A Simple, Efficient Method for Realistic Animation of Clouds

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
Incorrect Numbering of Reference
Typo
Incorrect
(Proceedings)
Correct
(CD-ROM)
3
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
Previous Work Simulation of Cloud Motion
Imple-ment
motion
speed
Simulation methods
Numerical simulation Kajiya84
Textured ellipsoids Gardener85
Diffusion process Stam93,Stam95
Fractal Ebert97
Qualitative simulation Neyret97
Particle system Kikuchi98
Stable fluids Stam99
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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
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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
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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

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Simulation Process
Basic idea

• Voxels correspond to cells

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Simulation Process
Basic idea

• Status of variables 0 or 1

by Boolean operations

• Simple transition rules

• cloud growth
• cloud extinction
• advection by wind

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Growth Simulation Nagel92
Cloud formation process
Water vapor becomes water droplets (clouds) due
to phase transition.
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Realizing Complex Motion
cld0
act0
hum0
State transition of a cell
cld1
act1
hum1
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Realizing Complex Motion
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Realizing Complex Motion
1 humid area (spheres/elipsoids in 3D)
2 change hum act from 0 to 1
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Advection by Wind

• Clouds move in one direction, blown by wind.

• Shifting all variables following the wind
  direction.

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Rendering Process
Basic idea
0 or 1
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Calculation of Density Distribution
Create continuous distribution using metaballs

• center density filtered value

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Rendering Clouds
Color of clouds (single scattering only)
sunlight
clouds
background color
viewpoint
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Rendering Clouds
Preprocess
compute billboard texture
metaball
virtual plane (billboard)
Calculation steps
1 calculate intensity reaching each metaball
2 clouds viewed from the viewpoint
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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
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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
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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
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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
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Step 2 Clouds Viewed from Viewpoint
1 Render background
metaball
2 Sort metaballs
viewpoint
3 Rotate billboards
4 Project billboards
frame buffer
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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
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Step 2 Clouds Viewed from Viewpoint
1 Render background
2 Sort metaballs
viewpoint
2
4
3 Rotate billboards
1
3
5
4 Project billboards
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Rendering Shafts of Light
Intensity at viewpoint
sun S
Isun
clouds
t'
P
t
V
a
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Rendering Shafts of Light
Intensity at viewpoint
sun S
Isun
clouds
t'
P
t
V
a
time-consuming
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Rendering Shafts of Light
Ieye
sun S
Is

clouds
1.Place spherical shells
2.Compute Isunb(t)F(a) b(t)
3.Map shadow texture
V
4.Draw shells with additive blending
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Rendering Shafts of Light
Ieye
sun S
Is

clouds
1.Place spherical shells
spherical shells
2.Compute Isunb(t)F(a) b(t)
3.Map shadow texture
V
4.Draw shells with additive blending
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Rendering Shafts of Light
Ieye
sun S
Is

clouds
1.Place spherical shells
spherical shells
2.Compute Isunb(t)F(a) b(t)
3.Map shadow texture
V
4.Draw shells with additive blending
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Rendering Shafts of Light
Ieye
sun S
Is

clouds
1.Place spherical shells
spherical shells
2.Compute Isunb(t)F(a) b(t)
3.Map shadow texture
V
4.Draw shells with additive blending
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Rendering Shafts of Light
Ieye
sun S
Is

clouds
1.Place spherical shells
spherical shells
2.Compute Isunb(t)F(a) b(t)
3.Map shadow texture
V
4.Draw shells with additive blending
35
Examples
rendering 10 sec
simulation 0.3 sec
image size 640x480
voxel size 256x256x10
(PentiumIII 733Mhz, NVIDIA GeForce256)
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Examples
shafts of light (daytime)
shafts of light (evening)
rendering 15 sec
simulation 0.5 sec
image size 640x480
voxel size 256x256x20
(PentiumIII 733Mhz, NVIDIA GeForce256)
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Example Animation (Video)
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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

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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