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Keymix

Keymix layers two images together using a specified Roto shape or image as a mask. It copies input A to input B where the mask is non-black. This follows the formula Aa + B(1-a). It is similar to the Over node, but for use with unpremultiplied images. The resulting image contains no alpha channel.

Inputs and Controls

Connection Type	Connection Name	Function
Input	A	The image sequence to merge with input B. By default, this input is copied to input B only where the mask is non-black.
	B	The image sequence to merge with input A.
	mask	The image to use as a mask. By default, the effect of input A is limited to the non-black areas of the mask.

Control (UI)	Knob (Scripting)	Default Value	Function
Keymix Tab
channels	channels	all	The channels to copy from input A. Any other channels are copied unchanged from input B.
mask channel	N/A	enabled	Enables the associated mask channel to the right. Disabling this checkbox is the same as setting the channel to none.
mask channel	maskChannel	rgba.alpha	The channel to use as a mask. By default, the effect of input A is limited to the non-black areas of this channel.
invert	invertMask	disabled	Inverts the use of the mask channel so that the effect of input A is limited to the non-white areas of the mask.
mix	mix	1	Dissolves between the original B input at 0 and the full Keymix effect at 1.
Set BBox to	bbox	union	How to output the bounding box: • union - combine the two bounding boxes. • B side - use the bounding box from input B. Any data from the A input that’s outside this box is clipped. • A side - use the bounding box from input A. Any data from the B input that’s outside this box is clipped.