幾何變形攻擊，包括裁切、旋轉、尺度縮放，甚至隨機變形等，所產生的同步問題，對於數位影像浮水印的偵測影響極大。本研究提出了一種基於特徵點擷取之數位浮水印方法，來抵抗對於靜態圖片的幾何變形攻擊。首先，尺度不變之特徵點將會被擷取出來做為定位點，此特徵點亦能存活於一般的訊號處理以及仿射轉換等攻擊。利用此類特徵點適當的強韌性，我們依據特徵點位置建構出多個局部幾何不變之格狀形區域，並於每個局部幾何不變區域中嵌入兩種訊號，第一種為隱藏資訊之數位浮水印，以及第二種做為同步機制的訊號，此訊號又稱為樣板訊號。樣版訊號可以確保較大範圍之局部幾何不變區域能夠被擴張建構出來以提供經過幾何攻擊後之浮水印偵測。較大的偵測區域將使得浮水印嵌入量獲得提升，且浮水印的可信度也隨之增加。在偵測浮水印時，我們對於每個特徵點所定位之格狀區域參數進行微調整，尋找局部之最佳可能區域，浮水印訊號將因此被更可靠地偵測。實驗結果顯示我們所提出的浮水印方法對於幾何攻擊具有強健性，並且能夠抵抗一般的訊號處理攻擊。Geometrical transformations, such as cropping, rotation, scaling or even random bending, cause the synchronization problem of detecting the digital image watermark. This research presents a feature-based watermarking scheme to deal with geometrical attacks in still images. First, the scale-invariant feature extraction is applied to locate the interest points that can survive the signal processing procedures and affine transforms. A local invariant region based on the scale-space features of an image is then acquired. At each invariant region, two signals will be embedded, {em i.e.} the watermark carrying the hidden information and the extended synchronization pattern or grid, which helps to ensure that a reasonably large invariant region be available for carrying the watermark payload and increasing the confidence of watermark extraction. The detection of the grid is based on the local search by adjusting the related parameters of the grid to match with the possible hidden pattern so that the watermark can be retrieved afterwards. Experimental results demonstrate that the proposed scheme is robust against common image processing and geometrical attacks.
