112 VIDEO EDITOROne of the most common and useful overlay key types is the Blue Screen. Thefollowing procedure is a simple example of how to use a Blue Screen effect inVideo Editor. The primary factor in determining the success of a Blue Screen effectis in the actual filming and not in post production. If you have a good solid coloredscreen and uniform lighting, creating a Blue Screen effect is easy. If you do nothave either of these things, it is nearly impossible to get a clean overlay.To use a blue screen effect in an overlay:1. Insert the video clip that you want to be in the background into the Video1track.2. Insert a video clip shot with a blue screen in the background into Video2 track.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 clip box.6. The underlying clip will appear behind the overlay clip, in the blue screen areasand in the Overlay preview box. (If the blue screen is not totally replaced bythe 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 involves usingAlpha Channels. This technical term simply means an area or areas of an imagethat have been selected to be transparent in an image or video. In many ways, anAlpha Channel is the same as a color key and could be based on a color in a clip.Many image editing programs allow you to create and save Alpha Channels and itis the technique employed by CG Infinity to control transparency. TGA and TIFimage files both support Alpha Channel transparency and make excellent overlays.Because you can control the level of transparency with 256 levels of gray, AlphaChannels are also used in Video Paint anytime you use a matte, as well as intransition effects such as wipes and dissolves. Most overlays in MediaStudio Protake advantage of Alpha Channel. Because this is such a powerful and effectivetechnique, it is always recommended that you insert your CG Infinity project filesdirectly into Video Editor (instead of rendering first and then inserting the resultingAVI file) and use the Alpha Channel for the overlay.
