effect on decoding and anti-forgery effect after coping. Due to there is no fixed watermark in personalized stamps, this study result can be used on the variable images for personalized stamp products. This personalized stamp will be the first and unique one with not only variable images but also fixed anti-counterfeiting watermark design.
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個人化郵票, 數位半色調, 網點位移, 網點面積百分比, Personalized Stamps, Digital Halftoning, Dot Shifting, Dot Area Percentage