---
title: "Image enhancement"
subject: "Multimedia System"
chapter: 3
key: "semester_8/multimedia_system:image-enhancement"
source: https://www.bctengineer.com/q/semester_8/multimedia_system/image-enhancement
site: BCT Engineering
---

# Image enhancement

Multimedia System · Chapter 3: Images and Graphics

- Asked in: 2072 Asoj, 3 marks
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_8/multimedia_system/image-enhancement
- Whole chapter: https://www.bctengineer.com/md/semester_8/multimedia_system/chapter-3

## Question

What is image enhancement?

## Short answer

Image enhancement improves an image's visibility for humans or machine analysis without adding information. Spatial methods include contrast stretching, histogram equalisation and filtering; frequency domain methods manipulate spectral components to reduce noise or sharpen edges.

## Answer

Image enhancement is the process of processing an image so that the result is more suitable than the original for a specific application, usually viewing by humans or further analysis. It does not add new information; it makes existing detail easier to see.

**Main aims**
- Improve contrast and brightness of dark, washed-out or low-contrast images.
- Reduce noise (random specks from the sensor or transmission).
- Sharpen edges and fine detail that look blurred.
- Highlight features of interest, such as tumours in an X-ray or roads in a satellite image.

**Two approaches**
1. **Spatial domain methods** work directly on pixels. Examples: contrast stretching, histogram equalisation, negative of an image, smoothing with an averaging or median filter, sharpening with a Laplacian filter.
2. **Frequency domain methods** transform the image (for example with the Fourier transform), modify the frequency components, and transform back. A low-pass filter smooths and removes noise; a high-pass filter sharpens edges.

**Example:** a photo taken in dim light has all its pixel values between 30 and 90. Contrast stretching maps this range to 0-255, so the picture becomes clearly visible.

Enhancement differs from image restoration. Restoration tries to undo a known degradation (such as a known blur), while enhancement is subjective and aims only at a better-looking or more useful picture.

## Check yourself

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

1. What does image enhancement do?
   A. Adds new information that was never in the image
   B. Makes the result more suitable than the original for an application, without adding information
   C. Exactly undoes a known blur
   D. Converts the image to vector form
   Answer: B. Enhancement makes existing detail easier to see for viewing or analysis; it adds no information and is subjective. (Key point: Meaning of enhancement)

2. Which describes frequency domain methods?
   A. Change pixel values directly, e.g. histogram equalisation
   B. Transform the image (e.g. Fourier), modify frequency components, transform back
   C. Only rotate or crop the picture
   D. Only compress the picture
   Answer: B. Frequency methods work on transformed data; a low-pass filter smooths and a high-pass filter sharpens edges. (Key point: Spatial vs frequency domain)

3. A dim photo has pixel values only between 30 and 90. What does contrast stretching do?
   A. Maps the range 30-90 to 0-255 so the picture is clearly visible
   B. Sets all pixels to 30
   C. Adds noise to the image
   D. Reduces the range to 0-10
   Answer: A. Stretching expands the narrow range to the full 0-255 range, improving contrast. (Key point: Contrast stretching)