一种基于近似点光源模型的光度立体视觉系统标定方法

A Novel Calibration Method for the Photometric Stereo System with Quasi-Point Light Sources

文章研究了一种由近似点光源组成的光度立体视觉系统的标定方法。由于光源自身的非均匀发光特性,传统的平行光和点光源模型不再适用,为实现精确的三维法向重建,必须对光源及光照场进行精确的标定。论文首先根据光源自身的发光特性建立其辐射度模型,进而提出了一种两步标定策略,对光源位置参数及主光轴参数进行标定。以相机作为参照系,提出了一种多球标定方法,用于计算各个光源的位置参数,该方法利用相机观察到的多个球面高光点图像坐标,结合球体轮廓线及球体半径实现了光源位置的精确计算。为实现光源主光轴参数的估算,提出了一种基于参考平面的标定方法,通过分析近似点光源照射下的平面亮度分布,结合等亮度线拟合策略实现了光源主光轴方向参数的计算。基于系统标定参数以及光源的发光模型,就可以对每个场景点的入射光条件进行精确建模,从而实现精确的法向计算。在实验部分,分别对标准几何物体和自由曲面物体进行三维重建实验,并与传统的平行光及点光源模型进行了对比。结果显示,所提出的标定方法能够在非均匀光照条件下获得精确的三维法向重建结果。

In this paper,a new calibration method for a specially designed photometric stereo system was presented.Different from conventional photometric stereo systems which adopt parallel or distant point light sources,the quasi-point light sources are used in the proposed system,which permits the system with compact size and short working distance.Subject to the non-isotropic radiance property of the quasi-pointlight source,the lighting field should be precisely calibrated so as to determine the intensity and direction for each incident light ray.The proposed calibration method was performed in two steps.The first step was to calculate light source position,where a multi-sphere-based method was introduced.By extracting the highlight image points formed from the light source on the specular sphere,and utilizing the sphere contour and radius,it shows that the light source coordinate under camera reference frame can be precisely estimated.In second step,a reference plane-based method was proposed to calculate the principle optical axis of light source.It proves that,the principle optical axis should cross the brightness point in the image of reference plane,and the brightness point can be estimated by fitting the iso-luminance curve in the image.With the calibration parameters,and considering the radiance model of light source,lighting condition for each scene point can be determined.The experiments were conducted with various target surfaces and compared with conventional light models to demonstrate feasibility and accuracy improvement by the proposed calibration method.