Absolute Scale Structure-from-Motion on Underwater Images with a Monocular Camera
- Renxia Wu (Ocean University of China) | Liya Duan (Qilu University of Technology) | Hui Fang (Ocean University of China)
- Document ID
- International Society of Offshore and Polar Engineers
- The 29th International Ocean and Polar Engineering Conference, 16-21 June, Honolulu, Hawaii, USA
- Publication Date
- Document Type
- Conference Paper
- 2019. International Society of Offshore and Polar Engineers
- Underwater SfM, refractive reconstruction, pose estimation
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- 19 since 2007
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Structure-from-Motion is a technique for estimating 3D structures from 2D image. Due to different refractive index of mediums, the light ray will bend during the underwater imaging process, making the perspective camera model invalid. In this paper we set a camera fixed in a glass housing and use a virtual camera geometry relationship to describe the imaging process, it’s analyzed that the relative camera transform between two images can be recovered and the 3D object points can be reconstructed with absolute scale. Under different thickness of refractive plane, synthetic reconstruction experiments are conducted and the result demonstrates good performance of proposed method.
In many underwater applications, three-dimensional structure of underwater object is needed to gain measurements. One of the significance methods to gain the 3D structure is Structure-from-Motion, which can recover the structure of object from two-dimensional image sequences captured with different camera poses. However, underwater images are often obtained by a camera fixed in a glass housing, thus the light ray coming from water will meet three different mediums: water, glass and air. Due to the different refractive index, refraction occurs. The fact that light ray will bend during imaging makes the perspective camera model invalid for Structure-from-Motion with underwater images. In order to deal with the problem, some try to simplify the model by assuming the glass to be very thin (Treuvutz, Schechner, Kunz and Singh, 2012) or drive the effect caused by refraction into the change of focal length and radial distortion (Lavest, Rives and Lapresté, 2000), some use underwater calibration targets to do reconstruction(Kunz and Singh, 2008; Telem and Filin, 2010) though it’s not applicable in real operation.
Agrawal et al.(2012) analyzed the geometry of multi-layer refractive system and modeled it with an axial camera, dividing the calibrating process into axial estimation, glass housing thickness estimation and so on. However, the system can be used to estimate pose of the camera on the basis of the 3D-2D correspondences given.
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