While working in Nuke, there are a few Nodes that you will be using repeatedly -one of the most common nodes being the merge node.
The merge node is used to layer images, so when compositing, this will be used repeatedly to modify the final outcome.
The merge node itself has two inputs: A and B. The B input should be connected to the background and anything on A will be layered on top of it.
The merge node has different merge operations which do different things. Some of the more commonly used ones are:
Over: Layers image A over B according to the alpha of image A, Used when layering a foreground element over the background.
Plus: The sum of A and B. This may result in pixel values higher than 1.
Multiply: A*B, Used to composite darker values from A with the image of B – dark gray smoke shot against a white background, for example.