Apple Motion 3 - Manual (Page 309)

Chapter 3 Basic Compositing 309
Behind
The Behind blend mode forces the object to appear behind all other objects and
groups, regardless of its position in the Layers tab and Timeline.
If multiple objects or groups are set to Behind, they appear behind all other groups
that are not set to Behind, in the order in which they appear in the Layers tab.
Alpha Add
The Alpha Add blend mode works similarly to the Add blend mode, but instead of
adding the color channels of overlapping objects, it adds their alpha channels together.
Try using this blend mode instead of Motion’s default method of alpha channel
compositing for a different treatment of overlapping areas of translucency.
Premultiplied Mix
The Premultiplied Mix blend mode is intended for compositing images that are
premultiplied, such as those generated by 3D modeling and rendering software. (A
premultiplied image’s red, green, and blue channels are multiplied by the image’s alpha
channel.) Premultiplied Mix is useful for images with translucent lighting effects such as
lens flares, volumetric lighting, fog and haze effects, and so on.
The Premultiplied Mix blend mode performs an unpremultiply composite—the
foreground image is assumed to be premultiplied. Artifacts may appear as a result of
unpremultiplying pixels whose RGB and alpha values are very small (resulting in pixels
with values of 255). In some cases, the hardware performs bilinear filtering and then
the blend mode unpremultiplies the alpha.
Drop Shadows
A drop shadow, by default, is a dark, translucent, offset shape that falls behind an
object, as if a light was shining on the object. Drop shadows are the same size as the
object to which they’re applied, although blurring a drop shadow may enlarge it
somewhat.