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Unpremult

By default, this node divides the input’s rgb channels by its alpha. If your input images are premultiplied, this can make color corrections more accurate.

When you color correct premultiplied images, you should first connect an Unpremult node to the image to turn the image into an unpremultiplied one. Then, apply the color correction. Finally, add a Premult node to return the image to its original premultiplied state - this last step is important if you want to merge the image over another one, as Merge nodes in Nuke expect premultiplied images.

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 premultiplied image sequence to receive the unpremultiply.

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