All images from Inspired 3D Lighting and Compositing by David Parrish, series edited by Kyle Clark and Michael Ford. Reprinted with permission.

The following is a continuation of the tutorial on Compositing Techniques and Methods from the new Inspired series published by Premier Press. Comprised of four titles and edited by Kyle Clark and Michael Ford, these books are designed to provide animators and curious moviegoers with tips and tricks from Hollywood veterans. The following is excerpted from Lighting and Compositing.

Edges and Blending
One of the primary concerns when layering images together is the edge quality of the elements. When placing an element over a background, the place for problems to be introduced and noticed is most likely the outer edge of the element being composited. As stated in the section describing the over function, a pre-multiplied image is necessary for many basic compositing functions. The process of multiplying the color channels by the alpha channel is necessary for an operation such as the over, but is not desirable for operations such as color corrections. Pre-multiplication changes the color channels in order to provide the proper blending levels for compositing functions. By changing those values, however, a different set of color channels is produced, affecting any future adjustments to color. If the over operator is the final step in the composite, the pre-multiplied image is what is needed. If color corrections are required on the original element, though, they should be performed before the pre-multiplication. If performed afterward, the edges of the image will be adjusted incorrectly. For instance, if an image has a portion of its matte edge with an alpha value of 0.5, during the pre-multiply process each color channel will be multiplied by 0.5. Each red, green and blue value will become one-half of its original value in those edge areas in which the alpha value is 0.5. A color operation, such as a brightness call, performed after that pre-multiplication adjusts a darker value of each color channel than in the original image. Brightness values of two will double the red, green and blue values for the majority of the image. In those edge areas in which the pre-multiplication has reduced the values of the color channels, however, the resulting colors will be darker than if the original color values had been brightened. If that image is then used in an over function, a dark edge will appear around the element. If the alpha edge is fairly abrupt, meaning the transition from values of zero to one happens in a very small amount of space, the artifacts introduced by pre-multiplying before a color correction may very well be imperceptible. The problem is most pronounced with soft-edged matte channels, in which the transition from values of zero to one happens over a larger distance. In any case, it is good practice to consistently make color corrections on un-pre-multiplied images. Even the smallest of lines around the edge of an element can balloon into larger problems by the end of a complex comp script.

The Earth mentioned in Chapter 6, “If You Can See It, You Can Light It,” presents a good example of layering several elements together. The elements combined in creating the Earth are the planet itself, the clouds and a smoke layer to be used on the outer edge. It is important to note with any composite script and any element, the image is not confined to only the images provided. A composite script may start with only three layers, but each of those three layers may be manipulated to create several additional layers. Each layer presents an opportunity to add realism to a scene. With a strong understanding of the desired look gained from studying reference images and footage, additional layers can be created and introduced into the comp script in ways to enhance the final output.

Base Layers
The first layer to be utilized for compositing the Earth is a render of a sphere with the continents and ocean textures applied (see Figure 36). The two texture layers of land and water are combined in the rendering stage. The clouds are rendered separately as their own layer, and are simply a texture map applied to the same sphere as the Earth element (see Figure 37). Step number one is placing a shadow of the soon-to-be added cloud layer on the earth. This is accomplished by taking the cloud layer and using it as a matte for the Earth. To do this, an alpha channel must be created which will outline the shapes of the clouds and provide transparency values for the clouds that are less dense. The alpha channel resulting from the render of the cloud layer is a solid outline of the entire Earth sphere.