光学学报, 2018, 38 (8): 0815005, 网络出版: 2018-09-06   

彩色复合条纹投影测量系统串扰消除方法 下载: 1475次特邀研究论文

Crosstalk Elimination Method for Color Composite Fringe Projection Measuring Systems
作者单位
河北工业大学机械工程学院, 天津 300130
摘要
由于从一个彩色图像中可以提取多个条纹,复合条纹投影技术在三维测量领域得到了广泛的研究和应用。为了覆盖所有的光谱范围,投影和成像系统颜色通道间存在着串扰,从而改变了条纹的形状,最终影响三维数据测量的精度。综述了彩色复合条纹投影三维测量系统中颜色通道间串扰的补偿方法。针对彩色条纹投影系统中相机和投影仪颜色通道间的串扰问题,分析了其形成原因。对彩色条纹投影三维测量系统的串扰消除方法进行了分类和总结,并具体比较了各种方法对串扰消除的效果。所综述的方法对选择合适的串扰消除方法、提高彩色复合结构光投影三维测量系统的精度具有重要的应用价值。
Abstract
The composite fringe projection techniques have been widely studied and applied in many fields because multiple fringe patterns can be extracted from single color image. Crosstalk between color channels exists in projecting and imaging systems in order to cover all the range of spectrum. Therefore, the obtained fringe patterns are deformed and the measurement accuracy of three-dimensional (3D) shape data is affected. The compensation method of crosstalk between color channels in color composite fringe projection 3D measuring profilometry is reviewed. The crosstalk between color channels is analyzed with respect to the camera and projector in color fringe projection system. The corresponding crosstalk elimination methods are classified and summarized. The effects of these elimination methods are also compared. The reviewed methods have important application value in choosing the appropriate crosstalk elimination methods and improving the accuracy of color composite fringe pattern projection profilometry.

1 引言

随着网络通信技术的发展和生产生活需求的扩张,各行业需要一种简单方便的物体三维形貌测量方法。三维测量技术主要通过机械或光学等方法来获取被测物体三维形貌信息。光学三维测量术作为一种结合光学、图像、信号处理等技术的非接触式、快速测量方法,是目前研究的热点[1-2]

基于各种结构光对物体表面进行三维形貌测量的技术不断涌现,并以各自的特点应用于不同的领域[3-7]。由于条纹图案设计和实现精度高、自动化程度高、易于识别,光学三维测量中投影图案主要趋向于使用条纹模式。条纹结构光投影法是一种基于三角测量原理的主动式光学测量方法。条纹有各种形状,如正弦条纹[8]、二值化条纹[9-10]、三角形条纹[11-12]、梯形条纹[13]和锯齿形条纹[14]等,其中正弦条纹和二值化条纹应用最为广泛。彩色投影图案可以增加基本代码的数量并且确保编码唯一,对运动物体进行重建[15],所以随着彩色高精度投影仪和电荷耦合元件(CCD)相机的发展,彩色复合正弦条纹投影技术被广泛的研究和应用[16-19]。彩色条纹的各个颜色通道可以携带更多的相位信息,从而减少图像拍摄次数,提高测量速度。但是彩色编码涉及到的彩色投影仪与彩色CCD相机的通道间颜色串扰问题,会直接影响相位的计算,降低物体三维形貌测量的精度。随着彩色复合条纹投影技术的发展和广泛应用,颜色通道间串扰消除问题的研究变得越来越重要。

本文综述了国内外学者提出的消除颜色串扰的各种方法,分析对比了各种方法的处理过程和技术特点。简要介绍了基于条纹投影的光学三维测量原理、彩色条纹投影技术和串扰生成的原因;对各种补偿串扰的方法进行了分类总结和对比分析;最后简要讨论了串扰问题研究的发展方向,并对全文进行总结。

2 条纹投影三维测量原理

2.1 基于条纹投影的三维测量

条纹投影三维测量通过投影仪将正弦条纹投射到被测物体表面,条纹受物体表面高度调制发生形变。由相机采集的变形条纹图的强度如下:

