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A Simple, Efficient Method for Realistic Animation of Clouds

A Simple, Efficient Method for Realistic Animation of Clouds. Yoshinori Dobashi ( Hiroshima City University ). Tsuyoshi Okita ( Hiroshima City University ). Kazufumi Kaneda ( Hiroshima University ). Tomoyuki Nishita ( University of Tokyo ). Hideo Yamashita ( Hiroshima University ).

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A Simple, Efficient Method for Realistic Animation of Clouds

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  1. A Simple, Efficient Method for Realistic Animation of Clouds Yoshinori Dobashi (Hiroshima City University) Tsuyoshi Okita (Hiroshima City University) Kazufumi Kaneda (Hiroshima University) Tomoyuki Nishita (University of Tokyo) Hideo Yamashita (Hiroshima University)

  2. [1] D. Blythe, “Advanced … [2] Y. Dobashi, T. Nishita, … [1] Y. Dobashi, T. Nishita, … [2] Y. Dobashi, T. Nishita, … [1] D. Blythe, “Advanced … [2] Y. Dobashi, T. Nishita, … [3] Y. Dobashi, T. Nishita, … [4] Y. Dobashi, T. Nishita, … Incorrect Numbering of Reference Typo: Incorrect (Proceedings) Correct (CD-ROM)

  3. examples: • clouds • fire • ocean waves Realistic animation of clouds • Goal: • movies/commercial films • flight simulators • computer games applications: Introduction • Simulation of natural phenomena using computer graphics

  4. Introduction Important elements for realistic animation of clouds • complex motion • easy to implement • fast simulation of cloud motion • photorealistic images • fast image generation

  5. Introduction Important elements for realistic rendering of clouds atmospheric effects (shafts of light) cloud color shadows

  6. Our method Previous Work: Simulation of Cloud Motion Imple-ment speed motion Simulation methods Numerical simulation [Kajiya84] Textured ellipsoids [Gardener85] Diffusion process[Stam93,Stam95] Fractal [Ebert97] Qualitative simulation[Neyret97] Particle system [Kikuchi98] Stable fluids[Stam99]

  7. Previous Work: Rendering of Clouds shafts of light shadows colors Rendering methods speed Ray-tracing method e.g. [Kajiya84] Scanline method [Ebert90] 2D texture mapping e.g. [Gardener85] 3D texture mapping [Stam99]

  8. Our method Previous Work: Rendering of Clouds shafts of light shadows colors Rendering methods speed Ray-tracing method e.g. [Kajiya84] Scanline method [Ebert90] 2D texture mapping e.g. [Gardener85] 3D texture mapping [Stam99]

  9. shadows on the ground • shafts of light through clouds Proposed Method • Simulation process • 3D cellular automaton • complex cloud motion • small amount of computation • Rendering process • hardware-accelerated • cloud color taken into account single scattering model

  10. Three logical variables at each cell ti ti+1 time time ‘hum’: there is enough vapor. hum :humidity phase transition is ready to occur. act ‘act’: :activation cld :cloud ‘cld’: clouds exist or not. Simulation Process Basic idea • Voxels correspond to cells

  11. ti ti+1 time time hum :humidity act :activation cld :cloud Simulation Process Basic idea • Status of variables: 0 or 1 by Boolean operations • Simple transition rules • cloud growth • cloud extinction • advection by wind

  12. Disadvantages • cloud extinction never occurs • complex motion cannot be realized ti+1 ti time time ：activation act hum cld ：cloud phase transition effects cld propagation of activation variables (cloud growth) act z z y y x x act=0 hum=0 cld=0 act=1 hum=1 cld=1 hum ：humidity Growth Simulation[Nagel’92] Cloud formation process: Water vapor becomes water droplets (clouds) due to phase transition.

  13. humid active/humid cloud empty supply ‘hum’ & ‘act ‘ Formation and extinction occur repeatedly. Realizing Complex Motion act=0 hum=0 cld=0 State transition of a cell: act=1 hum=1 cld=1

  14. 1 humid area (spheres/elipsoids in 3D) 2 change ‘hum’ & ‘act’ from 0 to 1 Realizing Complex Motion

  15. 3 clouds are formed 4 change ‘cld’ from 1 to 0 Controlling red area Controlling cloud shapes & motion Realizing Complex Motion 1 humid area (spheres/elipsoids in 3D) 2 change ‘hum’ & ‘act’ from 0 to 1

  16. hum act cld wind direction Advection by Wind • Clouds move in one direction, blown by wind. • Shifting all variables following the wind direction.

  17. filtering continuous image volume rendering 1 Calculating density distribution 2 Rendering clouds 3 Rendering shafts of light through clouds Rendering Process Basic idea 0 or 1

  18. q center density field function R effective radius metaball metaball ( cell width x 1.5 in our case) • radius: specified by user Calculation of Density Distribution Create continuous distribution using metaballs • center density: filtered value

  19. scattered light Splatting method using billboard technique Rendering Clouds Color of clouds (single scattering only) sunlight clouds background color viewpoint

  20. 1 calculate intensity reaching each metaball 2 clouds viewed from the viewpoint billboard Rendering Clouds Preprocess compute billboard texture metaball virtual plane (billboard) Calculation steps 1 calculate intensity reaching each metaball 2 clouds viewed from the viewpoint

