Transcript Cloud Rendering

Real-Time
Cloud Rendering
Alex D’Angelo
[email protected]
March 3, 2003
Intro
Clouds are a common part of nature.
 Several types (cumulus
 Relatively complex to simulate and
render.

Some Previous Techniques
Fluid Simulation
 Cloud Textures
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Fluid Simulation

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Fluid system is governed by simplified
Navier-Stokes and other fluid dynamics
equations.
2D/3D grid system of exchanging energy.
Realistic simulation and interaction but slow!
Simulation in software and rendered using
raytracing -- doesn’t take advantage of
graphics hardware.
Refer to papers by Stam and Fedkiw.
Cloud Textures
Cloud textures are blended with sky
plane or sky box.
 User cannot interact with clouds.
 Quick to draw but very unrealistic.
 Takes advantage of graphics hardware.
 Typically seen in games (i.e. Quake).

Proposed Technique
Hybrid fluid flow/texture solution.
 Clouds are represented as particles.
 Real-time and interactive.
 Simulates and renders clouds with
thousand of particles.
 Takes advantage of graphics hardware.
 Research by Dobashi (2000) and Harris
(2001)
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Basic Idea
Each cloud is a particle system.
 Cloud evolves based on cellular
automata.
 Two-pass lighting
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– Preprocess lights (sun, moon, etc).
– Forward scattering lighting at run-time.
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Clouds are rendered using imposters.
Cloud Particles
Particle represents a small portion of
the humid air which makes up the cloud.
 Properties:
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– Spherical shape
– Density (“albedo”)
– Extinction (humidity drop-off from center)
– Color
Cloud Formation Simulation

Physical formation based on cellular
automata.
 Course volumetric grid with 3 Boolean
properties at each location:
– Humidity
– Cloud
– Phase transition

External influences: wind, time.
 Refer to [Dobashi2000] for complete rules.
Cloud Formation Alternative
Artists can manually place clouds or
“spray” particles.
 Useful for strategically placing clouds in
games and animation.
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Lighting Idea (Pre-Process)
Simulate forward scattering
 Drawing particles from the light’s view
and reading back properties from the
image.
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Lighting (Pre-Process)

View particles from light’s point-of-view
 Sort particles by distance to light
 Clear the frame buffer
 For each particle in the cloud
– Calculate light intensity where particle will be
drawn.
– Particle color is function of intensity, density, area.
– Splat particle with particle color modulated by a
phase function.
Phase Function
We use a simple Rayleigh scattering
phase function
 Favors scattering in the forward and
backward directions
 P() = 3/4(1+cos2())
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Rendering Clouds
Particles are drawn as textured
billboards.
 Particles are sorted by distance to eye.
 Each particle is lighted by modulating its
color by the phase function, then drawn
using alpha-blending in the frame buffer.

Faster Clouds
Use imposters!
 Clouds are rendered to a textured and
shown as a viewer-aligned billboard.
 Error metric used to determine when
cloud’s imposter needs to be updated.
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Moving Objects Through Clouds
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Idea: Simulate a volumetric cloud using
our cloud rendering technique and allow
an object (i.e. model of a jet) to fly
through the cloud.
Moving Objects Through Clouds
Given: an object intersecting a cloud
 Algorithm:
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– Sort particles by distance from eye
– Render all particles behind object
– Render object
– Render all particle in front of object
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Supports any number of objects
Splitting Cloud With Objects
View
Direction
Object a
Back
Particle
Imposter
Object b
Middle
Particle
Imposter
Front
Particle
Imposter
Enhancements
Optimization by view-frustum culling per
cloud.
 Full sky and time-of-day simulation
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– Sun, moon movement.
– Cloud color takes on sky color.
– Lights have to be reprocessed when lights
are moved.
Questions?
Links
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Fluid simulation:
Jos Stam:
http://www.dgp.toronto.edu/people/stam/reality/Research/pub.html
Ron Fedkiw:
http://graphics.stanford.edu/~fedkiw/
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Cloud Simulation
Mark Harris
http://www.cs.unc.edu/~harrism/clouds/
Yoshinori Dobashi
http://www.eml.hiroshima-u.ac.jp/~doba/
Papers

[Harris2001] M. Harris and A. Lastra, "Real-Time
Cloud Rendering". Eurographics 2001 Volume 20
Number 3
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[Dobashi2000] Y. Dobashi, K. Kaneda, H. Yamashita,
T. Okita, and T. Nishita. "A Simple Efficient Method
for Realistic Animation of Clouds". SIGGRAPH 2000,
pp. 19-28