Edges
Edges in compositing go unnoticed if they’re done well, but can destroy a shot if not handled properly. The two most glaring problems a viewer normally spots in a poorly composited shot are lighting and edge problems. When an element’s lighting does not match the rest of the scene, the viewer’s eye is drawn to it as odd rather than a visual point of interest. Poor lighting can also accentuate problems with the edge of an element, making a tough problem even worse. If elements are created on the computer graphics side of things, there is no excuse for mismatched lighting. The CG lights can be set up to match any environment and made consistent from element to rendered element. The problem becomes more difficult when bluescreen or greenscreen elements do not match the lighting of the scenes with which they are to be combined. A bluescreen character with high fill levels and a key to the left can be extremely difficult to integrate into a live-action scene with the key on the right and low fill levels. For this reason, it is important in the planning stages of shots, particularly for bluescreen or greenscreen elements, to create a solid lighting plan and stick with it. Changes can and will be made down the line, but major shifts in lighting emphasis, such as switching the key light to the opposite side, can make for many long days in the compositor’s world.

The edges in bluescreen and greenscreen extractions can take a considerable amount of work. Element edges in compositing definitely fall into the final 10 to 20% category that can be the most challenging part of a shot. Fortunately, there are several powerful software options that provide excellent tools for simplifying this task (Ultimatte from the Ultimatte Corp. and Primatte from Photron Inc. are two examples). The ins and outs of extractions can be learned from the software manuals, from a mentor, or from trial and error. There’s no substitute for experience in this area, so practicing with bluescreen and greenscreen extractions is important. One good type of practice subject is a bluescreen or greenscreen element of a person with long hair, which can be extremely challenging due to the fine detail and transparency of the hair. Another practice scenario involves a blue-screen element shot with excessive blue light spilling across the subject. This blue spill needs to be removed or suppressed and replaced with colors matching both the subject and the scene. Before, during, and after each operation in the extraction process, it is necessary to pay careful attention to the edges of the element. Check for deterioration, discontinuities, loss of edge detail, excessive fuzziness or crispness, and ensure that the matte resulting from the extraction, accurately represents the shape of the object being composited.

Grain and Finishing Touches
After combining the elements and sorting out the layering processes, the final touches can be applied to the composite. These final touches vary depending upon the types of input and output the shot requires. They include adding grain to match film footage, carefully checking each color channel for details and inconsistencies, evaluating the black levels to ensure consistency between CG elements and film or video elements, and adding in additional renders or elements for small details.

[Figure 40] Noise field to simulate film grain, shown in red, green, blue and all color channels.

For film work, in which computer graphics elements are added to film footage, it is necessary to match the grain of the film. Grain is the noise variation due to the uneven distribution of the light-sensitive crystals, which capture the image. Each color has its own layer of crystals on the film, so each color has different grain properties. Additionally, each film type has its own distinctive type of grain due to its chemical composition, and the grain amounts can vary greatly from one roll of film to the next, based on the manufacturing run. The best way to view the grain is by zooming very close on the image and checking each individual color channel. In most cases, the blue channel has the densest grain, followed by green, with the least amount usually in the red channel. The grain appears as a subtle change to the image, but is more noticeable in moving footage. Grain is simulated in the compositing stage with a noise function. This noise function can be adjusted in terms of scale and intensity in each color channel to closely resemble the look of the background film image (see Figure 40). The scale and intensity of the grain in the example shown here are magnified to show the element more clearly. There is also grain in digital video footage that sometimes goes unnoticed. The light-sensitive chip in a video camera is also susceptible to manufacturing imperfections and electromagnetic interference, which can add noise to the image recorded digitally (see Figure 41). The digital footage of the beach and pier clearly show the grain effect in the sky of the blue channel. If grain is not added, computer graphics elements will stand out as too crisp and clean. Grain is one of the many elements utilized by the compositor to combat the perfect CG look.