---
title: "Dithering technique"
subject: "Multimedia System"
chapter: 3
key: "semester_8/multimedia_system:dithering"
source: https://www.bctengineer.com/q/semester_8/multimedia_system/dithering
site: BCT Engineering
---

# Dithering technique

Multimedia System · Chapter 3: Images and Graphics

- Asked in: 2074 Magh, 5 marks; 2072 Magh, 5 marks; 2073 Bhadra, 3 marks (asked 3 times)
- Video: [Multimedia P1( Quantization - Floyd Error Dithering - Ordered Dithering Method )](https://www.youtube.com/watch?v=cilkoTIwLHE) — Mohamed Farg
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_8/multimedia_system/dithering
- Whole chapter: https://www.bctengineer.com/md/semester_8/multimedia_system/chapter-3

## Question

Define and explain the dithering technique.

## Short answer

Dithering simulates missing colours or grey levels by arranging available colours in patterns so the eye blends them. It avoids banding in limited-palette displays; methods include ordered, random, and Floyd-Steinberg error diffusion.

## Answer

**Dithering** is a technique that simulates colours or grey levels that are not available in a limited palette by arranging pixels of the available colours in a pattern. The eye blends neighbouring pixels, so the image looks as if it has more colours or shades than the display really has.

#### Why it is needed

A display or file format with few colours (for example 1-bit black and white, or an 8-bit 256-colour palette) cannot show smooth gradients of a photograph. Direct reduction gives visible bands (false contours). Dithering trades spatial resolution for apparent intensity resolution.

#### Principle (black and white example)

A small block of pixels is used for one original pixel. A 2 x 2 block can show 5 grey levels depending on how many pixels are black:

```text
 level 0   level 1   level 2   level 3   level 4
  . .       # .       # .       # .       # #
  . .       . .       . #       # #       # #
 (white)                                  (black)
```

#### Methods

1. **Ordered (pattern) dithering.** Each pixel intensity is compared with a threshold from a repeating dither matrix, e.g. for a 2 x 2 matrix `[[0,2],[3,1]]` scaled to 0-255. If the pixel value is above the matrix threshold the output is white, else black. Fast but gives a regular pattern.
2. **Random dithering.** A random noise value is added before thresholding. No pattern, but grainy.
3. **Error diffusion (Floyd-Steinberg).** The pixel is rounded to the nearest available colour, and the error is spread to the neighbours not yet processed: 7/16 to the right, 3/16 below-left, 5/16 below, 1/16 below-right. Gives the best quality.

#### Uses and effect

- Showing photographs on 256-colour or monochrome displays, in GIF images, newspaper (halftone) printing and fax.
- Advantage: a better look with a small palette. Disadvantage: image looks grainy and dithered images compress poorly.

## Check yourself

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

1. What does dithering do?
   A. Increases the true number of colours of the display
   B. Arranges pixels of available colours in a pattern so the eye blends them into more apparent colours or shades
   C. Compresses an image without loss
   D. Removes noise by filtering
   Answer: B. Dithering simulates unavailable colours using patterns of available ones, trading spatial resolution for apparent intensity resolution. (Key point: Definition)

2. What problem appears if a photograph is reduced directly to few colours without dithering?
   A. The file becomes larger
   B. The image turns completely black
   C. Visible bands (false contours) appear
   D. Colours become brighter
   Answer: C. Direct reduction gives banding or false contours; dithering hides them. (Key point: Why needed)

3. How does Floyd-Steinberg error diffusion work?
   A. It adds random noise before thresholding
   B. It compares each pixel with a repeating matrix
   C. It averages the whole image
   D. It rounds the pixel to the nearest available colour and spreads the error to unprocessed neighbours
   Answer: D. The rounding error is spread to neighbours (7/16 right, 3/16 below-left, 5/16 below, 1/16 below-right), giving the best quality. (Key point: Error diffusion)