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图1为实验装置示意图,液晶平板显示器LCD显示标准余弦条纹图,广角镜头相机拍摄条纹图,相机的光轴垂直液晶显示器平板。拍摄到的畸变条纹图,如图2所示,图中虚线表示无畸变情况下的条纹图,实线表示畸变条纹图,C、D分别表示无、有畸变时场景中同一物点的像点位置,那么CD为该物点像的畸变量。
假设无畸变图像为
$C(x,y)$ ,畸变图像为$D(x',y')$ 。$(x,y)$ 和$(x',y')$ 分别是图像畸变前后对应的坐标对。两个坐标的关系可以表示为:$$\left\{ \begin{aligned} & x = {o_1}(x',y') \\ & y = {o_2}(x',y') \end{aligned} \right.$$ (1) 只需要用畸变前后图像上目标点到中心点
$({x_0},{y_0})$ 的距离来计算整张图像的畸变量。畸变图像坐标与无畸变图像坐标之间的关系可以简单地表示为:$$ r = o(r') $$ (2) 其中
$r(x,y) = \sqrt {{{(x - {x_0})}^2} + {{(y - {y_0})}^2}} $ 和$r'(x',y') = $ $ \sqrt {{{(x' - {x_0})}^2} + {{(y' - {y_0})}^2}} $ 是无畸变图像和畸变图像上点$(x,y)$ ,$(x',y')$ 分别到其图像中心的距离。如果能建立无畸变图像坐标与畸变图像坐标之间的映射关系,就可以实现图像的校正。假设
$C[r(x,y)]$ 是校正后图像点$(x,y)$ 的灰度值,该灰度值等于畸变图像点$(x',y')$ 上的灰度值$D[r'(x',y')]$ :$$ C[r(x,y)] = D[r'(x',y')] = D[r(x,y) + \Delta r] $$ (3) 式中:
$\Delta r$ 是相应的径向畸变量。因为图像的径向畸变是关于图像中心圆对称的,只需确定从图像中心点开始沿正方向的径向畸变量分布,便可进行图像畸变校正。需要留意的是广角镜头的径向畸变是径向压缩,校正后图像的坐标
$(x,y)$ 要大于校正前对应的畸变坐标$(x',y')$ ,也就是校正后图像的像素数要大于校正前图像的像素数,或者计算得到的$(x',y')$ 坐标不是整数,因此,有必要对那些多出的像素点或非整数点进行灰度插值。使用双线性插值算法,可以克服灰度不连续性的问题。假设${x_1}$ 和${y_1}$ 分别是小于或等于$x'$ 和$y'$ 的最近整数,而且,$(x',y')$ 落在$({x_1},{y_1})$ ,$({x_1} + 1,{y_1})$ ,$({x_1},{y_1} + 1)$ 和$({x_1} + 1,{y_1} + 1)$ 这四个像素构成的二维区域之间,双线性插值算法的计算公式为:$$ \begin{split} C[r(x,y)] =\; & (1 - \alpha )(1 - \beta )D[r({x_1},{y_1})] + \\ & \alpha (1 - \beta )D[r({x_1} + 1,{y_1})] + \\ & (1 - \alpha )\beta D[r({x_1},{y_1} + 1)] +\\ & \alpha \beta D[r({x_1} + 1,{y_1} + 1)] \end{split} $$ (4) 式中:
$\alpha = x' - {x_1}$ ;$\beta = y' - {y_1}$ 。 -
四幅相移量分别为0,π/2,π,3π/2的纵向余弦条纹图作为校正模板。一般情况下,广角镜头的径向畸变是中心圆对称的,只需要利用广角镜头相机拍摄到的畸变条纹图的中心行
$(y = {y_0})$ 条纹来测量整幅图像的径向畸变。中心行条纹可表示为:$$ \begin{split} {I_1}(x,{y_0}) =\;& G(x,{y_0})\{ A(x,{y_0}) + \\ & B(x,{y_0})\cos [\varphi (x,{y_0})]\}\end{split} $$ (5) $$ \begin{split} {I_2}(x,{y_0}) =\; & G(x,{y_0})\{ A(x,{y_0}) + \\ & B(x,{y_0})\cos [\varphi (x,{y_0})+\frac{{\text{π}}}{2}]\} \end{split} $$ (6) $$ \begin{split} {I_3}(x,{y_0}) =\;& G(x,{y_0})\{ A(x,{y_0}) + \\ & B(x,{y_0})\cos [\varphi (x,{y_0})+{\text{π}}]\} \end{split} $$ (7) $$ \begin{split} {I_4}(x,{y_0}) =\; & G(x,{y_0})\{ A(x,{y_0}) + \\ & B(x,{y_0})\cos [\varphi (x,{y_0})+\frac{{3{\text{π}}}}{2}]\} \end{split} $$ (8) 式中:
$G(x,{y_0})$ 为幅度调制函数;$A(x,{y_0})$ 为背景光强度;$B(x,{y_0})/A(x,{y_0})$ 为条纹图的对比度。$\varphi (x,{y_0})$ 为径向畸变条纹相位。通过对四张条纹图的中心行使用四步相移法进行相位解调,可以获得径向畸变条纹图的包裹相位为:$$\varphi (x,{y_0}) = \arctan \left[ {\frac{{{I_4}(x,{y_0}) - {I_2}(x,{y_0})}}{{{I_1}(x,{y_0}) - {I_3}(x,{y_0})}}} \right]$$ (9) 通过使用解包裹算法,可以获得实际的径向畸变条纹图的相位值。
