數位過網影像品質評估

Evaluation for Digital halftoning image Quality

本文以灰階形式之網點級數影像、實物影像、低暗調（Low-key)影像、人像影像、金屬反光物影像及戶外風景影像，使用調幅（AM）過網之分散式點陣抖動法（dispersed-dot ordered dither）與調頻（FM）過網法之誤差擴散法（error diffusion），進行數位過網二階影像產生製作及其品質比較。探討結果得知：一、分散式點陣法不會降低影像列印的解析度，漸層上會產生跳階現象，且在圖像在層次較平調區域（flat）有重複之圖案（pattern)出現在視覺上並不悅目，此乃演算法本質造成之現象。二、誤差擴散法所得之過網圖像階調品質，視覺上較點陣調散法為佳，且其網點為隨機自然散佈無錯網網花產生，在高頻處（邊界區域）表現較佳。

The objective of this research is to use screening algorithm to create binary image by the digital halftoning techniques. The methods used in this research are based on several digital halftoning techniques, including ordered dithering (Amplitude Modulation )and Floyd-Steinberg error diffusion (Frequency Modulation ), with certain production. There are six kinds of image type, is to be halftoned by different digital halftoning techniques. The research results showed binary image halftoned by Floyd-Steinberg error diffusion quality is better than ordered dithering quality. In the research, we had found an better halftoned quality for screening algorithm in binary images with the consideration of image type. The outcomes of this research project will benefit to the printing.