Yoshinori Dobashi (Hokkaido University) Yoshihiro Enjyo (Hokkaido University)

1. A Fast Rendering Method for Clouds Illuminated by Lightning Taking into Account Multiple Scattering Yoshinori Dobashi (Hokkaido University) Yoshihiro Enjyo (Hokkaido University) Tsuyoshi Yamamoto (Hokkaido University) Tomoyuki Nishita (The University of Tokyo)

2. Overview • Introduction • Previous Work • Basic Idea of Our Method • Precomputation of Basis Intensities • Real-time Rendering Process • Examples • Conclusions & Future Work

3. Introduction (1/3) • Realistic Image Synthesis • Visual Assessment, flight simulators, etc… • Assuming under fine weather conditions • Simulation under bad weather conditions • Rainfalls, windstorm, lightning, clouds, etc… • Flight simulation/computer games • Real-time rendering • Reality

4. Introduction (2/3) • Example image clouds illuminated by lightning

5. Introduction (3/3) • Multiple scattering • Enhancing reality • High computationcost • Difficult to achieve real-time rendering clouds viewpoint lightning

6. Features of Proposed Method • Real-time rendering of clouds illuminated by lightning • Arbitrary shapes of lightning • Anisotropic multiple scattering • Consisting of preprocess and real-time process • Use of wavelet transform for drastically accelerating intensity calculation • Assumption: Static clouds

7. Overview • Introduction • Previous Work • Basic Idea of Our Method • Precomputation of Basis Intensities • Real-time Rendering Process • Examples • Conclusions & Future Work

8. Previous Work on Lightning • Visual simulation of lightning [ Reed94] • Use of random numbers to create shape of lightning strokes • Probabilistic modeling of lightning [Kruszewski99] • Allowing user to control shapes • Physically-based animation of lightning [Kim04] • Calculating propagation of lightning via physical simulation • Focusing on modeling shape of lightning • Illumination on clouds are not considered.

9. Previous Work on Fast Rendering of Clouds/Smoke • Rendering of clouds using GPU [Dobashi00] • Clouds/atmosphere illuminated by lightning • Use of GPU for acceleration • Single scattering only • Precomputed radiance transfer [Sloan02] • Real-time rendering of clouds as well as glossy objects • Assuming light sources far from clouds • Lihgtning is not considered. [Dobashi01]

10. Overview • Introduction • Previous Work • Basic Idea of Our Method • Precomputation of Basis Intensities • Real-time Rendering Process • Examples • Conclusions & Future Work

11. Basic Idea of Our Method (1/7) • Shape of lightning • Polylines generatedby Reed’s method [Reed94] • Density distribution of clouds • Represented by 3D grid • Use of Dobashi’s method[Dobashi00] clouds lightning

12. point source Basic Idea of Our Method (2/7) • Intensity calculation of clouds • Point sources on strokes • Illuminations due to each point source • Multiple scattering • Accumulation of illuminationsdue to all point sources • Preprocess and real-time process clouds illuminate lightning

13. Basic Idea of Our Method (3/7) • Two separate grids clouds grid for clouds grid for simulation space simulation space

14. clouds virtual point source Basic Idea of Our Method (4/7) • Virtual point sources at each grid point grid for clouds grid for simulation space simulation space

15. intensity calculation taking into account multiple scattering virtual point source (intensity = 1.0) Basic Idea of Our Method (5/7) • Preprocess：computation of basis intensities save

16. Basic Idea of Our Method (6/7) • Preprocess：computation of basis intensities

17. Basic Idea of Our Method (6/7) • Preprocess：computation of basis intensities

18. Repeating for all grid points Basic Idea of Our Method (6/7) • Preprocess：computation of basis intensities basis intensities

19. multiplication intensity of virtual point sources generating lightning S × × × × • Fast but cost proportional to number of grid points • Further acceleration by using wavelets Basic Idea of Our Method (7/7) • Real-time process：rendering of clouds basis intensities accumulation

20. Overview • Introduction • Previous Work • Basic Idea of Our Method • Precomputation of Basis Intensities • Real-time Rendering Process • Examples • Conclusions & Future Work

21. clouds r virtual point source 1.0 intensity due to point source r2 simulation space Subdivision of Simulation Space • Contribution of virtual point source

