Donald Kline

CS 480

Lecture Notes on Fire Simulations

3-19-09

Particle based fire simulations.

Avoids the need for connectivity which increases performance. Most simulations can run in real time.

Visual effects are typically lower quality.

Not to say that all particle systems have low quality, the ghost rider movie sported some the best CG fire that has been created using Maya. However a lot of time and money went into creating that effect.

Typical algorithm creates particles in a designated spot. The particles are generally textured and semi transparent. They behave very simplistically in many simulations where the particles have a large upward velocity and a smaller side to side velocity which makes the particles spread out like this

Or come to a point like this.

To make the particles actually look life fire, a yellow/reddish texture is used, and it diffuses or dissipates after a certain amount of time.

Flame Blobs

·         Similar to the particle system, but with bigger particles that can take on different shapes.

·         Can look bubbly

Mesh based fire simulations.

·         Ability to create objects that can actually burn away and it looks realistic.

·         Decent simulations run on the order of minutes per frame.

All objects are modeled with a mesh system. Ridged objects use a different mesh than gases and solids, however with a wrapper they can be used together. This is particularly nice because it means that this fire simulation can be coupled with any other physical reaction that can be done on mesh based objects. Examples: flowing liquid, moving/rolling objects, friction, rigid and non-rigid objects, collision/self-collision.

2-d version of creating a mesh based circular object.

Start with a large grid system (top – left) then get rid of square that are completely outside the object, and keep any squares that are completely inside the same. Squares that are on the edge get split up (top-right). Then a red-green system is applied and the grid is created (bottom-right). The process is similar in 3-d.

The temperature of each grid cell is kept track of, and when something becomes hot enough to burn it changes color and releases heat until it gets to the point where it no longer exists and the grid must then be updated to compensate for the change in the shape.

To achieve a realistic fire, fire is treated like a liquid or gas. It is still a mesh but now the area of the fire can be rendered using Navier–Stokes equations, and or fluid dynamics.

Texture based fire simulations.

·         Textures can be handled quickly on the graphics card.

·         This has its limitations as there must already be fire images. The fire is not created from scratch.

Take two similar photographs of real fire, then compare the pixels to determine a synthesized image from the pair. Now create a motion field for the flame from what we know about how fire acts and the color of the flame.

Again the problem with this is that in order to dramatically change the look of the fire we have to first take photos of a different fire, and then recreate the simulation with parameters that match the new photos.

Here is a quick and complicated explanation from a research paper.

References:

Article: Melting and Burning Solids into Liquids and Gases by Frank Losasso, Geoffrey Irving, Eran Guendelman, and Ron Fedkiwel

http://www.frankpetterson.com/publications/melting/melting.pdf

Article: Wrinkled Flames and Cellular Patterns by Jeong-Mo Hong, Tamar Shinar, Ronald Fedkiw

http://physbam.stanford.edu/~shinar/papers/hong_2007_wrinkled_flames.pdf

This document discusses the historical and current research in the area of fire simulation

http://www.cs.umd.edu/~mount/Indep/Yulia/fire.html

Article discussing the fire effects in the ghost rider film.

Using maya to create a simple fire system.

http://www.cgarena.com/freestuff/tutorials/maya/firevideo/fire.html

Bubbly looking fire video.