How an image is represented as a series of pixels, represented in binary

The idea

A bitmap image is a grid of pixels. Each pixel stores one colour value as a binary code. The number of bits per pixel is the colour depth, so n bits per pixel allow 2ⁿ different colours. For example, 1 bit per pixel gives 2 colours, 3 bits gives 8 colours, and 8 bits gives 256 colours. The image file stores the binary codes for the pixels, usually row by row. Image resolution is the number of pixels in the width and height. File size in bits can be estimated by multiplying width by height by colour depth. For example, a 10 by 10 pixel image at 3 bits per pixel needs 10 × 10 × 3 = 300 bits. More pixels or more bits per pixel increase file size and can improve quality, but use more storage.

What earns the marks

• States that an image is made of pixels arranged in a grid.
• Explains that each pixel is represented by a binary code.
• Links the number of bits per pixel to the number of available colours using 2ⁿ.
• Describes how the binary codes for all pixels are stored to represent the image.
• Calculates image file size in bits using width × height × colour depth.
• Explains that greater resolution or colour depth increases file size and can improve image quality.

Common mistakes and how to put them right

• Treating a pixel as a single bit rather than a code of one or more bits.

  Put it right by saying each pixel uses a fixed number of bits called the colour depth.

• Confusing resolution with colour depth.

  Put it right by defining resolution as the number of pixels and colour depth as bits per pixel.

• Forgetting to multiply by colour depth when estimating file size.

  Put it right by using width × height × bits per pixel.

Exam tips

• Define pixel, resolution and colour depth clearly before using them in an explanation.
• Show the file-size calculation with units, for example 300 bits, and convert to bytes only if asked.
• Use the 2ⁿ relationship to justify how many colours a given colour depth provides.