ImagePremultiply

This node premultiplies (mult) the rgb channels by the alphaChannel when an image is connected to the alpha input. Otherwise, a is read from the input leaving the alpha channel unchanged.

Also see ImageUnpremultiply.

Connection Type	Connection Name	Function
Input	input	The image sequence you want to premultiply. If no alpha input is connected, this input should contain an alpha channel to use for premultiplying the color channels.
	alpha	An optional input for attaching a separate channel that is used to premultiply the input image. If this input is connected, you can use alphaChannel to select the channel to use.
	out_mask	An optional image to use as a mask. By default, the premultiplication is limited to the non-black areas of the mask.

Control (UI)	Default Value	Function
[2D node controls]	N/A	Set the controls for the stereo view. For more information, refer to the Common 2D Nodes Widget Type in Common Parameter Widgets.
ImagePremultiply parameters continued
alphaChannel	A	If a separate alpha input is provided, choose which of its channels to use to premultiply. If no separate alpha input is provided, the alpha from input is used and this control is disabled.