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Pyrotechnics: Creating Fire, Explosions, and Energy Phenomena in After Effects 7.0

  • Article is provided courtesy of Adobe Press.
  • Date: Nov 13, 2008.

Contents

  1. Firearms
  2. Sci-Fi Weaponry
  3. Fire
  4. Explosions
  5. In a Blaze of Glory

Article Description

Humans are so familiar with fire that we can tell when it looks wrong, even if we don't understand the physics of explosions and fireworks. By using Mark Christiansen's After Effects techniques, however, you can provide at the compositing stage what the filmmaker couldn't afford on set: realism.

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Sci-Fi Weaponry

Now we shift into a more imaginary area, albeit one with a few roots in our own world. How do you create the look of pure energy used for the blasters and lightsabers in Star Wars or the phasers and photon torpedoes in Star Trek (whose distinctions are familiar to me as an ex-Lucasfilm employee). These are now visual clichés, yet they tap into an area of visual effects that seems to come up regularly. The key seems to be that even these imaginary weapons of pure energy contain recognizable resemblances to phenomena from our own world: high-powered lasers and high-voltage electrical arcs, for example. The other key is that 32 bit per channel compositing makes the play of the super-bright forces in a scene more natural, provided that you know how to set them up.

The final key is really to take these effects beyond the tried and true, but that involves art direction concerns that are mostly beyond the scope of this article. The basic look is relatively simple: a hot white core surrounded by a luminescent glow.

Full Control

A couple of effects in the Render category of the Effects menu automatically create an element with a core and a surrounding glow. For your basic blaster or lightsaber effect, you might be tempted to reach for Beam. And why not, especially now that you can supercharge the look of Beam by working in 32 bpc?

True, a canned effect such as this surrenders artistic control for convenience, but it lays the groundwork necessary for what this section is all about by providing an element with built-in thresholding (albeit in 8 bit—in a moment, I'll review why that doesn't particularly matter). Other than the shape, it's customizable, as follows:

  1. Apply the Beam effect to a solid layer above the plate layer. Beam can be applied directly to the plate by checking Composite on Original, but working with the element in HDR requires that it be a separate layer.
  2. Extend the length to 100% and set the Starting and Ending points to the ends of the stick.
  3. Add some Thickness: 24 for Starting and 30 for Ending. (The non-uniform settings lend an artificial impression of three-dimensionality with 3D Perspective checked on, as it is by default.)
  4. The basic element is there, but it's not looking too cool. Switch the Project to 32 bpc mode and apply the Levels effect. Uncheck Clip to Output White and lower Input White way down to around 30%, and you'll see the beam get white hot.
  5. Finesse the look by manipulating the Softness and Outside Color settings in Beam and Gamma in Levels (see Figure 11).
Figure 11

Figure 11 If you're still unconvinced about the power of 32 bit per channel HDR mode, check out how good this effect looks even though it's based on a simple (cheesy, really) 8 bit per channel Beam effect, with a Color Profile Converter to make it linear (optional) and, most essentially, Levels set to push the threshold areas into over-bright land. (Source footage courtesy of MarkAndMatty.com.)

What happens next is what gives it the cool factor, and just offhand there are several available options:

  • Animate the lightsaber being waved around and enable motion blur
  • For other types of shots, add interactive lighting where needed, such as the glow on nearby faces and passing objects
  • Add bullet hits, either like those discussed earlier, or your own special version of energy coursing through the target
  • Create this effect for yourself, without Beam, but using the same basic formula: a bright (or white) core layer and a darker-colored, blurred duplicate (or several) to create the basic element, which is then made to glow in 32 bpc by using Levels (see Figure 12)
Figure 12

Figure 12 All kinds of trippy sci-fi looks can result from blending the type of "high energy" elements described in this section via HDR compositing.

Heat Distortion

Heat distortion, that strange rippling in the air that occurs when hot air is dissipated into cooler air, is another of those effects that compositors love. Like a lens flare, it's a highly visible effect that, if properly motivated and adjusted, lends realism to your scene rather than distracting from its story.

