A novel image edge detection algorithm developed to extract object region from a Thangka image by gradient magnitude and two-step gradient. The idea of our image edge detection algorithm is similar to the canny edge detection algorithm, but the implementation is rather different. By using this algorithm, we can realize the object region extraction by four steps. Firstly, the Gaussian algorithm is used to smooth the Thangka image. Then, the second order gradient amplitude and gradient angle for

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... he Thangka image are calculated. Thirdly, non-maximum suppression is used to detect the edge of the Thangka image. At last, the double threshold is performed to achieve the outline of the Tang image extraction.