Accurate feature detection for out-of-focus camera calibration

Abstract

For conventional camera calibration methods, well-focused images are necessary to detect features accurately. However, this requirement causes practical inconveniences to image acquisition for long- and short-distance photogrammetry. In this study, three active phase-shift circular grating (PCG) arrays are used as calibration patterns. The PCGs' centers are regarded as feature points that can be accurately extracted by ellipse fitting of 2π-phase points even though patterns are substantially blurred. In the experiments, Gaussian filters are utilized to blur pattern images, and different standard deviations are set for different fuzzy degrees. Pattern images with different defocusing degrees are also captured. The period and number of PCGs and noise are considered. Experimental results indicate that our method is accurate, reliable, and insensitive to image defocusing.

Camera Calibration Robust to Defocus Using Phase-Shifting Patterns

“…As it can be seen in Figure 8 d,f, the centers of ellipses were farther from the real centers as the angle increased but the projection located by our method coincided well with the real projection. Then, we can conclude that the location accuracy of the method is more accurate than the method used in our previous work [ 25 , 26 ]. Thus, the result of this experiment verified the precision of our method and partly illustrated that our work is valuable and a contribution.…”

“…Definitely, the above methods often required that multiple images should be captured at each camera pose and demanded more human interaction, which are laborious and inefficient. In our previous works [ 25 , 26 ], we proposed a method to calibrate the camera with defocused images, but directly used the center of projection ellipse as the real center of a phase-shift circular grating, which can be improved. Since the projective of a circle is not invariant, the circle center which is recovered directly from the center of the projection ellipse is not the real projection [ 15 ].…”

A Robust and Accurate Calibration Method for Out-of-focus Camera

“…Their method only worked when lens is under the ideal condition of slight, uniform out-of-focus. Wang et al [13] used three-step phase-shift circular grating(PCG) arrays as calibration patterns, and extracted the PCG's centers by ellipse fitting of 2π-phase points. Their method's accuracy is not comparable with well-focused calibration for the usage of the ellipse fitting.…”

“…In this paper, we propose a robust and accurate calibration method for out-of-focus camera using LCD monitor. Firstly, we estimate the defocus map by the temporal coded binary-shift patterns, which makes our method more accurate than [12] [13]. Secondly, based on the defocus map, we encode LCD pixel's coordinates into phase-shift patterns with optimal frequency and step properties, and deblur captured patterns.…”

An Iterative Distortion Compensation Algorithm for Camera Calibration Based on Phase Target

“…Ma et al [25] proposed a feature extraction method by using fringe pattern groups as the calibration target. Moreover, a phase target can effectively calibrate the camera with a small depth of focus and long working distance because the obtained phase from sinusoidal fringe has little influence on out-of-focus images [26,27]. Schmalz et al [18] made a comparison of camera calibration with a phase target and a classic planar target to show the superiority of the phase target.…”