Phong Shading

• Phong Shading interpolates lighting model parameters, not colours.
• Much better rendition of highlights.
• A normal is specified at each vertex of a polygon.
• Vertex normals are independent of the polygon normal.
• Vertex normals should relate to the surface being approximated by the polygon.
• The normal is interpolated across the polygon (using Gouraud techniques).
• At each pixel,
  • Interpolate the normal...
  • Interpolate other shading parameters...
  • Compute the view and light vectors...
  • Evaluate the lighting model.
• The lighting model does not have to be the Phong lighting model...
• Normal interpolation is nominally done by vector addition and renormalization.
• Several ``fast'' approximations are possible.
• The view and light vectors may also be interpolated or approximated.
• Problems with Phong shading:
  • Distances change under perspective transformation

    Can't perform lighting calculation in projected space

  • Normals don't map through perspective transformation

    Normals lost after projection

  • Have to perform lighting calculation in world space
  • Requires mapping position backward through perspective transformation
• Pipeline is one way and lighting information lost after projection

  Therefore, Phong not normally implemented in hardware