CMSC 635: Advanced Computer Graphics Global Illumination

Spring 1999

David S. Ebert Computer Science and Electrical Engineering Department University of Maryland, Baltimore County

Global Illumination

• Global illumination must consider 4 types of light transport between pairs of surfaces.

1. diffuse to diffuse

2. specular to diffuse -- handles caustics and refraction

3. diffuse to specular

4. specular to specular

• All must be considered at once for each light propagation path.

• Post processing radiosity solution with ray-tracing won't work!

Why?

1. some specular may be result of earlier diffuse reflection.

2. some diffuse may be re-reflected specular

• diffuse to diffuse correctly

• Ray tracing does:

• specular to specular correctly

• diffuse to specular empirically

• specular to diffuse can be added in by a 2 pass method:

Pass1 -- backward trace light from source to environment.

Pass2 -- normal tracing using illumination from Pass1.

Combining Ray Tracing and Radiosity -- Two pass method:

• Pass1: Enhanced Radiosity - View Independent [Rushmerier 90]

• handles:
diffuse - diffuse
specular-diffuse

• General Equation

• diffuse transmission:

where is backward diffuse transmission.

• * specular transmission: path j may specularly transmit
light-window form factor

• * specular-diffuse reflection: mirror form factor
add terms to handle seeing ``virtual'' through mirror surface .

* - The last two items are only for specular surfaces that can't see each other.

• Pass2: ``Enhanced Ray-Tracing'' -- View Dependent

• specular-specular -- normal ray tracing

• diffuse-specular:

• Integrate light arriving at reflection point by using a square pyramid in the direction of specular bump.

• Divide into grid and do a z-buffer at low resolution to see what is visible -- use radiosity calculated intensities here.

• can do recursively

• only handles special cases of specular transmission and specular-diffuse reflection.

Rendering Equation [Kajiya 1986]

where

• I(x,x') = transport intensity (from x' to x)

• g(x,x') = visibility between x and x'

• = transfer emittance from x' to x.
• = scattering or bidirectional reflectivity -- intensity of light at x' from x''scattered in direction x.

• s means all points on all surfaces in the scene.

Interpretation:

transport from x' to x =
sum of light emitted from x' in direction of x
plus the total light scattered from x' to x from all other surfaces.