Apple Motion 4 - Manual (Page 331)

Add
Add emphasizes the whites in each overlapping image and lightens all other overlapping
colors. The color values in every overlapping pixel are added together. The result is that
all overlapping midrange color values are lightened. Blacks from either image are
transparent, while whites in either image are preserved.
The order of two objects affected by the Add blend mode does not matter.
Suggested uses: The Add blend mode is useful for using one image to selectively texturize
another, based on its lighter areas such as highlights. You can also use Lighten, Screen,
Color Dodge, and Linear Dodge to create variations of this effect.
Lighten
Lighten emphasizes the lightest parts of each overlapping image. Every pixel in each
image is compared, and the lightest pixel from either image is preserved, so that the final
image consists of a dithered combination of the lightest pixels from each image. Whites
in both images show through in the resulting image.
331 Chapter 7 Basic Compositing