Opacity mask always blacks

Hi,

Since I switch on Unreal Engine 4.15, the opacity pass which I’m use to access from post process material via scene texture are always black.
It is also black in the buffer vizualisation.

I really appreciate any help you can provide.

Opacity pass used by SubSurface materials only (always been).
For Translucent materials use SeparateTranslucency.

4.14

4.15

Ok I was using it in the wrong way ^^
Thanks for the quick answer !

Can you say how you are supposed to use it?
I would like to know!

Thanks