# Error with Fresnel and Backfacing Polys

• Hi,

I seem to have a problem with using a fresnel blend when it applies to back facing polys

From this mat

I get this output

Notice the grey "stripe" on the back of the dress.

If I swap the fresnel for an edge blend

I get this

Am I doing something wrong with the fresnel blemd node? BTW it is the same if I use the poser FresnelBlend or the Cycles Fresnel node

• try the Fresnel node in the Cycles list. I think the one you are using is for a slightly different purpose. (could be wrong but it's worth checking out)

• @ghostship Thanks, but I tried that already with exactly the same results

BTW it is the same if I use the poser FresnelBlend or the Cycles Fresnel node

gives

angles are different and cloth has been re simmed but lighting etc is the same

• The Fresnel nodes should not be used for the back side of non-manifold geometry.

Why? Because the Fresnel nodes' job is to decide how much reflection happens vs. how much something else (usually Refraction, but in your shader, Diffuse) happens. In doing this it follows the laws of physics, subject to the approximation we use in 3D models. The approximation we use is that the solid material, such as a crystal ball, does not exist at all. Rather we have a mesh representing the BOUNDARY of that solid volume of matter (glass or whatever). The Fresnel effect actually only happens at the boundary anyway, so this works out for us. Now what is the Fresnel effect?

It says that there is a function that decides how much reflection happens at the boundary, and that this function largely depends on three things: the index of refraction, the angle of incidence of the light ray, and, finally, AND THIS IS VERY IMPORTANT .... whether the ray is entering or exiting the bounded volume.

The test for entering or exiting is to look which way the polygon is facing vs. the incident ray. When the ray is striking the "back" or "inner" side of the polygon, the function is very different than when it strikes the "front" or "outer" side of the polygon.

I'm assuming your dress is not a bounded volume at all, but a 2-dimensional sheet suspended in 3-dimensional space. For such an object, the use of the Fresnel equation on the back side is COMPLETELY INAPPROPRIATE. Doing so is decidedly "operator error".

Conversely, Edge_Blend and its Cycles brethren (Facing, etc.) do not make a distinction between back and front side and calculations applied to a zero-thickness 2-dimensional sheet (such as a dress, trans-mapped hair, curtains, a shawl, etc.) will produce what you expect.

• If you want to understand more than "don't do that" look up material regarding "total internal reflection".

Here's an example:

Snell's law and Fresnel's law go together. Fresnel gives the answer to how much and what sort of Reflection happens. Snell's law gives the answer to how much and what sort of Refraction happens. Both apply to boundaries of volumes.

When Snell's law says that from the back-side no refraction can happen, Fresnel's law says 100% reflection happens. This total "internal" reflection occurs outside Snell's window.

Your dress, being curved, is randomly displaying some parts within Snell's window and some parts outside it.

• @bagginsbill Thanks, that makes perfect sense and I will not do it again ;o)

BTW is the cycles Facing available in Poser, I saw it mentioned in a Blender reference but couldn't find it when I was looking in to this "error"

Amanda

• @amethystpendant Layer Weight Node