If you are looking for a completely free alternative to the Photoshop ecosystem, GIMP has a "Foreground Select Tool" that mirrors much of Knockout's original logic. Safety Warning
Please note that this is just a draft feature and you may need to modify it according to your specific requirements. Additionally, make sure to check the plugin's compatibility and system requirements before downloading and installing it. knockout photoshop plugin free download full
If you want to expand Photoshop's capabilities safely, check these verified free options: If you are looking for a completely free