While I am happy with the current filter mask, I believe there could be room for improvement. Here are some ideas of options -

1) Invert Alpha
2) Ignore Luminosity and/or hue
3) Ignore Lightness/Value
4) Apply to saturation value of and threshold. Anything below a value and threshold will be opaque
4) Binary Alpha where alpha value of 0 of black is fully transparent while alpha value of 1 is fully opaque
5) Feathering after binary alpha option

What are your thoughts?