Like so many other cautious and responsible young citizens, I'm fascinated by fire. A few weeks ago, I had the opportunity to go to the 2009 Fire Arts Festival at an organization called The Crucible. The Crucible is an industrial arts facility that offers classes and provides workspace for community artists. I brought my camera along, slapped on a fancy "Photography Okay" sticker, and set out, thinking about game development all the while. Here are a few things I noticed.
1. Fire is bright
This one is obvious, I know, but fire's brightness is one of the most striking differences between fire in real life and fire on a monitor. Current monitors can only display a limited range of light intensities. So, physical brightness (i.e. luminance) often has to be approximated with color lightness. For example, to make pure yellow look brighter, we will have to just make it whiter. But, in the real world, there is no such thing as pure yellow - yellow light can keep getting brighter and brighter. Our eyes may max out at some point, but that's far beyond the point at which standard monitors max out. In the real world, when we look at an explosion, we can see the individual flames of blinding yellow, orange, and red. But in a game, or a photograph, we have a dilemma. We can either have a bright, blank explosion, or a dimmer, but more colorful and intricate, explosion.
When games err on the side of colorful fire, I find it often looks rather benign. Scanning through my photos of Pyrokinetics' fire pendulum, I picked out the one above, even though its flames are blown out and overexposed, as my favorite. I think the pure white captures the heat and vitality of those flames better than a more detailed and colorful version could have hoped.
2. Hot things glow
One weird thing about fire is that as bright as it looks, it still manages to give off a soft, diffuse glow. A lot of light may be emitted from a fire, but it's emitted across a wide surface area, and it's emitted in all directions at once. Thus, the light is spread out and smoothly applied.
Without flames, red hot objects spread light even more thinly. They often look fuzzy and translucent. Not only is the emission spread across the surface of the object, the light comes from within as well. As the light passes through the solid, it bounces around, experiencing subsurface scattering. Subsurface scattering is a pretty hot topic in graphics these days. Its a big part of what is making cgi skin look more and more fleshy and alive, rather than plastic. For example, here's Nvidia's take on real-time subsurface scattering for skin rendering. Perhaps similar techniques could be used for thermal glow.
3. Fire lives in air
When playing 20 Questions as a kid, my favorite choice of mystery item was fire. Is it an animal? Vegetable? Mineral? No, no, no I would grin. Is it alive? Mayyybe. I could hardly contain my glee. Fire is hard to categorize. It's not really made of anything, is it?
Well, actually it is. It took me awhile, but eventually I figured out that flames are made of airborne particles and gases. In a manner of speaking, they are glowing air currents. Acknowledging this should be at the core of any good fire engine (fire graphics engine, that is). Wind both shapes and fuels fire. And fire returns the favor: Convection currents from the heat warp the air. In "Oscillation" above, the flames bellow as the spinning orb whirls oxygen around them. In "Fire Vortex," a circle of fans twists the flames into a slender tornado, and convection sends the gout soaring up to sixty feet into the sky.
What is the best fire you've seen in a game? How might fire in games be improved?