AQA A-Level Computer Science (7517)

## Learning Outcomes

- Understand how bitmap graphics and vector graphics are stored
- Explain why bitmaps are resolution dependent and vectors are resolution independent
- Calculate the uncompressed file size of a bitmap image in bits and bytes
- Compare bitmap and vector graphics in terms of quality, storage, processing, editing and use cases
- Identify when photographs, logos, icons and diagrams should use bitmap or vector formats
- Avoid common exam errors, especially confusing resolution with colour depth

## AQA A-Level Computer Science (7517) Syllabus

For the AQA A-Level Computer Science (7517), you are expected to understand how images are represented digitally, with a focus on the following syllabus points:

- Know that a **bitmap image** is stored as a grid of pixels, each with colour data
- Know that a **vector graphic** is stored as mathematical descriptions of objects such as lines, curves and polygons
- Explain the difference between **resolution dependent** and **resolution independent** graphics
- Understand how bitmap file size depends on width, height and colour depth
- Compare bitmap and vector graphics in terms of storage requirements, scaling and image quality
- Identify why bitmaps are usually better for photographs and vectors are usually better for logos, icons and diagrams
- Recognise that vector graphics often, but not always, use less storage for simple images
- Understand that vectors must be rendered from stored object properties, while bitmaps can be displayed from pixel data directly

## Test Your Knowledge

_Attempt these questions before reading this article. If you find some difficult or cannot remember the answers, look more closely at that area during your revision._

1. What is the difference between a bitmap image and a vector graphic?
2. Why does a bitmap image become pixelated when enlarged?
3. How do you calculate the uncompressed size of a bitmap image in bytes?
4. Why are photographs normally stored as bitmaps rather than vectors?
5. Give two situations where vector graphics are more suitable than bitmaps.

## Introduction

In AQA A-Level Computer Science, image representation is a high-value topic because it mixes definition questions, comparison questions, and calculation questions. Students often remember that bitmap graphics use pixels and vector graphics use shapes, but miss the deeper point: the way data is stored controls file size, scaling quality, editing options, and suitable uses.

This means exam questions are rarely just asking for a definition. They often ask you to explain why one method is better in a given situation, or to calculate bitmap storage and then link that calculation to image quality. To answer well, you need to understand not only what each format is, but how it works.

> **Key Term: bitmap image**
> An image stored as a grid of pixels, where each pixel holds colour data.

### How bitmap and vector graphics represent images

A **bitmap image** stores the image pixel by pixel. Each pixel is the smallest addressable part of the image, and each one has a binary value representing its colour. If an image is 1920 × 1080, the computer stores colour information for 2,073,600 separate pixels.

A **vector graphic** stores an image very differently. Instead of storing every pixel, it stores a list of objects and their properties. These objects are often called **vector primitives** and include lines, curves, circles, polygons and text. The file does not say “pixel 1 is blue, pixel 2 is blue, pixel 3 is red”. Instead, it says things like “draw a circle with centre at x,y, radius r, fill colour blue, outline black”.

> **Key Term: vector graphic**
> An image stored as mathematical descriptions of objects such as lines, curves and polygons rather than individual pixels.
>
> **Key Term: vector primitive**
> A basic object used to build a vector graphic, such as a line, curve, circle, polygon or text object.

This difference in storage method is the most important idea in the whole topic. Nearly every comparison point comes from it.

For example, imagine a simple company logo made from three coloured circles and one line of text.

- In a bitmap, every pixel across the whole image area must be stored
- In a vector file, only the definitions of the circles and text need to be stored

That is why vector files are often smaller for simple graphics. The amount of data depends more on the number of objects than on the number of pixels across a fixed grid.

However, be careful: vector files are not always smaller. If a scene is extremely complex and uses huge numbers of separate objects, the vector file can become large too. Examiners like careful wording here. Say vectors are **often** smaller for simple images, not **always** smaller.

A vector drawing list might contain entries like these:

- Circle: centre = (50, 50), radius = 20, fill = red, stroke = black
- Rectangle: position = (10, 10), width = 100, height = 40, fill = yellow
- Line: start = (0, 0), end = (120, 80), colour = blue, thickness = 2

That is enough for the computer to recreate the image whenever needed.

> **Key Term: polygon**
> A shape made from straight lines, such as a triangle, rectangle or hexagon.

A bitmap does not store drawing instructions like this. It stores the final result as a fixed grid of coloured dots.

