Render regions with alpha has white outline

[timAugustine]

Hi all,

My scene has a grassy foreground and some hills in the background. I'm rendering the grass separately with the alpha turned on. When I overlay the grassy layer on top of the mountains, I'm getting this weird white outline. Is there a setting I'm not aware of right now?

Render with the white outline

Render

Here are the settings I'm using:

Settings

Thanks in advance for any response!

[Andreas_Resch]

That seems to me an Premultiplied Alpha problem. I use an Alpha Convert node in the Compositor to fix that.

[timAugustine]

Thank you! I will try this tomorrow

[timAugustine]

Hi,

Is this something from a version of blender without Octane? When I try to bring up the covert alpha node I keep getting errors with the word Octane all over it. Even after I select Cycles as the renderer. Is there a way to do this within octane settings?

[Andreas_Resch]

Works in the custom build 28.9 Prime version.

[timAugustine]

I must have done something wrong. It works for me now. And it also fixed the issue for me. Thanks again!

[timAugustine]

Hi, I have another question about this! Is it possible to set the output images to be premultipled directly in Octane? Just curious!

Thanks

[skientia]

Hi, I have another question about this! Is it possible to set the output images to be premultipled directly in Octane? Just curious!

Thanks

Absoutely. Associated-encoding (the proper term) should be available somewhere, perhaps in the export tab, is it?
In Standalone it is located in all export tabs, not in the kernel settings.

[timAugustine]

I didn't find it under blender export, but I did find it under the octane export subsection. It was available only for exr export options (not for png)

[skientia]

PNG doesn't support associated "alpha". TIFF does, hence the strongly recommended and viable alternative to EXR when not suitable in certain situations where the latter (EXR) isn't supported, available or viable.
More information here.