However, variation of velocity is inevitable in practical applications. When slippage occurs between the pipe and the rotating rollers or when the movement of the vehicle is not smooth, the velocity will change in unpredictable ways. Fig. 11(a) and (b) illustrates the detection of true defects in an image sequence acquired in un-predefined varying velocity of the inspection. Fig. 11(a) shows the position of detected potential defect regions in image coordinate j, in relation to the frame index. Using the Shaos line-detecting method, no line is detected by the Hough transform. It leads to the failure of detecting the true defects. Fig. 11(b) shows the position of defected potential defect regions in image coordinates, both i and j, in relation to their frame index.