印刷品膚色色調分離評估模式之研究

A Study of Printing Posterization Prediction Model

陳郁娟(Yu-Chuan Chen) ； 孫沛立(Pei-Li Sun)

《中華印刷科技年報》 2009 (2009/03) Pp. 111-122

https://doi.org/10.30153/JCAGST.200903.0009

摘要

印刷複製品發生色調分離現象一直是令人困擾的問題，因為色調分離現象並無法在印前製作階段被發現，經常到了後端的印刷階段才顯現出來。這影響的不只客戶交期延遲，若重新印製更會造成印刷成本的嚴重損失。因此本研究的目的在於了解影像色彩複製流程中，色調分離發生的原因與嚴重程度，並據此設計一套「色調分離指數」演算法，在印前製作時，推測最終印刷品的色調分離嚴重程度。本研究首先經由一系列前導實驗獲得影像「檔案值」與實際「測量值」之間的色階差異比值，然後實施視覺對人像攝影的色調分離敏感度視覺評比實驗。本實驗請觀測者針對四種照明角度所拍攝的人像，選擇出所能接受的膚色色調分離容忍之程度，並利用Laplacian of Gaussian二階導數濾鏡，結合前導實驗獲得的色階差異比值，建立一套預測膚色色調分離程度的影像處理模式，所獲得的色調分離指數，與視覺評比結果的相關度達到0.6。

關鍵字

色調分離 ； 色彩管理 ； 色域對映 ； 影像處理 ； 影像品質評估

並列摘要

Posterization is an unpleasant color discontinuity phenomenon in image reproduction. It is frequently happened in hardcopies but difficult to be observed during their pre-press stage. It would result in delayed delivery and lose considerable money in reprint the reproductions. To solve this problem, the study aims to explore the source of skin tone posterization in image reproduction workflow. The findings will be used to design a model to show 'Posterization Index' of an original for predicting the phenomenon in the prepress stage. A visual experiment was conducted which asked observers to identify posterization areas on portrait photos with 4 different lighting directions. The results show that the lighting directions have something to do with the effect. Finally, we derived a model based on Laplacian of Gaussian filtering with the above experimental data to process the original photos to predict the degree of posterization. The proposed 'Posterization Index' has good correlation (coefficient about 0.6) to that of visual posterization.

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