基於穩態小波轉換的彩色半色調影像回復方法

Halftoning image descreening using SWT

戴裕豐(Yu-feng Tai)

指導教授 ： 范國清

國立中央大學/資訊電機學院/資訊工程學系碩士在職專班/碩士(2011年)

摘要

近年來由iPad、iPhone所掀起之數位閱讀風潮正席捲全球，消費者將紙本圖書掃瞄成數位檔案作為個人用途，將原本的紙本書籍化身為數位檔案於iPad中閱讀的方式日益普遍，也因此掀起了圖書數位掃瞄的商機。

印刷技術必須先將欲印刷的數位內容做半色調(halftone)的處理，如此一來才能由機器印刷出來，相反地，如果要將印刷品掃瞄成為數位化的圖檔，一些問題就會顯現出來，其中最嚴重的就是印刷網格的問題，使用者會發現印刷品掃瞄的圖檔畫面品質不佳，會有網格的細小圖案產生出來
，這種細小圖案造成影像不連續，嚴重影響人眼視覺感觀。

因此，本論文以此方向為題目，主要分為兩個研究部分，第一部份為半色調方法的原理以及其形成影像網點、錯網(moire)的原因 ，同時也提出了相關影像品質評估方法，利用客觀的數據來作為評估的標準；第二部份則利用小波轉換來處理半色調掃描影像，本研究論文中利用穩態小波轉換(SWT)的方法來改善這個問題，並比較了穩態小波轉換(SWT)與離散小波轉換(DWT)的差異，同時比較了一些常用於影像處理的小波函數對其效果的影響。

關鍵字

電子書 ； 網紋 ； 離散小波轉換 ； 穩態小波轉換 ； 半色調影像回復 ； 快速小波轉換 ； 掃描影像

並列摘要

Due to the rapid growth of 3C mobile devices, such as iPad and iPhone, the emerging of ebook becomes a trend accordingly. Users can scan traditional paper books into ebook and read it in iPad or iPhone. It further creates business opportunity and add-on value for traditional publishing industries.

In printing technology, digital contents have to be transformed into halftone format for printing purpose. However, it will result in some unwanted effects when we inverse the scanned image into its original digital image file. The most serious effects are moire and print grid. It hence deteriorates image quality and cause uncomfortable viewing quality. The reason is that the halftone fine patterns will result in image tone intermittent and thereby influence the
perception of human vision seriously.

In this thesis, we focus our research on improving the viewing quality of halftoning image. Firstly, we will describe the principle of digital halftoning and the forming of moire and rosette patterns. We will also present the objective method of estimating descreening image quality at the same time. Then, a method that inverses halftone images using wavelet transform is proposed.The comparison between stationary wavelet transform and discrete wavelet transform is also given. Last, experiments were conducted on various images to illustrate the influences of wavelet families that were used in halftoning process. Experimental results demonstrate the
superiority of our method.

並列關鍵字

fast wavelet transform ； scanned image ； halftome inverse ； stationary wavelet transform ； moire ； halftone image ； descreening ； discrete wavelet transform

參考文獻

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