representing images
4 detailed 50-minute lessons with teaching scripts, worked examples, parent guides, and assessment criteria.
Lesson Overview
Total Lessons: 4
Tier: Foundation and Higher
Duration: 50 minutes per lesson (200 minutes total)
Exam Boards: AQA, Edexcel, OCR, Eduqas, CCEA
Tier: Foundation and Higher
Duration: 50 minutes per lesson (200 minutes total)
Exam Boards: AQA, Edexcel, OCR, Eduqas, CCEA
Learning Objectives
- Explain the key ideas of representing images
- Apply representing images to exam-style questions
- Develop independent study skills
- Build cross-topic connections
Prerequisites
- Read the revision notes for representing images first (GCSE Revise summary)
- Key vocab to pre-teach: Exam Focus, GCSE Computer Science Exam Tips
- Basic skills: reading the summary notes and answering the practice questions there
Materials & Equipment
- Exercise book, coloured pens
- Ruler
- Printed revision notes (link below)
- Internet for videos (see Resources)
Lesson 1: Introduction: representing images
Duration: 50 minutes
Starter Activity (5 minutes)
Quick Recall
Write down everything you already know about representing images. Then check against the key terms: Exam Focus, GCSE Computer Science Exam Tips. Use a mini-whiteboard or paper.
Main Content (35 minutes)
Parent/Teacher Guide:
Before lesson: Read the script below. Pre-teach key vocab: Exam Focus, GCSE Computer Science Exam Tips.
If stuck: Re-read the revision notes (link above), then break the content into smaller steps.
Extension: See the Stretch & Challenge ideas in Lesson 4.
Before lesson: Read the script below. Pre-teach key vocab: Exam Focus, GCSE Computer Science Exam Tips.
If stuck: Re-read the revision notes (link above), then break the content into smaller steps.
Extension: See the Stretch & Challenge ideas in Lesson 4.
Teaching Script (35 mins):
Mins 0-5 - Hook: "Today: representing images. By the end you will be able to answer exam questions on it unaided. It connects to the rest of Computer Science because the ideas here recur across the spec."
Mins 5-20 - Direct Instruction: Work through the core ideas below one at a time; after each, ask your student to explain it back in their own words.
Mins 20-30 - Guided Practice: Model the worked example together, then let your student attempt the first practice question with guidance.
Mins 30-35 - Independent Practice: 2-3 practice questions from Lesson 3 below, with immediate feedback.
Mins 0-5 - Hook: "Today: representing images. By the end you will be able to answer exam questions on it unaided. It connects to the rest of Computer Science because the ideas here recur across the spec."
Mins 5-20 - Direct Instruction: Work through the core ideas below one at a time; after each, ask your student to explain it back in their own words.
Mins 20-30 - Guided Practice: Model the worked example together, then let your student attempt the first practice question with guidance.
Mins 30-35 - Independent Practice: 2-3 practice questions from Lesson 3 below, with immediate feedback.
First Look
Start with the revision notes summary, then attempt: An image has a resolution of 1920 x 1080 and a colour depth of 24 bits. Calculate the file size in megabytes.
Plenary (5 minutes)
Check Out
Your student states one thing they learned and one question they still have about representing images.
Lesson 2: Core Concepts: representing images
Duration: 50 minutes
Starter Activity (5 minutes)
Review Previous Lesson
Quick recap: write 3 key points from Lesson 1 on representing images. Check them against the notes below.
Main Content (35 minutes)
Definition: A bitmap image is made up of a grid of small squares called pixels (picture elements). Each pixel is assigned a binary value that represents its colour.
Definition: Resolution is the number of pixels in an image, expressed as width x height. For example, 1920 x 1080 means 1920 pixels across and 1080 pixels down.
Definition: Colour depth (also called bit depth) is the number of bits used to represent the colour of each pixel. More bits per pixel means more possible colours.
Important: There is always a trade-off between file size and image quality. Higher resolution and greater colour depth produce better images but require more storage space and bandwidth.
Exam Focus: The GCSE specification primarily focuses on bitmap images. However, you should understand the key differences between bitmap and vector representations.
GCSE Computer Science Exam Tips: File size formula: width × height × colour depth (in bits), then convert to bytes/KB/MB. 1 byte = 8 bits, 1 KB = 1024 B, 1 MB = 1024 KB. For resolution changes: doubling both dimensions quadruples pixels. Bitmap = pixels, Vector = mathematical shapes. Vector advantages: scalable without quality loss, smaller for simple graphics. Bitmap advantages: better for photographs.
| Term | Meaning | Example |
|---|---|---|
| 1-bit | 1 | 2 (black and white) |
| 8-bit | 8 | 256 |
| 16-bit | 16 | 65,536 |
| 24-bit | 24 | 16,777,216 |
| 32-bit | 32 | 4,294,967,296 |
| Increase resolution (more pixels) | Increases | Improves (more detail) |
| Decrease resolution (fewer pixels) | Decreases | Degrades (pixelation) |
| Increase colour depth | Increases | Improves (more colours, smoother gradients) |
Practice (10 minutes)
Q: An image has a resolution of 1920 x 1080 and a colour depth of 24 bits. Calculate the file size in megabytes.
