Apple MA261Z/A User Manual - Page 808

Using Keying to Isolate Foreground Elements There are two different methods used for keying: chroma keying and luma keying. Chroma keying is a method of keying on a particular hue of color. Although any color can be keyed on, the colors most frequently used for chroma keying are blue and green. Specific hues of blue and green with particular levels of saturation have been developed that provide the best results, and different companies have created commercially available paints, fabrics, and papers that use these colors. The color you use-blue or green-depends largely on the color of your foreground subject. If you're trying to create a key around a blue car, you probably want to use green as your background. Another advantage of using green, when possible, is that video formats generally preserve more information in the green component of the signal, resulting in slightly better keys. Luma keying is based on a particular range of luminance. Black is usually used, but you can also key on white. While keying out a white or black background may be more convenient in certain circumstances, it may be harder to correctly isolate your foreground subject because of shadows and highlights, which may have black or white values close to the luma range you're keying out. Shooting Footage That Keys Well Regardless of the keying method you use, it's important to start out with clips that key well. The decisions you make before and during your shoot affect how well your footage keys. Make sure that you:  Choose a video format with a minimum amount of compression that is ideal for shooting and keying  Light the background screen and foreground subject properly Choosing an Appropriate Video Format Ideal video clips for keying can be captured from uncompressed or minimally compressed video formats, such as Beta SP or Digital Betacam footage digitized with an uncompressed video capture interface, or DVCPRO-50 footage captured digitally with no additional compression added. Compression discards color information from a clip and can add artifacts around high-contrast edges in the picture (such as the edges surrounding the image to be keyed). If you use compressed video to create keying effects, you'll frequently lose details around the edges of the keyed image, including hair, translucent cloth, reflections, and smoke. 808 Part IX Effects