one of the tiny single-colour squares that make up an image. An image is a grid of pixels.

What is the resolution of an image?

the number of pixels in the image. Width in pixels × height in pixels.

What is the effect of increasing the colour depth?

more colours and a larger file. Each pixel needs more bits, so more colours are possible and there is more to store.

An image is 100 pixels by 100 pixels with a colour depth of 16 bits. What is its file size?

20 000 bytes. 100 × 100 × 16 = 160 000 bits, and 160 000 ÷ 8 = 20 000 bytes.

How many bits are needed for each pixel if an image uses 256 different colours?

8. 2 multiplied by itself eight times is 256.

Images · Cambridge IGCSE Computer Science 0478 revision

In plain words

A digital image is a grid of tiny squares called pixels. Each pixel is one colour, and the colour is stored as a binary number. From a distance your eye blends the squares into a picture.

Two numbers decide how good the image looks and how big its file is: how many pixels there are, and how many bits describe each one.

6 things to know

An image is a series of pixels that are converted to binary, which is processed by a computer.
The resolution is the number of pixels in the image: its width in pixels multiplied by its height.
The colour depth is the number of bits used to represent the colour of each pixel.
With a colour depth of n bits, each pixel can be one of 2ⁿ colours. One bit gives 2 colours, 8 bits give 256.
A higher resolution or a higher colour depth improves the quality of the image and makes the file larger.
File size in bits = width × height × colour depth.

Worked example

An image is 1024 pixels wide and 768 pixels high, with a colour depth of 24 bits. Calculate its file size in mebibytes.

Bits = 1024 × 768 × 24. Do not multiply it out yet.
To bytes, divide by 8: 1024 × 768 × 3 bytes.
To kibibytes, divide by 1024: 768 × 3 = 2304 KiB.
To mebibytes, divide by 1024 again: 2304 ÷ 1024 = 2.25 MiB.

Tips and tricks

Calculators are not allowed in the Cambridge papers. Cancel the 8 and the 1024s before you multiply, as in the example.
"More colours" comes from colour depth. "More detail" comes from resolution. Do not swap them.

6 questions, about 2 minutes.

It lands in your notebook with its questions as flashcards.

Images: 6 questions and answers

These are the quiz’s questions. Do the quiz first, then come back here for the ones that got you.

What is a pixel?
- the number of colours in an image
- one of the tiny single-colour squares that make up an image (the answer)
- a kind of compression
- the size of an image file
An image is a grid of pixels.
How many different colours can an image have if its colour depth is 3 bits?
- 3
- 6
- 8 (the answer)
- 9
2 × 2 × 2 = 8.
What is the resolution of an image?
- the number of bits for each pixel
- the number of pixels in the image (the answer)
- the size of the screen
- the number of colours used
Width in pixels × height in pixels.
What is the effect of increasing the colour depth?
- fewer colours and a smaller file
- fewer colours and a larger file
- more colours and a smaller file
- more colours and a larger file (the answer)
Each pixel needs more bits, so more colours are possible and there is more to store.
An image is 100 pixels by 100 pixels with a colour depth of 16 bits. What is its file size?
- 1600 bytes
- 10 000 bytes
- 20 000 bytes (the answer)
- 160 000 bytes
100 × 100 × 16 = 160 000 bits, and 160 000 ÷ 8 = 20 000 bytes.
How many bits are needed for each pixel if an image uses 256 different colours?Stretch
- 4
- 8 (the answer)
- 16
- 256
2 multiplied by itself eight times is 256.

6 questions

Tap an answer and you’ll see straight away whether it’s right, and why.

Worksheet

2 questions, 4 marks. Write your answers on paper, then check them.

Images

Cambridge IGCSE Computer Science 0478 · 4 marks · papermunch.org

Name ______________________________ Date ______________

An image is 800 pixels by 600 pixels with a colour depth of 16 bits. Calculate its file size in bytes.[2]
Show answerHide answer
960 000 bytes. 800 × 600 × 16 = 7 680 000 bits, and 7 680 000 ÷ 8 = 960 000.
I got this one
Explain the effect of reducing the resolution of an image.[2]
Show answerHide answer
There are fewer pixels, so the image shows less detail and its quality is lower. There is less data to store, so the file is smaller.
I got this one

Answers: Images

1. 960 000 bytes. 800 × 600 × 16 = 7 680 000 bits, and 7 680 000 ÷ 8 = 960 000.
2. There are fewer pixels, so the image shows less detail and its quality is lower. There is less data to store, so the file is smaller.