   About Fresnel term
  I'm writing an article about rendering.  I find that the term "Fresnel effect" is very confusing to me.  It make me very nervous about the usage of this term.When I read the information from the internet, most of them is referring it is an effect about reflection and refraction.  However, in most actual demonstration (either captured images or rendering program), the Fresnel effect is implemented as the linear interpolation between blurry reflection (sometimes just simple lighting) and mirror reflection with a fresnelPower as the weighing.  Why are they calling those opaque lighting demonstration as "Fresnel effect"?  Is Fresnel effect only applicable to transparent/translucent objects?   
  The linear interpolation you see in the code is probably the Schlick approximation of the Fresnel reflection factor. The Schlick approximation is this:Rf(theta) approx =  Rf(0) + (1 - Rf(0))(1 - max(cos theta, 0)) ^ 5But according to Real-Time Rendering 3rd ed., the Fresnel effect happens with a optically flat plane. What happens is when light rays "hits" this perfect plane it goes in 2 directions: the ideal reflection direction and the ideal refraction direction. As the light hits at increasingly glancing angles, more light is reflected than refracted. The radiant power to which the light is reflected is different for each surface and for what angle the ray hits at compared to the normal of the surface. Then the power to which its refracted at is equal to (1 - the radiant power of the reflection) because of energy conservation.It doesn't only apply to translucent/transparent objects though, and in fact you can apply it to basically any object if you use microfacet theory. Which basically is making the assumption that at the subpixel level of the object it is made up of some perfect planes that the Fresnel effect will apply to. Lighting models like Cook-Torrance I'm pretty sure use microfacet theory.   ;Quote:Original post by mikfigThe linear interpolation you see in the code is probably the Schlick approximation of the Fresnel reflection factor.The linear interpolation I said is not the one you described.  That Fresnel Reflection Demo as an example.  While the document discuss about the refractive index of a number of transparent medium (e.g. water, diamond, etc), the implementation of the Fresnel term is simply the linear interpolation between the mirror-reflection and the ordinary-material-color (weighted by Fresnel’s formula).Is the author trying to simulate the appearance of a dull surface with a thin layer of transparent medium coating?  Is it appropriate for me to describe Fresnel Reflection Demo as Fresnel Reflection?[Edited by - ma_hty on May 26, 2009 1:11:50 PM];  Ah okay, I took a look at the more detailed whitepaper that is associated with Nvidia's Fresnel Reflection Demo. I'm pretty sure I see which linear interpolation you are talking about. The reason they interpolate between the environment map and the normal material is simply because the fresnel reflectance factor tells you how much of the light is reflected.The light that isn't reflected is refracted. The diffuse color is supposed to be the light that is refracted into the surface, scattered, and then reflected out of the surface again. So when the fresnel factor is less, more of the diffuse color is reflected rather than the surface reflection. Although one thing you should be aware of is that in reality the fresnel reflectance factor is also supposed to affect the specular term too because the specular term is supposed to simulate surface reflectance. But I guess to keep it simple in this example, they have the whole material color simulate the refracted light and the environment map simulate the reflected light.Edit: Oh, I forgot to address your last two questions.1. Quote: Is the author trying to simulate the appearance of a dull surface with a thin layer of transparent medium coating? This method of simulating the fresnel effect only physically makes sense for smooth surfaces. So unless the coating made the surface really smooth, than the fresnel effect still wouldn't apply as far as I know.2. Quote:Is it appropriate for me to describe Fresnel Reflection Demo as Fresnel Reflection?Yes, this demo is simulating the fresnel effect that we are discussing.[Edited by - mikfig on May 26, 2009 10:34:59 PM];Quote:Original post by mikfig...But I guess to keep it simple in this example, they have the whole material color simulate the refracted light and the environment map simulate the reflected light....Thanks you very much.  I got it. ^^   ;  Glad I could be of assistance :D   