摄像机标定中的特征点提取算法研究与改进

为提高摄像机的标定精度,必须研究高精度的特征点提取方法。针对三种比较典型的标定图样:棋盘格,二维正弦条纹和高斯点阵,分析比较了相应的提取特征点算法和用于摄像机内参数标定的精度,得出基于傅里叶相位分析法的二维正弦条纹的特征点提取精度最高。并针对傅里叶方法处理二维正弦条纹不能准确提取边缘特征点的缺陷,提出了使用相移法提取正交相位以代替傅里叶方法得出正交相位的新方法。这种方法可以消除傅里叶变换滤波对点阵边缘的模糊作用,扩大了二维正弦条纹在摄像机标定中的适用范围。摄像机外参数标定实验表明该方法在处理包含边缘点的点阵时,重投影误差保持在原有水平,证明了其有效性。     

摄像机标定中的特征点自动提取方法

提出了一种在复杂背景下,无需人工干预、自动提取二维正弦标靶特征点的方法。以液晶显示器作为平面标靶,标定图像为正弦强度条纹。摄像机拍摄标定条纹图后,算法根据条纹调制度自动分离背景和有效条纹区域;先将水平和垂直相移条纹的截断相位进行相位展开,再将两个相位分布构造成新的二维正交条纹图,通过多项式拟合建立各相位分布与其像素坐标的关系,以正交条纹亮点(对应相位为2π)作为标定特征点,并对这些亮点进行定位操作,获取其对应的亚像素坐标;最后利用展开相位的级次与条纹周期信息自动计算出它们的世界坐标,用于后续的摄像机标定。与常用的二维棋盘格对比实验结果表明,该方法标定结果可靠,而且精度更高,最为关键的是,该方法无需人机交互的手动操作,自动易行,可在以二维正弦条纹为标靶的摄像机标定中推广应用。     

基于相位标靶的光学坐标测量方法

提出一种基于相位标靶的光学坐标测量方法,与有3个以上的标记点的辅助测量棒(传统标靶)相比较,由于相位标靶特征点数量的大量增多以及基于相位计算的精确的特征点的提取,使得用这种标靶进行测量时,其测最结果更为准确和可靠.在计算机屏幕上产生二维正弦条纹,并以此作为校准靶进行摄像机标定.设计制作的相位标靶由小型特征图象屏和测棒组成,采用相位测量技术及条纹处理方法计算特征图象屏各点的相位和面内坐标,进而计算测棒上测点的三维坐标.将该相位标靶用于光学坐标测量,提出了标靶移动时坐标变换的计算公式和移动距离的计算方法,分析了测量误差的成因.实验得到了准确的光学坐标测量结果,证明了该方法的有效性.     

光刻对准中干涉条纹相位解析研究

针对光刻对准中双光栅产生的具有多频率的干涉条纹,提出了一种基于二维解析小波变换进行条纹分析的方法。该方法首先通过二维小波变换的多尺度对条纹的多频率进行分析,并通过解析小波基函数将条纹的幅度与相位进行分离,最终通过二维小波脊方法提取出与偏移量相关的相位。在相位提取的同时通过二维小波脊所处点的角度分布来移除封闭条纹处理中常见的相位符号不确定性。数值模拟与实验验证了该方法的可行性并与传统的基于频域的相位分析方法进行了对比分析。结果表明,该方法能在获得所需相位信息的同时较好地滤除掉由光路抖动引起的噪声,具有很强的适应性。     

基于彩色交叉条纹的相机标定北大核心CSCD

针对相位标靶标定相机需采集多幅条纹图获取相位的问题,提出了一种基于彩色交叉条纹的相机标定方法。本方法在每个标定位置仅需一幅条纹图就可以完成特征标识点的提取;通过液晶显示屏显示一幅红蓝交叉条纹图像,由相机拍摄,采用傅里叶变换和多频外差法分别得到水平和垂直方向的绝对相位;根据相位标靶的空间位置与图像中绝对相位的精确对应关系,标定相机内参,采用非线性最小二乘法优化。基于重投影误差的实验结果表明,本方法可以快速精确的实现相机标定。     

