Lesson 8 of 10

Shade Faces, Vertices, or Pixels

Choosing a lighting formula leaves another question: where do you evaluate it? Shading methods choose where colors or normals are calculated and interpolated.

Three approaches

• Flat: Compute a constant face color using a representative normal and other values. Triangle boundaries remain distinct.
• Gouraud: Evaluate lighting at vertices and interpolate colors inside.
• Phong shading: Interpolate vertex normals, renormalize, then evaluate lighting per fragment.

Phong shading names a normal-interpolation method. The Phong lighting model from the previous lesson is a reflection formula. They are often combined but are different concepts.

Why a small highlight can disappear

Suppose a narrow highlight lies at the center of a triangle, while all vertices are dark. Blending only vertex colors with Gouraud shading can miss it. Phong shading evaluates lighting inside the triangle and can capture it.

Simple color interpolation works like this:

a, b, t = 0.2, 1.0, 0.25
print(round((1 - t) * a + t * b, 2))

The result is 0.4. If both endpoints were 0.2, every blend would remain 0.2. Interpolating vertex colors cannot invent a highlight absent from those values.

Check your understanding

Does smooth normal interpolation also make a low-polygon sphere’s outer silhouette round?