Ii(x,y)=I'(x,y)+I(x,y)cos[ϕ(x,y)+ϕi],(1)

式中:Ii(i=1,2,…,n)为第i步相移处的条纹强度值;I'为平均亮度;I″为调制亮度;xy为CCD的像素坐标;ϕ(x,y)为条纹图对应的折叠相位;ϕ为相移值。变形的条纹通过傅里叶变换[20-23]、多步相移法[24]、窗式傅里叶变换[25-28]、小波变换[29-40]等方法解调相位。通过标定相位和深度之间的关系,得到待测物体的三维形貌数据。

2.2 彩色复合条纹投影三维测量

彩色复合条纹由灰度正弦条纹图调制到彩色图像红绿蓝(RGB)三颜色通道后复合形成。软件生成的RGB正弦条纹图强度关系为

Ij(x,y)=Ia+Ajcos(2πx/pj+ϕ),(2)

式中:j取R,G,B分别对应红绿蓝通道;Ij表示投影仪投影的条纹灰度值;Ia表示平均强度;Aj为条纹幅值;pj为像素的条纹距;ϕ为相移;x,y分别为投影仪水平和垂直方向的像素索引值。CCD相机采集的彩色条纹图强度为

I'c(m,n)=I'a(m,n)+A'c(m,n)cos[φ'c(m,n)+ϕ],(3)

式中:I'c为相机采集的条纹图的强度;mn分别为CCD相机水平和垂直方向的像素索引值;I'aA'cφ'c分别为采集条纹图的平均强度、调制度和相位。采集一幅彩色变形条纹图可以提取出三幅包含相位信息的灰度条纹图,提高了测量速度。