基于条纹反射的光学表面疵病检测法

提出一种基于条纹反射的光学表面疵病检测方法,检测系统由液晶显示屏、CCD相机和计算机组成.检测时在显示屏上分别显示水平和垂直两个方向正交的正弦性条纹,用CCD相机记录经被测表面反射的正弦条纹像,通过相移技术得到两个方向正交的相位分布和对比度分布,通过对比度分布确定被测表面的疵病位置.被测面表面面形相关信息包含在相位分布中,其表面疵病细节信息包含在局部微观相位分布之中.对相位分布进行多项式拟合后相减得到疵病引起的微观相位分布,计算得到表面疵病的梯度分布,积分得其高度分布.实验验证了该方法的可行性.     

基于视觉测量的飞行器舵面角位移传感器标定

提出一种基于计算机单目视觉的飞行器副翼、襟翼、方向舵和升降舵角位移传感器的非接触标定方法.用共面的两个特征圆组成一标定靶固定在飞行器舵面上,用一台定焦数码摄像机对标定靶拍照,获得特征圆数字图像.经图像分析确定特征圆圆心及直径在像面上的透视投影位置和长度,根据透视投影逆变换原理建立物和像空间坐标关系的解算模型,进而导出靶面平面方程.飞行器舵面角位移则由标定靶面的法线方向余弦来表示.仿真结果表明,该方法具有标定过程快速、简单和准确的特点.不需事先标定摄像机内外部参量,其标定准确度优于0.2°.     

离焦条纹投影三维测量中正弦光栅的二值化方法研究

采用离焦二值条纹投影技术进行三维面形测量可以克服数字投影仪的非线性响应,同时有利于提高投影速度,实现高速测量.通过理论分析和实验研究了采用数字投影仪时二值条纹的基频、高次谐波能量分布与条纹周期的关系.结合相移算法,以相位测量误差为指标,衡量各种二值条纹的性能.研究结果表明:在周期小时,罗奇光栅和二维误差扩散算法生成的二值条纹离焦后能产生正弦性较好的条纹;在采用满周期等间隔奇数次相移时,罗奇光栅离焦能获得更高的测量准确度;在周期大时,采用优化脉宽调制方法得到的二值光栅离焦测量准确度最高.研究结果为离焦条纹投影三维测量中二值光栅的选择提供了依据.     

一种新的反向投影条纹生成方法研究

反向条纹投影技术是一种应用于在线或批量检测的快速而稳定的光学三维面形检测技术.提出了一种新的产生反向条纹的算法,新的算法建立投影器坐标系与摄像机坐标系的正向映射变换关系,通过投影器坐标系上一个像素点的两套相位值,找到其在摄像机坐标系中对应的位置,即产生投影器坐标系像素点在摄像机坐标系中的注册.由于期望在摄像机中观察到的条纹图像只是简单的正弦条纹图像,直接读取注册点的期望条纹相位,很容易产生反向条纹.计箅机模拟和反向条纹投影实验中的相位标准差分别达到7.044×10-6 rad和3.34×10-2rad,比以前的方法在精度上有了较大的提高,并简要分析了精度提高的原因.计算机模拟和实物测试实验都验证了该方法的可行性.     

基于载频条纹相位分析的畸变测量和校正

为了对图像畸变进行测量和校正,利用纵向朗奇(Ronchi)载频条纹作为测量模板,通过成像系统获取畸变光栅条纹.运用傅里叶变换对畸变条纹图像进行频谱分析、滤波提取基频信息,直接从畸变图像的中心无畸变区域提取理想条纹像信息.通过相位分析提取包裹相位并解包,获得畸变光栅条纹的径向畸变相位分布规律.将该分布规律转化为径向位置畸变分布规律,并结合双线性插值灰度重建对畸变图像进行校正.实验结果证明该方法是有效的.     

