Year 9 · Spring 2
Representations: going audio-visual · Lesson 2 of 5
Resolution and file-size calculation
TheoryTopic 5 Representing imagesTopic 6 Representing sound
Year 9 · Representations: going audio-visual · Lesson 2 of 5
You will learn to
- Explain what resolution is: the number of pixels across and down.
- Calculate an image's file size in bits: width × height × colour depth.
- Convert bits into bytes, kilobytes and megabytes, showing every step.
- Compare the file size of a phone photo and a small icon.
Year 9 · Representations: going audio-visual · Lesson 2 of 5
Success criteria
You have got it when you can say:
- I can calculate the size of a 32 × 32 icon with 24-bit colour.
- I can write my working with a unit on every line.
- I can explain why doubling the width and the height makes the file four times bigger.
Year 9 · Representations: going audio-visual · Lesson 2 of 5
Retrieval starter
Questions from before. Write your answers, then show them.
- Last lesson
In #00FF00, which light is on?
A BlueB RedC Green
C. Green
- Last unit
Which field makes a good primary key?
A HouseB StudentIDC FirstName
B. StudentID
- Earlier this year
Caesar shift 1: 'CAT' becomes…
A BZSB CAT1C DBU
C. DBU
Year 9 · Representations: going audio-visual · Lesson 2 of 5
Key words (1 of 3)
- Resolution
- Number of pixels: width × height.
- Pixel
- The smallest single-colour dot of an image.
- Bitmap
- An image stored as a grid of pixels.
- Colour depth
- Bits per pixel. Sets how many colours are possible.
Year 9 · Representations: going audio-visual · Lesson 2 of 5
Key words (2 of 3)
- Metadata
- Data about data, like an image's width and height.
- Vector
- An image stored as shapes and instructions.
- Analogue
- Smooth and continuous, with any value possible.
- Digital
- Made of separate, countable steps.
Year 9 · Representations: going audio-visual · Lesson 2 of 5
Key words (3 of 3)
- ADC / DAC
- Analogue-to-digital converter: turns a smooth signal into numbers.
- Sample rate
- Sound samples taken per second, in hertz.
- Bit depth
- Bits per sound sample. Sets how precisely each is measured.
The explanation · step 1 of 3
Resolution
Resolution = how many pixels, written width × height. Full HD = 1920 × 1080 = 2,073,600 pixels. Too few pixels and you see blocky squares.
The explanation · step 2 of 3
The maths: Image file size
The explanation · step 3 of 3
Calculating file size
An MP3 of the same song is about 3–4 MB because it's compressed.
The explanation · step 3 of 3
The maths: Sound file size
Year 9 · Representations: going audio-visual · Lesson 2 of 5
Quick quiz · 1 of 3
width × height × colour depth gives the size in…
- A bits
- B bytes
- C KB
A. bits
Each pixel needs d bits. Divide by 8 for bytes.
Year 9 · Representations: going audio-visual · Lesson 2 of 5
Quick quiz · 2 of 3
Which is metadata?
- A The colour of pixel 7
- B A pixel's red value
- C The image's width
C. The image's width
Width, height and colour depth describe the data. They aren't pixels themselves.
Year 9 · Representations: going audio-visual · Lesson 2 of 5
Quick quiz · 3 of 3
A logo must look sharp on a pen and on a billboard. Best stored as…
- A A vector
- B A bitmap
A. A vector
Vectors are redrawn from shapes, so they're sharp at any size.
Year 9 · Representations: going audio-visual · Lesson 2 of 5
Exit ticket
Answer on paper before you go. Then check.
A 2-bit colour depth gives how many colours?
A 8B 4C 2
B. 4 2 × 2 = 4.
Double the width and the height. The file becomes…
A 2 times biggerB 4 times bigger
B. 4 times bigger 2 × 2 = 4 times the pixels.