Author Topic: Reflections not respecting normals? Should it?  (Read 6829 times)

2015-04-03, 09:20:21

aTanguay

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I thought I noticed something a while back, and it's definitely become a thing for me. I noticed it first on car headlight lenses during the beta...their reflection seemed VERY bright and I had to artificially knock them down and just keep moving. But I've looped back around and tested this out again and it seems odd.

What I made here is just a simple geosphere inside another geosphere to simulate a thin wall like glass, or lenses. The inner sphere's normals are flipped so that the outside points out, the inside points in.

Rendered on the left is Corona. If you notice on the bottom of the Corona render, there are two reflections of the single square light in the scene...the inside sphere, and the outer facing sphere.
On the right is a Vray render of the same scene with roughly the same material. No doubled up reflection, I'm assuming because the second reflection faces outward.

Did I miss a setting somewhere or is this a bug or is this physically correct? This cant be right...objects don't reflect an outer facing surface this way. Riiiiight?  8-)
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2015-04-03, 09:24:14
Reply #1

aTanguay

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I look at this and I wonder...


2015-04-03, 09:26:00
Reply #2

romullus

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Left picture is correct one. This phenomena calls total reflection.
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2015-04-03, 09:58:28
Reply #3

aTanguay

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Total Reflection!

Wow, so Vray has been and is doing it wrong. Fascinating.

2015-04-03, 10:36:38
Reply #4

romullus

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Mental Ray has checkbox to enable total internal reflections, so, i guess,  Vray should have too.
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2015-04-03, 12:17:30
Reply #5

Ondra

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there might be some changes in handling of normals, but it is quite complicated topic, so I will need to do some research first
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2015-04-03, 15:03:43
Reply #6

maru

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I'm not sure about the new Vray but in 2.0 there were two things you need to do to make good looking glass:
-check "reflect on back side"
-set "cutoff" to 0
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2015-04-03, 22:05:44
Reply #7

aTanguay

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It's an interesting phenomenon that I've frankly never noticed before. It definitely seems that on a lot of surfaces the away facing surface has less reflectance...or maybe it's being scattered through the material a bit. Not too sure.

2015-04-04, 03:28:42
Reply #8

Stan_But

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I'm not sure about the new Vray but in 2.0 there were two things you need to do to make good looking glass:
-check "reflect on back side"
-set "cutoff" to 0

+1
exactly this way
and set up number of bounces to more than 10-15 in the material and in the render settings

2015-04-04, 03:56:38
Reply #9

Juraj

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I'm not sure about the new Vray but in 2.0 there were two things you need to do to make good looking glass:
-check "reflect on back side"
-set "cutoff" to 0

Damn myself, I never knew about the reflect on backside had effect on glass in Vray. I quickly went to give it a try, and behold, now it looks right. Why isn't such stupidity by default on there ? The renderer should decide when tracing them is necessary and when not based on visibility.

In 3.0 the default cutoff is thankfully 0.001 instead of older 0.01.

Well, that explains my confusion when I saw inner reflection for first time in Corona and wondered myself wtf it is :- ).  {it was with spherical glass vase which reflected a lesser circumsed sphere inside it, I was baffled).
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2015-04-04, 19:32:39
Reply #10

maru

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Damn myself, I never knew about the reflect on backside had effect on glass in Vray. I quickly went to give it a try, and behold, now it looks right. Why isn't such stupidity by default on there ?
I have no idea how to justify it too. Same for "specular" in photometric lights when rendering with Mental Ray. What's the reason for making default settings that you have to change each time you are doing anything? Trying to keep my fanboism to minimum, but Corona is awesome in this field.
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2015-04-05, 05:19:05
Reply #12

aTanguay

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Boy, now I don't feel so stupid for bringing this up. ;-)

A few months ago I simply didn't have time to delve into it and made car plastic lens materials and just turned down the reflection until it looked ok to me. Seems like something that needs to be able to be controlled a little though.
« Last Edit: 2015-04-05, 05:22:48 by aTanguay »

2015-06-08, 16:59:47
Reply #13

Ludvik Koutny

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This seems resolved as this is intended behavior.

Moving to resolved section.