Alpha expands

[hdace]

I'm attaching two screenshots of a render of a room with the floor missing to be filled with live action. Cleary, a good alpha channel would be helpful. In many recent renders I've found that the alpha channel has expanded, encroached, over the CGI wall that is supposed to meet the live action floor very precisely. Octane is rendering the wall correctly and it displays correctly (well, almost --there's often a weird jagged edge), but the alpha channel is creating a gap between the wall and the floor.

If you look carefully at the screenshots you'll see the bottom metal piece is narrower in height in the alpha render.

Please explain what is causing this and how to re-do these renders without the alpha expansion, thanks --Hal

[hdace]

Upon further inspection I've found that it's not the alpha that is expanding, it's the geometry itself. Wherever geometry is adjacent to alpha, Octane seems to be extending the geometry out into the alpha area. But the alpha channel is actually rendering correctly.

I did another test and found that turning on "Keep Environment" in the Path Tracing node appears to solve this problem. It's a little annoying because the documentation does not explain what this button is for. I would still appreciate an explanation of all this, thanks --Hal

PS - my renders are getting an ugly jagged edge in After Effects because of this phenomenon

[juanjgon]

This problem can be related to the "Premultiplied Alpha" parameter available in the imager node.

Thanks,
-Juanjo

[hdace]

Okay yes this seems to solve the problem. Thanks!