Distorted and double reflections on glass panels

[Suthriel]

Hello again, just wanted to let you know, until UE 4.25, even the documentation uses values between 0.01 to 0.1 as input for specular to get a more realistic refraction.

https://docs.unrealengine.com/en-US/...ing/index.html

Different values applied to the Specular input affect the index of refraction for the translucent material.



Dragging the slider shows the amount of refraction being applied from 0.01, 0.05, and 0.1.

This is the picture for 0.1, their highest value.

[Suthriel]

Oh, I see...

Anyways, while further testing I noticed that in the docs they use very low refraction values. Values like 0.04 in specular pretty much gives nice result for something like drinking glass etc, for glass panels in windows it's a bit too much but lower values can work(minimizing the bending).

What I'm confused about is, if the weird bending is expected behaviour due to some misconfiguration on user part, and also how to transfer proper IOR values to specular in unreal.

The bending is basically the refraction, which is expected with transparent materials, it just seems to be way to strong, with normal values. I have trouble with that myself, and i started to reduce the values by a factor of 10, to get more normal looking results, so it seems, that maybe there is some internal calculation error, that results in way to strong refraction, if normal values are used.

For the transformation from IOR to specular, there was a formula for this. i created a tiny IOR2Specular function for that, including a "debug" feature to massively weaken the result, and therefore the resulting refraction.
You can try it, smash this code into a new function, then use an IOR value as input, and connect the output to the specular input from the material. The "IOR to strong" is basically the weakener, it divides by 10. Maybe i should replace this constant with a parameter to make it dynamic and adjustable:

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I tried for days but couldn't get glass looking anything like this: https://www.youtube.com/watch?v=WTAB...nel=RyanBrucks

As the description says, this is a new experimental feature at work, so not released until now. Hopefully it gets released in the final version of 4.26, because yeah, it looks awesome.

[presto423]

So far, I haven't seen a guide or tutorial that teaches how to create that quality level of glass (from the video). It's difficult when starting from not knowing how UE works, or how it processes certain values for specular / refraction / etc. It's like a chemistry lab I've never used before with chemicals and containers, and I'm a rookie mad scientist.

[UElaci]

Well, I was saying it happens with non-RT. So, it could be something other than RT or whether Specular or Refraction inputs are used. I don't remember how I got the translucent, double reflections of certain actors to be removed. I'm definitely not understanding this enough yet either.

Oh, I see...

Anyways, while further testing I noticed that in the docs they use very low refraction values. Values like 0.04 in specular pretty much gives nice result for something like drinking glass etc, for glass panels in windows it's a bit too much but lower values can work(minimizing the bending).

Specular value 0 removes refraction and thus the bending of the reflection on backfaces, this works for some arch glass(windows etc..)

What I'm confused about is, if the weird bending is expected behaviour due to some misconfiguration on user part, and also how to transfer proper IOR values to specular in unreal.

I tried for days but couldn't get glass looking anything like this: https://www.youtube.com/watch?v=WTAB...nel=RyanBrucks I always get the visible backfaces looking kinda weird.

[presto423]

Well, I was saying it happens with non-RT. So, it could be something other than RT or whether Specular or Refraction inputs are used. I don't remember how I got the translucent, double reflections of certain actors to be removed. I'm definitely not understanding this enough yet either.

[UElaci]

I encountered the exact same in a project I'm testing refraction in. What I tried doing to fix the curvature of reflections so they're reflected like a rectangular mirror or window is connect a 3-vector (1, 1, 1) node to the Normal input. But I saw that it actually produces the effect of curving reflections. So I set it to (0, 0, 0) and it straightens things. I think you're correct that the problem is in the backfaces reflecting in a distorted shape, and when I tried a cone it did that too (consistent with a cone's curvature / stretching). Disabling two-sided, however, has the result of making it appear too thin and not as realistic as glass with refraction or solid thickness. I suggest trying to input a normal vector into the fresnel nodes with (0, 0, 0) set to start, make them parameters, and plug in another of the same in the Normal input of the main material node. I think that the refraction input doesn't fully take into account the normal of the object and the camera without a directive for it to do something with, as it would do since refraction is not easy to simulate for every 3D object and material. It may require more than that to get corrections to the reflections, but it's a starting point.

Actually refraction input doesn't do anything for raytraced refraction. Specular is controling refraction. It's a mess, I don't even understand how is this supposed to work.

[presto423]

I encountered the exact same in a project I'm testing refraction in. What I tried doing to fix the curvature of reflections so they're reflected like a rectangular mirror or window is connect a 3-vector (1, 1, 1) node to the Normal input. But I saw that it actually produces the effect of curving reflections. So I set it to (0, 0, 0) and it straightens things. I think you're correct that the problem is in the backfaces reflecting in a distorted shape, and when I tried a cone it did that too (consistent with a cone's curvature / stretching). Disabling two-sided, however, has the result of making it appear too thin and not as realistic as glass with refraction or solid thickness. I suggest trying to input a normal vector into the fresnel nodes with (0, 0, 0) set to start, make them parameters, and plug in another of the same in the Normal input of the main material node. I think that the refraction input doesn't fully take into account the normal of the object and the camera without a directive for it to do something with, as it would do since refraction is not easy to simulate for every 3D object and material. It may require more than that to get corrections to the reflections, but it's a starting point.

[UElaci]

If I get camera inside of a glass objects I get this spherical reflection. I think the problem could be solved If I could simply disable reflection on backfaces somehow.

[UElaci]

A part of it is refraction. Can you post the material graph of the glass? What Refraction Mode is the glass material using (Index of Refrac or Pixel Depth Offset)?

here's material.. Very basic - using index of refraction, pixel offset looks the same so do other shading models. I noticed this in 4.25.3 and now I'm using 4.26p3. I think it's the internal(backface) reflections that are bent like this.

[presto423]

A part of it is refraction. Can you post the material graph of the glass? What Refraction Mode is the glass material using (Index of Refrac or Pixel Depth Offset)?

[UElaci]

Hey guys, I get weird bent reflections when I get camera close to an object and also double reflections when using raytraced translucency.

This is always visible even in very basic materials and it doesn't matter if I use thin translucent or other translucent shading model. I first thought that I need to disable twosided option but that turns objects opaque.

here's an example


look at the reflected column on the left side. One reflection is at proper distance and straight as it should be and there's another weirdly bent one. This makes me think I'm forgetting some setting on the engine or materials but I'm out of ideas.