fresnel reflections, when to use them

[nvanherpt]

I usually set the appropriate glossy value, set reflection to white, click fresnel, unlock the IOR, and control the reflection strength via IOR value.

hey clifton, you do that for nearly every material?

[Clifton Santiago]

Hey Nik! long time... we should meet for a pint sometime

Yes, mostly. Except for low reflective things like pavement, matte wall paint, etc. In those cases I might use mid-grey for reflection, about 0.6 glossy, and click fresnel but leave the IOR locked at default 1.6. I haven't really experimented with IOR values for glossy reflections below 1.6. In theory you could control the reflection strength until you approach 1 which would be not reflective.

However, if I need short render times, I don't use reflections on low-reflectivity materials. Oh, and I always accelerate my glossy reflections with LC for glossies option (as opposed to using interpolation which I think is slower).

[nvanherpt]

cheers mate, been experimenting with this technique this am.

yep definitely a pint or six is in order whatchaupto next week?

[Clifton Santiago]

the usual... work might start getting busy though

I'll give you a call next week. I can only do Tuesday nights or weekends because I am trying to be good and go to the gym 4 times a week.

[pailhead]

Would anyone care to explain why are we using fresnel reflections on chrome and pavement since it's a refraction phenomenon?

[Clifton Santiago]

I'm not claiming that the reflections in all opaque materials are necessarily "correctly" fresnel, but they certainly do have a falloff. With almost all reflective materials, reflection is stronger on surfaces closer to parallel to the viewing angle than they are on surfaces perpendicular. I suppose you could get similar results with a perpendicular/parallel falloff map in the reflection slot, but I prefer the fresnel falloff instead. I think it looks better and I can control it numerically with IOR values.

[joconnell]

Would anyone care to explain why are we using fresnel reflections on chrome and pavement since it's a refraction phenomenon?

Cos it isn't. The light can be either transmitted or refracted. Transmitted eg the light energy becomes absorbed into the medium that the light hits and thus dies off from visibility, or refeacted as we know. Either of these two contribute to the deadening of the light that is reflected back.

[ene.xis]

ditto!

[zoranm]

As far as I know Fresnel is only applicable to dielectric materilas.

Dielectric definition:
noun
a material such as glass or porcelain with negligible electrical or thermal conductivity

Therefore chrom and other metals shouldn't use IOR. Setting IOR for chrom to 12 or more is so unrealistic that practicaly cancels the influence of IOR on that material.

Z.

[pailhead]

Exactly, i doubt that 12 is noticeable at all.

[glyph]

an IO Reflectance of 12 on chrome is noticeable by the fact that it is then not mirror-like shiny

(as a result I persoanlly set chrome and stainless steels to IOR OFF and then use the reflect level to tune it to a little below full reflection)

but but but I still think there is confusion..
I have a brass door knob at home - getting right up close to it I can clearly see more reflection in the glancing angles than straight on - ergo a non-dielectric material is showing fresnel reflectance!

I think Vlado needs to visit this post and comment - now where's the call-for-Vlado button?

[Clifton Santiago]

Ultimately its about what looks right... if it looks good, then it is. I personally find that glossy fresnel reflections on most surfaces look good...

[instinct]

There was a thread about that if i recall right. And what you notice on the doorknobs is not necessarily MORE or LESS reflection but more or less GLOSSINESS. As vlado discussed in that other thread, materials tend to be sharper on the sides then when looked straight on.

Regards,
Thorsten

[davision]

I'm not sure if it's the same thread -- but I remember a discussion about glossiness changing in a frensel way.

Essentially, the flatter (or more glancing) the reflective angle, the less glossy a reflection would appear... or to say it another way, surfaces that ran parallel (or nearly parallel to the camera's view would look sharper than those that were perpendicular.)

The example that Vlado showed was a plane with a fine noise bump on it. As it rotated from perpendicular (from camera view) to parallel the glossiness went from blurry to sharp. I assume it's sort of like taking a blurry photograph and shrinking it in one dimension... as it gets compressed it will appear sharper. When you rotate a plane from perpendicular to parallel -- it's the same type of effect.


In the case of the doorknob -- I would agree with Thorsten -- I think it's the apparent glossiness is changing and not it's amount of reflectivity. I would think the curvature of the doorknob would cause the reflections of nearby objects to appear more "compressed" and therfore sharper (which could be mistaken for increased reflectivity). But who knows, I guess I'd have to see the doorknob.

[joconnell]

The example that Vlado showed was a plane with a fine noise bump on it. As it rotated from perpendicular (from camera view) to parallel the glossiness went from blurry to sharp. I assume it's sort of like taking a blurry photograph and shrinking it in one dimension... as it gets compressed it will appear sharper. When you rotate a plane from perpendicular to parallel -- it's the same type of effect.

Exactly - the bumps in the surface that cause the roughness move closer together making a more smooth mirror like surface.