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 Ref: B.K.P. Horn. Robot Vision. The MIT Press, 1986.

Visibility of Rain (Cont ’ d) Raindrops and Exposure Time Rain visible region: Rain visible region

Visibility of Rain (Cont ’ d) Raindrops and Exposure Time  When : Ref: K. Garg and S.K. Nayar. Detection and removal of rain in videos. In CVPR04,2004.

Visibility of Rain (Cont ’ d) Rain and Depth of Field  Since raindrops fall at high velocity, we assume Ref: K. Garg and S.K. Nayar. Detection and removal of rain in videos. In CVPR04,2004.

Visibility of Rain (Cont ’ d) Rain and Depth of Field  When : 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.

Visibility of Rain (Cont ’ d) Camera Parameters and Volume of Rain  The variance due to a single layer of rain:

Visibility of Rain (Cont ’ d) Camera Parameters and Volume of Rain  The standard deviation due to a volume of rain:

Visibility of Rain (Cont ’ d) Camera Parameters and Volume of Rain  The visibility of rain decreases as with exposure time of the camera.

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.

Visibility of Rain (Cont ’ d) Camera Parameters and Volume of Rain  dependence with respect to distance of the focal plane.

Camera Parameters for Rain Removal Reducing Rain using Depth of Field  As decreases, decreases.

Camera Parameters for Rain Removal (Cont ’ d) Reducing Rain using Exposure Time  As increases, decreases.

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. Ref: R. Gunn and G.D. Kinzer. Terminal velocity for water droplet in stagnant air. J. Metero., 6:243–248, T. Wang and R.S Clifford. Use of rainfall-induced optical scintillations to measure path-averaged rain parameters. JOSA, 8:927–937, 1975.

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.