Chadebecq François, Vasconcelos Francisco, Lacher René, Maneas Efthymios, Desjardins Adrien, Ourselin Sébastien, Vercauteren Tom, Stoyanov Danail
Wellcome/EPSRC Centre for Interventional and Surgical Sciences (WEISS), London, UK.
School of Biomedical Engineering & Imaging Sciences, King's College London, London, UK.
Int J Comput Vis. 2020;128(5):1101-1117. doi: 10.1007/s11263-019-01218-9. Epub 2019 Nov 18.
Recovering 3D geometry from cameras in underwater applications involves the Refractive Structure-from-Motion problem where the non-linear distortion of light induced by a change of medium density invalidates the single viewpoint assumption. The pinhole-plus-distortion camera projection model suffers from a systematic geometric bias since refractive distortion depends on object distance. This leads to inaccurate camera pose and 3D shape estimation. To account for refraction, it is possible to use the axial camera model or to explicitly consider one or multiple parallel refractive interfaces whose orientations and positions with respect to the camera can be calibrated. Although it has been demonstrated that the refractive camera model is well-suited for underwater imaging, Refractive Structure-from-Motion remains particularly difficult to use in practice when considering the seldom studied case of a camera with a flat refractive interface. Our method applies to the case of underwater imaging systems whose entrance lens is in direct contact with the external medium. By adopting the refractive camera model, we provide a succinct derivation and expression for the refractive fundamental matrix and use this as the basis for a novel two-view reconstruction method for underwater imaging. For validation we use synthetic data to show the numerical properties of our method and we provide results on real data to demonstrate its practical application within laboratory settings and for medical applications in fluid-immersed endoscopy. We demonstrate our approach outperforms classic two-view Structure-from-Motion method relying on the pinhole-plus-distortion camera model.
在水下应用中从相机恢复三维几何形状涉及到折射运动结构问题，其中介质密度变化引起的光的非线性失真使单视点假设无效。针孔加失真相机投影模型存在系统几何偏差，因为折射失真取决于物体距离。这会导致相机姿态和三维形状估计不准确。为了考虑折射，可以使用轴向相机模型或明确考虑一个或多个平行折射界面，其相对于相机的方向和位置可以校准。尽管已经证明折射相机模型非常适合水下成像，但在考虑很少研究的具有平面折射界面的相机情况时，折射运动结构在实践中仍然特别难以使用。我们的方法适用于水下成像系统，其入射透镜与外部介质直接接触。通过采用折射相机模型，我们为折射基本矩阵提供了简洁的推导和表达式，并以此为基础提出了一种用于水下成像的新颖的两视图重建方法。为了进行验证，我们使用合成数据展示我们方法的数值特性，并提供真实数据的结果，以证明其在实验室环境中的实际应用以及在流体浸没式内窥镜检查中的医学应用。我们证明我们的方法优于依赖针孔加失真相机模型的经典两视图运动结构方法。
