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.

See also Premult.

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.

alpha

rgba.alpha

Divide the above channels by this channel (usually alpha).

invert

invert

disabled

Inverts the use of the alpha channel.