Precise optical method for three dimensional ship deformations measurement

A robust positioning algorithm of crosshair center based on grayscale weighting

“…We used the crosswind least-squares algorithm to position the center of the cross-hair based on the Sobel operator and compared it with the proposed method. The leastsquares center-positioning algorithm for the crosshair based on the Sobel operator (SobelLSCP) is a two-step method that first uses the Sobel operator for edge detection and then carries out least-squares straight-line fitting according to the four edges of the crosshair to locate the center [3].…”

“…As a particular cooperative target, the crosshair mark has been used as a carrier for information transmission (for instance, to represent direction) when performing measurements on images, such as the deviation in the center of an optical lens system [1,2], the three-dimensional (3D) deformation of images of ships [3], and angle-measuring instruments in digital metrology (such as the theodolite or the laser plummet) [4,5] . A fundamental requirement for performing such image measurements is the appropriate positioning of sub-pixels representing the center of the crosshair.…”

“…While the centroid localization algorithm obtains a least-squares linear fit based on the center of the cross-section or the vertical section at different locations in its second step, the sub-pixel edge detection algorithm obtains a linear fit with the four edges [11,12] by taking the intersection of two straight lines as the center of the crosshair. Examples of such fitting algorithms include the polynomial [10] and the Gaussian fitting algorithms [3].…”