改良式混合網點應用於數位浮水印之研究

Abstract

由於調幅網點( Amplitude Modulation, AM)與調頻網點( Frequency Modulation, FM)顯微結構的差異，過去在製作混合網點數位浮水印時，會因為兩種網點在印刷時所產生的網點擴張情形( Dot Gain)不一致，而影響浮水印隱藏效果，必須額外輸出濃度匹配導表，進行浮水印濃度校正，才能確保浮水印的隱藏品質。本研究為省去輸出導表之步驟以縮減製程，提出改良式混合網點數位浮水印技術，利用填入不著墨白點的方式分散AM網點顯微結構，使其與FM網點具有相同之顯微結構，藉此平衡兩者網點擴張率。並利用兩種網點不同的微結構及排列特性，形成複印機及掃描機取樣能力的差異，達到浮水印圖像於複印後顯現之效果。本研究採用實驗研究法，透過實際輸出濃度匹配導表，測試四分之一著墨AM網點與FM網點的網點擴張情形。最後將改良式混合網點浮水印與紙張載體整合設計，利用複印機驗證其成效。研究結果顯示本研究所建構之改良式混合網點，確實能達到平衡網點擴張的效果，並成功使潛藏之浮水印圖像於複印後顯現。未來利用此改良技術製作混合網點浮水印，不僅在製程中不需重複進行濃度校正，也可確保在不同印刷條件及長版印刷時，四分之一著墨AM網點及FM網點網點擴張率仍能維持一致，使浮水印隱藏品質獲得較佳的控管。此外，由於改良式AM調幅網點之網點散佈區域較以往略為增加，能提升低濃度區AM調幅網點之複製能力，使複印後浮水印呈現之效果更佳。

In this research, a novel method is proposed to design watermarks with modified hybrid halftone dots. Due to the diversity of microstructures, the amplitude modulation (AM) halftone dots and frequency modulation (FM) halftone dots create different dot gains while printing, which affect the hidden effect of watermarks. Therefore, a density calibration chart is required to help calibrate watermarks with selected parameters. In order to balance the dot gains between AM and FM halftone dots, this research modified the micro-structure of AM halftone dots to make it similar to that of FM halftone dots. However, the different patterns of arrangement can still cause the distinctions of sampling requirements of duplicating equipment, to make the hidden watermarks emerge after duplication. This study employs the experimental method. A density calibration chart were printed out to observe the dot gain effect of modified hybrid halftone dots, which are composed by 1/4 AM and FM halftone dots. At last, the watermark, constituting of modified halftone dots, were printed onto paper, and using a copy machine to testify its effect. The results show that this method successfully balances the dot gain, and makes the watermarks emerge after copied. This technique not only makes the calibration process no longer necessary, but also provides better control of dot gain in any printing condition and long-run printing, to improve the quality of watermark presentations. In addition, the increased dispersion of modified AM halftone dots enhances the detection capability within highlight area and attains the better effect of printed watermarks.