[Figure 35] A photograph with a narrow depth of field.

Depth of Field
Another depth cue, which often dictates whether the level of detail is discernable, is depth of field. A camera lens, along with the human eye, can only focus on a specific distance from the lens, and anything in front of or behind that point is not in precise focus. The range that appears in focus is dependent on the aperture and focal length of the camera recording the image. A shot taken from exactly the same point as in Figure 34 can yield a very different result with a much lower aperture setting, which results in a narrow depth of field (see Figure 35). This new image has a narrow depth of field with focus centered on the light fixture in the foreground. Notice that the range of focus is so narrow that even the foreground shrubs are now becoming blurred. This narrow range of focus can also serve to trick the viewer into thinking the midground trees on the opposite side of the waterway are farther away than they really are. Because the human eye often maintains a broad depth of field, the interpretation when viewing an image with a narrow depth of field can be that of a drastically increased distance between layers in the scene. Another difference between Figures 34 and 35 is in the level of detail on the foreground light fixture. At first glance, Figure 34 appears to represent the majority of the scene in focus, including the light fixture. When comparing it with Figure 35, however, it is clear that the level of detail in the foreground object is now much higher. The corrugation on the interior portion of the light is now evident, as are the screws at the base and the grain on the wooden support post. If the light fixture is the point of interest for this shot, then Figure 34 has failed to fully display its level of detail, despite the greater overall depth of field. The compositor must always keep in mind the intended emphasis of a shot and avoid operations that will diminish or obscure important elements in a scene.

Contrast
Contrast is an often overlooked depth cue. The level of contrast in an element and between elements in a scene helps the viewer place the objects in 3D space. Contrast is closely related to each of the previous categories, in that each can increase or decrease the perceived level of contrast within an element or between elements. Contrast is the amount of difference between the brightness of different elements. The greater the difference is between elements, the higher the level of contrast. Unfortunately, the human perception of contrast is affected by many variables which makes contrast a much more complicated topic. Outside factors, such as color, saturation, and illumination can change the perception of contrast within a scene. As a general rule, elements closer to the camera appear higher in contrast than those farther away. This is tied closely with level of detail, which allows the viewer to resolve an element in clearer focus and thereby perceive it as having crisper edges and higher contrast and color saturation values. Objects closer to the camera do not necessarily have higher contrast or saturation values, but the camera lens, as well as the human eye, is able to resolve closer objects with more detail than those farther away. Be aware that juxtaposition can also affect the perception of color and brightness. A solid white object appears brighter when placed on a solid black background, as opposed to a light gray background. Likewise, a blue element appears more saturated and vibrant when surrounded by its complementary color of orange, as opposed to a color such as purple or green. The Art of Color, by Johannes Itten, provides an excellent reference for studying the principles and perception of color.

Atmosphere
The last depth cue in this discussion is atmosphere. Looking back at Figure 33, notice the two most distant layers in the background, the tree-covered hill and the peak in the distance. Each of these layers displays the effects of atmosphere on elements far from the camera. Depending on the amount of moisture in the air, the atmosphere will reduce the contrast and clarity with which distant elements are perceived. The layer of background trees on the hills appears to have much less contrast than the trees in the foreground, due to the effects of the atmosphere on the resolution. The most distant peak in the photo shows even greater atmospheric effects, with almost no detail or contrast visible. Each of these elements also begins to take on the value of the area surrounding them — in this case the sky. The water and particles in the atmosphere reflect the color of the sky, and as the distance from the camera increases, this effect becomes more pronounced. With elements in this scene extending farther back in the distance, dark areas become bright or washed out and brighter areas become duller. Comparing the distant trees with the palm trees in front of them, the difference in perceived contrast is clear. The palm trees can be more clearly delineated, and the atmospheric hazing effect is less because they are much closer to the camera. In a comp script, the effects of the atmosphere on distant elements can be simulated by blurring the elements, decreasing their contrast levels and by adding a sky-colored haze. Atmosphere, along with each of the other depth cues, can help in adding a great deal of realism to computer graphics elements and composites.

Continue to Compositing Techniques and Methods — Part 2.

To learn more about lighting and compositing and other topics of interest to animators, check out out Inspired 3D Lighting and Compositing by David Parrish; series edited by Kyle Clark and Michael Ford: Premier Press, 2002 (266 pages with illustrations). ISBN 1-931841-49-7 ($59.99) Read more about all four titles in the Inspired series and check back to VFXWorld frequently to read new excerpts.

Author David Parrish (left), series editor Kyle Clark (center) and series editor Mike Ford (right).

David Parrish went straight to work for Industrial Light & Magic after earning his master's degree from Texas A&M University. During the five years that followed, he worked on several major films, including Dragonheart, Return of the Jedi: Special Edition, Jurassic Park: The Lost World, Star Wars: Episode I — The Phantom Menace, Deep Blue Sea, Galaxy Quest and The Perfect Storm. After five years with ILM and a short stay with a startup company, he was hired by Sony Pictures Imageworks to work on Harry Potter and the Sorcerer's Stone.

Series editor Kyle Clark is a lead animator at Microsoft's Digital Anvil Studios and co-founder of Animation Foundation. He majored in film, video and computer animation at USC and has since worked on a number of feature, commercial and game projects. He has also taught at various schools, including San Francisco Academy of Art College, San Francisco State University, UCLA School of Design and Texas A&M University.

Michael Ford, series editor, is a senior technical animator at Sony Pictures Imageworks and co-founder of Animation Foundation. A graduate of UCLA’s School of Design, he has since worked on numerous feature and commercial projects at ILM, Centropolis FX and Digital Magic. He has lectured at the UCLA School of Design, USC, DeAnza College and San Francisco Academy of Art College.