彩色复合条纹包含红、绿和蓝通道,是编码三个条纹图的载体[41],如图1所示。彩色复合条纹投影三维测量原理如图2所示。

图 1. 红绿蓝彩色正弦条纹图

Fig. 1. RGB color sinusoidal fringe pattern

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图 2. 彩色复合条纹三维测量原理图

Fig. 2. Three-dimensional measuring principle of color composite fringe

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利用不同编码方式对条纹进行编码可以形成各种形式的投影图[42-46]。Harding等[47]提出了彩色编码莫尔技术,采取单次拍摄的方式投影彩色编码条纹。Tajima等[48]提出一种获取三维数据的彩虹模式,利用线性可变滤波器在物体上投影连续彩色光谱,通过颜色匹配和三角法获取三维形貌,使得测量精度不受光照和物体表面颜色的影响。Geng[49]强化了这一概念,提出使用线性可变波长滤波器生成彩虹图案的方案。Salvi等[50]利用一幅一系列横竖条纹图实现了动态三维场景测量。Skocaj等[51]针对非均匀反射率物体的深度图像获取问题提出一种结构光距离传感器,为解决不能测量彩色物体的问题,另外拍摄了两张图:一张全照明的白色图案,一张无照明的黑色图案,以获取物体彩色信息;但是该方法降低了测量速度,不适于动态测量。Liu等[52]提出了一种快速高效的形貌测量方法,对二进制8颜色光栅投影条纹图进行彩色编码,但是该方法不能用于测量彩色物体。Zhong等[53]提出基于相移法和三焦点的测量技术,从三幅条纹图中获得物体的展开相位图,但是不能获得物体彩色纹理信息。Wang等[54]提出基于非编码红外结构光的方法,用于测量物体三维形貌。为了同时获得物体的形貌和彩色纹理,Sakashita等[55-56]提出利用颜色编码方法,同时结合红外通道和可见光通道采集三维信息。Ou等[57]使用一台红外相机测量三维数据,一台彩色相机测量彩色纹理,两个相机之间需要进行坐标变换,并且因为红外相机需要拍摄10幅条纹图用以计算三维信息,而彩色相机只需拍摄一幅纹理图,所以彩色相机的利用率不高。Jeong等[58]针对高速三维测量提出了一种彩色光栅投影莫尔条纹法。Huang等[59]提出一种使用数字视频投影仪的彩色编码数字条纹投影技术。Chen等[60]利用彩色方块的颜色信息,采用投影具有局部唯一性的二维彩色编码图案,从而确定物体上每一点的唯一位置;但其没有考虑彩色物体本身颜色对彩色编码图案的干扰,增强了相邻方块之间的颜色串扰影响。Zhang等[61]提出一种基于最佳三频率选择法求取展开相位的复合条纹投影系统。利用软件产生12幅具有最佳条纹个数的正弦条纹图,调制到三颜色通道,组合形成4幅复合条纹图,进行投影并采集。从4幅彩色复合变形条纹图中提取出12幅变形正弦条纹图后,条纹数相同的每4幅条纹图利用四步相移算法计算折叠相位。得到三幅折叠相位图后,由最佳三条纹选择法得到展开相位。Ma 等[62]在基于颜色编码正弦三步相移算法的实时轮廓测量中,利用红、绿、蓝单独通道颜色编码正弦光栅,使其产生相同的位移并组合成为一种颜色条纹图案。Chen等[63]利用三颜色通道中的一个通道预测相移轮廓曲线,另外两个通道生成二进制图案。每个条纹都有唯一的二进制序列识别,便于实现相位展开。利用通信领域中的通道编码方案对序列进行编码,并将编码嵌入到二进制编码的条带图像中,减小了物体表面颜色和照明噪声引起的干扰。Li等[64]提出一种彩色序列编码条纹深度检测方法。将彩色编码的De Bruijin序列表示的相位周期分配至两个通道中,并将第三个通道用于通道编码相位信息。这种方法通过通道覆盖提高了相位强度的质量,使得到的测量结果更为精确。Cheng等[65]提出一种利用几何约束法的彩色条纹投影三维测量相位解包裹法,利用彩色条纹投影扩大了测量深度范围。

2.3 颜色通道间串扰分析

为了覆盖所有的光谱,彩色光学三维测量系统所用投影仪与相机颜色通道间均存在串扰,改变了正弦条纹的形状,影响相位计算的精度。

2.3.1 相机对彩色复合条纹图的影响

彩色相机有两类:一类是三CCD相机,三基色红绿蓝中每种颜色都配置有一个CCD,使用三棱镜将三种颜色分别成像到三个CCD,最后得到三组数字原始图像组合的彩色图像;另一类是常用的单CCD彩色相机,工作时利用拜耳滤光片[66]将彩色图像过滤分布在单片CCD所有像素上,经过拜耳滤光片后每个像素上只产生红绿蓝中的一种颜色,最后通过插值算法计算得到其他两种颜色的分量[67]

进行图像处理时,三CCD相机将数字原始图像恢复到彩色图像的过程和单CCD相机去马赛克的过程都会导致颜色通道间的串扰[59,68]。通常为了避免频谱上的彩色盲区,图像处理单元的过滤器会对一个宽频谱作出响应,如图3所示。滤光器探测到的更宽波长的光导致相邻通道光谱重叠,这就是通道间的颜色串扰。CCD相机在采集彩色复合条纹时,一种条纹的颜色信息会出现在其他通道中[69-71]。三种颜色相互混叠,导致软件不能从相机采集的彩色条纹图的颜色通道中分离出正确形状的正弦条纹图,从而影响三维测量的精度[72-73]

图 3. 相机传感器响应示意图

Fig. 3. Schematic diagram of camera sensor response

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2.3.2 投影仪对彩色复合条纹图的影响

数字光处理器(DLP)投影机的核心元器件是数字微镜器件(DMD),投影机通过控制镜片的开关和旋转角度显示图像[74]。根据DMD的数量,DLP投影仪可以分为单片式、双片式和三片式。三片式DLP投影仪主要应用于高级工程、影院等。双片式投影仪应用较少。本研究主要讨论光学三维测量中常用的单片式DLP投影仪。

