use the key mix which can allows you to blend two input images together based on a third matte input.

1. Import Footage: Import the footage of the person walking in front of the green screen into Nuke. This will be your Foreground (A).

2. Import Background: Import the background image or footage that you want to replace the green screen with. This will be your Background (B).

3. Create a Matte: Use a Keyer node (like Primatte or Keylight) to key out the green screen and generate a matte that separates the person from the background. This matte will be your Matte

F_ReGrain

I need to match the grain pattern of CGI elements or other composited elements with the grain of the original footage.

Input: F_ReGrain takes several inputs:

• Source Image: The image or footage that you want to regrain.
• Reference Image: The reference image or footage from which you want to sample the grain.
• Grain Map: A grayscale map that controls the strength of the regraining effect in different areas of the image. White areas apply more grain, while black areas apply less grain.

when I need to add some visual effects, such as explosions, fire, smoke, or particle effects. Visual effects elements are often created digitally and may lack the natural grain present in live-action footage. F_ReGrain can be used to apply a matching grain pattern to the visual effects elements, ensuring they blend seamlessly with the original footage.