Phong Interpolative Shading
A way of shading in which we [see page 27, bilinearly interpolate] the normals of each of the three points across the surface of the polygon and then calculate an intensity for each pixel.
See here and [see page 41, here] if you're still confused. I.E. PIS [see page 42, interpolates] between vertex normals at as fine a resolution as you're rendering.
Costs
- Vector interpolation is 3 times more expensive than intensity interpolation.
- Normals need to be normalized at each step.
- You need to to lighting calculations for each pixel.