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I am new to graphics rendering and there are a lot of terminology. Can someone please explain what fresnel is?

I see fresnel in unreal engine when people deal with translucent objects such as glasses. What is it in the fresnel that everybody uses it for translucent object. When I googled the term I only find technical documentations that only some people can understand.

Could you guys can explain as you would explain to little child? So other new people, too, can understand?

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    \$\begingroup\$ Didn't downvote, but the "arrow down" specifically mentions "question does not show research effort". So maybe you could do some research and then highlight parts you did not understand? \$\endgroup\$ – Kromster Jun 18 '17 at 19:41
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The fresnel effect is the way light works when gets in contact with a translucent object.

You can actually try this, take a bowl fill it with water and look at it from above. You'll see the bottom of the bowl clearly. Now start looking at it from different angles. The closer you are to the plane of the water's surface (aka. The smaller the angle is between your head and the ground) the less you'll be able to see through the water and the more it will reflect the stuff behind it.

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To complete Bálint's answer:

In Mathematical terms, the Fresnel node is a Dot Product between the Camera Vector and the Surface Normal, where both vectors are normalized!
The resulting value determines the angle you are looking onto the surface. 0 means you are looking straight onto the surface and 1 means you are looking on the same plane as the surface.

Fresnel is often used in PBR, because it simulates the reflectivity in real life pretty well. Every surface is perfectly reflective when you are looking onto it on the same plane. This can be seen when you are driving with the car or looking onto a long road. You can see the reflection of cars on the streets when the road becomes convex, but the reflection disappears when it becomes concave again. This "reflection threshold" can be simulated by calculating the fresnel with an exponent > 1

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