Rotobrush 2: render to .mov has a blackish outline

[Ed.it.son]

I'm new to AE and rotobrush but im trying to render out something a roto with rotobrush 2.0 with transparency. When I place the clip overtop a different background but it shows blackish outline when I view it. 

Any tips on how to fix this? 

I thought I could render out a feather but it seems that feather has a black outline?

thanks! 
ed.it.son

[James Landy]

Please post a pic of your comp, layer and timeline panels and I'll see if anything obvious stands out. Thanks.

[Ed.it.son]

Thanks!!!   

[James Landy]

This looks like the expected behavior based on the screen shot (i.e., I'm not seeing the black outline). Take another shot, but this time place a light color solid layer underneath the roto layer, so I can see the outline better.  Thanks.

[Ed.it.son]

The black outline happens after I render it and I put it into a different editing software.

let me try putting a solid background in AE 

[edohwin]

So I looked into it more and people are saying the blackoutline may because I rendered it premultiplied. So I looked into changing that setting, but when I go to intepret footage, the alpha channel is greyed out and it does not let me change the premultiplied settings. 

I want to go straight alpha instead. Any idea how i do this?

[VictoriaNece · Accepted solution]

This definitely sounds like what's going on. You need to export using a codec that supports straight alpha (not all of them do).