Camera calibration with planar crossed fringe patterns

We introduced a novel crossed fringe pattern as the model plane for camera calibration. It can provide abundant control points, in extreme case each pixel on the CCD sensor can be taken as control points. Fourier Fringe Analysis is used to extract the phase distributions from the image of this pattern. Each control point consists of the locations of a pixel and the world coordinates, which can be calculated from the phase distributions. It avoids the complex extracting procedure of those classical control points and could produce abundant control points. Both computer simulation and real data have confirmed the proposed technique is easy to use, reliable and high accurate.     

Robust geometric,phase and colour structured light projection system calibration

This paper introduces a new comprehensive procedure for both geometric and colour calibration of structured light system. In order to perform both geometric and colour calibration procedure, a new calibration artifact is proposed. The intrinsic and extrinsic parameters of projector and camera are estimated by using an extended pinhole camera model with a tangential and radial distortion. Camera image plane coordinates are obtained by extracting features from images of a calibration artifact. Projector image plane coordinates are calculated on the basis of continuous phase maps obtained from a fringe pattern phase reconstruction procedure. In order to stereo calibrate camera-projector system, pairs of corresponding image plane points are calculated with subpixel accuracy. In addition, one of three pattern views is used in colour calibration. RGB values of a colour field pattern detected by camera and their reference values are compared. This comparison leads to derivation of a colour transformation matrix. The performance of the proposed method is tested by measuring plane, sphere and distance reference. Also 360 degrees complex object 3D model from a set of measurements is obtained. Residual mean errors for all tests performed are calculated.     

Constructing feature points for calibrating a structured light vision sensor by viewing a plane from unknown orientations

To calibrate a structured light vision sensor, it is necessary to obtain at least four non-collinear feature points that fall on the light stripe plane. We propose a novel method to construct non-collinear feature points used for calibrating a structured light vision sensor with a planar calibration object. After the planar calibration object is moved freely in the range of measuring of the structured light vision sensor at least twice, all the local world coordinates of the feature points falling on the light stripe plane can be readily obtained in site. The global world coordinates of the non-collinear feature points in the local world coordinate frame can be computed through the three-dimensional (3D) camera coordinate frame. A planar calibration object is designed according to the proposed approach to provide accurate feature points. The experiments conducted on a real structured light vision sensor that consists of a camera and a single-light-stripe-plane laser projector reveal that the proposed approach has high accuracy and is practical in the vision inspection applications. The proposed approach greatly reduces the cost of the calibration equipment and simplifies the calibrating procedure. It advances structured light vision inspection one step from laboratory environments to real world use.     

Three-dimensional shape measurement for an underwater object based on two-dimensional grating pattern projection

In this paper, a practical method using phase tracking and ray tracing algorithms is proposed for measuring the three-dimensional (3D) shape of an underwater object. A 2D projected sinusoidal fringe goes through the water and illuminates the tested object. Firstly, the phase tracking algorithm is employed to identify homologous points in phase distributions of the deformed fringe captured by the camera and these of the fringe pattern projected by the projector. The projector is regarded as a special camera as regards the stereovision principle. In the calibrated system, both ray directions of the homologous points can be easily figured out. Secondly, the ray tracing algorithm is used to trace the propagation path of each ray and to calculate the 3D coordinates of each point on the tested object's surface. Finally, the whole shape of the tested object can be reconstructed.     

An absolute phase to world coordinates method for the defocusing structured light shape measurement system

This paper proposes a flexible calibration method for the defocusing structured light three-dimensional (3D) imaging system. Neither a high accuracy translation stage nor the parameters of the projector is required. Therefore the method is more flexible in many practical applications. World coordinates for calibration are obtained based on the method of coordinates from known plane. The monotonic nonlinear relationships between the absolute phase and each world coordinate have been investigated. Different from other methods, in our method, the 3D shape is directly recovered by the absolute phase to world coordinates method. Experimental results show that, when polynomial curve fitting methods are used, the root-mean-squared (RMS) error for the flat plate reconstruction is less than 0.12 mm, and the measurement error of 3D points is less than −0.34%. Furthermore, when comparing the measurement results of our method with the method of coordinates from known plane based on camera calibration, the relative error is less than −0.05%. Therefore, camera calibration is considered as the major error source.     

