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Topic: Fresnel: You know... that lighthouse guy
posted by PEABODY121
archived on 1.5.2002
I'm having a bit of trouble here.
I set up my scene using the HDRI loader and I can get it to illuminate the scene, create the right shadows, etc.
But my reflective objects don't seem to be reflecting the environment in the right way. Look at the two attachments.. the first one shows a fully reflective object without fresnel reflections. You can see that the white where the illumination from the sky is intense. But look at the second attachment, where I checked the fresnel reflections checkbox. You can see that the white part where the sky is doesn't stand out at all. In fact, if you didn't know better, you wouldn't think that that's where the illumination is coming from.
I want it to have fresnel reflections so it will look correct, but I want that white to stand out at least somewhat more than the rest of the environment. Where do you think I'm going wrong in my HDRI setup?
I used the uffizi gallery light probe, converted to panorama with HDRShop.
If someone could show me what kind of settings I should put in to make this look right, or even upload an example scene (MAX 3.0) that would be really helpful.
 
Follow-ups
A: Basically, that scene is rendering totally correctly...
What is happening is that the fresnel reflections you have on are making the reflection strong at glancing angles but weak at more "full on" angles, and the resulting low contrast makes your sky look dull.
There are some issues to consider when tuning reflections...
Ok, to start with, fresnel reflections, while physically correct, great, and too often unknown to people making materials, *are not how all materials reflect*. Many materials do, notably glass, water, ceramics etc, and for these materials, fresnel reflections are a must. However, some materials do not reflect in a fresnel fashion. IIRC, metals do not have fresnel reflectance properties (I believe fresnel reflectance applies only to dielectric materials, that is, not metals). In addition to this, fresnel isn't a single kind of reflection that magically gives you physical correctness. Instead, it is a varying property, that changes with material IOR (index of refraction). Fresnel reflections can vary very widely with changing IOR - some IORs have little reflection except at glancing angles, and some have actually got more intense reflections *not* on glancing angles.
Anyway, it's a bit of a complex issue, but primarily, you should pick reflectance type based on your material type. Very reflective materials, with mirror like properties, like mirrors (well, duh :) and metals and so forth, should be created more through consistently intense reflections. That means fully reflective, highly reflective (sometimes it's more realistic to use a light grey, because almost nothing reflects light in a completely efficient way - in fact, if something reflects all light and other incoming radiation, it has a tendency to become extraordinarily cold - or a tinted colour. So if you are trying to make a gold material, try a gold tinted reflection colour.
Plastics, water, glass, ceramics, diamond, etc. should use fresnel reflections with the appropriate IOR - plastics vary a fair bit, choose a goodlooking one usually between around about 1 and 3 or 4. Water has an IOR of 1.333, glass of a varying amount, depending on the type of glass, but usually in the vague region of 1.6, though it can be below 1.5 and above 2 (1.6 usually looks great for glass renders), ceramics are tuned similarly to plastics - the difference is largely in the colouration and increased subtlety of diffuse illumination in ceramics. Diamond is in the 2 sort of range, ~2.4, IIRC. The MAX docs have a great IOR list, both in the printed manuals and in the online help - do a search for IOR in the help file.
Now, onto tinting:
Water, while often looking really blue, is mostly blue because it is reflecting the sky. Sure, it has a tendency to filter out non-blue wavelengths of light, but it is pretty colourless. Your glass of water doesn't look blue at all, and if it did, you would probably think there was blue gunk in it. So make your water either completely colourless or massively subtlely tinted - I mean a oh-so-slightly blue shifted colour.
Glass should have their refraction colour tinted to watever colour you want the glass to be - so a green glass should have a green refract colour. I'm afraid I don't remember if coloured glass tints its reflections too, but I don't know that it does - I think its only light traveling through glass that gets tinted. And clear glass is basically identical to water, but the IOR is different (1.6 or so instead of 1.333).
Ceramics and plastics have reflections that are the result of the glaze/clear surface layer, so green plastic does *not* have green tinted reflections - the colour is unmodified by the greenness, and the green effect is all from the diffuse underlayer. So make your reflect colour clear, and your diffuse colour the colour of your plastic. If doing glass, make your diffuse colour very bright, and probably a flourescent colour. Ceramics are usually a little dimmer, and act less as diffuse shading, but the main difference for trying to construct a shader approximation is that their diffuse shading is less colour saturated.
One of the major differences between the appearance of metals and other objects, as well as fresnel properties, is this reflection tinting:
Most nonmetals don't alter their reflection colours, but metals tint them.
So blue plastic has colourless reflections and blue diffuse, and blue metal has no diffuse, since it its appearance is completely from mostly focused reflections, which are tinted blue.
Now, for the material you are making, if you want a metal sphere, use the first material. It's about as accurate an approximation of metal as you can make with that stuff. The second material is very ceramicy, this is because it has a fairly dim, unsaturated diffuse shade, and subtle fresnel reflections. The sky looks dim and weak purely because it is in a low reflection area of the object - if you rotate your camera 90 degrees around the up direction of your scene and render, you will see the reflection become brighter as it gets at more glancing angles (towards the top of the sphere)... it is actually how a ceramic type object tends to look.
Hope this helps :)
Comment: Your explaniation on materials setup was VERY helpful. I understand now how to make nice metals and plastics such as:
without using a ton of useless fresnel reflections :
Comment: Though for a plastic earth you might want to put a fresnel reflection in - plastics do reflect in a fresnel way, and fresnel reflections are still a great tool for realistic glass, ceramics, and plastics, they just aren't really for metals... so that earth globe might look even nicer with an IOR 2 or 3 fresnel falloff on its blurry reflections...
But those are damn nice materials you did there - the metals look good enough to eat. Or something... :)
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