单片式DLP投影仪工作时,当光源通过会聚透镜后,高速旋转的色轮将复合光过滤成红、绿、蓝三基色光。同样为了覆盖所有的颜色,色轮上三个基色光谱间存在叠加。另外,由于色轮的转速受到限制,投影的图像会出现彩虹效应(图像中看到的物体边缘会有红绿蓝色的拖影)。当在光学三维测量中利用DLP投影彩色条纹图像时,红绿蓝三基色之间会相互影响,产生串扰,从而导致三维测量的精度降低。

3 颜色通道间串扰消除方法

彩色条纹投影中颜色通道间串扰的消除对于光学三维测量系统精度的提高有着非常重要的作用。现有的串扰消除方法有:串扰系数补偿法、滤波器补偿法、查找表(LUT)法、方程校正法、盲相位误差抑制法、经验模态分解(EMD)法和差频原理消除法等。对比总结后可以分为4大类:1)固定系数补偿,包括串扰系数补偿法和滤波器补偿法,根据通道间的信号耦合大小与光谱的重叠程度对颜色串扰进行补偿;2)函数关系补偿,包括LUT法、方程校正法,通过校正方程或LUT,建立测量值与真实值间的关系,由真实值对相应的测量值进行赋值来消除串扰影响;3)直接进行相位校正的盲相位误差抑制法,颜色串扰对测量系统的影响表现为对相位值的影响,所以可以直接对相位进行校正来间接补偿串扰影响;4)信号分解消除,包括EMD法和差频原理消除法,按照频率大小分解过滤信号以消除串扰影响。方法分类如图4所示。

图 4. 串扰消除方法分类图

Fig. 4. Crosstalk elimination method classification diagram

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3.1 固定系数补偿

对于已知的测量系统,相机和投影仪通道间的颜色串扰影响固定。因此,可以使用串扰系数补偿法和滤波器补偿法消除串扰。

3.1.1 串扰系数补偿法

串扰系数补偿法是指求出串扰方程的误差项系数,获得完整的串扰补偿方程,最后根据方程或矩阵求得采集图像各通道间的去耦强度,进行相位计算。

Huang等[59]针对通道间颜色耦合问题提出了串扰系数补偿法,该方法根据得到的条纹图的强度计算调制度,补偿三颜色通道间的串扰。仿真实验结果表明,该补偿方案有效地减少了耦合效应引起的误差。Pan等[75]提出了更为完善的串扰系数补偿法,该方法将纯色红绿蓝三幅条纹图投射到一个白色平板表面,根据相移公式求得各颜色条纹图的调制相位,计算各通道之间的串扰系数。进行实际测量时相机采集得到各通道带有串扰误差的亮度,然后根据串扰系数对各通道强度进行补偿。Zhang等[76]研究了每个通道投射不同数量条纹时的情况。每通道产生4幅90°相移图像,三通道共4幅全彩色图像被投射到白色平板表面。采集4个全彩色复合图像,并从红绿蓝通道分离得到12幅灰度图像。相互两通道间调制强度的比率代表串扰因子,组成串扰矩阵对通道间串扰进行补偿。该方法有效减小了通道间串扰的影响。

李华等[77]提出一种条纹分离与聚类方法,该方法优化了白色平板反射率的影响,校正了相机及投影仪的颜色误差,消除了系统颜色串扰,同时纠正了彩色物体自身的颜色对彩色条纹的影响。文献[ 77]首先利用软件补偿串扰误差。投射之前建立相机采集条纹光强与投影仪投射条纹光强之间的关系式:

Ic(λ)=Ip(λ)·K(λ)=i=R,G,BPi·Ipi(λ)·Ki(λ),(4)

式中:Ic为理论上相机采集的图像光强;Ip为投影仪投射的条纹图像强度;K为物体表面对不同颜色的反射系数;Pi为投影仪设定的不同颜色分量大小。相机实际采集的图像光强表示为

