Loading…
Free A-Level Computer Science guide
Keep image dimensions, colour depth and physical resolution separate. Build a small image, account for its stored bits, then choose pixels or geometric objects for a particular task.
Content owner: Michael Print · Written for A-Level learners · Checked against official specifications
Explore the idea
Choose dimensions and a palette, paint cells, then predict the stored size. Compare the same shape stored as pixels and as drawing instructions.
A bitmap records a code for each pixel. Width × height gives the pixel count; colour depth gives bits per pixel. A vector graphic instead describes objects and their properties, which a renderer turns into display pixels.
Calculate the stated payload first, convert bits to bytes, then include any stated header or padding. A formula for uncompressed pixels does not predict the actual size of every PNG, JPEG or vector file.
AQA 7517 · 4.5.3.1–4.5.3.2; 4.5.6.4–4.5.6.6
Useful foundations
By the end, you should be able to
AQA: interpret pixel dimensions, colour depth, physical resolution and object properties. File-size questions specify their own header, palette and packing assumptions.
Shared foundations and optional extensions are labelled separately; this page does not claim complete coverage of the qualification.
An 8 × 6 bitmap contains 48 pixels. With two bits per pixel, each pixel can select one of four codes, 00 through 11. The code needs a colour interpretation: an indexed image may store a separate palette.
A header can record dimensions, format and depth. File formats may also include a palette, row alignment and metadata. Only include the overhead described in a calculation; do not silently treat it as zero or invent a standard header size.
For fixed-width uncompressed codes, payload bits = width × height × depth. Divide by eight to express bits as bytes. If a final partial byte is stored, round up once for continuously packed data; separately padded rows require a row-based calculation.
A kB is 1000 bytes, while a KiB is 1024 bytes. MB/GB/TB use 10^6/10^9/10^12 bytes; MiB/GiB/TiB use 2^20/2^30/2^40. State which unit is requested and retain enough precision until the final conversion.
Pixel dimensions describe the sampled grid. At the same pixel count, changing only a print-size or pixels-per-inch setting need not change stored pixel data. Increasing the captured pixel dimensions can preserve smaller features but increases payload.
A 600-pixel-wide image printed at 300 pixels per inch is two inches wide; at 150 ppi it is four inches wide. Both calculations use the same 600 source pixels. Enlarging a bitmap cannot recover detail that was never captured.
A vector drawing can contain a circle with centre, radius, fill and outline, followed by a line with endpoints, colour and thickness. Object order can affect which shape is visible on top. Scaling changes geometry before rasterisation.
Vectors suit diagrams, logos and reusable shapes because the underlying geometry can be redrawn at different sizes. Bitmaps suit captured photographs and complex pixel-level detail. A vector file with many objects can exceed a simple bitmap; representation alone does not guarantee a smaller file.
| Order | Object | Properties |
|---|---|---|
| 1 | circle | centre (80,60), radius 20, blue fill |
| 2 | line | (20,20) to (100,20), black stroke, width 2 |
Worked example
| Image | Bits | Bytes |
|---|---|---|
| 8×6, 2 bpp | 96 | 12 |
| 16×6, 2 bpp | 192 | 24 |
| 8×6, 4 bpp | 192 | 24 |
4 original questions total 16 marks. Attempt each before opening the independently written indicative marking guidance.
A 20 × 12 bitmap uses four bits per pixel and a 32-byte header. Assume continuous packing, no palette or compression. Calculate its colour-code capacity, payload and total bytes.
Indicative answer
A 3 × 3 one-bit image has no header. Compare continuous packing with byte-aligned rows. Explain why the results differ.
Indicative answer
An image is 900 pixels wide. Calculate its printed width at 300 ppi and 150 ppi, and explain whether changing only this setting creates new image detail.
Indicative answer
A circle has centre (30,40), radius 10 and a blue fill. Scale its coordinates and radius by 3. Justify vector storage for a logo and bitmap storage for a photograph. Give drawing-list records for the original circle and a black line from (0,0) to (30,40).
Indicative answer
This guide addresses AQA 7517 4.5.3.1–4.5.3.2; 4.5.6.4–4.5.6.6. Check your examination year and the complete specification for the assessment scope.
These are independently written explanations and practice questions. CompSciTutoring.co.uk is not affiliated with or endorsed by an examination board. The marking guidance is indicative; always check the syllabus for your examination year.
Found an error? Email a correction.
Use the free examples first, then explore one-to-one tuition if guided practice would help.