Lights

• This is chapter 4 (sort of) in your book
  • I am referencing 4.1 right now.
  • But the commands in this section of the book are specific to opngGL 1.x
  • And WILL NOT WORK in webgl.
  • Lights have changed.
  • Or more appropriately been removed.
• A great Demo
  • I did not write this one.
  • But it is superb.
  • Here is the data for the plane.
    • Three vertex points
    • two texture points
    • The normal vector at this location (3 points)
• This represents two old lighting models
  • Gourand Shading, at the triangle level.
    • Set the model to sphere to see the difference.
  • Phong-Blin Shading at the pixel level.
• These models are polygon
  • Again, use the sphere model and look at the edges.
• For this discussion, consider a "point" light source
  • It really has no width or height, it is a point.
    • Nothing to "see" in the reflection.
  • It emits light in all directions.
  • But later we can control this.
• The idea behind the two models are essentially the same.
  • Light is made up of three different components.
    • Ambient light is light that just fills the room.
      • Natural background light.
      • This is the roundoff error.
      • Everything everywhere in the scene is light by this light.
    • Secular light or secular reflected light is light that is reflected by smooth surfaces.
      • This is the shiny spot
      • Perhaps the direct reflection of the light source.
      • This is related to the position of the viewer.
    • Diffuse light or diffuse reflected light is the light that is reflected from less reflective material.
      • This is the light that is not absorbed by the item.
  • The book has a great image for this
  • Source Eck, Introduction to Computer Graphics.
• These reflections are a combination of the properties of
  • The material doing the reflection
  • The the light being reflected.
• Light properties include
  • The position of the light.
  • The orientation of the light, spotlight vs
  • The color of the light
  • The attenuation or "fall off" of the light.
• Along with position we can assign properties to the model as well
  • Color is normally replaced with ambient, secular and diffuse color components.
  • In this model we also assign a shininess value to the material.
  • And we will defiantly need vertex normals
• Sometimes we allow a material to generate light
  • This is the emissive color of the material.
  • This does not change the color of nearby objects.
  • And is not part of the classical color model.
• Color related items are usually 3 vectors
  • These are now describing the reflectivity of the components.
  • <0.0, 1.0, 0.5> reflects no red light, completely reflects green, and absorbed/reflects 50% of the blue light.
• Normal Vectors
  • A polygonal shaded model uses one normal per polygon.
  • This is called flat shaded
  • A fragment or pixel shaded model uses an interpolated normal at each point.
  • This is called smooth shading
  • Normal vectors will need to be transformed with the object.
  • But be careful of non-uniform scaling
• The lighting model
  • The model we are about to discuss is a crude attempt to model lighting.
  • It does not include shadows, reflections, transparency, or other items.