Professional Digital Compositing: Essential Tools and Techniques

Chapter 3. Interpreting Alpha and Creating Mattes

"The cost-benefit of properly setting up a shoot with an evenly lit greenscreen can be poor when compared to having the compositing team fix the shot."

——Darren Poe, digital effects supervisor, Digital Domain, Venice, California

Alpha channels, and their associated matte information, are cornerstones of compositing. Alpha allows the combination of two or more images, whether the images are part of a layer within After Effects or a node output within Nuke. Although 3D programs generate alpha as part of the rendering process, motion picture and video footage does not contain alpha information. As such, it's necessary to create alpha values. One of the most common techniques for creating alpha is to "key out" blue or green from bluescreen or greenscreen footage. Both After Effects and Nuke offer tools designed specifically for this process. The tutorials at the end of this chapter will give you the opportunity to use these tools by removing a greenscreen from a digital video shoot. In addition, you'll have a chance to revisit the tutorials from Chapter 2 in order to refine alpha edge quality and the overall color balance of the composite.

Alpha Support and Premultiplication

Alpha is a channel of a digital image that is designed to store matte ...