本論文提出一個利用數位半色調影像做為機密資訊傳遞的共享機制。多張數位影像被當做載體並轉換為數位半色調影像，影像在轉換的過程中，我們利用半色調影像的特性，將相同大小的機密半色調影像嵌入於載體中，達成秘密通訊及資料共享的目標。此機制的數位半色調轉換基於多層次誤差擴散演算法，每次選擇適當的影像與位置放置白點，並根據機密半色調影像的內容讓同位置的白點個數滿足隱藏條件，研究的主要目標在於嵌入秘密影像的同時亦保持載體半色調影像的畫質。此外，我們提出數位半色調影像的互嵌機制，在不指定欲嵌入機密影像的情況下，能夠在參與共享的M 張影像中，利用任何M-1 張影像擷取剩餘的一張半色調影像。為了從資料庫中選擇適當的影像作為資料載體，我們提供選圖機制做為實作參考。實驗結果顯示所有數位半色調影像能夠維持良好的畫質，能夠成功地將機密半色調影像被嵌入與擷取，達到秘密通訊與資訊共享之目的。

This research presents a secret sharing scheme in halftone images. Some gray-level images of the same resolution are selected and transferred to halftone ones, which are responsible for carrying a secret halftone image. Given the pixels of secret image as the constraint, the host images are generated using Multi-scale Error Diffusion (MED). The original pixels of host images are examined and the modified MED ensures that the resulting pixels of the host images should satisfy the required conditions. After grouping all the processed halftone images, the secret image can be successfully revealed. The research objective is maintaining the quality of all the halftone images in this secret sharing scenario. Another proposed method is termed "mutual embedding," in which a halftone share can be decoded using all the other shares by modifying the initial setting in this secret sharing scheme. Besides, the approach of selecting host images is proposed so that suitable images can be chosen from an image database to ensure the quality of resulting halftone images. The experimental results and discussions demonstrate the interesting characteristics of the proposed scheme.

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