Two clear volumes in face contact

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lighting_freak
Posts: 233
Joined: Thu Jan 18, 2018 6:02 pm

Two clear volumes in face contact

Post by lighting_freak » Tue Apr 13, 2021 6:34 am

Hello all,

I've some troubles with two bodies made from clear plastics that touch each other and share a surface.
I was used to manage this situation with an intermediate surface with defined internal and external volumes.
This example shows this situation and the rendering looks wrong. The reflections are by far too high... and the optical effects too strong.
A simpl3 offsetted volume with the same thicknes everywhere should have almost no optical effect (slight shift of the image).
The volumes also share identical IOR so the reflection on the intermediate should be 0.
Test_Scene.zip
(1.63 MiB) Downloaded 75 times
I've been looking in the face normals several times and I can't find any mistake... (tried a lot)
Also already tested, the behaviour is the same with 2.4 and 2.5 luxcore. And its also retested on the 2.83 LTS and current 2.92.0 blender.

What's wrong with this? Thanks a lot - I really apprechiate your help.

EDIT:
current two bodies result
image.jpg
EDIT2:
single layer only
image_2.jpg
OS - Windows 7 X64
CPU - Intel CORE i7
GPU1 - Variants of notebook card from nVidia
GPU2 - Variants of notebook onboard card from Intel
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Jan
Posts: 36
Joined: Sun Jan 10, 2021 2:37 pm

Re: Two clear volumes in face contact

Post by Jan » Fri Apr 16, 2021 3:41 am

Hi,

I'm having artifacts too if the Image Surface Plane touches the Glass Surface Plane
surfaces contact.jpg

Artifacts gone after moving the Image Surface Plane around 0.01" away from the Glass Surface Plane
no surfaces contact.jpg

I cannot reproduce your single layer without image shift with your blender file
single layer and refraction.jpg

My best guess for your "current two bodies result" was because light rays diverges when it came out from the Volumes with an Intermediate Surface.

Here is my example:

Emissive Image Plane inside the Glass Volume (left) looks darker than Emissive Image Plane behind the Glass Volume (right) because light rays from the former diverge while the latter just merely shifted.
comparison.jpg

Jan
Posts: 36
Joined: Sun Jan 10, 2021 2:37 pm

Re: Two clear volumes in face contact

Post by Jan » Fri Apr 16, 2021 6:00 am

P.S.

IMO
refraction and reflection occurs between "two bodies" and the "intermediate surface" as you didn't indicate the IOR of the "two bodies" exterior volume and it's assumed to be 1 which differ from the 1.492 IOR of your "intermediate surface". They cannot be treated as one single entity if that's the case no matter how together they are.
Snap2.jpg
Snap3.jpg

lighting_freak
Posts: 233
Joined: Thu Jan 18, 2018 6:02 pm

Re: Two clear volumes in face contact

Post by lighting_freak » Fri Apr 16, 2021 9:20 am

Hello,

thanks for your feedback.
Jan wrote:
Fri Apr 16, 2021 3:41 am
I cannot reproduce your single layer without image shift with your blender file
The test-scene shows my two bodies issue.
The image with a single layer shows how it should like without errors.
Having two volumes with exact the same IOR, no additional refraction should happen.
The distortion and magnification while watching through that lens should be the same with two bodies as it is with a single one (not exactly, because of translation due to higher thickness).

Hope this image shows my setup with more details and easier.
VOLUME_Setup.jpg
With best regards.
OS - Windows 7 X64
CPU - Intel CORE i7
GPU1 - Variants of notebook card from nVidia
GPU2 - Variants of notebook onboard card from Intel
Lux - Latest possible relaease

Jan
Posts: 36
Joined: Sun Jan 10, 2021 2:37 pm

Re: Two clear volumes in face contact

Post by Jan » Sun Apr 18, 2021 4:55 pm

Hi,

The distortion and magnification while watching through that lens should be the same with two bodies as it is with a single one (not exactly, because of translation due to higher thickness).

