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 91-108

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.

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.

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.

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”

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.

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.

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

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

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

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

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

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

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

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,…

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.

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.

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

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.

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

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 25000 particles exist in the image.

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

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

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

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

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

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

4. Other Applications of Particle Systems 4.1 Fireworks Multicolored fireworks

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

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.

4. Other Applications of Particle Systems 4.3 Grass

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.

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. https://www.youtube.com/watch?v=6JDLr TmLo8M

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 91-108 42:30