结构光三维成像系统的计算机仿真

基于相位测量轮廓术和摄像机模型,提出一种结构照明三维成像系统的高精度计算机仿真算法。对于给定的物体三维模型,首先根据系统结构,采用Z缓冲技术消除遮挡和阴影部分,得到与摄像机像素点对应的物体表面采样点三维坐标,再使用统一的数学模型和方法处理投影过程。根据摄像机及投影仪的内、外参数,最终得到了摄像机像素点、物体表面采样点和投影仪像素坐标三者之间的对应关系,从而实现了结构照明三维成像系统仿真。为实际系统的结构设计、调整和参数校正提供了参考。     

相位测量轮廓术中三维坐标校准新方法

提出了一种新的用于相位测量轮廓术中的三维坐标校准方法。该方法用一个含有三个虚拟校准平面的标准块代替传统相位测量轮廓术校准方法中使用的标准平面，每个虚拟校准平面由多个高度相同但空间离散分布的子区域组成，相应的连续相位恢复是通过一个迭代过程实现的，同时各个子区域还被用于横向坐标的校准。由于各个虚拟校准平面间高度是垂直变化的，传统的相位展开不能得到正确的相对位置关系，基于等效波长概念，提出了一种准确的虚拟校准平面连续相位间相对位置恢复的办法。此校准方法的特点是：在校准过程中不需要标准平面的多次精确平移；仅需要一次测量就可以完成高度和横向坐标的同时校准。文中给出的实验结果说明了该校准方法的有效性和可行性。     

基于计算机视觉的大射电望远镜馈源舱动态定位系统

在FAST项目馈源舱扫描跟踪设计中 ,舱体的动态定位是实现系统闭环控制的基础。作者根据相机交汇测量原理提出了舱体空间动态定位的基本方法。推导了通过舱体上不共线三点描述舱体位姿的方法 ,介绍了相机交汇测量原理及系统标定方法 ,最后给出了大射电望远镜FAST 5 0m模型计算机视觉动态定位系统的组成 ,并讨论了CCD技术在FAST项目中应用的可行性     

Large depth-of-view portable three-dimensional laser scanner and its segmental calibration for robot vision

A portable 3D laser scanning system has been designed and built for robot vision. By tilting the charge coupled device (CCD) plane of portable 3D scanning system according to the Scheimpflug condition, the depth-of-view is successfully extended from less than 40 to 100 mm. Based on the tilted camera model, the traditional two-step camera calibration method is modified by introducing the angle factor. Meanwhile, a novel segmental calibration approach, i.e., dividing the whole work range into two parts and calibrating, respectively, with corresponding system parameters, is proposed to effectively improve the measurement accuracy of the large depth-of-view 3D laser scanner. In the process of 3D reconstruction, different calibration parameters are used to transform the 2D coordinates into 3D coordinates according to the different positions of the image in the CCD plane, and the measurement accuracy of is obtained experimentally. Finally, the experiment of scanning a lamina by the large depth-of-view portable 3D laser scanner used by an industrial robot IRB 4400 is also employed to demonstrate the effectiveness and high measurement accuracy of our scanning system.     

A novel 1D target-based calibration method with unknown orientation for structured light vision sensor

The problem associated with calibrating a structured light vision sensor is that it is difficult to obtain the coordinates of the world calibration points falling on the light stripe plane. In this paper, we present a novel method to address this problem by randomly moving a 1D (one-dimension) target within the sensor's view field. At each position where the target is set, the world coordinates with one calibration point on the light stripe plane that can be obtained based on at least three preset known points on the 1D target by a proposed two-stage technique. Thus, as long as the 1D target is at least set at three different positions, not less than three such calibration points can be obtained to perform the structured light vision sensor calibration. The simulation and real experiments conducted reveal that the proposed approach has an accuracy of up to 0.065 mm. The advantages of the proposed method are: (1) 1D target is easily machined with high accuracy, which reduces the cost of the calibration equipment; (2) the method simplifies the calibration operation and can be convenient in on-site calibration; (3) the method is suitable for use in confined spaces.