You can save by clicking on render window, but...unless g bufffer and alpha AND save to a file on disc is selected beforehand, you sitll can't save the rendered alpha on screen!
 jeesh.

So, youc an save an alpha of ALL elements rendered at any time,
but, to save an alpha of a specific thing you have GOT to have g buffer and save to a file done, before you render.

man that's a PIA!

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