为了测量像素点的畸变量,必须以无畸变的条纹图作为基准。
由于镜头拍摄的中心区域的图像几乎没有畸变。可以根据计算得到的径向畸变条纹图的相位分布的中心位置像素点的相位值拟合一个径向无畸变条纹图的相位分布作为基准。假设径向无畸变条纹图的相位分布为:
$$ {\varphi _{{\rm{undistorted}}}}(x,{y_0}) = kx + {\varphi _0} $$ (10) 式中:
$k{\rm{ = 2}}{\text{π}}{f_0}$ 和${\varphi _0}$ 分别为直线拟合的一次系数和常数系数,${f_0}$ 为基频。径向畸变相位可以表示为:$$ \Delta \varphi (x,{y_0}) = \varphi (x,{y_0}) - {\varphi _{{\rm{undistorted}}}}(x,{y_0}) $$ (11) 根据图2所示的几何结构,径向畸变量
$\Delta r$ 和其对应的径向畸变相位$\Delta \varphi $ 可以表示为:$$ \Delta r = \overline {CD} = \frac{{\Delta \varphi }}{{2{\text{π}}{f_0}}} $$ (12) 所以,使用条纹图相位分析的方法,可以确定径向畸变量分布。最后,根据公式(3)和公式(4)可以对畸变图像进行校正。
Wide-angle lenses distortion calibration using phase demodulation of phase-shifting fringe-patterns
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摘要:
文中提出了一种基于相移条纹图相位分析的广角镜头畸变校正方法。首先,用大尺寸液晶平板显示器显示四幅相移量为π/2的余弦条纹图作为校正模板。然后,用广角镜头相机拍摄该校正模板,获得四幅畸变条纹图,使用四步相移算法解调径向畸变条纹图的相位分布。由于经广角镜头成像的图像中心区域几乎无畸变,利用图像中心无畸变的相位值进行数值拟合得到径向无畸变条纹图的相位分布,作为求解径向畸变相位的基准,也就是径向畸变相位分布可以根据径向畸变条纹图的相位分布与径向无畸变条纹图相位分布相减得到,再将畸变相位转换成实际的畸变量。提出的方法不需要通过特征点或特征线确定畸变模型,可以直接计算畸变图像中每个像素点的畸变量。实验结果表明,提出的方法简单、有效,具有广泛应用价值。
† 贡献相同Abstract:A distortion calibration method for wide-angle lens was proposed based on fringe-pattern phase analysis. Firstly, four standard cosine fringe-patterns with phase shift step of π/2, which were used as calibration templates, were shown on a large-size Liquid Crystal Display screen, and captured by the camera with wide-angle lens to obtain four distorted fringe-patterns. A four-step phase-shifting method was employed to obtain the phase distribution of the radial distorted fringe-pattern. There was no distortion within the central region of the image captured by the wide-angle lens, so the phase distribution of radial undistorted fringe-pattern, as a benchmark for computing radial distorted phase, could be acquired by performing numerical fitting by the central undistorted phase value of the distorted image. It means that the radial distorted phase distribution was computed by subtracting the phase distribution of radial distorted fringe-pattern from the phase distribution of radial undistorted fringe-pattern. Finally, the distorted phase was transformed into the actual distorted variables. There was no need to establish any kind of image distortion model by lots of characteristic points or lines. Furthermore, the radial distortion variable at each point of the distorted image can be determined by the proposed method. Experimental results show that the proposed method is simple, effective, and has wide application value.
