可還原式浮水印技術的特點在於將影像的浮水印擷取後，仍可將已嵌入浮水印的影像無損的還原成原始影像，它適用於一些對影像品質較高的領域，例如醫學與軍事影像等。可還原式浮水印在嵌入浮水印的過程中, 極可能發生像素值溢位的問題, 過去大多數的可還原式浮水印方法都採用位置圖(location-map)來記錄像素溢位的位置，供還原影像時使用。雖然位置圖可以透過無損的演算法壓縮，但是它還是降低了浮水印的嵌入容量，並且增加了嵌入及擷取浮水印程序的複雜度。在本論文裡，我們提出一個不需要位置圖來當作參考資訊，便能夠完整的還原出原始影像的方法。我們首先將影像分成四個集合，並利用排序以及調整原始像素的預估值來增加浮水印嵌入的容量。接著我們根據預估差值計算而得預估門檻值，只要預估值在門檻值的範圍內，可保證在嵌入浮水印時不會產生像素值下溢位(Underflow)或上溢位(Overflow)的現象。所以我們不需要使用位置圖來記錄影像中下溢位或上溢位的像素，便能夠還原出無損的原始影像，並且得到更多的嵌入容量。

FUJIYOSHI的方法也是不需要location-map的方法。實驗結果顯示，我們提出的方法比FUJIYOSHI的方法多60%的浮水印嵌入量，並且在同樣的浮水印嵌入量下，我們的方法所得的嵌入浮水印影像，影像品質的PSNR平均約高5 dB。

Reversible watermarking techniques extract the watermark and recover the original image losslessly from the watermarked image. They have been applied to those sensitive fields, such as the medicine and the military. Since an embedding pixel value may exceed the limitation of pixel value during the embedding process, most of the reversible watermarking methods require a location-map to record those pixels for recovering cover images. Although the location-map can be compressed by a lossless compression algorithm, and then embed into the watermarked image, this lowers embedding capacity and increases the complexity of watermarking during the procedures of embedding and extraction.

In this thesis, we propose a reversible location-map free of watermarking algorithm. This algorithm first exploits the sorting and the correlation of neighboring pixels to increase the embedding capacity. Next we find thresholds from the predicted values. If the predicted value of an embedding pixel is within the thresholds, we can ensure that the pixel has no underflow or overflow problem during embedding process. Therefore, we can recover the cover image without any distortion.

The experimental results reveal that the performance of our proposed method outperforms that proposed by FUJIYOSHI et al. For example, the embedding capacity obtained by the proposed method is higher than that obtained by FUJIYOSHI et al. about 60%, and the PSNR of our scheme is higher than FUJIYOSHI et al. about 5 dB.

摘要 i
Abstract ii
Chapter 1 Introduction 1
1.1 Background 1
1.2 Reversible watermarking schemes 2
1.2.1 Reversible watermarking schemes using difference expansion 3
1.2.2 Reversible watermarking schemes using histogram shifting 4
1.2.3 Reversible watermarking schemes using prediction-error and histogram shifting 5
1.3 Location-map free reversible watermarking schemes 5
1.4 Motivation and objectives 7
1.5 Organization of the Thesis 8
Chapter 2 Related Works 9
2.1 Prediction and compute the deviation parameter d 9
2.2 Derive threshold parameter τ 11
2.3 Embedding and extraction algorithms 11
2.3.1 Embedding algorithm 12
2.3.2 Extraction Algorithm: 12
Chapter 3 Proposed Method 15
3.1 Prediction and sorting processes 15
3.1.1 Dividing image into four sets 16
3.1.2 Sorting according to local variance 16
3.1.3 Prediction-error Method: 17
3.2 Find the limit of predicted values 21
3.3 Watermark embedding and extraction 23
3.3.1 Embedding Process: 23
3.3.2 Extraction Process: 24
3.3.3 Obtaining Threshold: 25
Chapter 4 Experimental results 27
Chapter 5 Conclusions and future works 31
5.1 Conclusions 31
5.2 Future works 31
Reference 32

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