This creates a second major difference: editing. In a bitmap, editing is mainly at pixel level. You can change individual pixels, which is useful for photo retouching. In a vector graphic, editing is mainly at object level. You can move one shape, recolour one line, or resize one object without changing the rest.

> **Test Tip:** If the question says “compare”, do not write only about vectors or only about bitmaps. Make direct paired statements such as “bitmaps store pixels, whereas vectors store mathematical objects”.

### Resolution, colour depth and bitmap file size

Bitmap questions often include calculations, so you must know the core terms accurately.

> **Key Term: resolution**
> The number of pixels in an image, usually expressed as width × height.
>
> **Key Term: colour depth**
> The number of bits used to store the colour of each pixel.

Students often confuse these two. Resolution tells you how many pixels there are. Colour depth tells you how many bits are used for each pixel. They affect file size in different ways.

If resolution increases, there are more pixels to store.

If colour depth increases, each pixel needs more bits.

The key formula for an uncompressed bitmap image is:

File size in bits = width × height × colour depth

File size in bytes = (width × height × colour depth) ÷ 8

Also remember:

Number of possible colours = 2^(colour depth)

So:

- 1 bit gives 2 colours
- 2 bits give 4 colours
- 8 bits give 256 colours
- 24 bits give over 16 million colours

Worked example:

An uncompressed bitmap image has:

- width = 1920 pixels
- height = 1080 pixels
- colour depth = 24 bits per pixel

Step 1: find total pixels

1920 × 1080 = 2,073,600 pixels

Step 2: multiply by colour depth

2,073,600 × 24 = 49,766,400 bits

Step 3: convert bits to bytes

49,766,400 ÷ 8 = 6,220,800 bytes

So the uncompressed bitmap size is **6,220,800 bytes**.

If needed, you could convert further into kilobytes or megabytes, but only if the question asks. A common exam mistake is converting units when not required.

Another worked example:

A 600 × 400 bitmap uses 16 bits per pixel.

Step 1: total pixels

600 × 400 = 240,000 pixels

Step 2: total bits

240,000 × 16 = 3,840,000 bits

Step 3: convert to bytes

3,840,000 ÷ 8 = 480,000 bytes

So the image size is **480,000 bytes**, ignoring metadata and compression.

> **Key Term: metadata**
> Data about data; in an image file this can include width, height, colour depth, date created and GPS information.

Notice the phrase “ignoring metadata and compression”. Real files are often not exactly the same size as your calculation because image files usually include metadata and may also use compression.

AQA sometimes tests whether students can separate the factors affecting bitmap size:

- higher resolution increases size
- higher colour depth increases size
- metadata adds extra storage
- compression can reduce stored size

Do not say that resolution and colour depth are the same thing. They are linked to file size, but they describe different features.

> **Test Tip:** In calculation questions, write the formula first, substitute the numbers clearly, and show the conversion from bits to bytes. This picks up method marks even if your final number is wrong.

### Scaling, quality and processing differences

The best-known difference between bitmaps and vectors is what happens when the image is resized.

> **Key Term: resolution dependent**
> Quality depends on the number of pixels available, so enlarging the image can reduce apparent quality.
>
> **Key Term: resolution independent**
> Quality does not depend on a fixed pixel grid, so the image can be scaled without losing sharpness.

Bitmap images are **resolution dependent**. They have a fixed number of pixels. If you enlarge a bitmap, the image must spread those same pixels over a larger area. As a result, the pixels become more visible and edges look jagged or blocky.

> **Key Term: pixelation**
> The blocky appearance that happens when a bitmap image is enlarged and its individual pixels become visible.

This happens because enlarging a bitmap does not create new real detail. Software may estimate extra pixels by interpolation, but it is still guessing from existing data. The image may become smoother, but not genuinely sharper.

Vector graphics are **resolution independent**. When you enlarge them, the computer recalculates the mathematical descriptions and redraws the shapes at the new size. A circle remains a smooth circle. A diagonal line remains sharp. That is why vectors are ideal for logos, icons, fonts, line drawings and CAD-style diagrams.

The quality difference can be explained as a cause-and-effect chain:

- bitmap stores fixed pixels
- enlarging stretches those pixels
- fewer pixels per inch are available at the new display size
- edges appear blocky
- image quality falls

For vectors:

- vector stores shape properties
- enlarging changes coordinates or dimensions
- image is redrawn from the object definitions
- no fixed pixels are being stretched
- quality stays sharp

Students often stop at “vectors scale better”. That is only half an explanation. The stronger answer explains **why** they scale better.

