Pixaflux
Mix node
Left Toolbar > 
Compose Nodes > 
Mix
The Mix node mixes all input images into a single output image.
inputs
Img 1
Img 2
Img 3
output mix average
output comp over
The Mix Node receives multiple images as input. When an image is connected to the empty socket, a new empty socket is added to the bottom of the list.
Procedure
To Mix multiple images:
On the left toolbar choose Compose Nodes > Mix.
> Drag the Mix icon into the Node Graph.
Connect the first input image to the Img input socket.
Connect all other images into the new Img input socets.
Set the Mix Method attribute.
Inputs
Img
One or more images.
Outputs
Img
The composed Image.
Attributes Panel
Method
Sets the mixing method use by the Mix node:
* Average: Each input image contributes the same amount to the output image.
* comp_over: The input images are compose one on top of the other with an Over blending method.
* addition: All input images are added together.