一种适于海浪测量的立体摄影物理模型

摘要

由于在海面上布设控制点较为困难,本文发展了一种无需海面控制点的立体摄影物理模型.该模型依次通过相机固有参数标定、相对定向和基于平均海面的绝对定向三个环节实现,文中给出了各个环节的控制方程和具体算法.利用室内实验时拍摄的一个影像对,将共面方程和距离方程联立构成闭合方程组,实现相机固有参数的标定;利用外海测量时拍摄的一个影像对,采用共面方程实现相对定向参数的标定;利用外海测量时拍摄的一个影像对序列,在左相机坐标系中通过影像匹配确定平均海面,建立特定的物方坐标系,并实现绝对定向.室内实验检验了固有参数控制方程的收敛性并表明定标精度较高;外海实验成功地构建了海面高度场,这表明本文提出的相对定向和绝对定向方法是可行的.该模型只需将两个相机按一定方位架设在平台上即可进行海浪测量,免去了繁琐的海面控制点布设过程,降低了工作难度和成本,有利于立体摄影海浪测量技术的推广和应用.%Unlike the traditional models, a new stereo photographic model requiring no sea control points that are difficult to lay on a sea surface is developed in this paper. It is realized according to the order of camera intrinsic parameters calibration, relative orientation and absolute orientation based on average sea surface, and we give the governing equations and its algorithms. First of all, in the paper we present an imaging model that adopts non-measurement camera with considering only the radial distortion coefficients, and then give the method that calculates the camera intrinsic parameters by combining the co-planar equations and the distance equations to form a closed system of equations. For verifying the convergence of the governing equations, we lay four parallel sand ridges on a level ground in laboratory, measure seven lines of different heights and take a pair of the images by two cameras. The intrinsic parameters are successfully calculated by using the method, and further analysis shows that the results are highly precise. When observing the ocean waves in offshore sea, we need to recalculate the relative orientation parameters. At this time the governing equations are the co-planar equations, but the accuracy of the image matching is seriously related to the accuracy of the relative orientation parameters. However, as the mirror reflection of the sea surface, the matching accuracy of the sea image is often low, so we select 407 optimal matching points from among 1600 conjugate points and calculate the parameters. It needs a sequence of sea image pairs to obtain the absolute orientation parameters, 256 pairs in the paper. For each pair, we select firstly 300 × 300 feature points on the left image approximately evenly, then find the conjugate points on the right image by image matching, calculate the coordinates of the corresponding sea surface points in the left camera coordinate system, and then fit the points into a plane; thus a plane sequence can be obtained. We establish a special object coordinate system on the average sea surface that is obtained by summing the plane sequence, and calculate the absolute orientation parameters by the geometric relationship between the two coordinate systems. And it is proved that the methods of relative orientation and absolute orientation proposed in this paper are feasible by reconstructing sea surface height field. The model greatly reduces the difficulty in calibrating the stereoscopic photography for ocean wave measurement, which is beneficial to its popularization and application.