A Simple, Efficient Method for
Realistic Animation of Clouds
Yoshinori Dobashi* Kazufumi Kaneda** Hideo Yamashita**
Tsuyoshi Okita* Tomoyuki Nishita***
*Hiroshima
City University
**Hiroshima
University
***University
of Tokyo
Contents
 Introduction and Motivation
 Simulation
 Rendering
 Results
 Conclusion
Introduction
Problem Overview
 Realistic modeling and animation of
(cumulus-type) clouds
 Two sub-problems:
 Simulation of cloud formation, extinction
and advection by wind
 Rendering of the clouds, shadows and
shafts of light
Previous Work - Simulation
Two categories of simulation methods:
1. Physical process of fluid dynamics


Very accurate
Computationally expensive
2. Heuristic approach (procedural modeling)



Computationally inexpensive
Easier to implement
Parameters needed
Previous Work - Rendering
 Accounting for multiple scattering of light
 Computationally expensive
 Using 3-D textures for volume density
 Does not handle atmospheric effects such as
shafts of light
 Rendering shafts of light using ray-tracing or
a similar method
 Computationally expensive
Goals
 Simple and efficient simulation method
 Support of effects such as ...
 Cloud color by single scattering of light
 Shadows of clouds cast on the ground
 Shafts of light through clouds
 Hardware-accelerated rendering
 Visually convincing result
Simulation – Basic Idea
 Cellular automaton with binary states
Nagel‘s Method
 Water vapor turns to water to form clouds
 Use Nagel‘s method to simulate cloud
formation:
 Divide 3-D space evenly into 3-D cells
 Assign boolean variables to each cell:
 cld indicates whether cell contain clouds
 hum indicates whether cell has enough water vapor to
form clouds
 act indicates whether phase transition is ready to occur
Nagel‘s Method (cont‘d)
 Cell properties in the current animation frame
ti are used to compute the cell properties in
the next frame ti+1:
hum(x, y, z, ti+1) = hum(x, y, z, ti)  act(x, y, z, ti)
cld(x, y, z, ti+1) = cld(x, y, z, ti)  act(x, y, z, ti)
act(x, y, z, ti+1) = act(x, y, z, ti) hum(x, y, z, ti) act(x, y, z , ti)
act is a boolean function and its value is calculated by the
status of act in the surrounding cells.
Cloud Extinction
 Extension to Nagel‘s method:
cld(x, y, z, ti+1) = cld(x, y, z, ti)  IS(rnd > pext(x, y, z, ti))
hum(x, y, z, ti+1) = hum(x, y, z, ti) IS(rnd < phum(x, y, z, ti))
act(x, y, z, ti+1) = act(x, y, z, ti) IS(rnd < pact(x, y, z, ti))
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rnd : uniform random number
pext : probability of cloud extinction
phum : probability of vapor forming
pact : probability of phase transition occurence
Advection by Wind
 Clouds move, blown by winds
 Wind velocity is different depending on the
height from the ground
cld(x, y, z, ti+1) = cld(x – v(z), y, z, ti)
hum(x, y, z, ti+1) = hum(x – v(z), y, z, ti)
act(x, y, z, ti+1) = act(x – v(z), y, z, ti)
 v(z) : wind velocity, piecewise linear function
 Assumption: wind blows towards the direction of x-axis
Controlling Cloud Motion
 Ellipsoids simulate air parcels
 Vapor and phase transition probability:
 higher at center / lower at edge
 Cloud extinction probability:
 Lower at center / higher at edge
 Ellipsoids move in direction of wind
 Different kinds of clouds by controlling
ellipsoid parameters (sizes and position)
Fast Simulation using Bitfields
 Each cell state (cld, act, hum) can be
stored in a single bit
 Low memory requirements
 Fast computation of simulation process
 Problem: Random numbers
Solution: Precalculated look-up tables
Rendering – Basic Idea
 Smoothing and volume rendering
 Splatting method for clouds
 Spherical shells for shafts of light
Continuous Density
Distribution Calculation
 Simulation output is a binary distribution
 Continuous density distribution results from
smoothing the binary distribution
 Cloud density of a cell is the weighted
average of the surrounding cells
 Each cell contributes a density distribution
over an effective radius (Metaballs)
 Cloud density of an arbitrary point is therefore
a weighted sum of a simple basis function
Metaball Billboards
 Generate 2-D texture of metaballs
Rendering - Step 1
 Set up parallel projection with sun at
viewpoint and initialize framebuffer to 1.0
 Place billboards at centers of metaballs with
their normals toward the sun
 Starting with billboard closest to the sun,
project and blend billboards to framebuffer
 Read back value at projected billboard center
to get attenuation ratio between sun and
metaball
Rendering – Step 1 (cont‘d)
 After all metaballs have been projected,
framebuffer contains shadow texture
Rendering – Step 2
 Render all world objects except clouds
 Place billboards at centers of metaballs
with their normals toward the observer
 Project and blend billboards to
framebuffer starting with those farthest
from viewpoint
Rendering – Step 2 (cont‘d)
Shafts of Light
 Render using spherical shells (made of
polygons)
 Modify Rendering – Step 2 to:
 Calculate colors of vertices of shell
polygons (atmospheric conditions)
 Repeat for all shells (back-to-front):
 Render shell k with additive blending function
and shadow texture mapping
 Render billboards between shell k-1 and shell k
Shafts of Light (cont‘d)
Results
Results (cont‘d)
Conclusion
 Advantages:
 Simulation requires little computation
 Memory requirements are small
 Rendering is fast by making use of graphics
hardware
 Shadows of clouds and shafts of light can also be
rendered
 Possible improvements:
 Effects of terrain under clouds
 Level of detail
The End
Questions?