Reflection / Refraction / Transparency basics?

[rmehta]

I've read the pdf documentation provided with Vray for Rhino but am left without an understanding of how reflection/refraction/transparency.

My main questions are:

What does the color do for each?
How do these three setting interact?

To give an example- I'm starting with an off the shelf architectural glass. How would I increase its reflectivity? How would I increase its reflectivity without increasing transparency? etc.

Can anyone give a super simple explanation of the settings, or point to a thread that does?
Thanks!

[Artifex-Auctor]

Re: Reflection / Refraction / Transparency basics?

I'll give a shot at this . . . but I'm pretty new so it might not be dead on. I'll start with a simple explanation up top and move into more detail at the bottom if you like.

Basically, the difference between the reflection and refraction channels is one of SURFACE effects vs. DEPTH effects. Reflection controls the qualities of the outside surface and Refraction controls the qualities of the thickness of the material.

If you have a surface and not a solid, then a refraction map pretty much doesn't do much other than help to make it transparent. If you have a solid (closed polysurface) then the refraction map does a lot.

With both of these channels, the IOR values are pretty much the key. This controls the angle at which reflection or refraction begin to occur. Any rise in value from 0 will dramatically increase the degree to which reflection/refraction operate. This has to do with the angle of the viewer to the material. Water is 1. Glass is about 1.55 if straight glass and anything with a coating is basically about 1.8-2.0. Diamond is at about 2.5-2.8 depending on quality. Metals are REALLY high IOR like 20 and above.

Any refraction/reflection will reduce transparency. But if you want maximum transparency but still want the glass to have reflections when viewed on the oblique I recommend setting:

For Refraction:
IOR to something like right at 1, the fog color to white and with a value of .001 so there is no cloudiness, keep glossiness at 1 unless you want to make frosted glass. Make sure that the two check boxes for affect shadows and affect alpha are checked otherwise the material won't transmit light and objects behind it will be dark.

For Reflection:
use a Fresnel map, in the fresnel map you can make the IOR something like 1.8-2.0 if you want it to be really quite actively reflective glass so you get lots of ghosting/suspended reflected objects and optical illusions. If you want it to be less noticable than normal glass so the glass disappears set it to something like 1.3. If you want it true-to-life set it to 1.55.

In more detail for Reflection Channel:
In the Fresnel map: the color on the bottom is the color reflected when you look at the object straight on so setting it to black means it reflects nothing and is totally transparent. The top color is the one reflected when you look at the object on an angle. The amount of reflection at any given angle is controlled by the Reflection IOR: The lower this number, the more severe the angle is needed to get the material to reflect. The higher the number the sooner the angle the material reflects. So, if you have the top color as white, and an IOR of 1.55 then it will act KINDA like glass. But realistically, glass has a reflection tint that would influence the top color to not be white . . . experiment with this top color if you have glass with high-performance coatings such as semi-mirrored glass or other types of high-performance coatings . . . they all have a color to the reflection that you can set in this area. So, the fresnel map here deals with what is seen as the reflected values of the surface of the material and not it's depth. Setting the top color to something other than white or grey will also have the effect of reducing the color spectrum of light TRASMITTED. So everything in the scene behind the glass will get a tint to the diffuse channel that is the INVERSE of whatever color you place in the top color of the channel. This is what you want to use if you are using a colorized leaded glass. If you want to influence the color of the reflection only and not the color of the transmitted light you change the filter color in the main reflection color channel. This won't influence the color of light transmitted but will alter the color of your highllight. If you want really real glass you also have to alter the glossiness of it as real glass has a highlight glossiness value of about .97. This lets you set the reflection glossiness to the same value so that now you get that little bit of distortion you see in the reflection of other buildings in the windows of buildings across the street. Glass also has an anisotropic value of about .6 at an angle of about 45.

