Cross-center extraction with sub-pixel accuracy

In order to achieve automatic measurement function, laser scanning measurement system need to ensure that measurement system of the cross and the target center overlapped. To develop the obtaining accuracy of the cross central coordinate, a novel algorithm based on the combination of two-step Hough Transform and Sub-pixel curve fitting was presented. Firstly, Hough Transform was used to detect the straight line of long axis of the cross, and then the central coordinate value of the cross can be deduced roughly through straight line equations. Secondly, take this coordinate as a center, and a certain number of pixel values as the edge length, a region of interest (ROI) can be defined. In this ROI, the two short axes of the cross were obtained to detect the straight line roughly using Hough Transform again. Then, the coordinate value of short axis was acquired at the pixel level with Canny edge detection algorithm. According to the equations of the straight line, rough errors were removed through 3σ rule, and the coordinate values of short axis at the sub-pixel level with curve fitting algorithm is obtained. Finally, based at the sub-pixel level of coordinate values, straight line equations of two short axes was presented with LSM, and thus the coordinate of cross center would be accurately detected with the intersection of two short axes. Experimental results showed that the accuracy of the central coordinates value of cross approaches to sub-pixel level and can satisfy the measurement system requirements of high precision.