CHAPTER 4: ADDING EFFECTS AND TITLES 113To use an Alpha Channel in an overlay:Using Gamma correctionGamma refers to the gray values of a color which contribute to the levels ofhighlight and shading that appear in an image. The Gamma graph is a visual guidewhich allows you to change these values and thus affect changes in the highlightsand shadows, as well as change the midrange or midtone values. If you move thegamma value higher up the graph, you darken the clip whereas moving it lowerlightens it. You can move the thumbtacks that appear along both the X and Y axisto give you better control over which highlights and shadows are affected. The Yaxis thumbtacks represent the possible changes you can make to the grays of theoriginal clip. The X axis thumbtacks work similarly except they restrict you tochanging only the midtone grays of the original clip, leaving the areas with stronghighlights and shadows untouched.1. Background clip2. Alpha channel3. Overlay clip4. Result1. Place a video clip into the Video1 track.2. Place a video clip which was created with analpha Channel (for example, a CG Infinity file)into the Video2 track.3. Right-click on the clip in Video2 and selectOverlay Options to open up the OverlayOptions dialog box.4. In the Type drop-down box, select AlphaChannel.At this point, the areas specified in the channel areimmediately keyed out. If the preview image doesnot change, then the overlay clip does not containan Alpha Channel. Click OK.
