Year 9 · Spring 2 · Representations: going audio-visual

Resolution and file-size calculation

What this lesson covers

Topic 5 Representing images

KS3 Computing · A picture is a grid of tiny squares called pixels. Each pixel's colour is stored as a binary number. More pixels and more colours make a better picture, and a bigger file.

Topic 6 Representing sound

KS3 Computing · A computer measures the height of a sound wave thousands of times a second and saves each measurement as a binary number. This is sampling.

Computer SystemsCambridge Lower Secondary Computing 0860

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.

Success criteria

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.

Key words

Say each word, then read what it means. Every one is in the glossary too.

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.
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.
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

Read it here, or open the study page below to go one step at a time with things to try.

Resolution

Resolution = how many pixels, written width × height. Full HD = 1920 × 1080 = 2,073,600 pixels. Too few pixels and you see blocky squares.

Calculating file size

e.g.Example

An MP3 of the same song is about 3–4 MB because it's compressed.

Study it

Open the study page and go one step at a time.

Topic 5 · Year 9

Representing images

A picture is a grid of tiny squares called pixels. Each pixel's colour is a binary number. More pixels and more colours = better picture, bigger file.

The steps for this lesson

Topic 5, step 7: File size (Image file size)

Check yourself

One question at a time. The reason shows after each answer.

Saved on this device.