Fresnel, pronounced frenel, with a silent s is the effect where a reflective object's edge is highlighted as well as the angle of the view changing how reflective an object appears. Every surface has some amount of fresnel but rough materials tend to have a very diffuse reflection to them.
Similar to actual Fresnel, Fresnel in Blender will cause the edges of objects and surfaces facing perpendicular to the view to reflect brighter.
In Blender, the Principled BSDF automatically calculates fresnel the specular value will change the fresnel intensity and the roughness value will change the falloff. We can also directly access the fresnel value with either a fresnel node or a layer weight node.
In the rest of this article, we'll go into more detail about fresnel and how it works in Blender. First, we'll go over what fresnel is and a couple of examples of it in real life. Next, we'll go over what fresnel is in Blender. Finally, we'll go over how to control the fresnel using either the Principled BSDF or custom shader.
Have you ever noticed how on really smooth shiny objects the edges of them will have a brighter highlight than the middle? Maybe you've looked at a flat reflective surface and noticed if you look at it from a very shallow angle it will reflect even more.
This is what fresnel is, while it's most obvious on reflective objects all materials have a certain amount of fresnel. The angle of the surface relative to our view will change how bright that surface's reflections are.
So if we're looking at a surface from the side the intensity of its fresnel will be much higher. If we were to look at a surface from the top down the intensity of the fresnel will be very low. Every material in real life has some amount of fresnel but the roughness of that material affects its intensity.
A very polished wooden floor or a plastic bevel might have a high fresnel intensity while a piece of paper might have almost no fresnel.
Note that there isn't a difference in the fresnel intensity of surfaces its just that some surfaces have a bunch of bumps in them that cause the falloff of the fresnel to be quite diffuse.
In Blender, the fresnel effect takes into account the normal of a surface and compares it to the direction that the camera or viewport is facing. A surface that has a normal facing perpendicular to the camera will have a high fresnel intensity.
A surface that has a normal that is parallel to the view won't have a very intense fresnel. Note that fresnel will take into account all adjustments made to the normals of an object including custom normals and normal maps.
Most of the shaders in Blender already include some amount of fresnel, similar to real-life examples it highlights the edges of reflective objects and changes the reflectivity of an object relative to the angle of the camera.
Pretty much any combination of values with the Principled BSDF shader will result in some amount of fresnel. Both Eevee and cycles support fresnel but cycles fresnel is more physically accurate.
With the principled BSDF shader, the main method of changing the fresnel is to change the specular, roughness, or IOR value. In the case that we've made our material transmissive the IOR will affect the fresnel of the material.
As well as just adjusting the material we can also use a fresnel node with specular set to 0 to manually creates the fresnel effect. It's important to note that the principled BSDF shader isn't the end all be all of shaders and we could build different shaders depending on our needs.
Especially for more artistic control, we may want to create custom shaders and manually add fresnel to these shaders. We can access fresnel in the shader graph in a couple of ways, one is just to use the fresnel node. The other way to access fresnel is to use a layer weight node.
So which one do we pick? For physically accurate IOR values use the fresnel node. For easier use and access to the facing values use the layer weight node.
While using the Principled BSDF specular value for adjusting fresnel might work for basic materials we may want to exaggerate it and metallic materials do not have any fresnel which we may want.
To add a fresnel highlight to our Principled BSDF materials go to add > search and look for a layer weight node, Mix Shader, Glossy BSDF and place all three in the shader graph.
Connect the Principled BSDF output to the first input of the Mix Shader node and then connect the Glossy BSDF to the second input of the Mix shader node. Finally, connect the layer weight fresnel output to the fac input of the Mix Shader and connect the mix shader output to the Material output surface input.
Now we can use the blend factor of the layer weight node to change the fresnel falloff. We can change the color of the fresnel by changing the gloss color. Increasing the roughness of the gloss node will also change the intensity of the Fresnel.
Finally, for a more artistic effect, we could replace the glossy BSDF with an emission shader or we could just directly plug in the color we want for the fresnel to make our fresnel act like a highlight.
This Fresnel effect can also be applied to any shader we've setup up by just plugging it into the first input of the mix shader.
Fresnel is an effect we see in basically all materials so knowing what is it and how it works can help us better fine-tune our materials. As well as that we'll also be able to better recreate photorealistic materials and use fresnel for artistic effects that aren't necessarily realistic.
Thank you for your time.