Channel Key Node

The Channel Key node determines background objects from foreground objects by the difference in the selected channel’s levels.

For example in the YUV Color Model, this node is useful when compositing stock footage of explosions (very bright) which are normally shot against a solid, dark background.

Inputs

Image

Standard color input.

Properties

Color Space

This button selects what color model the channels will represent.

RGB, HSV, YUV, YCbCr

Key Channel

This button selects the channel, defined by the Color Space, to use to determine the matte.

Algorithm

Method to calculate the difference between levels.

• Max:

  Limit by the maximum of the other two channels other than the Key Channel.

  Single:

  Limit by the maximum of the selected Limiting Channel.

  • Limiting Channel

    The channel to use when computing the maximum, the options are defined by the Color Space.

High

Determines the lowest values that are considered foreground. (Which is supposed to be – relatively – high values: from this value to 1.0.)

Low

Determines the highest values that are considered to be background objects. (Which is supposed to be – relatively – low values: from 0.0 to this value.)

Tip

It is possible to have a separation between the High and Low values to allow for a gradient of transparency between foreground and background objects.

Outputs

Image

Image with an alpha channel adjusted for the keyed selection.

Matte

A black-and-white alpha mask of the key.