don’t know if it’s me misunderstanding how it’s supposed to work, but “Mask image by brightness” doesn’t behave as I’d expect. My suspect is that “blue” and “green” parameters into “Threshold types” are inverted. Check the following example:
Mask Image by Brightness is a Luma Key (which, in Red/Green/Blue mode, evaluates those color channels individually) and not a Chroma Key (which evaluates distance from a specified color); it’s not great for Chroma Keying. Our community members have had a previous discussion on Chroma Keying.
To elaborate, Mask Image by Brightness measures the brightness based on the Threshold Type you select — if you select Blue, it only measures the Blue channel; parts of the image that have no blue (such as the green background) are made transparent, and parts of the image that do have blue (such as the dog’s gray collar) are left opaque. If you select Green, then parts that have green are left opaque.