Tutorial 2: Throwing Computer Vision Overboard: How to Handle Underwater Light Attenuation and Refraction
Anne Jordt, University of Kiel, Germany
Kevin Köser, GEOMAR Helmholtz Centre for Ocean Research, Germany
Besides professional survey cameras mounted to autonomous or remotely
operated underwater vehicles, there is now a huge variety of DSLRs or
action cameras for divers or even waterproof cell phones for taking
photos or videos underwater. It might seem at first sight that putting
a camera in a waterproof housing (with a glass port to look outside)
would already allow to use it "in the usual way" e.g. for measuring,
mapping and reconstruction.
However, there are several challenges that have to be addressed. First
of all, due to different optical densities of air, glass and water,
light rays can be refracted at the interface ("port"), which then
violates the pinhole camera model. On top, the port can act as a lens
itself and change the geometry of the focus surface. Second, the
apparent transparency of water is valid merely for the blue/green
parts of the visible spectrum, while red, infrared and virtually all
other electromagnetic radiation is significantly attenuated or blocked
completely, leading to distance-dependent color corruption. On top,
back scattering and forward scattering of light can degrade image
quality if not considered properly.
In this tutorial we provide an overview of the challenges and review
approaches to solve them, in particular focused to geometric problems
related to imaging models, single camera based structure from motion
and mapping. We will start from the basics such that everybody should
be able to follow, however for the geometric parts, attendees should
have a basic understanding of classical multiple view geometry
(standard camera calibration, projection matrix, epipolar geometry and
ideally structure from motion).