22. clouds rl rl+1 Subdivision of Simulation Space • Adaptive subdivision simulation space

23. bil : intensity at grid point idue to virtual source l (b1l, b2l, …, bncld,l) calculation of multiple scattering ncld : number of grid points for clouds Precompuation of Basis Intensities • Intensity calculation due to virtual point source grid point i bil virtual source l

24. (b1l, b2l, …, bncld,l) calculation of multiple scattering ncld : number of grid points for clouds Precompuation of Basis Intensities • Intensity calculation due to virtual point source grid point i basis intensities due to virtual light l bil virtual source l

25. (b1l, b2l, …, bncld,l) wavelet transform (B1l, B2l, …, Bncld,l) calculation of multiple scattering Precompuation of Basis Intensities • Intensity calculation due to virtual point source grid point i basis intensities due to virtual light l bil virtual source l ncld : number of grid points for clouds

26. number of basis functions (= number of grid points for clouds) basis intensity at grid point idue to virtual source l coefficient after wavelet transform basis function (Haar basis) Precompuation of Basis Intensities • Basis intensities due to virtual source l

27. Precompuation of Basis Intensities • Basis intensities due to virtual source l • Many of Bijare nearly zero. • Discardingif |Bil| < z(z : threshold)

28. nl << ncld • Reducing memory requirement • Accelerating intensity calculation Precompuation of Basis Intensities • Basis intensities due to virtual source l • Many of Bijare nearly zero. • Discardingif |Bil| < z(z : threshold)

29. Overview • Introduction • Previous Work • Basic Idea of Our Method • Precomputation of Basis Intensities • Real-time Rendering Process • Examples • Conclusions & Future Work

30. distributing to neighboring grid points generating lightning placing point sources on strokes Real-time Rendering Process (1/7) • Calculation of intensities of virtual source

31. intensity el virtual source l Real-time Rendering Process (2/7) • Calculation of intensity of virtual source

32. e1 e2 e3 e4 basis intensities final image S Real-time Rendering Process (3/7) • Weighted sum of basis intensities

33. intensity Ii grid point i number of virtual sources with non-zero intensities virtual source l intensity el basis intensity Real-time Rendering Process (4/7) • Final intensity Ii at grid point i for clouds

34. basis intensity coefficient after wavelet transform Real-time Rendering Process (5/7) • Intensity Ii at grid point i for clouds number of virtual sources with non-zero intensities basis intensity

35. 1. Calculation of Hk 2. Inverse wavelet transform wavelet transform of intensity distribution of clouds Real-time Rendering Process (6/7) • Intensity Ii at grid point i for clouds

36. with wavelet inverse wavelet transform (k = 1, 2,…, nl) + nl << ncld (1/10) O(ncld) without wavelet (i = 1, 2,…, ncld) Real-time Rendering Process (7/7) • Calculation of Intensities at all grid pointsfor clouds

37. Overview • Introduction • Previous Work • Basic Idea of Our Method • Precomputation of Basis Intensities • Real-time Rendering Process • Examples • Conclusions & Future Work

38. simulation space 20x20x7 cloud volume: 128x128x16 • Memory requirement for basis intensities • without wavelet compression： 1137.5 MB • with wavelet compression： 86 MB Examples • Simulation condition intensity calculation： 32x32x4 compression to 1/13

39. previous method[Max94] proposed method same visual quality Previous Method vs. Proposed Method

40. 10% previous 0 proposed difference image (100 % = 255 difference in intensity) Previous Method vs. Proposed Method

41. 提案手法 Previous method Proposed method 2.12 sec. 0.2 秒 0.06 sec. 35 times faster Previous Method vs. Proposed Method • Computation time • Precomputation: 60 min (proposed method only) • Real-time process: computer:PentiumⅣ 3.5GHz,GeForece 7800 GTX

42. Various examples

43. Demo

44. Animation

45. Conclusions • Real-time rendering of clouds illuminated by lightning • Taking into account multiple scattering • Precomputation of basis intensities • Acceleration by using Haar wavelet transform • 16 frames per second on CPU

46. Future Work • Further acceleration using GPU • Real-time rendering of realistic rain • Dynamic clouds