How to render with clean alpha for compositing?

[MoGrafik]

Hey everyone,
Apologies if this has come up previously.
I'm rendering a shot where I'll be rendering with alpha and replacing the sky with a different on in After Effects. I've never been able to achieve a fully clean alpha out of the render with Octane, and I've always got by with matte chokers / blurs etc. However on this project that doesn't seem to be enough (quite a detailed skyline) - no matter what I do, there always seems to be white grain and jagged edges around the alpha edge.
I was wondering what the process is for you guys? Is it possible to do this with depth of field also?
I've tried using 'disable partial alpha' in the camera tag in the most recent release, but it doesn't seem to help, sadly.

Would really appreciate any input on this. Thanks

[jayroth]

I've had trouble with this in the past as well, but I have just cranked out a bunch of frames over the last week or so, and all of the alphas have been clean. I am using the most current build, and I am rendering with a linear response (camera tag, camera imager tab, with neutral response enabled and gamma before response enabled.) I'm using the path tracing kernel, with alpha channel enabled and keep environment disabled. That said, I have not rendered with motion blur active for these shots, so I don't know if that factors in your case or not. Regardless, the alphas have all been super clean... good luck!!

[MoGrafik]

Hey, thanks very much for getting back to me on this. I'll try rendering a few frames with these settings.
Having done some experiments I've got fairly clean alphas, but there's still an apparent halo / darkening on feathered parts of the alpha.
Thanks for your advice!

[MoGrafik]

Hey, Jayroth - thanks again for your help!
Going though your steps I think I figured out how to get this working, and what I was doing wrong. It seems like changing over to path tracing made a lot of difference - I was trying to get a quick render with GI diffuse, but I think it had problems with the alpha. Also, after effects automatically interpreted the alpha as pre-multiplied. Changing it to straight made the alpha clean!

[jayroth]

Glad to hear you have it sorted!

[ahclausen]

I had the same issue and read through your post here searching for a fix. I did what you guys talked about as well, but without any luck. For other that might come across this, I fixed it by checking OFF "Keep environment" under the kernels tab. Should be noted that I have only an HDRI Environment object as a light source and that I'm rendering out with premultiplied alpha (as in I have NOT checked the straight alpha option in C4D save render option) - but none the less, this fixed it for me.

[max1e]

Hello guys. probably i have same problem. If you have few free minutes look please at this topic. Thanks
viewtopic.php?f=30&t=65898