Data representation and security · GCSE Computer Science

Image representation

GCSE Computer Science images: pixels, resolution, colour depth, file size, metadata and compression.

UNDERSTAND→RETRIEVE→REMEMBER
THE MEMORY HOOK
Width × height × depth — pixels are numbers wearing colour.

The important bits

What you need to know

  1. 1

    Bitmap: grid of pixels; each pixel stores colour as binary number.

  2. 2

    Resolution: width × height in pixels (or DPI for print — say which you mean).

  3. 3

    Colour depth: bits per pixel — 1-bit mono, 8-bit 256 colours, 24-bit ~16.7 million.

  4. 4

    File size (bits) ≈ width × height × colour depth (+ metadata).

  5. 5

    More pixels or deeper colour → larger file, sharper or richer image.

  6. 6

    Metadata: height, width, colour depth so software interprets bits correctly.

  7. 7

    Compression: lossless PNG; lossy JPEG discards data humans notice less.

  8. 8

    Vector vs raster (if on spec): vectors scale without pixelation; rasters are pixel grids.

Quotations worth analysing

Short evidence. Real method.

“A bitmap does not store a picture; it stores a number per pixel.”
Data representation teaching line

Links to binary topic — images are data with interpretation rules.

Go deeper

Calculate an image

800×600 24-bit: 800×600×24 bits. Divide by 8 for bytes. If colour depth halves, file halves (approx).

WORKED EXAMPLE

See the idea in action

800×600 24-bit image: 800×600×24 = 11,520,000 bits = 1,440,000 bytes. Drop to 8-bit → divide by three colour-wise.

Exam technique

Turn knowledge into marks

Show W×H×depth. State assumption about metadata if ignored.

Common mistakes

Do not give these marks away

  1. 01

    Confusing resolution with colour depth.

  2. 02

    Lossy “no quality loss”.

  3. 03

    Forgetting metadata role.

QUICK RETRIEVAL

What increases bitmap file size?

AMore pixels and higher colour depth

BFewer pixels only

CSaving as text

DTurning off the monitor

Show the answer

More pixels and higher colour depth. More pixels = more data points; deeper colour = more bits per pixel.

Quick questions

If this is the bit you searched

What is colour depth?

Bits per pixel — determines how many distinct colours can be represented.