Ci=0Wfi(λ)Ic(λ)+Ai,(5)

式中:Wfi为相机对应的三颜色通道光谱大小;Ai为环境光对不同颜色通道的影响。将(4)式代入(5)式可以得到相机实际采集的图像强度与投影仪之间的关系。整理得到:

C=MKF(p)+A0,(6)

式中:M为串扰矩阵;F(p)为投影仪设定的像素值与实际投射的像素值的对应关系;C= [CRCGCB]T;A0= [ARAGAB]T。通过白色平板的测量,消除背景光影响,计算出反射率和耦合矩阵。由耦合矩阵、耦合逆矩阵和(7)式最终得到校正后的颜色信息:

C~=K-1M-1KMF(p)(7)

Kinell[78]提出了一种类似的分色方法。根据颜色串扰与通道强度的线性关系找到通道间的耦合,计算出各通道间的耦合系数后,变换矩阵形式对三通道条纹强度分色补偿。该方法在颜色校正过程中需要光学透镜和另外的9幅图来确定耦合矩阵,使用时需要转换光学镜头,所以这种方法不适用于动态和实时测量。Hu等[79-81]提出盲颜色分离法(BCI)计算解耦矩阵以校正颜色串扰。该方法无需系统的先验知识、预处理信号和另外拍摄图像,可以自适应地确定解耦矩阵。

3.1.2 滤波器补偿法

投影仪的光谱和相机滤波器的截止频率共同决定图像的颜色耦合效应。在采集图像前对相机和投影仪镜头进行优化,可以有效消除彩色条纹图像通道间串扰。

图 5. 滤波器设计示意图[75]

Fig. 5. Filter design sketch[75]

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Pan等[75]提出可以根据投影仪产生的光谱范围大小在CCD相机上设计相应的滤波器,使相机采集的条纹图各通道光谱有一定带宽,相邻两通道不产生重叠,从而消除颜色通道间串扰,如图5所示[75]。三个滤波器中心频率应分别为450,550,650 nm。设计要求各滤波器的截止频率不超过光谱交叉区域,以有效滤除重叠误差。设置合理的滤波器值,使三通道亮度均衡。实验验证表明,该方法消除了输出的三路信号的重叠部分,有效消除了颜色通道间的串扰。

虽然滤波器补偿法会滤除一部分可见光,但是只会对图像亮度产生影响,不会降低其他方面的精度。Zhang等[76]提出在标准的投影仪和相机上使用额外的滤波器(50%转换率的颜色滤波器)来减少串扰、提高相位分辨率。实验证明,该方法在使用标准投影仪并在相机镜头前配置滤光片的情况下,有效地消除了串扰影响。

3.2 函数关系补偿

建立函数关系也是一种有效消除串扰的方法。该方法通过构建LUT,或采用线性拟合建立校正方程来实现。

3.2.1 LUT法

LUT法是一种软件补偿串扰的方法。通过给出的测量强度求得源强度并建立二者之间的关系。因为颜色耦合的程度与通道的强度线性相关,而且很难在测量强度与源强度之间建立一个确切的等式,所以提出的方法不需要解任何等式就可以根据测量的通道强度求得去串扰的通道强度。

Pan等[75]提出了一种LUT方法。首先通过构建三个红绿蓝二维LUT,建立各通道测量值与原始真实值之间的关系:Ii=fi(I'R,I'G,I'B),设置每10个灰度级投影一幅条纹图至白色平板,由相机采集并记录到对应的LUT。然后开发一种搜索算法,从相应的测量值直接获得所对应的无串扰原始真值。根据LUT中的值,通过线性插值,拟合求出与给出的测量值相对应的无串扰值。图6(a)和(b)分别为LUT补偿前后被测平板相位图。从图中可以看出,LUT法有效地减小了相位失真程度。由LUT法可以求得任一点的去串扰误差值,但是用时较长,受物体颜色影响较大,精度较小。

