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What does the Fresnel node (in cycles) do? I know that it kind of does rim selection(similar to rim lighting), but I'm not exactly sure what it's doing. I've seen it used in many tutorials, but I can't find any clear explanations.

What's the explanation, and which situations make sense to use it?

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    $\begingroup$ 2 minutes of googling would give you some quite nice answers $\endgroup$ – zeffii Jun 6 '13 at 20:59
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    $\begingroup$ @zeffii I can't find any good explanations on Google. $\endgroup$ – CharlesL Jun 6 '13 at 21:46
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    $\begingroup$ an image search for fresnel shader can probably say more than trying to read about it. $\endgroup$ – zeffii Jun 6 '13 at 22:06
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    $\begingroup$ @zeffii that's a really good idea, thanks! $\endgroup$ – CharlesL Jun 6 '13 at 22:11
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The Fresnel node outputs which percentage of light would be reflected off a glossy layer, with the rest being refracted through the layer. The most common use is to mix between two BSDFs using it as a blending factor in a mix shader node.

For a simple glass material you would mix between a glossy refraction and glossy reflection. At grazing angles more light will be reflected than refracted as happens in reality.

For a two layered material with a diffuse base and a glossy coating, you can use the same setup, mixing between a diffuse and glossy BSDF. By using the fresnel as blending factor you're specifying that any light which is refracted through the glossy coating layer would hit the diffuse base and be reflected off that.

For the Cycles node it is assumed that this glossy layer is a simple dielectric material with an index of refraction. Different and more advanced Fresnel equations exist but are not currently implemented. More advanced layering models exist but fresnel mixing is a commonly used approximation.

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  • $\begingroup$ note that there are many different implementations of Fresnel equations. you should use it carefully. $\endgroup$ – eriawan Jun 7 '13 at 4:07
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    $\begingroup$ But what is the logic behind using it as a blend factor, what if the glossy input is in the first shader and the diffuse in the second shader. Would it go other way around(as opposed stated above and go like:" any light which is refracted through the diffuse coating layer would hit the glossy base and be reflected off that."? $\endgroup$ – bzal Jan 18 '16 at 13:46
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What it does is that it distinguishes between those areas that would be totally reflective with the given index of refraction and those that wouldn't, letting you tweak your shader accordingly. I assume you know what index of refraction is, if not you should read up on it on wikipedia.

In most general cases you'd rather want to use the glass BSDF directly, but there are non-standard uses in which the fresnel value can be used for other purposes than to simulate translucent materials of differing density.

I seem to recall someone using it to tweak a car paint shader once, but having read about that months ago I'm afraid I don't have the link at hand. Also months ago, also without a link to it, I recall someone creating an ice shader utilizing fresnel input. Both SHOULD be in some thread over at Blender Artists if you're curious enough to go looking.

Short version: for advanced materials it's sometimes desirable to access the fresnel value outside of the default glass shader.

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