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