illumination model

Illumination Models 

• Motivation: 

In order to produce realistic images, we must simulate the appearance of surfaces under various lighting conditions

 • Illumination Model: Given the illumination incident at a point on a surface, quantifies the reflected light.

Illumination Model Parameters

• Lighting effects are described with models that consider the interaction of light sources with object surfaces 

• The factors determining the lighting effects are: – 

--The light source parameters: 

• Positions

 • Electromagnetic Spectrum 

• Shape

 – The surface parameters 

• Position 

• Reflectance properties 

• Position of nearby surfaces

 – The eye (camera) parameters 

• Position 

• Sensor spectrum sensitivities

Ambient Illumination

 • Assume there is some non-directional light in the environment (background light)

 • The amount of ambient light incident on each object is constant for all surfaces and over all directions

 • Very simple model, not very realistic

 • OpenGL default Ambient Illumination

 • The reflected intensity Iamb of any point on the surface is:

                                           Lamb  = Kala

Ia - ambient light intensity Ka ∈ [0,1] - surface ambient reflectivity 

• In principle Ia and Ka are functions of colour, so we have I R amb, I G amb and I B amb

Diffuse Reflection

 • Diffuse (Lambertian) surfaces are rough or grainy, like clay, soil, fabric

 • The surface appears equally bright from all viewing directions

 • The brightness at each point is proportional to cos( θ ) θ N L Diffuse Reflection

• Brightness is proportional to cos( θ) because a surface (a) perpendicular to the light direction is more illuminated than a surface (b) at an oblique angle a

Phong model :-

A more accurate method for rendering a polygon surface is to interpolate the normal vector and then apply the illumination model to each surface point. This method developed by Phong Bui Tuong is called Phong Shading or normal vector Interpolation Shading. It displays more realistic highlights on a surface and greatly reduces the Match-band effect.