Light Source and Direction


Often, when you need to match elements to a source, the steps outlined in Chapter 5, "Color and Light: Adjusting and Matching," for matching brightness and color are sufficient. In many scenes, however, there is clearly more involved with light than brightness and color. In some cases, the direction of the light plays a role, especially where the quality of the light is hard (direct) rather than soft (diffuse).

There is such a huge variety of light situations possible in a shot, and in an infinite array of combinations, that it becomes difficult to make any broad statements stand up about lighting. This section, however, tries to pin down some general guidelines for manipulating the light situation of your scene.

Location and Quality

You may have specific information about the lighting conditions that existed when your plate footage was shot. On a set, you can easily enough identify the placement and type of each light; this information is contained to some extent in a camera report also. If the source shot was taken only with natural lighting, you're seeking the position of the sun relative to the camera (Figure 12.1). This information can help you puzzle out highlights and shadows when it's not clear how to match the lighting of the plate.

Figure 12.1. Three shots lit only by the sun; in each case shadows tell you the light is coming from behind and to the right of camera, but as the sky becomes more overcast, the light becomes more diffuse and its direction more difficult to discern.


Sometimes the location and direction of light is readily apparent, but surprisingly often, it's not. As I write this, I'm looking out the window on an overcast day. The sunlight is coming from the south (on my left), but as I look at objects in my backyard, it seems to have no direction at all, because it's not direct, it's diffuse. Furthermore it keeps changing.

The quality of light in this scene is the most subjective and elusive of criteria. Hard, direct light casts clear shadows and raises contrast, and soft, diffuse light lowers contrast and casts soft shadows (if visible at all). That much seems clear enough. But these are broad stereotypes, which do not always hold as expected. For example, hard light aimed directly at a subject from the camera's point of view flattens out features, effectively decreasing contrast. And when multiple lights combine to light a subject, hard shadows can be diffused, a typical situation with artificial light (Figure 12.2).

Figure 12.2. To simulate the artificial lighting of a baseball stadium at night, this scene was lit from both sides with nearly equivalent key lights, causing light and shadow areas to overlap and cancel one another out to some degree. (Image courtesy of Tim Fink Productions.)


Heightening Drama with Light

Although the color and contrast of the scene can be nailed down precisely, light direction and quality can be slippery, surprising, changeable, and difficult to re-create. All true, but there's still plenty you can re-createor get away withif you follow a few basic guidelines:

  • Use elements with a similar quality of light. Matching hard- and soft-lit elements is generally going to be difficult, if not impossible. Sometimes you can raise contrast on soft shadows, or even lower it to soften existing shadows, but there's always a limit. If you can, start with elements that were shot, rendered, or painted with a similar quality of light to the plate.

  • Changing apparent light direction is just about impossible, particularly in a hard-lit scene (Figure 12.3). Incorrect light direction, in cases where it's evident, is one of the big giveaways of bad compositing, although for whatever reason it isn't always noticed by the audience the way matte lines and other such basic compositing mistakes are noticed. If you minimize its effects, this is one category where you can get away with something.

    Figure 12.3. Trying to convince the viewer that this figure was not key-lit from the upper-right of frame? Good luck. (All Newton images courtesy of Tim Fink Productions.)


  • Shadows are often broken up by indirect or reflected (bounced) light in artificially lit scenes. This is the phenomenon that gave rise to radiosity rendering in 3D rendering programs. Study the world around you, and you start to notice that some of the most interesting things that light does are the result of reflected light mixing with direct light. A clear, black cast shadow is often not nearly so interesting as one contaminated by secondary light, and this can free you from the need to faithfully play it straight with cast shadows.

  • Natural light can change rapidly over time. Clouds overhead, trees rustling, an open flame, the sunlight reflected off of a shimmering poolall of these create dramatic, interesting interactive lighting in your scene, helping bring it to life (or distracting the viewer if there's too much of it). If they're part of the story, you want these elements, but there are only limited ways you can add them in 2D (Figure 12.4). Some of these are explored in this chapter.

    Figure 12.4. Trying to re-create the effect of dappled natural light caused by an overhanging canopy of trees blowing in the breeze? Good luck. Although with planning, you can fake this lighting on set using the old trick of a bicycle wheel with pieces of paper woven through its spokes. Spinning the wheel slowly back and forth in front of the key light re-creates the effect of the moving canopy.


  • Foreground elements that you add can change the lighting situation. Be aware that if the element you're adding to your scene is highly reflective (and obviously, if it is self-illuminating), you must account for its interacting with the other elements.

Mastering the use of light in a scene is no simple matter. For centuries, painters defined themselves as much by their observation and use of light as anything; a new school of thought would develop, typically driven by a "master" who had observed something novel and revolutionary about how light works, and the course of art history would be changed. So instead of looking for the lazy quick fix, you are encouraged as always to keep shooting reference and scouring it for details you can steal.

Neutralizing Light Direction

If your source was shot with a light direction that is incorrect for the composited shot, that's a pretty big problem, depending on how hard and directional the light actually is. The solution is generally to neutralize, rather than to try to fix the discrepancy.

For the purposes of discussion, consider a situation in which shadows and highlights give a clear indication from where the light is coming. Also assume that a simple quick fix, such as flopping the shot, is not possible, which it usually isn't.

In such a case, you would first look for light direction clues that you could remove from the element; for example, look for cast shadows falling on an area of the shot that can either be removed or replaced (probably via matting and rotoscoping, as in Figure 12.5). In areas of the footage that can't be removed or replaced, use the Levels control to reduce contrast, raising Output Black to neutralize shadows and Output White to knock down highlights. There will likely be unwanted side effects, so you must do your best to strike a happy medium (Figure 12.6).

