When Does a Camera See Rain?
Kshitiz Garg, Shree K. Nayar
ICCV’ 05
Outline
Abstract Dynamic Weather And Vision Visibility of Rain Camera Parameters for Rain Removal Camera Based Rain Gauge Conclusion
Abstract
The Intensity fluctuations depend on the camera parameters, the properties of rain, and the brightness of the scene.
We use the camera parameters to remove rain in images and videos.
Dynamic Weather And Vision
Dynamic weather (rain and snow) introduces sharp intensity fluctuation in images and videos.
Dynamic Weather And Vision (Cont’d) Analysis of Visibility of Rain
The square of the raindrop size The brightness of the background scene Exposure time The depth of field
Dynamic Weather And Vision (Cont’d) Camera Parameters for Removal of Rain
Exposure time F-number Focus setting
Dynamic Weather And Vision (Cont’d) Camera Based Rain Gauge
Camera parameters can be set to enhance the visual effects of rain.
We can build a camera-based rain gauge to measure rain rate.
Visibility of Rain
Variance at a pixel over time can be used to measure the visibility of rain and can be used as a quantitative measure of it.
Visibility of Rain (Cont’d)
Camera and Intensity of a Raindrop
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Visibility of Rain (Cont’d)
Raindrops and Exposure Time
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Visibility of Rain (Cont’d)
Raindrops and Exposure Time When :
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Visibility of Rain (Cont’d)
Rain and Depth of Field
Since raindrops fall at high velocity, we assume
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Visibility of Rain (Cont’d)
Rain and Depth of Field When :
When :
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Ref: K. Garg and S.K. Nayar. Detection and removal of rain in videos. In CVPR04,2004.
Visibility of Rain (Cont’d)
Camera Parameters and Volume of Rain We partition the volume into thin layers of rain of
thickness .
The variance due to a variance of rain is then the sum of the variances due to the different layers.
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Visibility of Rain (Cont’d)
Camera Parameters and Volume of Rain The variance due to a single layer of rain:
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Visibility of Rain (Cont’d)
Camera Parameters and Volume of Rain The standard deviation due to a volume of rain:
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Visibility of Rain (Cont’d)
Camera Parameters and Volume of Rain The visibility of rain decreases as with
exposure time of the camera.T
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Visibility of Rain (Cont’d)
Camera Parameters and Volume of Rain initially increases rapidly with F-number and
then reaches saturation for higher F-number. r N
Visibility of Rain (Cont’d)
Camera Parameters and Volume of Rain dependence with respect to distance of the f
ocal plane.r 0z
Camera Parameters for Rain Removal Reducing Rain using Depth of Field
As decreases, decreases.N I
Camera Parameters for Rain Removal (Cont’d) Reducing Rain using Exposure Time
As increases, decreases. IT
Camera Parameters for Rain Removal (Cont’d) Reducing Rain using Multiple Parameters
Camera Parameters for Rain Removal (Cont’d) Reducing Heavy Rain
Camera Parameters for Rain Removal (Cont’d) This method will not be able to reduce rain in
scenes with very fast motion and when objects are very close to the camera.
Camera Based Rain Gauge
A camera-based rain gauge - a device that measures rain rate.
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Ref: R. Gunn and G.D. Kinzer. Terminal velocity for water droplet in stagnant air. J. Metero., 6:243–248, 1949.T. Wang and R.S Clifford. Use of rainfall-induced optical scintillations to measure path-averaged rain parameters. JOSA, 8:927–937, 1975.
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Camera Based Rain Gauge (Cont’d)
Conclusion
The visibility of rain is affected by factors such as camera parameters, properties of rain, and the brightness of scene.
This method is not as effective in reducing rain from scenes with very heavy rain or scenes with fast-moving objects that are close to the camera.
Conclusion (Cont’d)
The visibility of rain can be enhanced to build an inexpensive and portable camera-based rain gauge that provides instantaneous rain rate.
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