In this case, the image was a good candidate for this technique because the area you
needed to key out was a large bright area with little variation in color, and it was significantly
different in color than the rest of the picture. For the sky, this image was good to use
because there is a clear distinction between the buildings and the sky. There is little haze
and a strong variation in brightness and color.
Foreground Keying
Not all images are easy to use for keying and compositing. For images that are more complex,
or for those times when you want more control, LightWave offers you the Foreground Alpha.
An alpha is a grayscale image that is used to tell a program where certain things should
happen. In the case of a surface texture, an alpha image tells the surface where to apply a
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Chapter 16 Compositing and Postprocessing Techniques 669
texture map. It could, for example, tell a surface where to be transparent and where to be
opaque. And in the case of a foreground image, the alpha image determines where the
image appears and where it doesn??™t.
Exercise 16.4 Using Foreground Alpha
This exercise uses a feature available in LightWave that enables you to key out portions of
an image for foreground compositing.
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