網點細微結構產生數字形錯網之研究

Abstract

摘要 印刷品上，當網屏角度搭配不當時就會有錯網情形產生。在本研究中，利用錯網的原理，設計適當的網點結構和遮罩，使兩者重疊在一起時在一定角度下能呈現出數字形狀的網紋。本研究首先以數字“5”的形狀做例子，設計出數字“5”形狀的網點，並分別以不同的網點結構來構成單色影像和彩色影像，比較彼此之間錯網情形。為了能達到更好的資訊隱藏效果，本研究嘗試對網點的結構進行破壞分解，使影像在放大鏡觀察下無法直接得知數字的內容，但與遮罩重疊後仍能出現數字“5”形狀的網花。最後本研究對單色影像進行掃描複製再次輸出，比較原始影像與複製品之間錯網的差異。本研究所得結論如下： 一、不同的網點結構有不同的錯網現象 在單色影像部分，90度和六角形的網點結構與遮罩重疊後可清楚呈現出數字“5”形狀的網花，其間網花大小和位置會隨著重疊的角度而改變，但多重線的網點結構與遮罩重疊後無法呈現出數字的網花；在彩色影像部分，四色網屏不同角度與四色網屏同角度兩種結構的影像與遮罩重疊後，在一般觀測環境境下皆無法清楚看出數字形狀的網花，但四色網屏同角度的影像可在透射光源照射下看出數字形狀的網花。 二、分解後的網點結構有較佳的資訊隱藏效果 網點結構經分解後的影像，在放大鏡觀察無法直接觀察得數字的形狀，但與遮罩重疊在一定的角度下，可以清楚呈現出數字形狀的網花效果。因此，這種網點結構有較佳的隱藏資訊效果。 三、本研究之網點結構可有效防止掃描複製 複製品和原稿影像與遮罩重疊後，複製品上的網花很明顯地均比原稿上的網花模糊，尤其是網點結構經分解後的影像，數字形狀的網花糢糊情形較為嚴重，且能呈現網花角度範圍較小。 最後本研究之結果可做為一種隱藏資訊於影像中的方法，並提供便利的方式做資訊辦別及檢查影像是否遭到偽造。Abstract On the printings, when the screens can’t fit well with one another, moir phenomenon is easily appeared. In this thesis, we utilize this phenomenon to design suitable dot structure and mask, which displays patterns of digital shape when they superpose under certain angles. In this thesis, at first, we design the shape of dots --- take digit ‘5’ for example, and under different structures we compose we compare monochromes moir phenomena and chromatic moir phenomena as well. Second, for better effect of hiding data, we decompose dot structure, so people can’t receive digital data from the shape of dot observed under magnifier. However, superposing the picture with the mask, we can display moir of digital shape again. Finally, we scan and reproduce monochromes, then compare them with the originals. The conclusions are as follows: 1. Different dot structures have different moirs phenomena. In monochromes, superposing orthogonal and hexagonal pictures with masks we can clearly observe the moir of digit ‘5’ shape, and their sizes and positions will be altered when superposition angle is changed. However, multi-screen picture that superposes with mask cannot display moir of digital shape. In chromatic pictures, the moir of digital shape can’t be clear under either four screens with the same angle or four screens with staggered angles. Even so, chromatic picture made up from screens that have the same angle, pattern of moir can be recognized when they are observed under transparent light. 2. Decomposed dot structures have better effects in hiding data. In the pictures, dot structures have been decomposed, so people can’t recognize digital shape under magnifier observation. Nevertheless, we can display moir of digital shape when superposing with masks around some fixed angles. So, This dot structure has better effects in hiding data. 3. Originals will be hard to reproduced by the method. Moir on reproductions is more blurring than the originals, especially in the reproductions which structure has been decomposed. Besides, moir of digital shape can be found between smaller angles. Finally, this thesis can provide a method of hiding data in the picture, and also a convenient way for authentication and inspect the counterfeit.