I’ve had the same problem. After a search on Google, I found that a combination of Minimax and Mosaic should do the trick.
So tried some things and came up with a combination of Minimax first, then Mosaic and then Levels (channel set to Alpha). You probably need to tweak the parameters to fit your layer emitter.
Oh and apply the effects to the precomped layer, not the precomp itself!
Stef Prein
director/motion designer at StudioFets.
Utrecht, The Netherlands
https://www.studiofets.com