Figure 13 shows the fabricated results of heat distortion in a scene. Its rippling effect adds to the dynamism or chaos of the scene. When your eye sees heat distortion, it understands that the environment is dynamic, containing an abrupt mix of hot air with cold air. This understanding adds to the visceral reality of the shot, whether it's a desert exterior, a day at the racetrack, jet engine exhaust, or all three (which describes the pod race sequence from Star Wars pretty well). When the fire itself is in the shot and you can see through it or anywhere above it, you expect the fire to heavily distort whatever is visible behind its heat.

Figure 13

Figure 13 The effect of heat haze has been added to the general mayhem being issued by the helicopter (left). A comparison with an unaltered shot of the building (right) emphasizes the amount of distortion. (Images courtesy of The Orphanage.)

What's Actually Happening?

Stare into a swimming pool, and you can see displacement caused by the bending of light as it travels through the water. Rippled waves in the water cause rippled bending of light. There are cases in which our atmosphere behaves in the same way, when ripples are caused in it by the collision of warmer and cooler air—a medium that isn't quite as transparent as it seems.

As you may know from basic physics, hot air rises and hot particles move faster than cool ones. Air isn't a perfectly clear medium but rather a translucent gas that can act as a lens. This "lens" is typically static and appears flat, but the application of heat causes an abrupt mixture of fast-moving hot air particles rising into cooler ambient air. This action creates ripples that have the effect of visually displacing and distorting what's behind the moving air, just like ripples in the pool or ripples in the windows of an old house.

You might assume that a physics model devised in 3D animation software would be more accurate than faking this effect in 2D. However, 3D software generally doesn't take into account the role of air in a shot. In any case, this effect behaves like an overlaid distortion on top of whatever lays beyond the hot air, from the point of view of the camera. Therefore, it's a perfectly appropriate compositing effect, useful when there's an object in the scene capable of generating a significant amount of hot air.

How to Create Heat Distortion

Following are the basic steps for creating heat distortion in After Effects from an invisible source:

  1. Create a basic particle animation that simulates the movement and dissipation of hot air particles in the scene.
  2. Make two similar but unique passes of this particle animation (one to displace the background vertically, the other to displace it horizontally), and pre-compose them.
  3. Add an adjustment layer containing the Displacement Map effect, which should be set to use the particle animation comp to create the distortion effect, and apply it to the background (see Figure 14).
    Figure 14

    Figure 14 A subtle effect, the heat displacement has been laid over a checkerboard, where it's clearly visible traveling up the center of the image.

    Setting up the particles is potentially the trickiest part, and of course the settings are unique for each scene. Particle Playground is perfectly adequate for this type of use, and it's included with After Effects Professional. Unfortunately, Particle Playground is generally slow to render and cumbersome, full of not-quite-intuitive properties. The help documents are thorough, but there are dozens of pages of documentation. Thankfully, this effect requires only a few simple adjustments:

    1. Under Cannon, move Position to the source in the frame where the heat haze originates (in this case, the bottom center, but it could even be out of frame).
    2. Open up Barrel Radius from the default of 0.0 to the width, in pixels, of the source. I chose 50, only because 720, the width of the frame, would require many more particles and slow down the example.
    3. Boost Particles Per Second to 200. The larger the Barrel Radius, the more particles needed.
    4. Under Gravity, set Force to 0.0 to prevent the default fountain effect.

    The default color and scale of the particles is fine for this video resolution example, but you might have to adjust them according to your shot. A larger format (in pixels) or a bigger heat source might require bigger, softer particles.

  4. Now duplicate the particles layer and set the color of the duplicated layer to pure green. To vary it slightly so that the particles don't overlap, raise Direction Random Spread and Velocity Random Spread to 25 from the default setting of 20.
  5. The heat animation is almost complete; it only needs some softening. Add an adjustment layer with a moderate Fast Blur setting of 4.0.
  6. Now, to put the animation to use: Drag it into the main comp, and turn off its visibility. The actual Displacement Map effect is applied either directly to the background plate or preferably to an adjustment layer sitting above all the layers that should be affected by the heat haze. Displacement Map is set by default to use the red channel for horizontal displacement and the green channel for vertical displacement; all you need to do is select the layer containing the red and green particles under the Displacement Map Layer pull-down (see Figure 15).
Figure 15

Figure 15 The particles used to create the displacement. By default, Displacement Map uses red for horizontal displacement and green for vertical displacement. A slight amount of blur was added to soften the effect of each individual particle. The effect is exaggerated (to show up in a still figure) at the left of the plane wing.

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