图 6. 补偿前后被测平板相位图[75]。(a)补偿前;(b)补偿后

Fig. 6. Phase diagrams of the plate measured before and after compensation[75]. (a) Before compensation; (b) after compensation

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Ma等[82]提出了一种基于LUT的混合矫正方法。首先使用文献[ 61]的方法估计串扰矩阵,利用文献[ 75]的算法建立补偿图,改进投影条纹图。然后基于文献[ 59]的方法,每10个灰度作为一级,共26帧图拟合各自通道的非线性调制函数。通过对逆非线性调制函数的计算,预先编码计算机生成的相移补偿图,由串扰矩阵和补偿图校正条纹图。将校正后的条纹图调制到三通道,最后使用三步相移得到展开相位。LUT混合校正技术需要多项式拟合、预编码操作、丢弃饱和响应区,否则,LUT混合校正方法就会失效。

3.2.2 方程校正法

通过校正函数对彩色复合条纹图像红绿蓝通道各颜色分量关系式拟合校正,得到消除串扰的彩色条纹图的方法称为方程校正。

Wang等[83]针对三基色图像的红层、绿层和蓝层提出了一种“污染校正算法”用于粒子图像测速技术,以减少该颜色对其他两颜色的污染程度。每种颜色层上都显示颜色的强度值,将不同颜色下的成像特性结合在一起生成一组校正函数。该校正函数结合不同颜色光的校正功能,同时解决了三个颜色层的强度校正问题。Yu等[84-85]运用了一种类似的方法解决了三维形貌和速度测量过程中红蓝通道间颜色串扰问题。相机采集被测物表面红蓝复合图像,建立两通道中红蓝分量强度的关系,并通过需要实现的校正功能确定离散的校正函数。通过对离散的校正函数进行线性拟合求出拟合参数,最后得到校正的红蓝通道图像[68],有效地降低了红蓝通道间颜色串扰。

3.3 直接相位校正

彩色条纹图像通道间串扰会造成折叠相位和展开相位失真。直接对采集的彩色复合条纹图的相位进行失真纠正的方法称为直接相位校正法。

Ma等[82]提出了一种相位校正方法:盲相位误差抑制法。此方法基于各向同性N维条纹图标准化和载体挤压干涉(CSI)技术,不需要预先处理串扰系数,经过两个步骤就可以消除串扰引起的误差。Padilla等[86]分析了红绿蓝相移条纹投影形貌测量技术中,条纹通道间不同程度的串扰影响及补偿方法。对于无串扰和低串扰影响的测量系统,利用系统预处理消除串扰,或结合相位解调算法对串扰进行补偿。对于严重的通道间串扰,利用挤压干涉法进行相位解调是最为可靠的失真补偿方法。Padilla等证明了该方法可以恢复高质量的测量结果。Je等[87]提出了一种在彩色投影系统中纠正串扰和非线性引起的相位失真的方法,有效补偿了串扰引起的相位失真。为使相位高于某一阈值、排除低强度像素干扰并使均匀分布的颜色相位相等,对颜色进行了采样排序、划分等级,重新调整定义每一级的相位值。失真校正之前和归一化处理校正之后的相位图如图7所示。Flores等[88]针对彩色三相移正弦条纹图投影系统中的串扰问题,提出了一种具有任意相移值的广义相移算法,显著降低了颜色串扰的影响。

图 7. 相位失真校正实验结果[87]。(a)校正前的相位图; (b)校正后的相位图

Fig. 7. Experimental results of phase distortion correction[87]. (a) Phase diagram before correction; (b) phase diagram after correction

