These are the more common compositing operators found in software packages. During everyday compositing, the over function will likely be used more than all of the other operators combined. The over function is a powerful tool and is definitely worth studying in depth. Understanding the mathematics behind the operation, as described in the Appendix provides the compositor with an excellent point of reference for gaining control over the process of layering images.

Depth Cues and Contrast
As mentioned in Chapter 5, “Tools of the Trade,” depth cues are a primary method for clarifying the three-dimensionality of a scene within a two-dimensional image. Depth cues provide the viewer with information to indicate which elements in the scene are closer and which are farther from the camera. These cues can be occlusions of objects by another, relative sizes of objects, reduced clarity, blurring, haze and atmosphere, to name a few. All of these cues can be introduced or enhanced during the compositing stage of a shot. The compositional basics presented in Chapter 5, “Tools of the Trade,” also play an important role in implementing these depth cues. The first step (you’ll start to see a pattern here) is observation. As lighting requires the study and observation of how the human eye perceives illumination, compositing requires those same steps to understand how to combine layers in a realistic manner. Once the rules are learned, they can be stretched or broken to serve the intent of the artist. A strong understanding of how camera lenses and the human eye interpret the world and its three-dimensionality will give the compositing artist a visual vocabulary with which to build images. Compositing is a construction process with images, elements, and compositing operations being the layers with which you build.

Observing the world around you is the first step, so studying and breaking down the depth cues in a traditional photograph is a good starting point. The photo in Figure 33 shows many of the depth cues seen every day but in many instances, taken for granted. The specific depth cues examined here are the following:

Overlap
Scale
Level of detail
Depth of field
Contrast
Atmosphere

[Figure 34] A photograph displaying various depth cues.

Overlap
There are several distinct layers represented in this photograph (Figure 34), with multiple foreground elements, a distinct midground, a background, and a distant background. Starting with the foreground and working backward into the image, look at the oblong light mounted on the wooden post. The most basic indication that this object is in the foreground is the fact that it is obscuring other parts of the scene. The light fixture overlaps the shrubbery, water and dock retaining wall. Based on experience viewing the world, our eyes interpret this overlap as a clear indicator that the light fixture is closer to the camera than the other objects. With a photograph, the elements automatically overlap each other correctly according to their placement in the scene. In a composited image, however, the compositor chooses the layering order of elements. If the light fixture is element A and the shrubbery is element B in a composite script, placing A over B is just as easy as placing B over A. Layering the shrubbery over the light fixture would be possible, and due to the close proximity of the two layers in the scene, it might even look correct. It would, however, drastically change the composition and emphasis of the shot, with just a small portion of the light fixture peeking over the shrubs. A compositor must always maintain a clear understanding of the relationships between objects in three-dimensional space and attempt to layer the elements accordingly.

Scale
Another depth cue for this foreground element is its scale. The size of the light fixture, relative to the other objects in the scene, gives the viewer a clue to its proximity to the camera. If an element is larger in frame relative to another assumed comparably sized object in the scene, then it appears to be closer to the camera. This effect takes into account a viewer’s background knowledge and sense of scale with known objects. For instance, even though it may not be of a type specifically seen before, a light fixture has a certain size in most people’s minds. Scale has a great deal to do with human perception and experience, and the way we are accustomed to seeing things. Because the world is taken in through the eyes and processed with the brain each day, familiar items are categorized in terms of their scale. The trees in the midground of the photo are a good example. The trees relative to the scale of the light fixture are actually smaller in this photograph. The mind interprets the scale of the trees to mean they are much farther away from the camera than the light fixture.

Level of Detail
Along with scale, it is important to note the level of detail of each layer within a scene. Every portion of a scene displays details to help the viewer understand how far it is from camera. Certain objects display a greater level of detail, giving a clue as to their distance from the viewer. In Figure 33, the plant life along each bank of the waterway provides a good comparison of level of detail. The plants on the bank nearest to the camera can be clearly resolved into leaves, flowers and branches. The plants on the opposite bank, however, can only be seen as a green color and a basic outline. When rendering elements for a computer graphics scene, this type of depth cue not only helps a composition come together more realistically, but also presents an opportunity to save rendering time. The objects nearest camera need attention to detail in their textures, lighting and resolution. The same object placed farther away in the scene requires less attention in each of those areas. Whereas a foreground object may require rendering at a resolution of 2k, with high-resolution textures and large shadow maps in the lights, a midground object may only require a 1k render, with medium resolution textures and much smaller shadow maps. Each of those optimizations can save a tremendous amount of render time. Attention to detail in composition is important but not in areas in which it goes unnoticed. Spending time on the texture and lighting quality of foreground objects will usually be more important to establishing the quality level of a shot than using the same attention to detail in midground and background elements. There are a couple of exceptions to this: when a foreground element moves quickly through the scene and is highly motion-blurred, or when a midground or background element will be used in another shot in which it appears closer to camera.