Rotated checker‐pattern projection and its cross‐point extraction and tracking for 3‐D object profilometry

AbstractIn the measurement of 3‐D shape by projecting a regular pattern on the object surface, it is required to extract feature points with a high accuracy and to determine their mutual relations. In this paper, 45‐deg rotated checker‐pattern projection is proposed, and a method for extracting and tracking the crossing point is presented in which the geometrical properties of the rotated checker pattern are utilized. If the pattern projector and the camera are on the same horizontal plane, a large brightness difference is produced between the horizontal and nonhorizontal (vertical and slant directions) neighbors of the crossing‐point pixel. By utilizing this property, crossing points are extracted. The projection profile of the brightness pattern along the vertical direction has a triangular form, and the location of the extremum of the profile gives the vertical coordinate of the crossing point. By utilizing this profile, the crossing points are classified (segmentation). On the basis of the result of segmentation, the crossing points are successively tracked and their mutual relations are determined. To demonstrate the usefulness of the method, the experiment of 3‐D shape measurement was made. The error of the measurement was less than 2 mm, which is due to the error in extracting the crossing point (i.e., 1 pixel at the maximum). In the measurement of a cylinder, the measurement was made along the circumference. It is shown that the measurement of the position of the crossing point on the reference plane is not required if the rotation of the object is utilized.