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Define illumination and surface rendering

Short answer

Illumination model computes reflected intensity at a point using light sources and surface properties, e.g., Phong: I = ka·Ia + IL[kd(N·L) + ks(R·V)^n]. Surface rendering applies this model to generate pixel colors, using methods like flat, Gouraud, or Phong shading.

Define the terms illumination and (surface) rendering, with the illumination model.

Illumination

Illumination (lighting) model is the set of equations used to calculate the intensity of light reflected from a surface point towards the viewer. It considers the light sources (position, intensity, colour), the surface properties (colour, reflection coefficients, texture, transparency) and the position of the viewer.

Basic (Phong) illumination model for a point source:

I = ka·Ia + IL [ kd (N·L) + ks (R·V)^n ]

with ambient, diffuse and specular terms (N = normal, L = light direction, V = view direction, R = reflection direction, n = shininess).

Surface rendering

Surface rendering is the process of using an illumination model to compute the intensity of every pixel of a surface and display it. Rendering methods apply the model either to every pixel or to selected points and interpolate:

  • Constant (flat) shading: one intensity per polygon.
  • Gouraud shading: intensities calculated at vertices and interpolated.
  • Phong shading: normals interpolated, intensity calculated per pixel.
  • Ray tracing / radiosity: global illumination methods.
Light source + surface properties
        |
 Illumination model  ->  intensity at a point
        |
 Surface rendering (flat/Gouraud/Phong) -> pixel colours

The illumination model gives the intensity of a point; the rendering method decides how this is applied to the whole surface.

✓ Answer checked by Claude Opus 5.5 (high effort)

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