Answer: File size = 1920 x 1080 x 24 = 49,766,400 bits = 6,220,800 bytes = 6,075 KB = 5.93 MB
Plenary (5 minutes)
Explain Back
Your student teaches the key points back to you without looking. Fill any gaps immediately.
Lesson 3: Application: representing images
Duration: 50 minutes
Starter Activity (5 minutes)
Quick Recall
Recall the key terms: Exam Focus, GCSE Computer Science Exam Tips. Define each in one sentence.
Main Content (35 minutes)
Parent/Teacher Guide: Let your student attempt each question alone first, then compare with the model answer. Award method marks for correct working even if the final answer is wrong.
Q1: An image has a resolution of 1920 x 1080 and a colour depth of 24 bits. Calculate the file size in megabytes.
Answer: File size = 1920 x 1080 x 24 = 49,766,400 bits = 6,220,800 bytes = 6,075 KB = 5.93 MB
Q2: What is the effect on file size if you double the width of an image but keep the height and colour depth the same?
Answer: The file size doubles, because total pixels = W x H. Doubling W means 2W x H = 2 x (W x H), so twice as many pixels.
Q3: An image uses 8-bit colour depth. How many different colours can it represent?
Answer: 2 8 = 256 colours
Q4: Explain the trade-off between image quality and file size when increasing colour depth.
Answer: Increasing colour depth allows more colours to be represented, producing smoother gradients and more realistic images, but each pixel needs more bits, so the file size increases proportionally.
Q5: A 500 x 400 image has a file size of 1,600,000 bits. What is the colour depth?
Answer: Colour depth = 1,600,000 / (500 x 400) = 1,600,000 / 200,000 = 8 bits per pixel
Plenary (5 minutes)
Error Review
Review any questions answered incorrectly. Identify whether the error was knowledge, method, or reading the question.
Lesson 4: Exam Practice: representing images
Duration: 50 minutes
Starter Activity (5 minutes)
Command Words
Review what these command words require: state (one point), describe (say what happens), explain (say why), compare (both sides), evaluate (judgement).
Main Content (35 minutes)
Extended Answer
Extended question: Full-Mark Response A bitmap image is 1024 × 768 pixels with 24-bit colour depth. Calculate the file size in megabytes. If the colour depth is reduced to 8 bits, calculate the new file size and the percentage reduction. [4 marks] <div class="
Original file size: Number of pixels = 1024 × 768 = 786,432 File size in bits = 786,432 × 24 = 18,874,368 bits File size in bytes = 18,874,368 ÷ 8 = 2,359,296 bytes File size in MB = 2,359,296 ÷ (1024 × 1024) = 2.25 MB Reduced colour depth (8-bit): File size = 786,432 × 8 ÷ 8 ÷ (1024 × 1024) = 0.75 MB Percentage reduction = (2.25 - 0.75) ÷ 2.25 × 100 = 66.7% Reducing colour depth from 24-bit to 8-bit reduces the file size by approximately 66.7%.
Exam Tips: Always show your working in file size calculations - marks are given for each step | Remember the formula: File size = W x H x colour depth | Don't forget to convert between bits, bytes, KB, and MB when asked | If asked about quality vs file size, mention the trade-off | When asked about doubling resolution, calculate the new pixel count, don't just guess | Be precise: say "colour depth" not just "bits", and "resolution" not just "size"
Common Errors: ✗ Thinking higher resolution always means a better image regardless of colour depth ✓ Image quality depends on BOTH resolution (pixels) and colour depth (bits per pixel). A high-resolution image with 1-bit colour looks worse than a lower-resolution 24-bit colour image. ✗ Confusing pixel resolution with image file size ✓ File size depends on resolution × colour depth. Two images with the same pixel dimensions can have different file sizes if they use different colour depths. ✗ Forgetting that increasing resolution or colour depth increases file size ✓ Doubling the resolution (width and height) quadruples the number of pixels, and therefore quadruples the file size (assuming same colour depth)
Stretch & Challenge (Grade 8-9):
- Synoptic links: explain how representing images connects to another Computer Science topic you have studied
- Real-world: research one real-world use or example of representing images
- Critical: "What are the limitations of the models used in representing images?"
Plenary (5 minutes)
Assessment Criteria
- Got it: Confident explanation + correct worked examples
- Getting there: Main points OK, needs support with detail
- Not yet: Confused on key concepts - re-run Lesson 2
Homework & Consolidation
- Consolidation: Re-answer any Lesson 3 practice questions answered incorrectly (20 mins)
- Retrieval: Write flashcards for the key terms: Exam Focus, GCSE Computer Science Exam Tips (10 mins)
- Exam practice: One past-paper question on representing images from the board websites (15 mins)
- Extension: Explain representing images to someone else in your own words (10 mins)