Types of lighting sources

Light sources used in computer graphics:

1. Point source: emits light equally in all directions from a single position (like a bulb). L differs for each surface point, and the light is attenuated with distance by f(d).
2. Directional (infinite, distant) source: a point source very far away (like the sun). All rays are parallel, so L is the same for the whole scene and there is no distance attenuation.
3. Spotlight: a point source with a direction and a cone of half-angle θ_l. The intensity is greatest along the cone axis and falls off towards the edge of the cone: I ∝ cos^a(α), where α is the angle from the cone axis. There is no light outside the cone (α > θ_l).
4. Area (distributed) source: light comes from a surface of finite size (such as a tube light), giving soft shadows. It is modelled as many point sources.
5. Ambient light: uniform, non-directional light that comes from the surroundings.

Effect on the illumination model

• The light direction L, used in the diffuse term (N·L) and the specular term, depends on the source position (for a point source) or is constant (for a directional source).
• The intensity IL is modified by the attenuation f(d) for point sources and by the angular factor for spotlights.
• Contributions from several sources are added: I = ka·Ia + Σ f(d_l)·IL_l·[kd (N·L_l) + ks (N·H_l)^n].
• Ambient light adds a constant term ka·Ia.
• The type of source also affects shadows and highlights.