Specularity, Fresnel Terms

Specular Highlight on Opaque Sphere

Here I have used the Blinn-Phong approximation that involves computing the half-vector. The half vector is the sum of the Light and View Vectors, normalized. Dot product of this with the normal, raised to a power (specular coefficient) gives the value of the specular highlight at any point. The higher the power, the more concentrated the spot becomes.


Specular highlight + refraction

You can see that the specular highlight has become slightly dull because the sphere is transparent. Some of the light gets transmitted and some is reflected. This is implemented as a weighted average of the reflected and refracted light, where the weights are the coefficients of transmittance and reflectance.


Constant transmittance on a green sphere

This image shows a sphere with refractive index 1.01, and a transmittance of 0.9 (90% light goes through). The amount of light that gets transmitted is constant all over the sphere.


Fresnel transmittance

This image shows the same sphere, now rendered using Fresnel equations for reflection coefficient. Transmission coefficient is simply 1 - reflection coeffecient. The net effect is that when light is incident perpendicular to a surface, more light gets transmitted than if the ray hits the surface almost parallel to the surface. This can be seen in real life on a water surface. Water from a grazing angle is highly reflective, but when you look at it straight from the top, you can see right through the water. In the above image, this causes the edges of the sphere to be visible, while the rest of the sphere is highly transparent. On increasing the refractive index, the sphere becomes less transparent.