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Climate: Air, Water, Smoke, Clouds in After Effects 7.0

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If you think of a color key as an edge matte surrounded by two holdout mattes (one for the core, one for the background), how do you combine these mattes in Keylight? Mark Christiansen explains.

Even if you're not called upon to re-create extreme weather events such as those depicted in The Day After Tomorrow, climate conditions along with natural elements of water and wind are a constant in dramatic storylines. You may need to re-create these effects from scratch, or you may be called upon for subtle tweaking of what's already in the shot.

Why would you want to do this? Simply because the crew couldn't get the conditions the story required on the day of the shoot? In extreme cases, yes. But actually, any large exterior shot will exhibit some sort of meteorological influence, and everything in the shot (and sequence) is interrelated.

This article investigates phenomena that you can influence or even replace wholesale, including the following:

  • Particulate matter in the air. The look of particles in the air can offer important clues to a scene. Is it ever complicated to deal with particulate matter? Where does it not apply?
  • Sky replacement. This task comes up regularly. What is the sky, after all, but a big bluescreen? What else is involved?
  • Clouds of fog, smoke, or mist. Motion, color, and even depth inhabit this element. How can you create these properties in After Effects?
  • Billowing smoke. Thick plumes of smoke need to be created with some sort of complex 3D dynamics system, right? Wrong.
  • Wind. How do you re-create something you can't see? Cheap and easy ways that show its presence via secondary animation can really sell a shot.
  • Water and precipitation. The presence of water can influence a shot, even if offscreen. How do you handle rain and snow?

It's rare indeed that weather conditions cooperate on location, and even rarer that a shoot can wait for perfect weather. Transforming the appearance of a scene by using natural elements is one of the most satisfying things you can do as a compositor. The before-and-after comparison alone can be stunning, and the result can be worthy of a blockbuster film.

Particulate Matter

Particulate matter in the air influences how objects appear at different depths. What is it? Fundamentally, it's water—and other gas, dust, or visible particulate usually known as pollution.

Every ideal, pristine, pollution-free environment has water in the air—even in the dry desert. The amount of haze in the air offers a variety of clues to the viewer:

  • Distance to the horizon and of objects in relation to the horizon
  • Basic type of climate (aridness or heaviness of the weather)
  • Time of the year and the day's conditions
  • Stagnancy of the air (think Blade Runner)
  • Location of the sun (when it's not visible in the shot)

The color of the particulate matter offers clues to how much pollution is present, what it is, even how it feels: dust, smog, dark smoke from a fire, and so on (see Figure 1).

Figure 1

Figure 1 The same location under varied weather conditions. This type of study reveals environmental subtleties, such as how backlighting emphasizes even low levels of haze and reduces overall saturation, or how more diffuse conditions desaturate and obscure the horizon while emphasizing foreground color.

Essentially, particulate matter in the air lowers the apparent contrast of visible objects; secondarily, objects take on the color of the atmosphere around them and become slightly diffuse (see Figure 2). This is a subtle yet omnipresent depth cue: With any particulate matter in the air at all, objects lose contrast further from camera; the apparent color can change quite a bit, and detail is softened. As a compositor, you use this to your advantage, not only to re-create reality, but to provide dramatic information.

As an example, consider Figure 2, shot with a long lens. Long (or telephoto) lenses bring background elements more prominently into the frame; and a long lens is sometimes employed when a background element is meant to loom large or seem menacing. Anything far away that appears not only large but crystal clear, however, will simply look wrong. With the right amount of haze for the weather conditions, even a shot highly compressed with a very long lens will be something the viewer simply believes.

Figure 2

Figure 2 This shot was taken with a long lens. The structures one block away retain a good deal in the black of their shadows; buildings a mile or two away, far less; and the foothills ten miles away are so desaturated they begin to fade right into the sky—at first you don't even notice them.

Match an Existing Shot

Imagine the same aircraft flying through the scene as a toy model in the near foreground, a low-flying daredevil in a full-size airplane a block or two away, and high in the sky, miles away (see Figure 3).