In more color for Refraction Channel:
The IOR value here has less to do with reflection than it does for distortion and internal reflection. Principally, any jewel/diamond like thing will need a nice high value here so that you get max internal reflection or distortion. Also, the color of the refraction channel will affect the color of the entire thickness of the material and not just the color of the reflection. Likewise with the fog. This is where you get the effects like looking at normal green glass on the edge how it seems a deep emerald and opaque but deep color. This effect of glass is actually a fog color of aquamarine that is very minimally saturated with sat values of like 30-60 and a fog intensity of like .1 for something like Starfire glass or a low-iron glass and like .18 for something like normal low-quality glass. In terms of the glossiness value for refraction the distortion and fuzziness is part of the thickness of the material. This value is what you set if you want to have frosted glass.

[rmehta]

Re: Reflection / Refraction / Transparency basics?

Thanks this was very helpful I appreciate it.

I have a few follow up questions... What does the color next to reflection do? When I hover over it it says "specify the reflection intensity" does that mean that for a given reflection IOR, this color with increase or decrease the amount of reflections present?

Secondly, regarding material "thickness". I'm modeling a high rise bldg with floor plates and a skin. The skin is a closed polysurface right now. Does this mean that vray considers it to be solid glass? I can 't see a huge difference if i explode it. Should I model the glass realistically (ie 1/2" thick planes all around) or will this just slow down render times unnecessarily?

Thanks

[Gijs]

Re: Reflection / Refraction / Transparency basics?

if you don't have a map in the reflection slot (by default, there is a fresnel map assigned) then this color is the reflection color, otherwise this color is inactive.

for the skin: yes, vray considers this to be solid glass. Even if you explode it, that does not matter. When the first surface is raytraced, the light will be bent, when the second surface that has the same material is raytraced, the ray will be bent back. FOr the outer surface, you can possibly get away with a material with complete transparent diffuse, and a fresnel reflection layer.

[rmehta]

Re: Reflection / Refraction / Transparency basics?

With the approach that you describe: complete transparent diffuse, (100%white under diffuse>transparency) and fresnel reflection (and no refraction?) - would I be relying entirely on the reflections to define the surface? Since this bldg is not surrounded by many tall bldgs, I presume that I would then be relying on a jpg or hdi in the environment channel to provide those reflections? This sounds like a realistic approach to the problem (true to life...) but a hard one to pull off.

I've usually done this in photoshop, but if there is a good way to define the surface via reflections in the render, can someone give some tips to tweaking reflections so that they show up well?

Thanks everyone. I've tried to do my homework before asking these questions but can't find too many answers in the documentation.

[Gijs]

Re: Reflection / Refraction / Transparency basics?

shouldn't be too hard: grab one of the sky probes from debevec and you should be ready to go. the material doesn't need a refraction layer.

[Micha_cg]

Re: Reflection / Refraction / Transparency basics?

for the skin: yes, vray considers this to be solid glass. Even if you explode it, that does not matter. When the first surface is raytraced, the light will be bent, when the second surface that has the same material is raytraced, the ray will be bent back.

Do you mean, if I want to render a glas cube, than the surfaces can be exploded? That would be phantastic, but my experience is, that only joined surfaces will be recognized as solid objects. In the past I got the problem, that I want to render a transparent plastic soild object, one side glossy and the other blurry (sandblasted). I found no easy solution, only modelled offset surfaces helped.

[Gijs]

Re: Reflection / Refraction / Transparency basics?

my bad.. you're right. it does matter, since it is a different object, vray considers it to be a new object inside the first object. I expected that vray would only consider a new material when hitting the next surface, but apparently this is not the case. It is part of how Vray works to make rendering liquids in a glass easier to model, but in this case it is working against us...

btw: In your case, the only way I can think of to get a bumped inside surface is to 'unwrap' the UV's and apply a partial bumpmap to the surface. Other tools are required for this than Rhino, but in theory this should work.

[Micha_cg]

Re: Reflection / Refraction / Transparency basics?

Right, unwarp could be a way, but not easy per Rhino. Maybe the development team could add an option or could "disable volume fog" help here? This option can be used for faking dispersion per multiple refraction layers, maybe it could be used for independent surfaces of the same material.

[ludvigwitte]

Re: Reflection / Refraction / Transparency basics?

I have had the same problem but I can´t get the reflections right. I placed a HDI jpg in the Enviroment/reflection channel (sperichal) in 10000
But the reflections are really big and insharp.

Is there a way to controll de size and the sharpnes of the enviroment.


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Best regards / Ludvig