Strange falloff on reflection which shouldn't be there

[Art48]

Hi,

I got a simple scene, a floor, and a huge VRayLight (Plane) at the end of it, looking at it with a camera.
The shader has some faked fresnel-like reflection curve as you can see here:


Now what renders, is this:


You notice the strange darkening right next to where it should be brightest, at the light itself.

If I pull all maps, and the plain shader looks like this, then the effect is ALSO there oO.




I can not understand why this happens, any pointers?

The light is not special, just a plane, with normal, physically correct settings (well obviously 1000000 watts may be not, but its a huge wall )

[Art48]

Scene download link:
https://www.dropbox.com/s/es0odw4vwg...floor.zip?dl=0

[Dave_Wortley]

It's actually correct, reflections at the extreme angles of fresnel when you have bump/micro-facet-details get darker.

Think of this analogy... Instead of a plane with bump imagine you had a flat ground with thousands of small buildings on, all roughly about the same height. You'd be looking across your city and you would mostly be seeing rooftops, but at the extreme far distance you would stop seeing the tops so much and instead you would see just the faces of the building facing towards you. The light is only hitting the other side of these buildings at the top of the buildings so you start to see more of the dark side instead.

Make sense?

[Art48]

It does make sense from a fresnel standpoint. But not in this case. See the attached screenshots. In the simple shader fresnel isn't event activated. Also the ReflectionFilter elements looks unicolored, no gradients at all.

[vlado]

What you get follows directly from the way the reflection BRDFs are defined. Some of the other BRDFs (Phong, Blinn, Ward) might work better for what you want. Or you might map the reflection glossiness with a falloff map that pushes the glossiness to 1.0 at grazing angles.

Best regards,
Vlado

[Art48]

Alright, thanks for the explanation, I will propably try changing the BRDF first.

[im.thatoneguy]

Do you have specular on your light enabled? I found a bunch of weirdness I was seeing in the VrayMtl was because it was rendering the reflection of a light... and then also adding a specular function on top giving you a double specular highlight. You might be seeing layer 1 of specularity in the reflectivity and then a second layer of "Specular" on top of the raytraced specularity contributing to the falloff. This is obviously physically incorrect since you're getting "correct" speculars from the reflections and then double reflections from the blinn shader but that's the default.

[Art48]

I fixed it by increasing grazing angle reflectance on GGX with Falloff Map.