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I have a glass plane mixed with a diffuse fresnel (and a .png texture):

Glass plane

This glass plane shows up diffuse when viewed at an angle. When viewed straight on, it looks fine, like a regular glass plane. How can I get rid of this diffuse effect?

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    $\begingroup$ You are seeing the fresnel effect at an angle (which you have your diffuse set to use). Instead of a diffuse node try a glossy node. Keep in mind your texture will also use the glossy node, I am not sure how you want it to look , just keep that in mind. $\endgroup$ – icYou520 Mar 16 '18 at 21:44
  • $\begingroup$ If I start again and create the same setup (like this youtube.com/watch?v=1Y1Ny6MTPD4), I do not get this fresnel effect. Also, replacing the diffuse node by a glossy node doesn't work. $\endgroup$ – EmielBoss Mar 16 '18 at 22:02
  • $\begingroup$ I guess I need to understand what exactly you want it to look like. The fresnel effect needs a glossy shader to be physically correct. If you dont want to see the diffuse shader when looking at an angle you need to get rid of it. If you are going for photorealism, I would use the principled shader, then you dont have to worry about it. $\endgroup$ – icYou520 Mar 16 '18 at 22:07
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    $\begingroup$ Check your normals - looks to me like it’s flipped and causing it to behave as IOR of 1/1.450 rather than the intended 1.450. $\endgroup$ – Rich Sedman Mar 16 '18 at 22:16
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    $\begingroup$ Also, does it have thinkness or is it just a plane? Add a solidify modifier or extrude to give it thickness. $\endgroup$ – Rich Sedman Mar 16 '18 at 22:19
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The problem is due to a flipped normal - this can occur when extruding a plane, depending on which way the extrusion is applied.

In the following example one plane is extruded 'upwards' (the left-hand one) and the other is extruded 'downwards' (the right-hand one). The 'upwards' plane has correct normals but the other has inverted normals.

glass examples with opposite normals

When adding a Glass material the glass acts based on the Index of Refraction (IOR). For normal glass in air the IOR is usually around 1.45 - ie, the default setting for the glass. However, with the flipped normals the mesh actually behaves as if it is a cavity of air within a glass solid and this causes the "surface" to refract unexpectedly and behave as if the IOR is the inverse (in the case of an IOR of 1.45 it behaves as if the IOR is 1/1.45 - ie, 0.69).

The solution to this problem is to either Flip Normals or Recalculate Normals so that the normals are pointing 'out' of the mesh.

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