An improved line detection method, the fast GPI method, is presented in this paper. This method improves the GPI method, reduces computational time and memory storage. After the image is filtered by Sobel operator, the feature points are stored and a sequence of projection angles is calculated by these points. A preliminary GPI matrix is calculated by these angles and then the preliminary projection angle is obtained; more detailed calculation in the vicinity of the preliminary projection angle is performed. So the final GPI matrix can be calculated, and search the extreme of the final matrix, the straight line can be located in the image. This method can detect the position of the line more quickly than the GPI method and avoid massive calculations, and is useful in image understanding, especially for robot seam tracking in robotic welding.
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