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Representing images
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Every photo on your phone is really a grid of numbers. Learn how pixels and colour depth decide how good a picture looks and how much space it takes.
Pixels and resolution
A bitmap image is made of tiny dots called pixels (picture elements).
Resolution is the number of pixels, given as width × height, such as 1920 × 1080.
More pixels show more detail, but make a bigger file.
Resolution is the number of pixels, given as width × height, such as 1920 × 1080.
More pixels show more detail, but make a bigger file.
Colour depth
Each pixel's colour is stored as a binary number. Colour depth is the number of bits used for each pixel.
File size
File size in bits = width × height × colour depth
Divide by 8 for bytes.
Example: 10 × 10 pixels with 2-bit colour = 10 × 10 × 2 = 200 bits = 25 bytes.
Real image files also store metadata, such as the width, height and date taken.
Divide by 8 for bytes.
Example: 10 × 10 pixels with 2-bit colour = 10 × 10 × 2 = 200 bits = 25 bytes.
Real image files also store metadata, such as the width, height and date taken.
An image is 40 × 30 pixels with a colour depth of 8 bits. How big is it in bytes?
- Bits = 40 × 30 × 8 = 9600.
- Bytes = 9600 ÷ 8 = 1200.
Answer: 1200 bytes
Bitmap images are grids of pixels. Resolution is width × height; colour depth is bits per pixel, and n bits give 2 to the power n colours. File size in bits = width × height × colour depth.
The interactive lesson includes the diagrams for this topic.
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