The color of each polygon is constant. Could be produced by the Phong model with directional light sources.

Interpolated shading - Each point of the polygon has its own color. These models aim at a smooth transition from a polygon to its neighbor.

Gouraud Interpolation

• Compute the normal in each vertex of the polygon. Average the normals of all the polygons for which the point is a vertex.
• Compute the color in each vertex based on the average normal, for example, using the Phong model.
• Interpolate the color inside each polygon.

Color with Gouraud Normals

• Compute the color in each vertex based on the average normal.
• Compute the color in each point inside the polygon by a linear interpolation based on the colors of the vertices.
• If c₁, c₂, and c₃ are the colors computed in P₁, P₂, and P₃ respectively, and the color in Q is c,
• Q=(1-t)((1-s) P₁ + s P₂) + t P₃
• c = (1-t)((1-s) c₁ + s c₂) + t c₃

Phong Shading

• Compute the normal in each vertex of the polygon. Average the normals of all the polygons for which the point is a vertex.
• Interpolate the normal in each point inside a polygon based on the normals of the vertices.
• Compute the color in each point based on the interpolated normal, for example, using the Phong model.
• For the case of the triangle, compute the normals N₁, N₂, and N₃ in each vertex as before. The the normal N in the point Q is
•      N = (1-t)((1-s) N₁ + s N₂) + t N₃
• This model produces smoother results than the Gouraud model, but it's also slower.

• non-interpolated, each polygon has a constant color:

glShadeModel(GL_FLAT);
• interpolated color:

glShadeModel(GL_SMOOTH);

Smooth Color in OpenGL

Specifying the normal before each vertex in any sequence of geometrical commands. All transformations apply equally to the normals.

glBegin(GL_QUADS);
...
glNormal3f(nx, ny, nx);
glVertex3f(vx, vy, vz);
...
glEnd();

• Customize a light source:

glLightfv(GL_LIGHT0, GL_POSITION, light_position);
glLightfv(GL_LIGHT0, GL_AMBIENT, light_ambient);
glLightfv(GL_LIGHT0, GL_DIFFUSE, light_diffuse);
glLightfv(GL_LIGHT0, GL_SPECULAR, light_specular);

Spotlight in OpenGL (Projection Light Source)

• After defining the position, ambient, diffuse, and specular components of the light source,
• define the angle of the spotlight:
  

  glLightf(GL_LIGHT0, GL_SPOT_CUTOFF, 60.0);

• Define the shininess of the spotlight:
  

  glLightf(GL_LIGHT0, GL_SPOT_EXPONENT, 60.0);

Material Properties in OpenGL

• glMaterialfv(side, type, array_of_values);
  glMaterialf(side, type, value);
• Side: GL_FRONT, GL_BACK, GL_FRONT_AND_BACK .
• Type: GL_SPECULAR, GL_AMBIENT, GL_DIFFUSE, GL_SHININESS.
• Once the material properties are defined, they replace the usual calls to glColor3f(...);

Lights and glColor

• When the lighting is turned on, to be able to use the glColor, one must enable some general material properties:
  glEnable(GL_COLOR_MATERIAL);
  glColorMaterial(GL_FRONT, GL_AMBIENT_AND_DIFFUSE);
  GL float spec={0.3, 0.3, 0.3, 0.1};
  glMaterialfv(GL_FRONT, GL_SPECULAR, spec);
  
• After this, the material properties for each object can be redefined with a simple glColor3f(...); call.