Plane with Light material casts shadow on rough metal

[Beck]

Simple testscene:
Lightmaterial (no emitting light) with a picture on a plane in front of a plane, sun from behind.

-If the plane is nonmetal all is as expected, no shadow from the lightplane.
-If the plane is metal and the roughness is increased, the lightplane casts a shadow on the metalplane.

i think this should not happen? Even with a corona compositing tag and "cast shadow" unticked it casts a shadow on the plane.

I noticed this in an archviz with a lightplane in front of the scene for the foregroundreflections on the glass/windows with metalframes.

(corona-10-c4d-daily-2023-01-23-mac)

[Beanzvision]

Hi there, I believe this is already a known issue. Out of curiosity, may I ask what you're trying to do? Have you tried using a decal as a replacement? Would that work? If you can share a test scene, that would be great.

[Beck]

As i wrote, i used this setup for an architectural vizualisation of a building where i wanted the foreground landscape reflected in the windows.
I had to use planes in front and back of the scene for fore- and backgroundimages instead of environment overrides because i wanted to use global volume (fog)

The frames of the windows are rough metal so i noticed the shadow of the foregroundplane on parts of the metalframes making them too dark.
I solved the problem, just wanted to show you the problem.

Attaches previewrenders: (without plane for background) the frames on the right side are near black because of the shadow of the foreground plane and even shadow on the terrain (without scattered grass)

[Stefan-L]

we have similar problem here, hope this can be fixed.

sadly the occlude other light option is greyed out when emit light is turned off. when i turn on emit light and turn off occlude other lights it works ok, but this then adds light tot he scene which isnt wanted.

@beck what workaround did you find?

[Beanzvision]

As mentioned, this is a known issue/limitation. More on the issue can be found here under the ''Black highlights'' topic. A workaround has also been noted in the article. I hope that helps you both.
https://support.chaos.com/hc/en-us/articles/4528311915025-Known-bugs-and-limitations

[Beck]

@Stefan-L:

Sorry, not a workaround really, i had to change the angle of the sun and moved the foregroundplane not to cast shadow on the building.

[Beanzvision]

Did you manage to try the workaround from the thread mentioned in the article?

[Beck]

I just tried that, but that doesn't work. If emit light is on, the reflection of the plane is not visible.
It cast's no shadow though.

[Stefan-L]

yes this eliminates what the setup would be needed for! (to have no light but visible reflections from it)