Lighting and Shading

• The next section discusses how to add light to the scene.
• This is not perfect
• Nor will our discussion be complete.
• How does light work in the real world?
  • Light sources emit light rays
  • These rays either reflected (bounce off) or are absorbed by surfaces.
  • In the model, we assume that different frequencies are either reflected or absorbed differently by the individual surfaces.
    • The material they are constructed from determines this
  • And the angle might change based on the type of surface
    • Smooth surfaces reflect well
    • Not smooth surfaces (rough) reflect less well.
    • This also contributes to the color of the surface.
  • Finally, the angle with the viewer makes a difference as well.
    • Think of how light reflects off a curved "shiny" surface.
• Ideally we would trace rays of light from each light source, bouncing off each surface, until they either
  • Hit window in the viewplane
  • Exit the scene.
• Unfortunately, this is not possible
  • Infinite rays from a single source.
• Radiosity is one attempt to duplicate this
  • But it is computationally intensive.
  • And beyond the scope of this class.
• Ray Tracing is another attempt
  • It is also computationally intensive.
  • But the idea is simpler
  • From each pixel in the scene
    • Shoot a ray out into the scene
    • Have it interact with objects until
    • It reaches a light source.
    • Or it exits the scene
    • Shadows, reflections are all good.
• Another attempt is the illumination model that we will discuss next.
  • Attempt to model the physics of the scene at each pixel to be rendered.
  • Shadows and reflections are awful (not part of the model)
  • But it is fast
  • And it is mostly good.
• Surfaces generally have different properties/attributes
  • A specular property defines a direct reflection of light.
    • Highly specular surfaces reflect light almost perfectly
  • A diffuse property defines how much the surface absorbs or scatters light.
    • A perfectly diffuse surface will scatter light equally in all directions.
  • A transparent/translucent property defines how much light passes through a surface.
  • Finally, an ambient property defines how much the light comes from the surface.
    • This is a fudge factor in the model.
    • But we may need to control it.
• Lights also have properties
  • They have a position
  • The have a r,g and b component. describing the light emitted.
  • They may have an attenuation or fall off property.
  • They may also have a direction
  • Or other properties.