Adding an animated layer of translucent clouds to a shot is easily enough done in After Effects. You can create the basic element by applying the Fractal Noise effect to a solid, and then use a blending mode such as Add or Screen to layer it in with the appropriate Opacity setting.
Fractal Noise, at the default Fractal Type setting of Basic, already looks smoky (Figure 13.8); you can refine the look of it somewhat by switching the Noise Type setting from the default, Soft Linear, to Spline. Go ahead and check out the other Fractal Types if you're curious; most of them are more synthetic looking, although some can be appropriate for alternative effects, such as lava. The main thing you must add is motion. Figure 13.8. Fractal Noise (shown here at the default setting) is a decent stand-in for organic-looking fog. You can try varying the Fractal Type or Noise Type to get different looks, and you must animate the Evolution if you want any billowing of the element.There are two types of motion you typically may want to add: an Evolution animation to cause internal movement, and possibly Transform animations to cause the overall layer to move, as if being blown by wind (Figure 13.9). There is no need to create a huge layer and animate its Position value. The Offset Turbulence setting under Transform will do the job for you, and you can reposition it infinitely without running out of space to move. The question of how much the settings should change over time is largely one of trial and error according to what your shot demands. Figure 13.9. The highlighted controls are keyframed; these are the keys to putting your fog element in motion. Note that you can also add Scale and Rotation animations.The other main settings to consider are those affecting the apparent scale and density of the noise layer. To influence these, first adjust the Scale (under Transform) and Contrast settings. You will not, generally speaking, want to stray too far from the defaults for Complexity and Sub Settings; these also affect apparent scale and density, but with all kinds of undesirable side effects that make the smoke look artificial.
So now you have an animated smoke, fog, or mist element whose motion you can continue to tweak; the only remaining question is how you layer it in. In virtually no case can you simply blend a single Fractal Noise layer over footage and end up with the look you're after. Instead, you will more effectively sell the effect by combining several layers. Time to examine how that's done. Masking and AdjustingIf you're planning on covering your whole scene evenly with smoke or mist, you will achieve a more realistic look by using two or three separate layers of Fractal Noise. You can even use the same settings, choosing different portions and time offsets and positioning them in 3D space to set the scale as needed (Figure 13.10). Figure 13.10. The smoke in this shot is made up of one large rendered fractal noise element that was sliced up, staggered, and animated in pseudo-3D using the technique featured at the end of Chapter 10, "Expressions." This is a flyover, and there needs to be enough discrete overlapping elements for the smoke to hold its position but give convincing depth. (Final image courtesy of ABC-TV.)The unexpected byproduct of layering 2D particle layers in this manner is that they take on the illusion of depth and volume. The eye perceives changes in parallax between the foreground and background, and automatically assumes these to be a byproduct of full three-dimensionality, yet you save the time and trouble of a 3D volumetric particle render. Of course, you're limited to instances in which particles don't interact with movement from objects in the scene; otherwise, you instantly graduate to some very tricky 3D effects. The best method for blending these particle layers with your background is typically to apply each one as a luminance matte of a solid. That way, you can make the solid whatever color you choose for your particles without having to anticipate what a blending mode choice will do to them. Often you will want to hold out the particle effect to a particular area of your frame. If you are trying to add smoke to a generalized area of the frame, a big elliptical mask with a high feather setting (in the triple digits even for video resolution) will do the trick; if the borders of the smoke area are apparent, increase the mask feather even further (Figure 13.11). Figure 13.11. This mask of a single smoke element from the shot in Figure 13.10 has a 200-pixel feather, despite that the resolution of the shot is D1 video (720 by 486). The softness of the mask helps to sell the element as smoke and works well overlaid with other, similarly feathered masked elements.Moving Through the MistThe same effect you get when you layer several instances of Fractal Noise can aid the illusion of moving forward through a misty cloud. That's done simply enough, but how often does your shot consist of just moving through a misty cloud? Most of the time, you will be adding clouds of smoke or mist to the ground plane of an existing shot. You can use the technique for emulating 3D tracking (see Chapter 10) to make the smoke hold its place in a particular area of the scene as the camera moves through (or above) it. To make this work, keep a few points in mind:
The shot dissected in Figures 13.10 and 13.11 features just such an effect of moving forward through clouds. It is difficult to convey with still images how the effect works in motion, but it combines the tracking of each shot carefully into place with the phenomenon of parallax, whereby overlapping layers swirl across one another in a believable manner. Mist and smoke seem to be a volume but they actually often behave more like overlapping, translucent planesindividual clouds of mist and smoke.
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