VIDEO EDITOR: CHAPTER THREE 193To use a blue screen effect in an overlay:1. Insert the video clip that you want to be in the background into the Va orthe Vb track.2. Insert a video clip shot with a blue screen in the background into anoverlay track, V1 for example.3. Right-click on the overlay clip (the blue screen clip) and select OverlayOptions. The Overlay Options dialog box opens up.4. Click on the Type list and select Blue Screen.5. Click the Eyedropper on the blue portion of the clip in the Overlay clipbox.6. The underlying clip will appear behind the overlay clip, in the blue screenareas and in the Overlay preview box. (If the blue screen is not totallyreplaced by the underlying clip, increase the Similarity value.)7. Click OK and preview the clip.Understanding Alpha ChannelsOne of the most powerful methods of creating transparent overlays involvesusing Alpha Channels. This technical term simply means an area or areasof an image that have been selected to be transparent in an image or video.In many ways, an Alpha Channel is the same as a color key and could bebased on a color in a clip. Many image editing programs allow you to createand save Alpha Channels and it is the technique employed by CG Infinity tocontrol transparency. TGA and TIF image files both support Alpha Channeltransparency and make excellent overlays. Because you can control the levelof transparency with 256 levels of gray, Alpha Channels are also used inVideo Paint anytime you use a matte, as well as in transition effects suchas wipes and dissolves. Most overlays in MediaStudio Pro take advantage ofAlpha Channel. Because this is such a powerful and effective technique, it isalways recommended that you insert your CG Infinity project files directlyinto Video Editor (instead of rendering first and then inserting the resultingAVI file) and use the Alpha Channel for the overlay.
