Extraction of central positions of light stripe in sub-pixel in 3D surface measurement based on light sectioning method

In order to extract the central positions of a light stripe and to ensure the accuracy of surface measurement based on light-sectioning method, an approach of the light stripe central position extraction in sub-pixel is investigated. By taking the gray level of an image as a 2D function of coordinates, the directional derivative and the gradient of the image of light stripe are calculated. In the image, the pixel gradient represents the direction of the change in gray level, and it is assumed as the normal direction of the light stripe curvilinear. Pixels at the positions of light stripe can be extracted by setting a threshold for both absolute values of the gradient and the gray level. The sub-pixel central positions of the light stripe are equal to the mean value of the gray-level-weighed positions of the pixels in the stripe's adjacent region along the gradient direction. With this method, the normal direction of the stripe can be simplified to be the gray level gradient, which avoids complicated steps to calculate a Hessian matrix to get normal directions. Experimental results indicate that the elapsed time of the method is only 10% that of the traditional way, and the error (3σ) of real measurement is 0.057 m in the range of 200 mm×200 mm. It can satisfy the requirements of accuracy and real-time measurement.