本論文提出一種強健式、不需原圖即可作萃取的影像浮水印方法，此方法應用了中央極限定理於大型區塊(如 )作離散餘弦轉換(DCT)所得到的係數，也就是把來自於不同子影像的DCT係數當作隨機變數，並且藉由更改樣本平均值來藏入浮水印各位元。實作上先對各個大區塊子影像計算出DCT係數，再挑選出中、低頻部分的係數，為了提升強健性，我們又導入了係數的位置變換和加權等處理機制，然後去計算跨子影像的係數平均值，再依據一個給定的門檻值去調整該平均值。經由實驗驗證，本方法在藏入浮水印後仍然可保有相當高的 值(如 )，並且對於JPEG壓縮、低通濾波、剪裁、改變亮度及銳化等攻擊方式，均可表現出良好的防禦能力。

In this thesis, we study and propose a robust image watermarking method that does not need the original image to extract the embedded watermark. This method applies the central limiting theorem to the coefficients obtained from discrete cosine transformation (DCT) of large-scale (e.g. 50x50) subimage blocks. In implementation, DCT coefficients for each large-scale subimage are computed first. Next, the low and medium frequency coefficients are selected. To enhance the robustness of the method, two mechanisms, called “coefficient-position translation" and "coefficient weighting", are proposed and used here. Then, a mean value of the coefficients sampled from different subimages is computed. In terms of increasing or decreasing the mean value to a target threshold value, a watermark bit can be hid. According to the results of our experiments, an image after embedded with a watermark can still keep high value of PSNR (e.g. 40.67dB). Also, our method is shown to be very robust against such attacks as JPEG compression, smoothing, blurring, sharpening, cropping, and gamma changing of light-intensity.

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