Issue with Alpha and Blackbody Emission

[alecmaassen]

Hi,

Whenever I have an object with a blackbody emission over it that crosses in front of another geometry, the alpha gets extremely screwed up, which makes it harder for correct Z passes, etc.

Take a look at this photo below. I've got a beam of light ( just a long capsule with a blackbody emission over it) that crosses over regular geometry as well as the infinite background (alpha). The thickness of the light actually changes, which screws up my Z passes and is very very noticeable in Post. The light beam itself isn't touching any other geometry at all. There also appears to be a 1 pixel black line around the light as well.

This is rendered as a 16-bit EXR with :Alpha Channel" checked, "Keep Environment" unchecked. I've tried it with "Keep Environment" checked and I get the same result.

Any feedback would be greatly appreciated! Thanks!

[bepeg4d]

Hi alecmaassen,
I would try with a white visible background, with only Backplate option active, in order to not change the lighting and reflections, and the Keep Environment option active in Kernel panel.
ciao beppe

[alecmaassen]

Could you elaborate on the backplate option active? I’ve never heard of this setting.

[bepeg4d]

Hi,
sorry for the confusion, I was meaning Visible mode of Environment node:
Environment Maps - HDRI Environments
ciao beppe

[alecmaassen]

Okay so I played around with the visible environment and unfortunately that doesn't work because the render passes are still a different thickness.

Correct me if I'm wrong but this seems like a bug within Octanes ability to put Blackbody emissions on geometry without the geometry increasing in size. Refer to the images below where one of the images is with the blackbody emission at a power of 12 and the other at a power of 0 (0.001). The actual thickness is increased, and the render passes are utilizing the non-increased geometry. So even if I don't use an alpha channel and use a renderlayermask for alpha, it still wont work. Visible environments dont work either.

I feel like I've tried every trick/workaround in the book and just cant get it right. I've even tried changing materials like blackbody vs texture emission, guassian spectrum, etc.

The only workaround I can think of, which I dont want to do is to actually fake the emission by putting regular arealights on the spline, turn off blackbody emission, object ID the lights out and make them white in post and then use the invisible lights to actually light the scene.

[alecmaassen]

anyone?

[bepeg4d]

Hi alecmaassen,
what about the Disable partial alpha option in the Camera Imager panel?
Have you tried to reduce the power of the emission to reduce the the difference in exposition?
ciao beppe