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Key words:
- distortion calibration /
- wide-angle lens /
- fringe-pattern phase analysis
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[1] Cai Ping, Li Xiaoyan, Tang Yujun, et al. Improved distortion correction method for spacial large aperture tracking cameras [J]. Optics and Precision Engineering, 2019, 27(10): 2272−2279. (in Chinese) doi: 10.3788/OPE.20192710.2272 [2] Lu Lijun, Liu Meng, Shi Ye. Correction method of image distortion of fisheye lens [J]. Infrared and Laser Engineering, 2019, 48(9): 0926002. doi: 10.3788/IRLA201948.0926002 [3] Brown D C. Close-range camera calibration [J]. Photogrammetric Engineering, 1971, 37: 855−866. [4] Kakani V, Kim H, Kumbham M, et al. Feasible self-calibration of larger field-of-view (FOV) camera sensors for the advanced driver-assistance system (ADAS) [J]. Sensors, 2019, 19(15): 3369. doi: 10.3390/s19153369 [5] Sun Junhua, Cheng Xiaoqi, Fan Qiaoyun. Camera calibration based on two-cylinder target [J]. Optics Express, 2019, 27(20): 29319−29331. doi: 10.1364/OE.27.029319 [6] Wang Xianmin, Liu Dong, Zang Zhongming, et al. The regularized phase tracking technique used in single closed interferogram phase retrieval [J]. Chinese Optics, 2019, 12(4): 719−730. (in Chinese) doi: 10.3788/co.20191204.0719 [7] Zhang Minmin, Tian Zhenyun, Xiong Yuankang, et al. Research on incoherent self-interference digital holography imaging technology [J]. Infrared and Laser Engineering, 2019, 48(12): 1224001. (in Chinese) doi: 10.3788/IRLA201948.1224001 [8] Zuo Yang, Long Kehui, Liu Jinguo, et al. Analysis and processing of Morié fringe signals based on non-uniform sampling [J]. Optics and Precision Engineering, 2015, 23(4): 1146−1152. (in Chinese) doi: 10.3788/OPE.20152304.1146 [9] Wang Jianhua, Yang Yanxi. Double N-step phase-shifting profilometry using color-encoded grating projection [J]. Chinese Optics, 2019, 12(3): 616−627. (in Chinese) doi: 10.3788/co.20191203.0616 [10] Yang Chuping, Weng Jiawen, Wang Jianwei. Distortion measurement and calibration technique based on phase analysis for carrier-fringe pattern [J]. Acta Photonica Sinica, 2010, 39(2): 316−319. (in Chinese) doi: 10.3788/gzxb20103902.0316 [11] Yang Chuping, Weng Jiawen, Liu Jianbin. Using dilating gabor transform to fringe analysis in distortion measurement [J]. Opto-Electronic Engineering, 2010, 37(1): 13−18. (in Chinese) [12] Takeda M, Mutoh K. Fourier transform profilometry for the automatic measurement of 3-D object shapes [J]. Applied Optics, 1983, 22(24): 3977−3982. doi: 10.1364/AO.22.003977 [13] Zhong Jingang, Zeng Huiping. Multiscale windowed Fourier transform for phase extraction of fringe patterns [J]. Applied Optics, 2007, 46(14): 2670−2675. doi: 10.1364/AO.46.002670 [14] Zhong Jingang, Weng Jiawen. Phase retrieval of optical fringe patterns from the ridge of a wavelet transform [J]. Optics Letters, 2005, 30(19): 2560−2562. doi: 10.1364/OL.30.002560 [15] Zuo Chao, Feng Shijie, Huang Lei, et al. Phase shifting algorithms for fringe projection profilometry: A review [J]. Optics and Lasers in Engineering, 2018, 109: 23−59. doi: 10.1016/j.optlaseng.2018.04.019
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