Figure 3

Figure 3 The difference between a toy model airplane flying close, a real airplane flying nearby, and the same plane in the distant sky is conveyed with the use of scale, but, just as importantly, with levels that show the influence of atmospheric haze.

In this case, each plane of depth has pretty good reference to gauge how much atmospheric haze is in play. The street sign in the foreground has no haze (and, as an additional bonus, contains little color), making it a basic reference for black, white, and gray levels. Buildings a block or two away, particularly those in neutral colors, show the black and white levels becoming muted, but only slightly. Out on the horizon, nothing is even close to pure black; the blacks and whites take on some of the blue color of the sky as well.

For more of a challenge, try replicating the same principles (greater contrast in the foreground, much less in the far distance) with the image shown in Figure 4, which contains only mid-ground reference. The technique used here has the additional twist of understanding how atmospheric haze influences the color of the scene. Knowing how this works from studying a scene like the one in Figure 3 helps you to create it from scratch without good reference, in a scene like the one in Figure 4.

The plane as a foreground element seems to make life easier by containing a full range of monochrome colors.

Figure 4

Figure 4 The dome provides a great grayscale reference. But what about placing an item in the immediate foreground, or in the background sky, where no reference is visible? The same approach as in Figure 3 applies, but you must adjust according to an understanding of the phenomenon, rather than by checking reference right in the shot.

When matching a more colorful or monochrome element, you can create a small solid and add the default Ramp effect. With this element, it's simpler to set levels to add the proper depth cueing, and then apply those levels to the final element (see Figure 5).

Figure 5

Figure 5 If a foreground element doesn't contain enough of a range of values to make matching easy (for example, it's monochrome), you can use stand-ins (top) and apply the levels adjustments to the stand-ins (center), matching contrast channel-by-channel. Slam the result to check the accuracy of levels settings (bottom).

Creating a New Shot

What about creating a new background from scratch, as with a matte painting or 3D-rendered background? In either case, there's no reference built into the shot, but that doesn't mean that you can't use reference if you need it; a photo containing the necessary conditions will get you started.

To re-create depth cues in a shot, you must somehow separate the shot into planes of distance. If the source imagery is computer-generated, the 3D program that created it can also generate a depth map for you to use (see Figure 6). If not, you can slice the image into planes of distance, or you can make your own depth map to weight the distance of the objects in frame.

Getting reference is easy for anyone with an Internet connection, thanks to such sites as http://flickr.com/ for browsing—and of course http://images.google.com/imghp?hl=en&tab=wi when you need something specific.

Figure 6

Figure 6 A depth map of a cartoonish 3D city. This map can be applied directly to an adjustment layer as a Luma Inverted Matte (inverted in this example because the most distant objects should be most affected), and then you can dial in any contrast (via levels) and softening (via Fast Blur) effects. They're weighted to affect the background more than the foreground, and the contrast of the map itself can be adjusted to change the relative weighting. (Image courtesy of Fred Lewis/Moving Media.)

There are several ways in which you can use a depth map, but the simplest is probably to apply it to an adjustment layer as a Luma (or Luma Inverted) Matte, and then add a levels or other color-correction adjustment to the adjustment layer. With the depth matte in Figure 6, the heaviest level adjustments for depth cueing would be applied to the farthest elements, so by applying this matte as a Luma Inverted Matte and then flashing the blacks (raising the Output Black level in Levels), you would instantly add the effect of atmosphere on the scene.

Depth data can also be rendered and stored in an RPF file, as in Figure 7. RPF files are in some ways crude, lacking even thresholding in the edges (let alone higher-bit depths), but they can contain several types of 3D data, as listed in the 3D Channel menu.

This data can be used directly by a few effects to simulate 3D, including Particle Playground, which accepts RPF data as an influence map.

More extreme conditions may demand actual particles and the phenomena that accompany them. These conditions are examined in the following sections.

Figure 7

Figure 7 Look closely at the edges of RPF data and you'll see jagged pixels along the diagonals, but a depth map doesn't always have to be perfectly pristine. The 3D Channel Extract effect allows After Effects to work with RPF data. (Created by Fred Lewis; used with permission from Inhance Digital, Boeing, and the Navy UCAV program.)

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