Image Representation: Pixels, Colour Depth, Resolution, and File Size

Computers represent images as grids of tiny coloured squares called pixels. This lesson covers how images are stored digitally, including the concepts of colour depth, resolution, metadata, and file size calculation. This is part of OCR J277 Section 2.6.

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What Is a Pixel?

A pixel (picture element) is the smallest individual element of a digital image. Each pixel stores a single colour value as a binary number. When millions of pixels are displayed together, they form a recognisable image.

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Colour Depth

Colour depth (also called bit depth) is the number of bits used to represent the colour of each pixel. More bits means more possible colours:

Colour depth	Number of colours	Use
1 bit	2^1 = 2	Black and white
2 bits	2^2 = 4	Simple graphics
4 bits	2^4 = 16	Basic colour icons
8 bits	2^8 = 256	GIF images
16 bits	2^16 = 65,536	High colour
24 bits	2^24 = 16,777,216	True colour (photos)

24-bit colour is the standard for photographs. It uses 8 bits each for red, green, and blue (RGB):

• Red: 0-255 (8 bits)
• Green: 0-255 (8 bits)
• Blue: 0-255 (8 bits)

This is why web colour codes like #FF0000 use hex pairs — each pair is one 8-bit channel.

OCR Exam Tip: The number of colours available is 2^(colour depth). If the colour depth is 8 bits, there are 2^8 = 256 colours. You must be able to calculate this in the exam.

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Resolution

Resolution is the number of pixels in an image, typically expressed as width x height (e.g., 1920 x 1080).

Term	Meaning
Image resolution	Total number of pixels (width x height)
Screen resolution	Number of pixels a display can show
Print resolution	Pixels per inch (PPI) or dots per inch (DPI)

Higher resolution means more pixels, which means:

• More detail in the image
• Larger file size (more pixels to store)
• Longer processing time

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File Size Calculation

The formula for calculating the file size of an uncompressed image is:

File size (bits) = width (pixels) x height (pixels) x colour depth (bits)

To convert to bytes: divide by 8 To convert to kilobytes: divide by 1024 To convert to megabytes: divide by 1024 again

Worked Example

An image is 800 x 600 pixels with 24-bit colour depth. Calculate the file size.