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Premult

By default, Premult multiplies the input’s rgb channels by its alpha (in other words, premultiplies the input image). You may need this node when:

• Merging unpremultiplied images. Because Merge nodes in Nuke expect premultiplied images, you should use this node before any Merge operations if your input images are unpremultiplied. This avoids unwanted artifacts, such as fringing around masked objects.

• Color correcting premultiplied images. When you color correct a premultiplied image, you should first connect an Unpremult node to the image to turn the image into an unpremultiplied one. Then, perform the color correction. Finally, add a Premult node to return the image to its original premultiplied state for Merge operations.

Typically, most 3D rendered images are premultiplied. As a rule of thumb, if the background is black or even just very dark, the image may be premultiplied.

Inputs and Controls

Connection Type	Connection Name	Function
Input	unnamed	The unpremultiplied image sequence you want to premultiply.

Control (UI)	Knob (Scripting)	Default Value	Function
Premult Tab
multiply	channels	rgb	The channels to multiply (usually rgb). You can use the checkboxes on the right to select individual channels.
by	N/A	enabled	Enables the associated channel to the right. Disabling this checkbox is the same as setting the channel to none.
by	alpha	rgba.alpha	Multiply the above channels by this channel (usually alpha).
invert	invert	disabled	Inverts the use of the alpha channel.