Cambridge IGCSE Computer Science (0478) · lesson 6 of 35 · 25 min
How Computers Store Images
A digital image is made up of a grid of tiny squares called pixels (picture elements). Each pixel stores a colour value as a binary number. The way these values are stored depends on three key properties: resolution, colour depth, and file size.
Resolution
Resolution is the number of pixels in an image, usually expressed as width × height (e.g. 1920 × 1080) or as pixels per inch (PPI/DPI). Higher resolution means more pixels, more detail, and a larger file size.
- A 100 × 100 image has 10,000 pixels.
- A 1920 × 1080 image has 2,073,600 pixels (≈ 2 megapixels).
Colour Depth
Colour depth (bit depth) is the number of bits used to represent each pixel’s colour. More bits = more possible colours = better quality = larger file.
- Colour depth
- Colours possible
- Use case
- 1-bit
- 2 (black/white)
- Simple icons, QR codes
- 8-bit
- 256
- GIF images, simple graphics
- 16-bit
- 65,536
- Older displays
- 24-bit
- 16,777,216
- True colour — photos, modern displays
- 32-bit
- 16,777,216 + alpha
- Transparency in graphics
Calculating File Size
Uncompressed image file size formula:
File size (bits) = width × height × colour depth
Divide by 8 for bytes, 1024 for KB, 1024² for MB.
Worked Examples
Calculate the file size of a 640×480 image at 24-bit colour depth
Pixels = 640 × 480 = 307,200
Bits = 307,200 × 24 = 7,372,800 bits
Bytes = 7,372,800 ÷ 8 = 921,600 bytes
= 900 KB (÷ 1024) ≈ 0.88 MB
A 200×200 image uses 8-bit colour depth. What is its uncompressed size in bytes?
- 40,000
- 320,000
- 5,000
- 160,000
Answer: 40,000
200×200×8 = 320,000 bits ÷ 8 = 40,000 bytes.
Doubling the colour depth of an image will:
- Double the file size
- Quadruple the file size
- Halve the file size
- Have no effect
- Answer: 0
File size is directly proportional to colour depth.
What does “resolution” mean for a digital image?
- The number of colours used
- The number of pixels in the image
- The file format used
- The brightness of the image
Answer: The number of pixels in the image
Resolution = total number of pixels (width × height).
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