I don't really think so. IMO there will be visible fracture lines when you try to piece back two pieces despite cleanly cut glass bodies together. Likely more scattering of light rays and blurring however original distortion will still be there but not as pronounce as if from one single body.

You can shine a Laser to check how the light rays reflects and refracts.
laser.gif
BR
Attachments
t2.blend
(942.67 KiB) Downloaded 68 times

lighting_freak
Posts: 233
Joined: Thu Jan 18, 2018 6:02 pm

Re: Two clear volumes in face contact

Post by lighting_freak » Mon Apr 19, 2021 1:40 pm

Hello Jan,
Hello everyone,

I appreciate your work and I really like the result of that laser beam.
The difference between the your setup and my issue is that there is no IOR difference on the intermediate surface.
Both volumes share the value of 1,492.

Having equal IOR means that there is neither refraction nor fresnel reflection on that intermediate face. Therefore the appearance of an object (the cube in my example file) should be (almost) the same with single layer and double layer volume.

Another difference is, that I don't have any scattering in my volumes, but that is not the matter of fact.

From my point of view the issue remains open.

EDIT:
I modified your scene to make it fit to the described issue.
I think the result shows exactly the problem; the refraction on the rear surface is missing.
t2_mod.blend
(996.19 KiB) Downloaded 69 times
image_3.jpg
With kind regards.
Last edited by lighting_freak on Mon Apr 19, 2021 6:29 pm, edited 1 time in total.
OS - Windows 7 X64
CPU - Intel CORE i7
GPU1 - Variants of notebook card from nVidia
GPU2 - Variants of notebook onboard card from Intel
Lux - Latest possible relaease

Jan
Posts: 36
Joined: Sun Jan 10, 2021 2:37 pm

Re: Two clear volumes in face contact

Post by Jan » Mon Apr 19, 2021 2:49 pm

But you didn't specify Pmma1 and Pmma2 external volumes IOR which is 1 by default. Light rays will go thru 1 IOR interface after exiting Pmma1 then 1.492 interface (thin middle layer) then 1 interface again before entering Pmma2 then finally 1.492 interface. Lot of back reflections in it and that's why it look so dark IMO.

Jan
Posts: 36
Joined: Sun Jan 10, 2021 2:37 pm

Re: Two clear volumes in face contact

Post by Jan » Mon Apr 19, 2021 4:30 pm

As I said in my previous post strange artifacts happens if two surfaces overlap. A tiny gap of 0.1mm resolve the issue.

There are internal reflections at the interface.
internal reflection.jpg
Best Regards
Attachments
t2_b.blend
(918.53 KiB) Downloaded 62 times

lighting_freak
Posts: 233
Joined: Thu Jan 18, 2018 6:02 pm

Re: Two clear volumes in face contact

Post by lighting_freak » Mon Apr 19, 2021 5:47 pm

Hi,

... but I don't want this tiny gap. :D

The two materials are really molded on each other (multi component molding), there is really no air gap.

If you have a closer look to the models (mine and your's modified), there are no intersecting surfaces. I erased one of those faces in one of the bodies and defined the remaining intermediate face with the internal and external volume.

Your modified model shows exactly what I could observe on my model. Last refraction intersection is missing or fails. Don't you agree?

With kind regards
OS - Windows 7 X64
CPU - Intel CORE i7
GPU1 - Variants of notebook card from nVidia
GPU2 - Variants of notebook onboard card from Intel
Lux - Latest possible relaease

User avatar
Dade
Developer
Posts: 5524
Joined: Mon Dec 04, 2017 8:36 pm
Location: Italy

Re: Two clear volumes in face contact

Post by Dade » Mon Apr 19, 2021 6:12 pm

Jan wrote:
Mon Apr 19, 2021 4:30 pm
As I said in my previous post strange artifacts happens if two surfaces overlap. A tiny gap of 0.1mm resolve the issue.
It is just 32bit floating point (lack of) precision as I have already explained lighting_freak multiple times in the past. You can not have overlapping surfaces, you have to use a single surface and appropriate interior/exterior volumes.
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