Bit-Mapped Graphics (SQA National 5 Computing Science): Revision Note

Bit-mapped graphics

What is a bit-mapped graphic?

• A bit-mapped graphic is an image that is made up of small squares called pixels (picture elements)

• Each pixel represents one tiny part of the image and is stored as a binary value (a pattern of 1s and 0s) in the computer’s memory

• The collection of all these pixel values forms the complete image

• This method of storage is sometimes described as storing the image as an array of pixels

How is it represented in memory?

• Each pixel’s binary value identifies the colour of that pixel

• The more bits used to store each pixel, the greater the number of colours that can be represented

• The computer stores these binary values in a grid pattern that matches the layout of the image on screen

Resolution

• Resolution refers to the number of pixels that make up the image

• It is calculated using the width × height of the image in pixels

• A higher resolution means more pixels are stored, which gives more detail but increases the file size

Colour depth

• Colour depth is the number of bits used per pixel

• This determines how many different colours each pixel can represent

• For example:

  • 1-bit → 2 colours (black and white)

  • 4-bit → 16 colours

  • 8-bit → 256 colours

  • 24-bit → 16,777,216 colours (True Colour)

• The number of colours available can be calculated using 2ⁿ, where n is the colour depth in bits

Factors affecting file size

• The file size of a bit-mapped image depends on:

  • Resolution – more pixels increase the amount of data stored

  • Colour depth – more bits per pixel mean more data is needed for each pixel

• As either resolution or colour depth increases, so does the file size

• A balance must be found between image quality and the amount of storage space required

Common bit-mapped file formats

• Different bit-mapped file types are used depending on the required features:

  • JPEG – uses compression to reduce file size, suitable for photographs

  • PNG – supports transparency, often used for logos or web graphics

  • GIF – supports simple animation and a limited colour palette