現今資訊科技與影像複製設備日益進步，要獲得一份高品質的印刷複製品已不再是困難的事，也使得文件安全受到考驗。過去應用於印列輸出影像之數位浮水印技術仍有諸多限制，而未能有效地實際應用。我們提出將隨機點圖樣做為加密資訊，並以其反覆的幾何變換與部份重疊來形成具有數位浮水印功能的半色調影像，同時此密碼圖樣的幾何變換也可視為一種繪圖筆刷來描繪出影像的階調與輪廓，加密後的影像具有非擬真影像的視覺效果。而密碼驗證部分，因局部影像皆有足夠的密碼資訊存在，本研究利用電腦視覺中的部分對應方法—幾何雜湊法，解決可能的幾何轉換與影像定位問題，達到機器閱讀的自動化密碼驗證之目的。本論文提出了一種創新影像加密方法，同時此方法可將灰階影像轉成半色調影像，以供印列輸出之用途，由於藏密過程中加入的隨機參數以及混合搭配代表不同鍵值的密碼圖樣，使本研究的編碼具有一定的複雜度，不易被破解；此外，對於印列輸出的加密影像透過取像重建回數位影像後，可利用部分對應的方式對局部影像作影像定位與密碼驗證，除了增加密碼驗證的效率外，更可突破先前技術在面臨裁切與旋轉等幾何攻擊上的限制。實驗結果可看出本研究適合搭配數位印刷設備製作藏有不同密碼序號的底紋影像，以達到文件控管或身份認證之功能，本研究在防制影印機、掃描器與印表機的複製行為上也有不錯的表現，並具有實際應用的可行性。

As the developments of the information technology and duplication devices, it is easy to obtain a high quality duplication of printed document. Recently, it becomes a serious problem that the modern graphics art techniques, such as scanner or color copiers, are used to duplicate counterfeiting documents. Nowadays, security printings require more and more robust and anti-counterfeiting techniques to resist the guilty abuse. Previous works about watermarking/data hiding for printed images still lack of capability in practice. A novel method is proposed to apply on printed images to make its security features be machine-readable automatically. A random dot patterns are considered as a secret key and its geometric transformation are used to generate a halftone image. Since the partial halftone image is conducted by random dot patterns, the partial image is sufficient for authentication. After acquiring the digital image of printed document by scanner or other devices, a partial matching method, geometric hashing, is used for solving the localization problem automatically and secret key authentication.
The proposed method is not only a watermarking method, but also a process for halftoning. Due to added random parameters and at least two combined patterns in the encoding process, it is too complex to decipher the encoded code. In addition, decoding the partial image not only increases the efficiency of authentication, but also conquers the previous limitation of cropped and geometric transformations, such as rotation and scaling. The proposed method suits for combining the digital printing devise and generating the background image with various secret keys as fingerprinting. Experimental results reveal that the proposed method can resist the duplicating manner from scanner or color copiers in certain situation and be applied on print-and-scan documents in practice.

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