When Does a Camera See Rain?

1. When Does a Camera See Rain? Kshitiz Garg, Shree K. Nayar ICCV’ 05

2. Outline • Abstract • Dynamic Weather And Vision • Visibility of Rain • Camera Parameters for Rain Removal • Camera Based Rain Gauge • Conclusion

3. 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.

4. Dynamic Weather And Vision • Dynamic weather (rain and snow) introduces sharp intensity fluctuation in images and videos.

5. 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

6. Dynamic Weather And Vision (Cont’d) • Camera Parameters for Removal of Rain • Exposure time • F-number • Focus setting

7. 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.

8. 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.

9. Visibility of Rain (Cont’d) • Camera and Intensity of a Raindrop Ref: B.K.P. Horn. Robot Vision. The MIT Press, 1986.

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

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

12. 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.

13. Visibility of Rain (Cont’d) • Rain and Depth of Field • When : • When : Ref: K. Garg and S.K. Nayar. Detection and removal of rain in videos. In CVPR04,2004.

14. 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.

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

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

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

18. 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.

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

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

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

22. Camera Parameters for Rain Removal (Cont’d) • Reducing Rain using Multiple Parameters

23. Camera Parameters for Rain Removal (Cont’d) • Reducing Heavy Rain

24. 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.

25. 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, 1949. T. Wang and R.S Clifford. Use of rainfall-induced optical scintillations to measure path-averaged rain parameters. JOSA, 8:927–937, 1975.

26. Camera Based Rain Gauge (Cont’d)

27. 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.

28. 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.