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Hello,

I made a specular material that represents rough plastic, used as a clear ceiling which lets sunlight pass through. I would expect this to result in an overcast-like lighting, since rays would be scattered due to the material's roughness, but the shadows remain sharp. Fake shadows are enabled, and I'm using Path Tracing. I added a medium node with forward scattering, still no change.





Not sure if this is a good example, Maybe my assumption is completely wrong

Hi Phil_RA,
using SSS shader, increases render times, noise etc. my advice is to use specular shader with roughness and increase sun size, the shadows will be soft as overcast.
Regards
Paride

Hello paride,

This is a specular material with roughness. I used scattering just to see if it made any difference.

My point is, physically speaking, the roughness represents how the microsurface affects the direction in which the light is reflected, so I would expect that it would have a similar impact when light exits the medium than what happens when the ray is reflected as seen with specular reflections. The ray first bounces on the microsurface any number of times, and then into the medium or is reflected from it, but in the former case it means it enters the medium in a different direction than the direction the ray had before bouncing.

So basically, just as specular reflections appear blurred due to roughness, I would expect blurred shadows as well, since the likelihood that a ray will exit in the same direction as the original direction it had before entering the medium would be relative to the surface roughness on entry and exit.

Note I'm not talking about anisotropic scattering, which I don't think any commercial ray-tracing app supports yet anyway. I'm only talking about the result of the surface roughness on the entry/exit.

Again, maybe I'm wrong, or maybe this effect is already happening but is not noticeable enough due to the medium being continuous. I'd be curious to hear a dev on this.

edit: Thinking on it, it would probably just modulate the shadow intensity, not blur it, since the rays are simply being scattered more regardless of proximity to the edges.

Hi Phil,
have a look at this example with a Specular material with 0 roughness, and medium node: If you change the Phase value, the light is scattered in the correct direction: ciao Beppe

Your examples are completely different. The room is lit by a big area light therefore the shadows are soft. In your Cube example the lighting comes from a tiny lightsource therefore the shadows are sharp. Your SSS Cube can only soften the shadows of objects which are behind it.

I made a little scene for example: The shadows behind the SSS Wall are soft, but the shadows of the wall itself are sharp:

I'll try again tonight, in my original test (not pictured) I had a large rough circular ceiling, and the sunlight going through it, which would do the equivalent as having a large light source as light would scatter through it like in your example, yet it was resulting in sharp shadows for the objects under the ceiling.