Figure 12.5. The most immediate indication that Sir Isaac has been lit from the left is his long cast shadow, but you have the option of including it or not when pulling the key.


Figure 12.6. Removing the shadow and raising the black levels helps to eliminate at least the first impression of the strong direction of the light, even in a case as extreme as this.


If adjusting the image directly to soften highlights and shadows isn't looking so good, you can get creative in adding the equivalent of a soft filter on the image. To do this

1.

Duplicate the image.

2.

Set a modest Fast Blur (say, 10 pixels for a film-resolution source) and an Add (or Screen) blending mode to the duplicate.

3.

Knock down Opacity to 30%.

4.

Make a second duplicate with the new settings, and change the blending mode of the top layer to Multiply (or Overlay).

These blurred layers soften the highlights and shadows, respectively. With Screen mode, the overall levels will not change; with Add, they will brighten slightly (Figure 12.7).

Figure 12.7. The effect of a soft lens filter is created with two extra duplicates of the source, each blurred and then set to Add (or Screen) and Multiply, respectively, each with an Opacity of approximately 30%.


Far from being the only such adjustment you can make, this is one pretty much devised on the fly to give a specific example; different shots might well require different settings. More generally, the principle of combining an image with blurred and matted versions of itself can be a highly effective way to change not only its lighting qualities but also its overall look.

Leveling Uneven Light and Hotspots

There is a simple trick that you can use to even out the lighting in cases where it should appear uniform. The basic idea is simple: Create a counter gradient and use it to weight to your image adjustment by bringing up the shadows, bringing down the highlights, or both. What's surprising is how powerful the technique isand to how many situations it applies.

You can create the gradient to do this by eye. In some cases, such as scenes with low contrast, you can use the inverted, blurred source itself to create the matte. But as a compositor used to looking at light, you should be able to discern where the hotspot is, and how far it reaches (Figure 12.8). You might even find this kind of fun.

Figure 12.8. This scene is lit by a single key light aimed at the center of the scene, causing a distracting hotspot on the torso of the second bunny from the left.


The next step is to create a white-to-black gradient using the Ramp effect that matches your perception of the hotspot in the scene (Figure 12.9). You could apply the result directly to the scene with a blending mode, but you'll have more control if you apply it as a luma track matte to an adjustment layer containing a Levels or Curves effect. This allows you to select whether you're adjusting highlights and shadows or gamma.

Figure 12.9. A simple gradient created to match the offensive area of the hotspot. The center and edge of the radial gradient, created with the Ramp effect, have been positioned by eye to match Figure 12.8.


The result won't necessarily obliterate all evidence that there was hot lighting in some area of your scene (Figure 12.10), but as always, the goal is not only aesthetic beauty but also the viewer's focus. If there seems to be a distracting spotlight on the middle of some part of the scene, there's a problem. The same technique can even help when you're attempting to pull a key from an unevenly lit set, although tools such as Keylight already compensate for these types of image defects. Should the camera move during the shot, you can even consider marrying the gradient start and end points to a tracker with an offset, either via expressions or parenting. (See Chapter 8, "Effective Motion Tracking," for more on this.)

Figure 12.10. It's not as though all traces of the single hard key light have been eliminated, but that was not the aim of this adjustment. Instead, the effect of the light has been reduced so that it no longer puts the viewer's focus in the wrong place.


Conversely, you can also create a lighting effect this way; a radial gradient at the center of the frame, with the corners slipping away to darkness, creates a vignette or eye light effect, often associated with projected or heavily treated film and with handheld low-light shooting. The easiest way to do this is not with Ramp but with a heavily feathered and inverted elliptical mask applied to a black solid (Figure 12.11).

Figure 12.11. Reference of film footage that was shot and processed with a vignette, and addition of the equivalent vignette effect in After Effects, by double-clicking the Elliptical Mask tool to fill the frame, then opening the mask controls (MM), checking Invert, and setting a very high Mask Feather (500 for this 2 K resolution source) and Mask Expansion (50 pixels).


Geek Alert: Inverse Square Law

3D artists and lighting directors out there will be aware that a linear gradient is not the perfect model for light falloff and that light's intensity diminishes proportionally to its distance from the source squared. So if I'm twice as far from a single light source as you are, I will be illuminated by one-quarter the amount of light.

So I suppose you want a gradient that behaves this way? Simple. Duplicate your layer with the radial gradient, and set the upper of the two layers to a Multiply blending mode. There you have it: inverse square dissipation of intensity (Figure 12.12). Instead of using a gradient layer directly as a luma matte, put the two combined gradients into a single pre-composition, and you're good to go.

Figure 12.12. The same gradient as in Figure 12.9, but duplicated with the duplicate layer set to Multiply mode, creating the inverse-square fall-off that is a phenomenon of light. Because the example in Figure 12.8 has only a single light that would fall off in this manner, this gradient was used to create Figure 12.10. Ordinary incandescent lights exhibit this phenomenon.


Who knows, maybe you'll even find a situation where you notice the difference; the number of shots taken with a single light source is not high, but the same adjustment would work with any single light that is too hot. If nothing else, you've just avoided paying money for one more third-party plug-in that offers one more capability you can easily create for yourself.




Adobe After Effects 6. 5 Studio Techniques
Adobe After Effects 6.5 Studio Techniques
ISBN: 0321316207
EAN: 2147483647
Year: 2006
Pages: 156

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