Processing calibration-grid images using the Hough transformation

When the angular displacement method is used in stereoscopic particle image velocimetry, a projection algorithm is required to map the image coordinates onto physical coordinates. One difficulty is to find an accurate mapping function which describes the projection. A new approach for determining a mapping function from calibration-grid images is presented. The image of a Cartesian grid is processed using a windowing technique. The image plane is covered by small overlapping windows. The Hough transformation is used to detect straight line segments in each window. The straight line segments correspond locally to the possibly curved grid lines in the image. Then the data of these line segments are used to compute the coefficients of the mapping function. The algorithm is robust and requires only a little interactive control. Thus, an almost automatic evaluation of the calibration-grid images is possible.