I am trying to creat a green screen filter. I would like to have the option of passing it a background image, but if no image was passed, then to just make the background transparent. How would I go about checking if a background image was passed to the input?
By default an image texture with no associated file will output a completely black value. You can use this to drive the alpha value of your background, using a "Math" node set to "Greater Than 0" in order to have full transparency when there is no image texture and zero transparency when there is an associated image texture.