  21. 1 Divide billboard into mesh • cumulative density • attenuation ratio 2 Assume a ray passing through each mesh element 3 Compute cumulative density and attenuation ratio 4 Store the values as billboard texture Preprocess: Computing Billboard Texture 1 Divide billboard into mesh 2 Assume a ray passing through each mesh element 3 Compute cumulative density and attenuation ratio metaball 4 Store the values as billboard texture billboard

  22. billboard 2 Sort metaballs 1 3 2 3 Place billboards 4 5 4 Initialize frame buffer Initialized to 1.0 Step 1: Light Reaching at Each Metaball sun 1 Set camera at the sun, parallel projection 1 Set camera at the sun, parallel projection 2 Sort metaballs 3 Place billboards 4 Initialize frame buffer metaball 5 Project billboards 5 Project billboards frame buffer

  23. attenuation, sun metaball 1 • multiply attenuation read intensity • read pixel value multiply attenuation Step 1: Light Reaching at Each Metaball sun 5 Project billboards • multiply attenuation • read pixel value 1 3 6 Store shadow texture 2 4 5 projection

  24. attenuation, sun ground Step 1: Light Reaching at Each Metaball sun 5 Project billboards • multiply attenuation • read pixel value 1 3 6 Store shadow texture 6 Store shadow texture 2 4 5

  25. 1 Render background render background 2 Sort metaballs 2 4 3 Rotate billboards 1 3 5 Step 2: Clouds Viewed from Viewpoint 1 Render background metaball 2 Sort metaballs viewpoint 3 Rotate billboards 4 Project billboards frame buffer

  26. multiply attenuation • add color projection • multiply attenuation • add billboard color • attenuate background • add cloud colors Step 2: Clouds Viewed from Viewpoint 1 Render background metaball 2 Sort metaballs viewpoint 2 4 3 Rotate billboards 1 3 5 4 Project billboards

  27. rendered image • multiply attenuation • add color Step 2: Clouds Viewed from Viewpoint 1 Render background 2 Sort metaballs viewpoint 2 4 3 Rotate billboards 1 3 5 4 Project billboards

  28. b b : attenuation, atmosphere : attenuation, atmosphere ò Is = Ieye= I dt b g g : attenuation, clouds : attenuation, clouds ( t ) b g F ( a ) ( t' ) ( t' ) s F F : phase function : phase function I sun Is b, F are obtained analytically Rendering Shafts of Light Intensity at viewpoint sun S Isun clouds t' P t V a

  29. b b : attenuation, atmosphere : attenuation, atmosphere ò Is = Ieye= I dt b g g : attenuation, clouds : attenuation, clouds ( t ) b g F ( a ) ( t' ) ( t' ) s F F : phase function : phase function I sun Is sample points b, F are obtained analytically ray-tracing Rendering Shafts of Light Intensity at viewpoint sun S Isun clouds t' P t V a time-consuming

  30. ò I dt b ( t ) s b g ( t' ) ( t' ) F ( a ) I sun Rendering Shafts of Light Ieye= sun S Is = clouds 1．Place spherical shells 2．Compute Isunb(t’)F(a) b(t) V 3．Map shadow texture 4．Draw shells with additive blending

  31. ò I dt b ( t ) s b g ( t' ) ( t' ) F ( a ) I sun vertices Rendering Shafts of Light Ieye= sun S Is = clouds 1．Place spherical shells spherical shells 2．Compute Isunb(t’)F(a) b(t) V 3．Map shadow texture 4．Draw shells with additive blending

  32. ò I dt b ( t ) s b g ( t' ) ( t' ) F ( a ) I sun shadow texture Rendering Shafts of Light Ieye= sun S Is = clouds 1．Place spherical shells spherical shells 2．Compute Isunb(t’)F(a) b(t) V 3．Map shadow texture 4．Draw shells with additive blending

  33. ò I dt b ( t ) s b g ( t' ) ( t' ) F ( a ) I sun shadow texture texture mapping Rendering Shafts of Light Ieye= sun S Is = clouds 1．Place spherical shells spherical shells 2．Compute Isunb(t’)F(a) b(t) V 3．Map shadow texture 4．Draw shells with additive blending

  34. ò I dt b ( t ) s b g ( t' ) ( t' ) F ( a ) I sun shadow texture texture mapping Rendering Shafts of Light Ieye= sun S Is = clouds 1．Place spherical shells spherical shells 2．Compute Isunb(t’)F(a) b(t) V 3．Map shadow texture 4．Draw shells with additive blending

  35. Examples simulation: 0.3 [sec] rendering: 10 [sec] image size: 640x480 voxel size: 256x256x10 (PentiumIII 733Mhz, NVIDIA GeForce256)

  36. Examples shafts of light (daytime) shafts of light (evening) simulation: 0.5 [sec] rendering: 15 [sec] image size: 640x480 voxel size: 256x256x20 (PentiumIII 733Mhz, NVIDIA GeForce256)

  37. Example Animation (Video)

  38. Conclusion Simulation using 3D cellular automaton • complex motion • easy implementation • fast simulation by Boolean operations Realistic rendering of clouds • uses graphics hardware • two-pass method using billboards for colors and shadows • virtual spherical shells for shafts of light

  39. Future Work Further acceleration for real-time animation • use of LOD techniques • hierachical representation of voxels Creating various kinds of clouds • taking into account effects under terrain • handling multiple wind direction, wind field

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