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Year 7 · Summer 2 · Raid: Code for a Cause

Image editing: cropping, layers and file formats

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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.

Computer SystemsCambridge Lower Secondary Computing 0860

You will learn to

  • Use cropping and layers to edit a photo and put text on top.
  • Explain the difference between a bitmap image and a vector image.
  • Compare the file size of the same image saved as PNG and as JPEG.
  • Calculate the size of an image before compression, from its width, height and colour depth.

Success criteria

  • I can crop a photo and add a text layer over it.
  • I can say when to use JPEG, PNG or GIF.
  • I can work out an image's size in bits and bytes with the formula.

Key words

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

colour depth
Bits per pixel. Sets how many colours are possible.
Pixel
The smallest single-colour dot of an image.
Bitmap
An image stored as a grid of pixels.
Resolution
Number of pixels: width × height.
Metadata
Data about data, like an image's width and height.
Vector
An image stored as shapes and instructions.

The explanation

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

The simplest image: black and white

If a pixel is only black or white, 1 bit is enough: 1 = black, 0 = white. A 5 × 5 letter T:

1 1 1 1 1
0 0 1 0 0
0 0 1 0 0
0 0 1 0 0
0 0 1 0 0

Stored row after row: 25 bits. The computer also needs to know the width (5) to rebuild the rows.

More colours = more bits per pixel

Bits per pixel = colour depth. Same rule as before: n bits give 2ⁿ colours.

Colour depthColoursUse
1 bit2Black and white
2 bits4Very early games
8 bits256GIFs, retro graphics
24 bitsabout 16.7 millionNormal photos

Calculating file size

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

Try it

Check yourself

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

Saved on this device.