Abstract: Free difference operation breaks the template mode of classical differential operators. It turns the 2-D difference computation of image into 1-D difference operation. Therefore, the computing direction of difference operation can be decided by the actual problem flexibly. Based on the idea that any curve can be approached by a certain polygon, orthogonal Snake model turns the computation of gray gradient along the normal direction of a curve into the analysis of the relative or absolute variation of gray along the orthogonal line of each side of polygon by free difference operation. During each stage of curve development, this model can associate the contour variation with the gray variation by making difference operation along the orthogonal line, directed by the characteristics of the object curve and relied on the basic polygon. So the characteristics of the extracted contour approximate the characteristics of the object contour step by step. Compared with the conventional Snake model, the orthogonal Snake model stresses the affiliation between high-level information and low-level mechanisms, and its information processing is in closed state. Experiments on the contour extraction of malformed objects generate ideal result.