2.2.2.2. Pre-processing techniques for a line-pattern image

As lines in a calibration image are curved under radial distortion, using well-known line-detection methods such as Hough transform-based approaches are not always feasible. A simple method for detecting curved lines has been developed for Discorpy 1.4 where points on each line are detected by locating local extrema points on an intensity-profile generated by plotting across the image (Fig. 12). Many intensity-profiles are generated to detect points on lines at different locations, then they are grouped line-by-line (Fig. 13).

Fig. 12 Process of locating points belong to a line. (a) Intensity-profile extracted along the blue line. (b) Local minimum points located on this profile.

Fig. 13 Full routine of detecting lines. (a) Multiple crossing-lines are used to locate extrema points. (b) Points extracted. (c) Points after grouped into lines.