Synthesizing fire from scratch remains an advanced topic within the upper reaches of visual effects research and development. I was employed at The Orphanage during its work on Hellboy, a story featuring a leading character whose preternatural gift is to create lots and lots of fire when upset. There was no way to use photographed fire for those shots because they required too much interaction with the rest of the scene. I was amazed at what the team responsible for these effects was able to pull off.
Within After Effects, fire synthesis is way too hot to handle. If fire is at all prominent in a shot, it will require elements that come from somewhere else—most likely, shot with a camera. Let's take a look at what's involved.
Creating and Using Fire Elements
Figure 16 shows fire elements filmed for effects usage. The big challenge when compositing fire is that it doesn't scale very realistically. You might think that any fire that you shoot with a camera will look better than something you create on a computer, but a fireplace fire looks like it came straight from your hearth, no matter how it's scaled or retimed.
The bigger the fire you need to shoot, however, the more expensive it's likely to be because you really need the fire to be shot in negative space—against a black background—so that you can composite it using Blending modes. Fire is obviously capable of illuminating the entire environment around it, so if that environment has specific detail, it can be all but impossible to isolate the fire from the environment.
Figure 16 Fire elements typically are shot in negative (black) space, or occasionally in a natural setting requiring more careful matting. By adjusting Input Black in Levels, you can control the amount of glow coming off the fire as it's blended via Add mode, lending the scene interactive lighting for free. (Images courtesy of Artbeats.)
This, then, is a case where it's worth investing in proper elements. In many cases, stock footage companies such as Artbeats have anticipated your needs. Whether you require burning half-height miniature trees or a flamethrower, you're sure to find solutions for a wide variety of shots in these stock collections. As a result, the scale and intensity may be more correct than what you can easily shoot on your own.
When employing a fire element shot against black, it's best in 8 bpc mode to employ Screen or Add Blending modes (not a luma matte) to permit the background to be seen behind the fire. The Unmult plug-in from Knoll Light Factory will also effectively transform the black background into transparency. Neither of these methods by itself, however, is enough for a final composite (see Figure 17).
Figure 17 Using luma matte fire—for example, with Extract (top left)—will never look good. Add mode tends to appear oversaturated in 8 bpc mode (top right), while it and Screen (bottom left) both are initially more transparent than they should be. Unmult gets the transparency more correct by removing black pixels, but the result lacks contrast (bottom right). (Fire courtesy of Artbeats.)
If you simply lay the fire layer over the background and apply Screen mode, the effect may look rather weak. There are several ways to strengthen it, but this is a case where linear blending and Add mode are ultimately the best combination. To firm up a fire, flare, or other bright element, you could try this:
- Apply Unmult in addition to setting a Blending mode (Screen or Add).
- Fine-tune the result with a Levels effect, pushing in on Input White and Black (as well as color-matching overall).
- Overlap multiple occurrences of the fire element to create a raging inferno.
However, the more elegant solution is to switch your project to 32 bpc linear and use Add; the result is far more natural and less of a kludge. Additionally, as with fog elements, the "secret sauce" is a Compound Blur effect on the background, using the Luminance of the fire element (see Figure 18).
Figure 18 Add mode behaves more naturally with linear blending via 32 bpc mode (top left). To really sell the effect, however, requires the addition of a compound blur effect to the background (top right). The combination is immediately more believable (bottom).
Of course, a final composition will often involve multiple fire elements as well as smoke (see Figure 19). As always, things go better in linear floating-point.
Figure 19 The background plate was stock footage of a raging fire (left), but it wasn't raging quite enough. Layering extra raging inferno footage helped the opaque look of such intense fires to break through the overlaid extra layers of smoke (right, courtesy of ABC-TV).
Provided that your camera isn't moving too much, a 2D fire layer should read adequately as being fully three-dimensional. If it's not looking believable in your scene, the problem is likely a lack of interactive light. As mentioned earlier, fire tends to illuminate everything around it with a warm, flickering glow.
There are a few ways to add this glow. First of all, as Figure 20 shows, your fire element may include a certain amount of glow that you can use. Raising Input Black when adjusting Levels tends to eliminate this glow, so that control is an effective way to dial the glow in and out.
Note, however, that fire glow isn't anything particularly unique or special; you can re-create it either via a heavily blurred duplicate of the source fire or by using a masked and heavily feathered orange solid, with perhaps a slight amount of wiggle added to the glow opacity to cause a bit of interactive flickering.
Figure 20 Depending on the Levels settings applied to the source fire element (specifically Input White and Black on the RGB and Red channels), you can end up with a lot of extra glow or omit the glow entirely.
Into the Third Dimension (and Beyond!)
You can pull off the illusion of fully three-dimensional fire, especially if the camera is moving around in 3D space, directly in After Effects. I was frankly surprised at how well this worked in the shot featured in Figure 21. The background plate is an aerial flyby of a forest. Because of the change in altitude and perspective, this shot clearly required 3D tracking. The keys to making this shot look fully dimensional were to break up the source fire elements into discrete chunks and then stagger those chunks in 3D space so that, as the plane rose above them, their relationship and parallax changed (see Figure 22).
Figure 21 Before-and-after sequential stills of a flyover shot. Because of the angle of the aerial camera, the shot required 3D motion tracking, originally done using http://www.2d3.com/product/?v=1". (Images courtesy of ABC-TV.)
Figure 22 A top view of the 3D motion-tracked camera from Figure 21 panning past one set of fires (of which the final composition had half a dozen). The pink layers contain fire elements, the gray layers smoke.
It's easy to get away with any individual fire element being 2D in this case. Because fire changes its shape constantly, there's nothing to give away its two-dimensionality. Borders of individual fire elements can overlap freely without being distracting, so it doesn't look cut out. The eye sees evidence of parallax between a couple dozen fire elements, and doesn't think to question that any individual one of them looks too flat. The smoke elements were handled in a similar way, organized along overlapping planes. Smoke's translucency aids in the illusion that overlapping smoke layers have dimensional depth.