Notes
Inputs
A: Asset in front
B: Asset in back
Operation/Modes
TIP: Once an operation has been selected from drop-down menu, using the up/down keys can cycle through the list to visualize each of their impact in the Viewer.
Math operation key
A: A input image
B: B input image
a: Alpha channel of the A input
b: Alpha channel of the B input
1: Constant of White
Visual Reference
Guide: https://www.youtube.com/watch?v=QwFPd_nF2uM&ab_channel=SenecaVFX
Merge Operations
Open topic with navigation When layering images with the Merge node, you need to select a compositing algorithm that determines how the pixel values from one input are calculated with the pixel values from the other to create the new pixel values that are output as the merged image.
learn.foundry.com
Over
A + B * (1 - a)
Merge using the alpha channel
Plus
Merges without looking at alpha channel
Brings in more highlights than Screen mode
Max
Setting to be used for merging Rotos to verify that alpha values do not go over 1.0.