Shading Models

Shading Models

Shading determines the color of surfaces based on lighting, material properties, and viewing direction. Different shading techniques trade quality for performance.

Flat Shading

Each polygon is assigned a single color computed from its face normal. Simple and fast, but produces visible polygon boundaries.

Flat Shading:

  1. Compute face normal: n = (V1-V0) × (V2-V0)
  2. Compute lighting once per face
  3. All pixels in the face get the same color

      +--------+        +--------+
      | color1 |        | color1 |  Entire face
      | color1 |        | color1 |  same color
      | color1 |        | color1 |
      +--------+        +--------+

  Pros: Fast
  Cons: Faceted appearance

Gouraud Shading

Computes lighting at each vertex, then interpolates colors across the triangle. Produces smooth gradients but can miss specular highlights.

Gouraud Shading:

  1. Compute normal at each vertex (average adjacent faces)
  2. Compute color at each vertex using lighting model
  3. Interpolate colors across triangle using barycentric coords

      V0(color0)---------V1(color1)
         \                   \
          \  interpolated     \
           \  colors           \
            \                   \
      V2(color2)-------------------

  Color at pixel P:
    C(P) = λ0*C(V0) + λ1*C(V1) + λ2*C(V2)

  Pros: Smooth appearance, moderate cost
  Cons: Misses per-pixel highlights

Phong Shading

Interpolates normals across the triangle and computes lighting at each pixel. Produces the most accurate highlights but is computationally expensive.

Phong Shading:

  1. Compute normal at each vertex
  2. Interpolate normals across triangle:
     n(P) = λ0*n(V0) + λ1*n(V1) + λ2*n(V2)
  3. Compute lighting at each pixel using interpolated normal

      V0(n0)-------------V1(n1)
         \                   \
          \  interpolated     \
           \  normals          \
            \                   \
      V2(n2)-------------------
      For each pixel: compute lighting with local normal

  Pros: Best visual quality, accurate highlights
  Cons: Most expensive per pixel

Comparison

Method      | Per-Vertex | Per-Pixel | Quality | Speed
------------|-----------|-----------|---------|------
Flat        | No        | No        | Low     | Fastest
Gouraud     | Yes       | No        | Medium  | Medium
Phong       | Yes       | Yes       | High    | Slowest

Note: "Phong shading" ≠ "Phong reflection model"
  - Phong shading = interpolation technique
  - Phong reflection = lighting calculation