📚 Knowledge Library — Topic 1.3B — Data Representation

Image Representation

Understand how digital images are built from pixels, how resolution and colour depth affect quality, and why image files can become large.

Start LearningWorked Example
1. Invitation

A digital image is just a grid.

When you zoom into a digital image, it is made from tiny coloured squares called pixels.

Each pixel stores one colour value. The computer stores those colour values as binary.

💡 Remember: a pixel is the smallest individual element of a digital image.
Figure 1.1
Pixel Grid
⬛ 🟦 🟦 ⬛
🟦 🟩 🟩 🟦
🟦 🟩 🟩 🟦
⬛ 🟦 🟦 ⬛
Each square is one pixel.
2. Big Idea

More pixels usually means more detail.

Resolution is the number of pixels in an image. It is often written as width × height.

A higher resolution stores more pixels. More pixels can make an image sharper, but it also increases file size.

💡 Resolution = width × height in pixels.
Figure 2.1
Resolution
800 × 600
↓
480,000 pixels
Width multiplied by height.
3. Bridge

Think of a mosaic.

A mosaic is made from many small tiles. Close up, you see the tiles. From far away, they form a picture.

A digital image works in the same way. Pixels are the tiles, and each tile has a colour.

💡 More tiles can create more detail, but they also need more storage.
Figure 3.1
Mosaic Model
Tiles
↓
Pixels
↓
Image
4. Worked Example

Calculating total pixels.

To calculate the number of pixels in an image, multiply the width by the height.

Image resolution: 800 × 600

Width
800
Height
600
Total
?
800 × 600 = 480,000 pixels
Figure 4.1
Pixel Calculation
width × height
↓
total pixels
Always show the multiplication.
5. Colour Depth

Colour depth controls how many colours are possible.

Colour depth is the number of bits used to represent the colour of each pixel.

More bits per pixel means more possible colours, smoother gradients, and more realistic images.

8-bit colour depth

2⁸ = 256 possible colours
Figure 5.1
Bits per Pixel
1 bit → 2 colours
8 bits → 256 colours
24 bits → 16M+ colours
6. Exam Tip

Metadata is not the picture.

Metadata is data about the image file. It describes the file, but it is not part of the visual image itself.

🎯 Exam Tip: Good metadata examples include resolution, colour depth, file size, date/time taken, camera model, and file format.
Figure 6.1
Image Metadata
Image file
+
resolution
colour depth
date taken
7. Common Mistake

Colour depth is not the number of pixels.

Students often mix up resolution and colour depth.

⚠️ Common Mistake: resolution is about how many pixels there are. Colour depth is about how many bits are used for each pixel's colour.
Figure 7.1
Do Not Mix Them Up
Resolution
= pixels

Colour depth
= bits per pixel
8. Summary

Image representation in one screen.

Digital images are stored as grids of pixels.

Resolution affects the number of pixels. Colour depth affects the number of possible colours.

Higher resolution and higher colour depth usually improve quality, but they also increase file size.

💡 Key formula: file size in bits = width × height × colour depth.
Figure 8.1
Image File Size
width
× height
× colour depth
↓
bits