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3.4 信号分解消除

多通道间颜色串扰可以看作是信号之间的耦合效应。可以从信号频率和波长不同的角度消除信号的耦合问题。

3.4.1 EMD法

EMD可以按频率的高低将信号分解,自适应地将信号中的基本模式分量(IMF)提取出来,并且结合希尔伯特求解包络以及瞬时频率,应用于三维测量中条纹图的处理。

Huang 等[89]提出了一种新的分析非线性和非平稳数据的方法——希尔伯特-黄变换(HHT)信号分析方法。这种方法以EMD为基础,将任意复杂的数据集分解为一个有限的少量固有模式函数IMF。HHT信号分析法是一种自适应和非常高效的分解方法。邹海华等[90-91]将EMD法用到了彩色条纹轮廓术中。对颜色信号使用EMD分解各频率分量之后,按频率展开相位,最后使用傅里叶变换解调相位就可以恢复出三维形貌。图8(a)和(b)分别为未使用EMD分解信号时所采集的变形条纹图红色通道信号和EMD分离后对应的低频载频项。从图8中可看出,EMD较好地处理了串扰引起的相位信号失真的问题。

图 8. (a) EMD信号分离前的红色通道信号; (b) EMD分离后的低频载频项[90]

Fig. 8. (a) Red channel signal before EMD separation; (b) low frequency carrier frequency items after EMD separation[90]

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3.4.2 差频原理消除法

通过对投影条纹的三颜色通道分别进行一维傅里叶变换,可以得到频率光强分布。Zhang等[76]提出了一种基于差频原理的串扰消除方法。因为通道间存在能量泄露,所以在任意一通道的一维傅里叶图中都存在泄露进来的能量。以绿蓝颜色通道串扰消除为例,将不同频率的条纹调制到不同的颜色通道。为了提高测量范围,需要保证条纹图中两两通道频率存在较大间隔[92]。这种基于傅里叶变换的方法,可以在获得折叠相位信息的同时消除串扰的影响。但是傅里叶变换过程中存在频谱泄露,对于形状陡峭物体的测量精度有待提高。

3.5 抗串扰方法对比

以上讨论的抗串扰方法都可以不同程度地实现颜色串扰补偿,但是处理过程、使用方法的特点和处理的结果各有不同。几种常见串扰补偿过程中用到的方法的步骤比较如表1所示。表1横向对应上文所提出的各种补偿方法,纵向为对比指标。分别对比是否需要求取串扰矩阵,测量前是否需要预处理(硬件处理、数据处理),是否需要滤波(硬件、软件),是否需要拟合、迭代运算,投影条纹是否需要均值化、标准化处理。串扰系数补偿法在测量前需要利用白色平板求解耦合矩阵作预处理。BCI消除串扰的原理同样是根据耦合解耦矩阵进行补偿。相比于Huang等[59]提出的系数补偿法,BCI具有更复杂的解耦模型,通过希尔伯特变换提取谐波的一阶分量,消除背景光的影响,对条纹进行标准化、均值化处理。滤波器补偿法测量前需要根据测得的光谱波长,在CCD相机上设置相应的硬件滤波器。针对不同通道设置不同的强度值,对条纹作均值化处理。LUT法在测量前需要建立LUT,根据LUT拟合求值。方程校正测量之前需要使用相机采集白色平板的彩色串扰图像,以此来确定校正函数,再利用线性或二次曲线拟合确定校正方程,系统较简单和计算效率较高时适合使用线性插值,系统复杂时使用二次差值。盲相位校正法使用希尔伯特转换,抑制高阶分量,消除一阶系数的影响,对条纹作标准化处理并利用软件进行滤波。EMD法利用软件校正串扰,并对条纹作均值化处理。差频原理校正测量前需要求取红绿蓝各通道三条纹的最大最小瞬时频率,确定条纹频率间距,保证无串扰或较少的颜色串扰。

表 1. 串扰补偿处理过程比较

Table 1. Comparison of crosstalk compensation processes

Contrast indicatorCoefficient compensationBCIFilter compensationLUTEquation correctionBlind phase suppressionEMDDifference frequency
Crosstalk matrix××××××
Preproccess×××
Smoothing×××××
Fitting××××××
Equalization×××××
Standardization××××××
Iteration×××××××

