This is totally cool. I was playing around in Photoshop the other night and needed to duplicate a mask and then invert it. I remember that you can copy a mask using two fingers and the mouse button so that means it’s some combination of the Control, Alt and Shift keys – that’s all you really need to remember – the rest is experimentation.
Well, I did the two finger drag using Shift and Alt and this must be a keystroke combination I haven’t used before because lo and behold! it not only copies the mask but it inverts it too!
So, to copy a mask, drag it using Control + Alt (Command + Option on the Mac). To copy and invert the mask, Shift + Alt (Shift + Option on the Mac).