Represent digital images with pixels
Connect pixels, resolution and colour depth to image quality and uncompressed file size.
This sequence follows Cambridge IGCSE Computer Science 0478, 2026–2028 syllabus Version 6. It covers subject content sections 1.1–1.3 only. The separately published 2029–2031 syllabus is not mapped here. Practice is original, self-checked and does not award an official Cambridge grade. Full timed papers and human subject-teacher review remain outstanding.
Before you begin
- No programming language is required. Be comfortable with place value and multiplication; each lesson introduces the computer-science notation it uses.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
What is a pixel?
What you will learn
- Explain that images are stored as pixel data
- Distinguish resolution from colour depth
- Calculate an uncompressed bitmap size from the given dimensions and depth
Each pixel stores a colour code
A digital image can be represented as a grid of pixels. Resolution is the number of pixels in the image; for a width and height, multiply the two dimensions to find that total.
Apply and check the rule
Colour depth is the number of bits used to represent each pixel's colour. More pixels or more bits per pixel generally increase file size and can improve image detail or colour range. A bitmap-size calculation begins with width×height×colour depth bits.
Pause and explain
Which change directly increases the number of pixels in an image?
Worked example
Find the uncompressed size of a 640×480 image at 8 bits per pixel, in bytes.
Show the worked solution
- Pixels: 640×480=307,200.
- Bits: 307,200×8=2,457,600 bits.
- Bytes: 2,457,600÷8=307,200 bytes.
Answer 307,200 bytes
Try a topic test
Choose Easy, Normal or Difficult. Save your answers and open worked review after finishing.
Test this topic →From guided practice to a new situation
Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
An image is 320×200 pixels with 4-bit colour depth. Find its uncompressed size in bits.
Give me a hint
Multiply pixel count by bits per pixel.
Compare my reasoning
- Pixel count: 320×200=64,000.
- Size: 64,000×4=256,000 bits.
256,000 bits
Look for these in your work
- Pixel count correct
- Depth applied
- Unit is bits
A 256×256 image changes from 8-bit to 16-bit colour depth. What happens to its raw size?
Give me a hint
Resolution stays constant, so compare bits per pixel.
Compare my reasoning
- The number of pixels does not change.
- Bits per pixel double.
- The uncompressed size doubles.
The raw image size doubles.
Look for these in your work
- Resolution distinguished
- Proportional size change stated
A designer must choose between a 400×300 image at 8 bits per pixel and a 200×150 image at 24 bits per pixel. Which has the larger uncompressed file and by how much?
Give me a hint
Calculate pixel count×colour depth for both.
Compare my reasoning
- First: 400×300×8=960,000 bits.
- Second: 200×150×24=720,000 bits.
- The first is larger by 240,000 bits.
The 400×300, 8-bit image is larger: 960,000 bits versus 720,000 bits, a difference of 240,000 bits.
Look for these in your work
- Both calculations correct
- Comparison correct
- Difference and units stated
Common mistakes
- Treating colour depth as the number of colours rather than bits per pixel
- Forgetting to convert bits to bytes when the question asks for bytes
What can you explain now?
What happens to an uncompressed image size if colour depth doubles and resolution stays fixed?
Compare with the explanation
The size doubles
Each pixel needs twice as many bits.
After trying it yourself, choose your next review. This is your self-assessment.
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Make it stick
What are the two factors in the basic uncompressed bitmap-size calculation?
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.