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Noise behind a piece of glass

Posted: Mon Jun 21, 2021 4:03 pm
by naruto123sdo
Hi, I'm a new user, and I'm trying to render an optically correct scene. The scene itself is simple, just a lens attached on (or partially intersect with) a plane, and there will be a projector (a spotlight with image in Luxcore) illuminating on top of the plane. The issue I found is that the noise behind the glass is huge, and it does not give me optically correct results. Using denoiser will not help as well because it may harm the accuracy. I have attached my .blend file here. Any suggestion on such optical simulation would be helpful, thanks in advance!

Re: Noise behind a piece of glass

Posted: Wed Jun 23, 2021 11:24 am
by Dade
I loaded the .blend but I have sincerely only a vague idea of what I'm looking at or what is the expected result.

Anyway, you are using a "Two side" material with glass on one side and "Null" on other: it can not work. You are building an asymmetrical transparent material material that can not work or produce correct results.

Re: Noise behind a piece of glass

Posted: Wed Jun 23, 2021 5:42 pm
by naruto123sdo
Dade wrote: Wed Jun 23, 2021 11:24 am I loaded the .blend but I have sincerely only a vague idea of what I'm looking at or what is the expected result.

Anyway, you are using a "Two side" material with glass on one side and "Null" on other: it can not work. You are building an asymmetrical transparent material material that can not work or produce correct results.
Hi Dade, thank you very much for your reply. I apologize that I could not explain what to expect, but I knew the rendering was wrong through some optics related calculation. The rendering looks more reasonable after I dropped the "Two side" material node.

I guess my major question is, is LuxCore (with either bidir tracing or path tracing) capable of rendering a double refraction effect as shown in the attached diagram? You see, when the light enters the lens, it refracts once; and after it is reflected and exits the lens, it refracts for the second time, and then finally gets seen by the camera. Thanks.

Re: Noise behind a piece of glass

Posted: Wed Jun 23, 2021 5:54 pm
by B.Y.O.B.
What is the material of the bottom ground plane in this scenario?

Re: Noise behind a piece of glass

Posted: Wed Jun 23, 2021 9:12 pm
by naruto123sdo
B.Y.O.B. wrote: Wed Jun 23, 2021 5:54 pm What is the material of the bottom ground plane in this scenario?
It's Matte material.

Re: Noise behind a piece of glass

Posted: Wed Jun 23, 2021 9:59 pm
by naruto123sdo
It seems the render cannot get the 2nd back refraction through the lens. In the attached rendering result, I can see the Refraction 1 is rendered correctly, and the spotlight is able to be concentrated. However, when it is reflected back, we cannot see the reflection through the lens (see the blank area in the side view and the front view). I have attached my .blend file here with the updated side view camera. Is there anything wrong with my material setup?

Re: Noise behind a piece of glass

Posted: Wed Jun 23, 2021 10:41 pm
by Dade
naruto123sdo wrote: Wed Jun 23, 2021 9:12 pm
B.Y.O.B. wrote: Wed Jun 23, 2021 5:54 pm What is the material of the bottom ground plane in this scenario?
It's Matte material.
It is an SDS path (Specular-Diffuse-Specular). It can not be rendered by a normal Path tracer or even BiDir:

Image

See the caustic reflected on the mirror ? It is not visible with normal Path tracing or BiDir.

You have to enable Caustic cache to be able to render that kind of light paths.

Re: Noise behind a piece of glass

Posted: Wed Jun 23, 2021 11:18 pm
by naruto123sdo
Dade wrote: Wed Jun 23, 2021 10:41 pm
naruto123sdo wrote: Wed Jun 23, 2021 9:12 pm
B.Y.O.B. wrote: Wed Jun 23, 2021 5:54 pm What is the material of the bottom ground plane in this scenario?
It's Matte material.
It is an SDS path (Specular-Diffuse-Specular). It can not be rendered by a normal Path tracer or even BiDir:

See the caustic reflected on the mirror ? It is not visible with normal Path tracing or BiDir.

You have to enable Caustic cache to be able to render that kind of light paths.
Wow, this is eyeopening. But I'm not sure if I configured the caustic cache correctly. See my rendering result, I still cannot see the back refraction. Is it correct to first change the engine to "path", and then check "Caustic Light Cache"?

Re: Noise behind a piece of glass

Posted: Wed Jun 23, 2021 11:45 pm
by B.Y.O.B.
You might need to adjust the lookup radius to match your scene size.

Re: Noise behind a piece of glass

Posted: Thu Jun 24, 2021 5:06 pm
by naruto123sdo
B.Y.O.B. wrote: Wed Jun 23, 2021 11:45 pm You might need to adjust the lookup radius to match your scene size.
Thanks a lot, LuxCore is amazing. I can now get the back refraction rendered.

But the noise is an issue. I checked the wiki page and your Youtube video on engine setting tutorial, and play around with lookup radius and sample numbers, but I still cannot get a clean render. I found that setting the lookup radius as 0 m (which will automatically change to 0.00001m) is suitable for my scene, because if the lookup radius is set higher, the render will look blurred. The attachment is a rendering result after 7,000 samples, which still looks noisy. Is there anything else that I'm missing to configure? Thank you.