Illumination Models

What is Illumination?

Illumination is the transport of luminous flux from light sources through both direct and indirect paths.

What are the different types of illumination?

Local Illumination:

Local illumination only models the light arriving directly from the light source. Tricks and multiple rendering passes are used to add such things as interreflections and shadows.

Global Illumination:

Global Illumination deals directly with the interaction with the environment. Such as how one surface affects another. It extends the local illumination model to include reflection, refraction, transparency, shadows, and antialiasing.

What are the two types of illumination models?

Empirical:

Empirical illumination models are not physically based. They are simple approximations to observed phenomena.

Physically-Based:

Physically based illumination models are more complex and model the actual physics of light interactions.

What are the two components of illumination models?

Light Sources:

The light sources include the emittance spectrum of the light (colour of the light), the geomettry of the light (where the light is located), and the directional attenuation.

Surface Properties:

The surface properties include the reflectance spectrum (the colour of the relected light), the geometry of the surface (where is the surface, how is the surface oriented, and the mirco-stucture of the surface) and the absorption of the surface.

Local Illumination Models:

Emprical:

Ambient, Diffuse, Specular:

Combine the ambient, diffuse and specular reflection functions. It is used in OpenGL.

Model Function:

There may be a number of point sources. The following equation expands on the basic model above to include multiple light source. This is known as the standard local illumination model. It considers only light sources and surface properties. The approximations are based on appearance.

The following equation is a local illumination model that includes (1/s+k) where s relates to eye-object distance and k is some constant for the scene. A transperancy term has also been included in the equation.

It should be noted that three equations are used in applying the mode to computer graphics, one for each R, G, B value.

Physical Models:

Bi-directional Reflectance Distribution Function (BRDF)

The BRDF model describes the fraction of light that is reflected for all combinations of incoming (light) and outgoing (viewing) directions.

Global Illumination Models:

Applying BRDF to Global Illumination - Kajiya's Rendering Equation:

The viewer is at the point x, and is looking toward the point x'.

I(x,x'):Determines the amount of light arriving at x from x'.

g(x,x'):Is a geometry term. It is equal to 0 when x' is occluded, otherwise it is equal to the attenuation factor of 1/r^2 (or 1/(s+k)).

ε(x,x'):Is the amount of light emitted from x' to x.

ρ(x,x',x''): Is the fraction of light that is reflected and scattered off x' to the point x from the point x''.

The integral S is over all such points (x'') on all surfaces.

What are the two types of Global Illumination Methods?

View Dependent Methods:

The global illumination is dependent on where the eye is located, calculate the view from the camera. The above model, BRDF is a view dependent model. Ray Tracing is another example of a view dependent model.

View Independent Methods:

In view independent methods, the lighting is solved for the entire scene. Radiosity is a view independent solution.