Representing Images (Cambridge (CIE) IGCSE Computer Science)

Revision Note

Robert Hampton

Written by: Robert Hampton

Reviewed by: James Woodhouse

Bitmap Images

What is a bitmap?

  • A bitmap image is made up of squares called pixels

  • A pixel is the smallest element of a bitmap image

  • Each pixel is stored as a binary code

  • Binary codes are unique to the colour in each pixel

  • A typical example of a bitmap image is a photograph

Humidity sensor bitmap image
  • The more colours and more detail in the image, the higher the quality of the image and the more binary that needs to be stored

Resolution & Colour Depth

What is resolution?

  • Resolution is the total amount of pixels that make up a bitmap image

  • The resolution is calculated by multiplying the height and width of the image (in pixels)

  • In general, the higher the resolution the more detail in the image (higher quality)

  • Resolution can also refer to the total amount of pixels horizontally in a display, such as:

    • Computer monitors - 1440p means 1440 pixels horizontally compared to 4K which is 3840 pixels (roughly 4 thousand)

    • TVs - HD (high definition) channels have a resolution of 1080p, 1080 pixels horizontally compared to newer UHD (ultra high definition) channels with 3840 pixels (4K)

    • YouTube - The quality button allows a user to change the video playback resolution from 144p (144 pixels horizontally) up to 4K

What is colour depth?

  • Colour depth is the number of bits stored per pixel in a bitmap image

  • The colour depth is dependent on the number of colours needed in the image

  • In general, the higher the colour depth the more detail in the image (higher quality)

  • In a black & white image the colour depth would be 1, meaning 1 bit is enough to create a unique binary code for each colour in the image (1=white, 0=black)

Example 1 bit black and white image
  • In an image with a colour depth of 2, you would have 00, 01, 10 & 11 available binary codes, so 4 colours

Example 2 bit Mario image
  • As colour depth increases, so does the amount of colours available in an image

  • The amount of colours can be calculated as 2n (n = colour depth) 

Colour Depth

Amount of Colours

1 bit

2 (B&W)

2 bit

4

4 bit

16

8 bit

256

24 bit

16,777,216 (True Colour)

What is the impact of resolution and colour depth?

  • As the resolution and/or colour depth increases, the bigger the size of the file becomes on secondary storage

  • The higher the resolution, the more pixels are in the image, the more bits are stored

  • The higher the colour depth, the more bits per pixel are stored

  • Striking a balance between quality and file size is always a consideration

Seesaw to demonstrate the relationship between quality and file size of an image

Worked Example

1. Define the term Pixel [1]

2. If an image has a colour depth of 2 bits, how many colours can the image represent? [1]

3. Describe the impact of changing an images resolution from 500x500 to 1000x1000 [2]

Answers

  1. The smallest element of a bitmap image (1 square)

  2. 4

  3. The image quality would be higher [1] the file size would be larger [1]

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Robert Hampton

Author: Robert Hampton

Expertise: Computer Science Content Creator

Rob has over 16 years' experience teaching Computer Science and ICT at KS3 & GCSE levels. Rob has demonstrated strong leadership as Head of Department since 2012 and previously supported teacher development as a Specialist Leader of Education, empowering departments to excel in Computer Science. Beyond his tech expertise, Robert embraces the virtual world as an avid gamer, conquering digital battlefields when he's not coding.

James Woodhouse

Author: James Woodhouse

Expertise: Computer Science

James graduated from the University of Sunderland with a degree in ICT and Computing education. He has over 14 years of experience both teaching and leading in Computer Science, specialising in teaching GCSE and A-level. James has held various leadership roles, including Head of Computer Science and coordinator positions for Key Stage 3 and Key Stage 4. James has a keen interest in networking security and technologies aimed at preventing security breaches.