---
title: "Define illumination and surface rendering"
subject: "Computer Graphics"
chapter: 8
key: "semester_5/computer_graphics:define-illumination-rendering"
source: https://www.bctengineer.com/q/semester_5/computer_graphics/define-illumination-rendering
site: BCT Engineering
---

# Define illumination and surface rendering

Computer Graphics · Chapter 8: Illumination and Surface Rendering methods

- Asked in: 2078 Bhadra, 2 marks; 2076 Chaitra, 2 marks; 2069 Asar, 4 marks (asked 3 times)
- Video: [Illumination Model in Computer Graphics | CG | Lec-47 | Bhanu Priya](https://www.youtube.com/watch?v=vMsjhletTzo) — Education 4u
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_5/computer_graphics/define-illumination-rendering
- Whole chapter: https://www.bctengineer.com/md/semester_5/computer_graphics/chapter-8

## Question

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

## 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.

## Answer

#### 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.

```text
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.

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. What does an illumination model compute?
   A. Intensity of light reflected from a surface point towards the viewer
   B. Pixel positions
   C. Depth of polygons
   D. Mesh vertices
   Answer: A. It uses lights, surface properties and viewer position. (Key point: Illumination model)

2. What is surface rendering?
   A. Removing hidden surfaces
   B. Using the illumination model to compute pixel intensities of a surface
   C. Building the mesh
   D. Clipping lines
   Answer: B. It applies the model to pixels directly or by interpolation. (Key point: Surface rendering)

3. Which pairing is right?
   A. Gouraud interpolates normals; Phong intensities
   B. Both interpolate depths
   C. Gouraud interpolates intensities; Phong interpolates normals
   D. Both give one intensity per polygon
   Answer: C. Flat shading gives one intensity per polygon. (Key point: Gouraud vs Phong)

4. What decides how the intensity applies over the whole surface?
   A. The illumination model alone
   B. The frame buffer
   C. The normal only
   D. The rendering method
   Answer: D. The model gives a point's intensity; rendering spreads it. (Key point: Division of roles)

## Related in Computer Graphics

- [Phong illumination model expression](https://www.bctengineer.com/question.md?key=semester_5%2Fcomputer_graphics%3Aphong-illumination-model-derive) — Computer Graphics, chapter 8, asked 8×
- [General illumination model](https://www.bctengineer.com/question.md?key=semester_5%2Fcomputer_graphics%3Ageneral-illumination-model) — Computer Graphics, chapter 8
- [Model curved surface using polygons](https://www.bctengineer.com/question.md?key=semester_5%2Fcomputer_graphics%3Acurved-surface-polygons-tables) — Computer Graphics, chapter 6
- [Define terms; Phong intensity equation](https://www.bctengineer.com/question.md?key=semester_5%2Fcomputer_graphics%3Adefine-terms-lambert-phong-equation) — Computer Graphics, chapter 8
- [Polygon tables to model Dharahara](https://www.bctengineer.com/question.md?key=semester_5%2Fcomputer_graphics%3Apolygon-vertex-edge-table-dharahara) — Computer Graphics, chapter 6

## Related in other subjects

- [Colour rendering and CRI in luminaire selection](https://www.bctengineer.com/question.md?key=semester_6%2Findustrial_power_distribution_and_illumination%3Acolour-rendering-cri) — Industrial Power Distribution and Illumination, chapter 6
- [Compare software models; propose a fourth model](https://www.bctengineer.com/question.md?key=semester_5%2Finstrumentation_ii%3Asw-models-compare-fourth) — Instrumentation II, chapter 9
- [Equivalent-π model of a long line](https://www.bctengineer.com/question.md?key=semester_4%2Fpower_system_analysis_i%3Aequivalent-pi-long-line) — Power System Analysis I, chapter 5