對於不同時間、不同影像感測器、不同觀點所拍攝的影像于以疊合，為重要的影像套合課題。幾何上，影像套合校準了參考與待測兩影像。多年來，考慮不同變數與不同問題上，發展出各類型的套合方法。當一張待測影像相較於參考影像發生位移、旋轉與縮放時，其傅利葉頻域上會有相對應的關係。傅利葉轉換的位移定理是一個基本的概念，利用phase correlation可以強健的估算出位移參數。接著，使用對數極座標傅利葉轉換後，空間中旋轉與縮放關係被簡化成頻譜中的位移關係，使其亦可以phase correlation方法解出。由於涉及傅利葉轉換的部份皆以FFT演算法來求取，故整個方法被稱為FFT-based影像套合方法。
FFT-based影像套合方法的其中一種限制為，當參考影像與待測影像的取景大小差異過大時，無法將兩影像于以套合。在本篇論文中，我們提出了一種基於特徵點的方法，來粗略的標定出待測影像中最相近於參考影像的區域。首先，以Harris corner detector偵測出特徵點為方法的第一步。再來，我們調整了廣義霍夫轉換，使其能根據兩影像的特徵點，找出待測影像相似於參考影像的區域。最後，我們提出了結合以上基於特徵點與傅利葉頻域的影像套合方法。於實驗中，我們證明了準確的套合結果是可達成的。

Image registration is a fundamental task that matches the images taken at different times, from different sensors, or from different viewpoints. The registration geometrically aligns two images - the reference and sensed images. Over the years, a variety of techniques has been developed for various types of data and problem. When a sensed image is translated, rotated, and scaled with respect to the reference image, it has a counterpart in the Fourier domain. The basic notion is the shift property of the Fourier transform, which allows robust estimation of translations using phase correlation. Using log-polar Fourier transform, rotation and scaling are reduced to translation, which are also estimated by phase correlation. The method is called FFT-based image registration because of the utilization of the FFT algorithm to compute Fourier transform.
One constraint of FFT-based image registration is that it can not be registered if the size of view between reference and sensed images differs significantly. In this thesis, we propose a feature-based method to coarsely locate the region of the sensed image which is supposedly the most similar to the reference image. The first step of the algorithm detects characteristic points by Harris corner detector followed by modifying the GHT method to extract similitude based on the detected feature points. At the end, the combination of feature based and Fourier based image registration can be achieved. Our experiments show that the accuracy of the resulting registration can be reached.

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