1. Particle Systems - A Technique for Modeling a Class of Fuzzy Objects
William T. Reeves
Lucasfilm Ltd
ACM Transactions on Graphics
Vol 2, No 2,
April 1983
Pages

2. Overview: This paper introduces particle systems
A method for modeling fuzzy objects such as fire, clouds, and water.
The application of particle systems to the wall of fire element from the Genesis Demo sequence of the film Start Trek II: The Wrath of Khan is presented.

3. 1. Introduction
Modeling phenomena such as clouds, smoke, water, and fire has proved difficult with existing techniques.
These “fuzzy” objects do not have smooth, well-defined, and shiny surfaces; instead their surfaces are irregular, complex, and ill defined.

4. 1. Introduction
The representation of particle systems differs in three basic ways from normal representations:
1) An object is represented not by a set of primitives (polygons) that define its boundary, but as clouds of particles that define its volume.

5. 1. Introduction
The representation of particle systems differs in three basic ways from normal representations:
2) A particle system is not a static entity – it changes
New particles are “born” and old particles “die”

6. 1. Introduction
The representation of particle systems differs in three basic ways from normal representations:
3) An object represented by a particle system is not deterministic. Instead stochastic processes are used to create and change an object’s shape and appearance.

7. 1. Introduction Advantages: 1) Particles are simple:
so you can process more of them, and
they are easy to motion blur
2) The model definition is procedural and is controlled by random numbers
Fast design
3) They model objects that are “alive”
That is, they change form over a period of time.

8. 2. Basic Model of Particle Systems
For each frame:
1) new particles are generated into the system
2) each new particle is assigned its individual attributes
3) any particle that has existed past its prescribed lifetime is extinguished
4) the remaining particles are moved and transformed according to their dynamic attributes

9. 2. Basic Model of Particle Systems
2.1 Particle Generation (two ways)
1) Actual number of particles generated is:
NPartsf = MeanPartsf + Rand() * VarPartsf;
Rand() returns a uniform in [-1.0, 1.0]
MeanPartsf is the is the mean number of particles
VarPartsf is the variance

10. 2. Basic Model of Particle Systems
2.1 Particle Generation (two ways)
2) Number of particles generated is dependant upon the screen size:
NPartsf = (MeanPartssaf + Rand() * VarPartssaf )
* ScreenArea;
MeanPartssaf is the is the mean per screen area
VarPartsf is the variance
This method controls the level of detail

11. 2. Basic Model of Particle Systems
2.2 Particle Attributes
For each new particle generated:
Initial Position,
Initial Velocity (speed AND direction),
Initial size,
Initial color,
Initial transparency,
Shape,
Lifetime

12. 2. Basic Model of Particle Systems
2.2 Particle Attributes
Particle systems have
An origin,
Rotation angles,
A generation shape
They implemented a sphere, circle, and a rectangle

13. 2. Basic Model of Particle Systems
2.2 Particle Attributes
Typical particle system with spherical generation shape

14. 2. Basic Model of Particle Systems
2.3 Particle Dynamics
Individual particles move in 3D space and also change over time (color, transparency, size,..)
Moving is easy:
Add the velocity vector to the position.
You can also have an acceleration factor
To simulate gravity

15. 2. Basic Model of Particle Systems
2.4 Particle Extinction
When generated a particle is given a lifetime (in frames)
This could be modified to be based upon color, transparency,…

16. 3. Using Particle Systems to Model a Wall of Fire and Explosions
The Genesis Demo sequence from the movie Star Trek II: The Wrath of Khan was generated by the Computer Graphics project of Lucasfilm Ltd.
The sequence depicts the transformation of a dead, moonlike planet into a warm earthlike planet by an experimental device called the Genesis bomb.

17. 3. Using Particle Systems to Model a Wall of Fire and Explosions
The wall-of-fire element in the Genesis Demo was generated using a two-level hierarchy of particle systems.
The top-level was centered at the impact point of the bomb. It generated particles which were themselves particle systems.

18. 3. Using Particle Systems to Model a Wall of Fire and Explosions
Distribution of particle systems on the planet’s surface

19. 3. Using Particle Systems to Model a Wall of Fire and Explosions
The number of new particle system generated in each ring was based on the ring’s circumference (and a density parameter)
The second-level systems began generating particles at varying times on the basis of their distance from the impact point.
This gave the effect of an expanding wall of fire.

20. 3. Using Particle Systems to Model a Wall of Fire and Explosions
The second-level particle systems were modeled to look like explosions

21. 3. Using Particle Systems to Model a Wall of Fire and Explosions
Initial Explosion
One large system and 20 smaller ones about its base.
About particles exist in the image.

22. 3. Using Particle Systems to Model a Wall of Fire and Explosions
Expanding wall of fire.
200 systems, 75,000 particles

23. 3. Using Particle Systems to Model a Wall of Fire and Explosions
Wall of fire over limb of planet
200 explosions, 85,000 particles

24. 3. Using Particle Systems to Model a Wall of Fire and Explosions
Wall of fire about to engulf camera
400 systems, 750,000 particles

25. 4. Other Applications of Particle Systems
4.1 Fireworks
Control parameters vary more widely, and streaking is more predominant.
The next 3 figures show overlapping systems

26. 4. Other Applications of Particle Systems
4.1 Fireworks
Two red fireworks

27. 4. Other Applications of Particle Systems
4.1 Fireworks
Green and blue fireworks

28. 4. Other Applications of Particle Systems
4.1 Fireworks
Multicolored fireworks

29. 4. Other Applications of Particle Systems
4.2 Line Drawing Explosions
Particle systems were used to model exploding objects in a computer simulated tactical display for a scene in Return of the Jedi.
Motion blur is achieved through the drawing of lines instead of points

30. 4. Other Applications of Particle Systems
4.3 Grass
Instead of drawing particles as streaks, the parabolic trajectory of each particle over its entire lifetime is drawn
Thus the motion is used to make the shape.
Each particle becomes a simple representation of a blade of grass, and the whole system becomes a clump of grass.

31. 4. Other Applications of Particle Systems
4.3 Grass

32. 5. Ongoing Research in Particle Systems
We are investigating the modeling of clouds. But they are difficult because:
1) Clouds are complex: depending upon wind direction, temperature, terrain, and humidity
2) clouds can throw shadows on themselves.
3) the number of particles will be very large.

33. 6. Conclusions
The authors presented particle systems, a method for modeling a class of fuzzy objects
And have shown how they were used in making the fire element of the Genesis Demo sequence for the movie Star Trek II: The Wrath of Khan.
TmLo8M

34. Particle Systems - A Technique for Modeling a Class of Fuzzy Objects
William T. Reeves
Lucasfilm Ltd
ACM Transactions on Graphics
Vol 2, No 2,
April 1983
Pages :30