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各抗串扰方法有各自的特点。文献[ 59]中提出的系数补偿法忽略了背景光的影响。Li等[77]提出改进后的解耦矩阵补偿法校正了白色平板反射率造成的误差,消除了环境光的影响,进一步提高了测量精度。环境和物体颜色纹理对补偿效果会产生很大影响,所以这些消除方法在测量不同的物体时都需要重新对串扰进行标定。盲源颜色分离法通过建立非线性串扰模型,利用BCI迭代算法求解耦矩阵,可以消除干扰和非线性影响。滤波器补偿法需要定制带有滤波器的相机,造价高,会损失一些红绿蓝的亮度信息,但是总体效果较好。LUT法建立LUT比较繁琐费时,使用白色平板建立LUT,会使得测量有颜色或者反射率不同的物体时产生误差,最好与硬件滤波器分步补偿。盲相位误差抑制法不需要预先校正颜色串扰。EMD法在信号处理时对极值点依赖性强,对噪声也敏感,需要补偿非线性影响和校正非正弦条纹,避免高次谐波影响。为提高EMD法的抗噪性能,解决被测物体表面变化剧烈时测量精度不高的问题,可结合双目视觉等其他的测量方法来弥补。差频原理校正法是根据不同颜色通道的频率大小关系来避免颜色混叠,物体的最大深度变换值与不同颜色频率大小有关,所以三维测量范围将受到频率的影响。

各种方法在串扰消除的处理结果上也有着不同的表现。Hu等[81]提出的基于BCI算法的三维重构技术使得测量结果接近真值。Li等[77]提出的条纹分离与聚类法使系统识别受干扰的彩色条纹的正确率达92%。根据实验测量结果可知,滤波器硬件补偿的方法比软件补偿的方法效果好。LUT法可以将相位误差减小一半,但串扰消除仍不够彻底。Yu等[84]提出的方程校正法使得明显的颜色重叠几乎完全被消除,将物体重建的相对误差控制在了0.15%左右,为动态变形三维测量提供了新途径。基于BCI算法的盲相位校正法将物体重构的标准差从3.8874 mm减小到了0.6303 mm,比LUT法的测量精度提高了4.8倍[78] 。邹海华等[90]提出的基于EMD的彩色条纹轮廓术,实现了单次拍摄下的相位解调和展开,使得相位解调标准差小于0.0417 rad。EMD方法是动态物体测量的一种手段,但是在形状陡峭、纹理复杂处的测量精度较低。在三维测量系统中,可以根据各方法的特点和处理过程的需要,结合已有的测量仿真结果表现,选择合适的串扰补偿方法来提高相位计算精度。

4 未来研究方向

光学三维测量系统中的彩色相机和投影系统带来的颜色通道间的串扰问题,在设计阶段就已经形成,无法提前预见和评估。在未来抗串扰的研究上,可以考虑在对彩色通道信息获取不产生较大影响的情况下,在相机和投影仪设计阶段之前对它们的串扰进行消除或补偿。另一方面可以对DLP投影仪色轮和DMD芯片进行进一步的优化研究。再者,考虑串扰信号的时间性和几何性,在使用相机进行测量时使相邻通道信号到达的时间有一定的延迟,或者尽量不使用相邻通道进行信号传输。研究投影条纹的编码、物体表面的颜色对彩色条纹投影测量系统通道间串扰的影响,有利于扩大彩色复合条纹投影的应用范围。此外,抗噪性能好的EMD方法也可以在很大程度上提高测量的精度。最后,与EMD法有许多共性的变分模态分解法(VMD),在某些方面能够克服EMD法的不足,所以可以尝试将VMD应用于彩色复合条纹投影测量系统以解决多通道串扰影响。

5 结束语

对彩色投影三维测量系统中颜色通道间串扰消除和补偿的方法进行了综述。简单介绍了各种补偿方法的原理,对各方法处理过程中用到的技术进行了对比分析,总结了几种常用方法的特征性能与存在的问题。本研究可以为彩色光学三维测量系统消除串扰方法的选取提供参考。

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