[Figure 41] Digital video frame on the top and the blue channel isolated on the bottom to show the grain.

[Figure 42] Separate diffuse and specular renders of a beach ball element.

The following steps demonstrate techniques used in placing a simple computer graphics beach ball into the scene of Figure 41. The beach ball is rendered in two separate passes, one for the diffuse light contributions and a second for the specular (see Figure 42). By rendering these elements separately and then compositing them with the add operator, the specular highlight can be quickly and easily increased or decreased.

[Figure 43] Background darkened through mattes created with the shadow and drop shadow render passes.

Shadows
Two additional elements for this scene are shadows. When combining computer graphics elements with live-action footage, it is often necessary to render separate shadow passes. The shadows to be rendered must be cast upon a surface in the CG scene. This surface is a ground plane placed to match the ground plane of the live-action scene. With rudimentary pushing and pulling of points on the ground plane model, a rough or undulating surface can be simulated. The background image can be used as a guide in the 3D software package. Keep in mind that the ground surface need not match precisely, because the shadows are not the focus of most shots. Providing enough variation in the ground surface to break up the general shape of the shadow will suffice. The rendered shadow pass can then be used as a mask channel for darkening the background image (see Figure 43). Notice also the darker area on the inside left of the shadow. This is the second shadow render pass, referred to as the contact shadow. This pass represents the darker area closest to where the element comes into contact with the ground. In most scenarios, the ambient light in the scene lightens the shadow as it gets farther from the object casting it. This depends on a variety of external conditions, such as the amount of bounce light, additional light sources, and the intensity of the key light, but as a general rule, the contact shadow will help integrate a CG element more cleanly. The contact shadow pass is often blurred more than the full shadow pass, and is used to make that area slightly darker than the original shadow.