RGB to Alpha for scene render

I am trying to make thumbnails for a widget. but I am stuck on a problem of creating a alpha when using scene render. The normal alpha on the scene render seems to be inverted and doesn’t allow post processing?

so I made a post process material to try and make a alpha from the rgb channels.


this is the best I could come up with is this above. which kind of works . but the multiply causes a lot of bloom which gives it a black outline.

I am looking to turn the rgb channel into a purely 0/1 , black/white image that matches with the border of the rgb channel. Does someone know how to do this?