Explosions are a great subject for effects animation. Why? Well, they are very dynamic and by nature, very non-uniform. The residuals of an explosion are another thing that makes them interesting as this often include flying debris, smoke, shockwaves, and even secondary explosions. There are so many different kinds of explosions too. The often unrealistic and overused effect in Hollywood is the fiery explosion, but this is usually the result of a volatile source like barrels of gasoline. However watching grenades go off is a totally different effect. In this column will cover what goes into an explosion and how that ties in with creating synthetic ones. It doesn't really matter what 3D package you use to make the effects as the techniques are not platform specific.
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All explosions happen for a reason. Maybe it's two volatile chemicals mixing or it's the kinetic energy of something massive crashing into another. Either way it all begins with a flash. The intensity of the flash is usually proportional to the size and heat of the blast. Notice how the sky lights up like a small sun and lingers for bit when a nuclear weapon is detonated. On the flip side, light a firecracker and you'll just get a quick flash. One way of producing the flash is with a post effect like a glow or flare. The flare route is often over used and sometimes inappropriate. A simple way to emphasize the effect is to replace one of the initial explosion frames with a white matte frame. Though it displays for a split second, the white frame helps convey the action.
f u e l
Once an ignition has occurred the fuel burns. The speed and intensity at which the fuel burns usually defines the force of the blast. In essence an explosion is a lightning fast burn. Different fuels have different burn rates and generate differing amounts of heat. This is something that also can effect the color of an explosion. One thing that can help define the scale of the explosion is the definition of the fire or smoke. The more detail, the larger the explosion. The motion of the fire or smoke is also important. Fast moving flames usually tell the eye small scale, yet a large fireball will linger and burn much longer, billowing out slowly. Also, fire moves upwards over time so making the flames drift is important too, especially if they linger.
Creating realistic fiery explosions with CGI is not a simple task. There are many approaches and they have their strengths and weaknesses. There are many tricks that an artist can do with geometry and clever texturing to make explosions. In this case, fractals are your friend. Take a sphere for example and animate it scaling outwards uniformly. To simulate drag, set the timing of the scale to slowdown over time by using either bezier function curves or TCB controls. Add some fractal displacement to the sphere to breakup its profile over the course of the explosion. By using layers of self-illuminated fractal noise maps you can get a somewhat convincing fiery look. Control the speed at which the fractals move to help convey scale.Particles are another tool useful for making explosions. Many packages have particles integrated into the toolkit that you can use to create blasts of polygons or points. Sometimes you can get away with using square facing particles with a red/yellow fractal noise pattern. The important thing here is to break up the edges of the facings with another noise map or gradient. Mixing up the particle types is another way to do this. Some packages let you use spheres or tetrahedrons as well. Variation in the particle velocity is important too. Uniformly moving particles look too much like fireworks. One of the hardest parts about relying on particles for explosion effects is that the farther away from the origin they move, the more space is left between them, thereby causing the effect to fall apart. For this reason, high particle counts are often necessary. Another way around this is to scale the particle geometry over time.
Volumetrics are one of the best tools for making realistic looking explosions. Their main drawback however is their complexity and render speed. By nature they render slower because they have volume or depth unlike geometry which is made up of surfaces. Often controlling the look of volumetrics is a slow trail and error effort because the results are not immediate and have to be rendered Often packages use a helper apparatus to define the boundary of a volumetric effect. Another approach to this is volumetric particle shading. This is nice because it's a best of both worlds combination of using particles to define motion and volumetrics to define the look. With this technique you get the instant feedback of animating particles and more realistic looking fire from volumetric rendering. 3D Studio MAX now has a trio of commercial plugins that do volumetric particle shading. The one I am most familiar with, Afterburn, has a viewport preview feature that builds geometric boundaries around particles in the scene and properly represents the extents of the effect. This is a real time saver because you don't need to constantly test render the effect to see where the boundaries are, they're just there in the viewports.
Though there are many ways to create the fiery look of an explosion, no one technique is perfect. I often use combinations of the above techniques.
d e b r i s
An explosion without debris is a rare thing. It's an effect without consequence and doesn't necessarily carry the message as well. Blowing up geometry can be a simple or complex process depending on the amount of realism you are trying to achieve. Sometimes it's as easy as taking a copy of your spaceship model and splitting it into several pieces. At the time of the explosion replace the ship with the pieces and animate them blasting away. For a really cool addition add particles trailing the fragments. 3D Studio MAX has a powerful particle system called Particle Array that I often use for destroying objects. This particle system procedurally detonates your model by emitting particles from the surface of the mesh. These particles can be just about anything, but the best results come from using object fragments. This tool will break apart the model into pieces (like a puzzle) and blast them outward with control over velocity and rotation.
The rate at which debris explodes away can often be used to present the scale of the effect. Slower moving debris would rain down from a massive explosion, while fast moving shrapnel from a grenade would bounce around and deflect inside a small room (often causing secondary effects). Drag again, is another important factor here. An explosion in space has no drag, so debris would continue to travel at the same velocity and often would inherit the velocity of the exploding object 100 percent. When you introduce atmosphere, this changes things a bit. As debris blows away from the origin of the explosion, it should slow down according to it's velocity. This doesn't mean that it needs to come to a standstill, but it does need to meet some resistance from the air.
s m o k e
Where there's fire, there's smoke. The amount of smoke is often defined by the size and composition of the explosion. This also includes dust, which also accounts for debris, but still is an integral part of the aftermath of an explosion. A surface explosion blows volumes of earth into the air, not only chunks but fine dust. This is why some explosions aren't always fiery. Airbursts are a bit different because they don't interact with the ground as much, thus they don't produce nearly as much fallout. Look at where your explosion is occurring to judge when you need more smoke and debris. Another thing to think about is that smoke tends to linger much longer than the flames themselves, but this too is a scale defining effect. A small blast will have smoke dissipating quickly, while smoke from a massive event will linger for hours.
Creating the smoke follows along similar lines to creating the fire. Volumetrics and particles are usually the best tools for the job since geometry doesn't work well here. Volumetrics are best for very dense smoke with self-shadowing and also when you need to pass the camera through them. Particles can't be beat when you need swirling clouds or just more varied control over the motion of the smoke.
s h o c k w a v e
Depending on the size and location of the explosion, sometimes a visible shockwave is produced. Actually the most noticeable shockwave is one that has an effect on the objects in the scene. If you look at a nuclear blast, you can see the shockwave leaving a trail of dust and debris as it emanates away from the origin point. A lot of times though, a shockwave is just an artistic effect. Airbursts rarely produce noticeable shockwaves. If they do, they tend to be spherical and loose their uniformity over time. A surface blast produces a very visible shockwave. Sub-surface or underwater blasts sometimes produce a mix of the above effects. Geometry tends to be the best tool for creating shockwaves. One simple way to create shockwave is to use an animated sphere or torus radiating outwards. Sisyphus makes a freeware MAX plugin called Ringwave that consists of a procedural object that creates shockwaves. Using particles for shockwaves can be tricky. Again, it often requires high particle counts because the further the shockwave travels, the more apparent the gap between particles becomes.f i n a l n o t e
Reference footage is important to have for any subject you are looking to mimic and explosions are no different. If there ever was a good reason to rent a cheesy action flick, looking for good explosion footage is one of them. For the grandest of explosive reference, check out Peter Kuran's Trinity and Beyond. It's an incredible documentary on the history of nuclear testing in America and abroad. It features some valuable footage of nuclear explosions and their effects. Check out www.vce.com for more information.
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