Apple Shake 4 - Manual (Page 436)

436 Chapter 15 Image Processing Basics
Nodes That Affect Premultiplication
The following nodes can change the premultiplication status of an image, although
anything that operates an image’s mask differently from the RGB channels changes the
effect.
Node Description
Color–ColorCorrect This color corrector has an option in its Misc tab to specify when
the incoming image is premultiplied. If it is, set the premultiplied
parameter to yes. MDiv and MMult are internally inserted into the
computation.
Color–Reorder This node allows you to switch a channel into the alpha channel,
and may disrupt your premultiplication status. However, it isn’t
often used on images in the normal chain of color correct, position,
and composite operations—but it is more of a utility node.
Filter–DilateErode This node is typically used to add to or chew the matte by a few
pixels. Set your channels to just “a.” Because you then modify the
matte separately from the RGB, you make the image
unpremultiplied.
Key–LumaKey, DepthKey,
DepthSlice These keyers all have the option to immediately set your image to
a premultiplied state.
Key–KeyLight This keyer can output either a premultiplied or an unpremultiplied
version, or just the alpha channel with the RGB untouched.
Key–Primatte This keyer can output either just the alpha or a premultiplied
version. Setting it to unpremultiplied requires an external MDiv.
Layer–AddMix This modified Over node allows you to manually break the
premultiplied relationship by tweaking curves that specify how the
matte multiplies the foreground and background images. It was
inherited from another package, where it was used extensively
because this particular system had no control over
premultiplication.
Layer–SwitchMatte This node copies a channel from the second image to the alpha
channel of the first image. You have the option to immediately
premultiply the image (enabled by default).
Other–DropShadow This node should be applied to premultiplied images only.
Non-Black Premultiplied Images
Some packages output images that are considered premultiplied, but in which the
background is not black. This is mathematically bizarre. To work with these images,
try the AEPremult macro included Chapter 32, “
The Cookbook ,” on page 989.