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This has been a nightmare every time I try to compose multi pass with alpha using octane, but learning from experience can help you a long way.

try to use exr (octane) as much as you can, (half float if you dont do heavy grading) use proexr plugin in after effects is free and helps you extract all channels in one go https://www.fnordware.com/ProEXR/ (also enables to use the magical cryptomatte if you have the latest versions of octane) UPDATE: version 2.5 and its automatic pass extraction is already included in aftereffects 2020 but dont update during a project in course.

try to not use the c4d save option in the renderer settings (dont untick it but leave the filename and path there empty), the one in the octane plugin works perfect and produce correct images (save you beauty and your passes there)

now a workaround

I suggest is, work as normal, in the octane setting dialog, uncheck keep environment, you can use premultiply in the camera imager if you like or not, import your sequence in after effects then interpret the footage and set the alpha channel to the option you used in the renderer (premultiplied if you ticked that in octane and straight if you dont), this should set the correct alpha interpretation for that border.

I think the correct term for the white border is fringe, if the interpretation doesn't work apply a matte choker on the culprit or any other defringing method.

I hope this helps you.

Best Regards.

Just saw this...

Since you are compositing in After Effects, have you tried adding the effect under Channel—Remove color matting? This will sometimes correct that problem.

Just a thought.

Thanks for those tips. None of them worked for my Linear workflow. I tried the matte chokers and remove color matting and nothing works. I found a workaround but it's not great. I chose the crypto color I wanted to matte, set my layer to Luma matte, and added a Gaussian Blur and Exposure and brighten the heck out of the matte. It helped get rid of the clamped matte, but it's certainly not the solution. Still hoping someone has this figured out.

Make sure that your environment color is black, otherwise it will contaminate the anti-aliased pixels (black does too, but unmult operators assume black, so it works better.)

@jayroth2020 Good advice. In my situation, I have 2 environments. 1st is my HDRI lighting set to primary. 2nd is set to just straight Black to hide the HDRI and set to Visible > Backplate. I will have to experiment with disabling both environments or just using one that is Black. If that's the case, that seems like a very important thing to know and should be in their Help manual.

To be clear, the Environment color I was referring to is found in Live Viewer > Octane Settings > Settings tab > Env. tab, Default environment color option. Any other environments added into your scene will typically supersede the default environment. Regardless, if you wish to have a clean alpha, best to have a black environment.

oh gotcha, yeah both are black so have no idea what the issue is. One thing I did notice was Depth of Field seemed to make it worse. But even with zero DoF it still shows up. I'll post a screenshot of the issue and my settings in a bit.