1. Definition

Polygon shading is a technique used to determine the color and brightness of a polygon in a 3D scene based on light interaction and surface properties.

• Polygons are the basic building blocks of 3D models.
• Shading improves the visual realism of polygons by simulating light and material effects.
• Depending on how lighting is calculated, polygon shading can be flat, Gouraud, or Phong.

2. Types of Polygon Shading

A. Flat Shading

• Assigns a single color to the entire polygon based on one normal vector (usually the face normal).

• Steps:

  1. Compute the polygon’s normal.
  2. Apply lighting model (ambient + diffuse + specular) at one point (typically the centroid).
  3. Fill the polygon with that color.

• Pros: Simple, fast.

• Cons: Faceted appearance, edges of polygons are visible.

B. Gouraud Shading

• Computes lighting at vertices and interpolates colors across the polygon.

• Steps:

  1. Compute vertex normals (usually by averaging adjacent face normals).
  2. Apply lighting model at each vertex.
  3. Interpolate vertex colors across the polygon during rasterization.

• Pros: Smooth shading, simple highlights on curved surfaces.

• Cons: Can miss small specular highlights (e.g., tiny bright spots).

C. Phong Shading

• Interpolates normals across the polygon and computes lighting per pixel.

• Steps:

  1. Compute vertex normals.
  2. Interpolate normals for each pixel inside the polygon.
  3. Apply Phong illumination model per pixel.

• Pros: Realistic highlights, smooth appearance.

• Cons: More computationally expensive than Gouraud or flat shading.

3. Shading Pipeline for Polygons

1. Vertex Processing: Compute vertex positions and normals.
2. Lighting Calculation: Apply ambient, diffuse, and specular components.
3. Interpolation: Depending on shading method, interpolate color or normals across polygon.
4. Rasterization: Fill polygon pixels with computed colors.

4. Summary Table

Key Points:

• Polygon shading determines how polygons appear under lighting.
• Choice of shading affects realism, performance, and visual quality.
• Flat → Gouraud → Phong represents increasing realism and computational cost.