Structured Light in Scattering Media |
 | Virtually all structured light methods assume that the scene and the
sources are immersed in pure air and that light is neither scattered nor
absorbed. Recently, however, structured lighting has found growing applications
in underwater and aerial imaging, where scattering effects cannot be ignored.
In this project, we conduct a comprehensive analysis of two representative
methods - light stripe range scanning and photometric stereo - in the presence
of scattering. For both methods, we derive physical models for the appearances
of a surface immersed in a scattering medium. Based on these models, we present
results on (a) the condition for object detectability in light striping and (b)
the number of sources required for photometric stereo. In both cases, we
demonstrate that while traditional methods fail when scattering is significant,
our methods accurately recover the scene (depths, normals, albedos) as well as
the properties of the medium. These results are in turn used to restore the
appearances of scenes as if they were captured in clear air. Although we have
focused on light striping and photometric stereo, our approach can also be
extended to other methods such as grid coding, gated and active polarization
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Publications
"Structured Light Methods for Underwater Imaging: Light Stripe Scanning and Photometric Stereo,"
S.G. Narasimhan and S.K. Nayar,
IEEE/MTS Oceans ,
pp.00, 2005.
[PDF] [bib] [©]
"Structured Light in Scattering Media,"
S.G. Narasimhan, S.K. Nayar, B. Sun and S.J. Koppal,
IEEE International Conference on Computer Vision (ICCV),
Vol.I, pp.420-427, Oct, 2005.
[PDF] [bib] [©]
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Images
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Experimental Setup:
This picture shows the apparatus used in our light striping and photometric
stereo experiments. The tank is made of transparent anti-reflection coated
glass and contains the scattering medium (for example, dilute milk) and the
scene. In the case of light stripe range finding, a projector (Infocus LP120)
is used to sweep a light plane across the scene. A video camera (Canon XL-1S)
views the scene as well as the medium with the effects of scattering. In the
case of photometric stereo, the same tank setup is illuminated by 11 light
sources and is observed by a digital still camera (not shown in image).
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Light Stripe Range Finding in Scattering Media:
This picture shows the results of applying our scene and medium recovery
algorithms to two scenes. The detection of the object intersections and the 3D
reconstruction obtained under different densities of scattering (dilutions of
milk) compare well with the results obtained in clear air. Despite the strong
effects of scattering, we are able to remove them completely to restore the
original scene contrast. A comparison with the floodlit images shows that
simply using bright sources does not enhance visibility in scattering media,
and that new structured lighting methods that are designed to focus light on
the scene to alleviate blurring and backscattering must be used.
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Photometric Stereo in Scattering Media:
This picture shows the results of applying our photometric stereo algorithm
that is adapted to work in scattering media. Scattering effects cause
significant decrease in scene contrast and color. Hence, applying the
traditional algorithm directly on these images results in poor performance. Our
modified method requires 5 images (instead of the usual 3) to correctly recover
not only the surface normals and albedos, but also a depth map of the scene,
which is not possible in the original method.
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Videos
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ICCV 2005 Video (use Apple Quicktime 6.0):
This video is a compilation of the main results of this project (30 MB). (With
narration)
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Light Stripe Range Scanning:
This video shows the recovery of the 3D structures of objects immersed in a
scattering medium using the light striping method developed for scattering
media (23 MB). (With narration)
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Photometric Stereo:
This video shows the recovery of surface normals, depths and albedos of scene
points using the photometric stereo algorithm developed for scattering media (6
MB). (With narration).
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Slides
ICCV 2005 presentation With videos (zip file)
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Database
WILD: Weather and Illumination Database
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Vision through Fog and Haze
Participating Media: Single Scattering Model
Participating Media: Multiple Scattering Model
Detection and Removal of Rain
Selecting Camera Parameters for Rain Removal
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