Light

Tom Kelliher, CS 320

Mar. 25, 1998

Announcements:

From last time:

1. 3-D maze.

Outline:

1. Real world lighting.

2. A lighting model.

3. Shading.

4. The Phong reflection model.

Assignment: Read Chapter 14.

Lighting in the Real World

1. Viewer, lights, objects.

2. Light properties?

3. Material properties: Translucence, reflectance (specularity), scattering (diffusion). Examples? Color of an object.

4. How do lights and materials interact?

5. The rendering equation. Calculation for each point in a scene.

6. Need a balance between accuracy and efficiency.

7. Local vs. global lighting. The graphics pipeline.

A Lighting Model

1. General illumination function for a light source: .

2. Types of modeled light sources:
   1. Ambient light

   2. Point sources

   3. Spotlights

   4. Distant light sources

Color Sources

1. Illumination function is a continuous function of wavelength.

2. Complex computation, vision model.

3. Luminance function:

   

Ambient Light

1. Uniform light --- ``background'' light.

2. Model:

   

Point Sources

1. Emits light equally in all directions.

2. Assume point source at . Color vector:

   

3. Illumination at due to ? Depends upon square of distance:

   

4. High contrast harshness due to shadow effects: umbra, penumbra.

5. In practice, replace inverse square term with

   

   where d is the distance and a, b, and c are constants chosen to soften.

Spotlights

1. Simple spotlight: point source with light emitted only through narrow range of angles.

2. Consider the source at to be restricted by the cone described by and .

3. For accuracy, distribution within the cone is modeled by .

Distant Light Sources

1. Re-calculating the -- vector.

2. If the distance is ``large'' how much does the vector change?

3. Replace source location with source direction:
   1. Near source: (a point)

   2. Far source: (a vector)

Shading

1. Flat shading: each point on a polygon assigned same color.

2. Gouraud (smooth) shading: assign colors individually to vertices, interpolate.

The Phong Reflection Model

1. Consider an object point, and a light source .

2. Important vectors:

   

   1. l: vector to light source.

   2. n: surface normal.

   3. v: vector to COP.

   4. r: reflection vector.

3. The light from source to object can be described by:

   

   (theoretically wrong but, in practice, right)

4. Using material properties, distance from source, orientation of surface and direction of source a reflection matrix can be constructed:

   

5. (Simplified) Illumination at :

   

   A global ambient term may be ``thrown'' in.