**Prerequisite –** Basic Illumination Models

**Phong model of reflection :**

When we look at illuminated shiny surfaces, such as glittering surfaces, polished metal sheets, apple etc, we found a kind of bright spot at certain viewing point locations. This phenomenon is called specular reflection.

Look at the following figure :

N = Normal vector L = Point light source V = Viewing direction R = is representing the unit vector directed towards the ideal specular reflection ∅ = Viewing angle relative to the specular reflection direction R. θ = Angle made by L & R with N.

For ideal reflector surfaces(perfect mirror), incident light is reflected only in the specular-reflection direction. So, in this case, we could be able to see reflected light when **vectors V & R coincides(viewing angle(∅=0))**.

A Shiny surface has a narrow specular reflection range, while a dull surface has a wider reflection range. An empirical model for calculating the specular reflection range, invented by the Phong Bui Tuong is also known as **Phong specular reflection model**. This model sets the intensity of specular reflection directly proportional to the **cos ^{n}_{s}(∅)**. The range of angle ∅ can lie between

**0 ≤ ∅ ≤ 1.**Where

**n**is a

_{s}**specular reflection parameter**whose value is determined by the type of surface to be displayed. The value of

**n**for brighter(shiny) surfaces could be 100 or more whereas for dull surfaces its value is 1 or less than 1. The intensity of specular reflection depends on the object(Material) properties of the surface & the angle of light incidence, as well as other factors such as the polarization and color of the light incident.

_{s}We can control the intensity variation of the light through**, specular-reflection,** using spectral-reflection function **W(∅)** for each surface. Where **∅** the value lies in the range of **0 ≤ ∅ ≤ 1. **In general** W(∅) **tend to increase as the angle of incidence increases, at ∅=90* W(90*)=1, and in this case, all the light incidents on the surface of the material is reflected. So, using the spectral-reflection function W(∅) we can write the **Phong specular reflection model as : **

For many opaque material surfaces, specular reflections are nearly constant for all incident angles. So, in such case, we can replace **W(∅) with a constant coefficient(Ks)**, and the value lies between** 0 & 1,** for each surface:

Since, V & R are unit vectors so, |V|=|R|=1 : V * R = |V|*|R|*cos(θ) , V * R = cos(θ)

So, we can write just the previous equation as:

Here, **R **can be calculated by the projection of **L** onto the direction of the normal vector is obtained:

R + L = (2*N.L)*N So, using the above equation specular-reflection vector is obtained, R = (2*N.L)*N - L.

Combined ambient, diffuse and specular reflections in the Phong model can be represented as the following equation with multiple light sources:

So, for a single point light source, we can model the combined & specular reflections from a point on an illuminated surface as :

And, for **n** point light source, the equation will be: