Apple Shake 4 Tutorials - Manual (Page 103)

Chapter 3 Depth Compositing 103
To color correct the balloons image:
1 In the Node View, select balloons, then insert a Color–Add node.
2 In the Add1 parameters, click the Color control to open the Color Picker.
3 In the Viewer, scrub over the dark orange horizon in the city_bg image to add an
orange tint to the balloons image.
Everything from the balloons to the background turns bright orange—and check out
the artifacts around the edges! This is obviously not good.
The entire image is brightened because you’ve color corrected a premultiplied image.
The Color–Add node adds whatever color you choose to the pixel values of all channels,
including the alpha. This returns incorrect pixel values—and the corresponding
transparency values—when multiplied with the alpha channel.
The result is a compromised alpha which generates a tinted matte, rather than the full
transparency we need for the balloons image.