There is also a processing difference. Bitmaps usually need less processing at display time because the image data is already present as pixels. A vector graphic must be rendered first from its list of object properties. So vector images can require more processing when displayed, even though the file may be smaller.

This leads to a subtle comparison:

- bitmaps often use more storage but less rendering work
- vectors often use less storage for simple images but more rendering work

That distinction can help on longer explain questions.

### Best uses, limitations and exam-style comparisons

The easiest way to choose between bitmap and vector graphics is to ask: what sort of image is this?

Bitmaps are best for **photographs** and other highly detailed images with gradual changes in colour and shading. A digital photograph may contain millions of tiny variations in colour across skin, sky, shadows and reflections. A pixel grid handles this naturally because each pixel can store its own colour.

Vectors are poor for most photographs because a photo is not really made from a few simple shapes. To recreate it as vectors, you would need huge numbers of objects and colour regions. That makes the file difficult to create and may make it very large.

Vectors are best for:

- logos
- icons
- fonts
- clipart
- diagrams
- technical drawings
- CAD/CAM designs
- some animation frames

These images are often built from clear shapes, edges and text. They need to stay sharp at different sizes, from a small app icon to a large poster.

Bitmaps are best for:

- digital photographs
- scanned images
- realistic paintings
- textured scenes
- detailed colour images with fine shading

A useful comparison is object-level editing versus pixel-level editing.

With vectors, you can easily:

- move one shape
- resize one object
- rotate an object
- recolour one shape
- separate the image into editable parts

With bitmaps, you can easily:

- edit individual pixels
- retouch photographs
- blur, sharpen or recolour selected areas
- work directly with detailed image data

This is why photo-editing software is based mainly on bitmap graphics, while illustration and logo software often uses vector graphics.

A strong exam comparison might look like this:

- A bitmap stores every pixel, whereas a vector stores mathematical descriptions of objects.
- A bitmap is resolution dependent and may pixelate when enlarged, whereas a vector is resolution independent and stays sharp when scaled.
- A bitmap is suitable for photographs because it can represent subtle colour variation, whereas a vector is better for logos and diagrams made from simple shapes.
- A bitmap often needs more storage because every pixel must be recorded, whereas a vector often needs less storage for simple images because only object properties are stored.
- A bitmap can usually be displayed directly from its pixel data, whereas a vector must be rendered from the stored object definitions.

Common student errors:

1. Saying vectors are always smaller  
   This is too absolute. Say **often smaller for simple images**.

1. Saying bitmaps lose data when enlarged  
   The file itself does not necessarily lose data. The issue is that the fixed pixels are enlarged, so the image appears blocky.

1. Confusing resolution with colour depth  
   Resolution = number of pixels. Colour depth = bits per pixel.

1. Saying vectors are made of pixels too  
   When displayed on a screen, everything eventually appears through screen pixels, but the **stored representation** of a vector graphic is not pixel-based.

1. Forgetting the use-case link  
   You must connect features to applications. For example, “vector is good for logos because the logo may need printing at many sizes without quality loss”.
   > **Test Tip:** For “explain why” questions, use the pattern “because... therefore...”. Example: “A vector stores shape instructions rather than fixed pixels, therefore it can be redrawn at any scale without pixelation.”

## Key Point Checklist

_This article has covered the following key knowledge points:_

- A bitmap image is stored as a grid of pixels, with each pixel holding colour data
- A vector graphic is stored as mathematical descriptions of objects such as lines, curves and polygons
- Bitmap graphics are resolution dependent
- Vector graphics are resolution independent
- Enlarging a bitmap can cause pixelation because fixed pixels are stretched over a larger area
- Enlarging a vector does not reduce quality because the image is redrawn from object properties
- Uncompressed bitmap file size in bits is width × height × colour depth
- Uncompressed bitmap file size in bytes is (width × height × colour depth) ÷ 8
- Resolution means the number of pixels in the image, usually written as width × height
- Colour depth means the number of bits per pixel and determines how many colours can be represented
- Bitmaps are usually better for photographs and detailed realistic images
- Vectors are usually better for logos, icons, diagrams, fonts and technical drawings
- Vector files are often smaller for simple images, but not always
- Bitmaps support pixel-level editing, while vectors support object-level editing

## Key Terms and Concepts

- bitmap image
- vector graphic
- vector primitive
- polygon
- resolution
- colour depth
- metadata
- resolution dependent
- resolution independent
- pixelation


Data Representation - Vector and bitmap graphics comparison