The application of standard computer vision techniques to underwater imaging
involves dealing with additional problems. This is mainly due to the transmission
properties of the medium. The optical properties of different water bodies
depend on the interaction between the light and the aquatic environment.
This interaction includes basically two processes: absorption and scattering (see
the Figure on the right).
Absorption. It is the process whereby light energy is converted to a different form
(principally heat). Therefore, light "disappears" from the image-forming process.
Scattering. It is produced by change of direction of individual photons, mainly due
to the different sizes of the particles forming the water. It is nearly independent of
wavelength. Scattering can be further divided into backscatter and forward scattering:
- Backscatter appears when the light is reflected in the direction of the imaging device.
- Forward scattering is produced when the light reflected by the imaged object
suffers from small changes in its direction. This effect normally produces a blurring of
the object when viewed from the camera.
Backscattering is normally reduced by increasing the distance (l) between the light
source and the imaging device, and forward scattering can be attenuated by decreasing the
distance Z to the sea floor (or the imaged object).
Resulting images
For all the transmission properties of the media described above, processing of underwater images suffers from the following problems:
- Lack of image features. They often lack distinct features (e.g. points, lines or contours) that are commonly exploited in terrestrial vision systems for tracking, positioning, navigation, etc. There are two main reasons for this absence of features: firstly, the sea-floor lacks well-defined contours. Even man-made objects such as pipes or cables loose their straightness due to the proliferation of marine life. Secondly, the light reflected by the objects suffers from forward scattering [Jaf91] (as described above), which causes a blurring of these elements in the image.
- Limited range. The range is limited due to light absorption and the need for artificial light introduces many new properties to the image, such as low contrast and non-uniform illumination.
- Clutter and lack of structure. Sub-sea scenes frequently present little structure and high clutter in the regions of interest for exploration.
- Marine snow. Quite often, small observable particles floating in the water show up as marine snow making feature extraction difficult (backscattering). This effect is due to suspended particles, as much as the own water molecules [Car94].
- Image distortion. A first ray diffraction is produced at the water-camera housing, and a second one occurs at the air-camera housing interfaces, reducing the effective focal length of the camera.
