Vray fresnel reflection problem

[joconnell]

I think the issue is that the fresnel in vray IS accurate, but that's the problem - you want to get a technically inaccurate result, but one that looks right to you! In your video you were boosting the specular up quite high using the rgb level of one of the maps in your shader which in theory breaks the accuracy of the material but it's giving you a result you want. There have been a few other times when vlado's been asked to put in features in materials that might break them on the physics side, but they're very handy for artists to have that little bit more control.

[Bigguns]

No I'm not trying to get an innacurate result.. I try to match the reality. Have a look at this effect ( it's a pic of GSP, while watching it I found it has good fresnel reflection) http://s1.ibtimes.com/sites/www.ibti...ks_ufc_167.jpg and this one http://o.static-rs.foxsports.com/con...45_640_480.JPG

with the sss2 it's easy to get this, look , do a simple test yourself, put the sss2 with the fresnel on , on one of your head character, and try to match it exactly with a blend mtl, good luck. ^^ Anyway Vlado know about it and he will expose it in Vray 3, I hope it gonna be a map or something that we can use in the vray mtl also, not just a part in the skin shader.

[joconnell]

Hmm - with those pictures though I'd say you'd probably have very powerful lights quite far away, so the diffuse lighting is normal due to the falloff distance, but your spec and reflection is really high since the distance from a light affects it's intensity less?

[Bigguns]

most of the fresnel is coming from the ground of the fighting ring.. it's gray, try to match the sss2 fresnel with a vray mtl you'll see what I mean...

[trixian]

Terminology getting mixed up here?
I'm guessing you mean most of the reflection is coming from the ground?
As for your examples, maybe using something CG would be better, as most people here won't be bothered recreating those fighting images just to try to make the regular vray reflection match what SSS2 has.
Make a scene and post it, and we can fiddle and tweak to see how close we get.

[joconnell]

most of the fresnel is coming from the ground of the fighting ring.. it's gray, try to match the sss2 fresnel with a vray mtl you'll see what I mean...

I see what you mean now - most fresnel things should go quite close to 100% reflective at 90 degrees to the camera so you should be able to get a bit of that, what does your reflection filter element look like on those renders? It should be up at pure white for those edge pixels, maybe it's something else misbehaving?

[adamlewis]

The issue you're having is not necessarily anything wrong with Vray, it's the BRDF being used. Right now Vray has Phong, Blinn, and Ward - and while all models are overall good and speedy, they are incapable of reproducing certain qualities seen in real-world materials. For example, they appear dimmer than expected at glancing angles, which is why you're having such a difficult time getting a proper looking bright fresnel highlight. Some BRDFs suffer a lot less from these issues such as Cook-Torrance, which is generally considered a fair bit more accurate than Blinn or Phong. As it happens a user implemented just such a BRDF in the form of an OSLMaterial, and having tested it I can say out of the box it looks much better than either Blinn or Phong no matter how you tweak them. But for complex reflections it is very hard to sample so it's not too practical to use at the moment. As I understand, it's not sufficient to merely add a BRDF, as it also needs to be optimized, so some type of native implementation with Vlado's coding magic is needed. As Vlado has mentioned before, he is looking into new BRDFs for Vray, so we'll see what comes of that. All I can say is that for now, it's going to be very hard to get the kind of reflections you want without either a more accurate BRDF, or some fudging with additive blends and such.

For further reading, check out this paper: Experimental Analysis of BRDF Models. They compare several different BRDFs with actual measured materials and talk about the differences. Useful stuff to know.