📄 Paper 2 · 4.5 Data Representation
4.5.6a Representing Images (Bitmap)
AQA 7517 · A-Level Computer Science · ~14 min read

Bitmap Images

A bitmap (raster) image stores a grid of pixels. Each pixel is represented by a binary number encoding its colour. The image is stored as metadata (width, height, colour depth) plus the pixel data.

Key Properties

PropertyDefinitionEffect
ResolutionNumber of pixels in width × height (e.g. 1920×1080)Higher resolution → more detail → larger file size
Colour depthNumber of bits used per pixelMore bits → more colours → larger file size
PixelSingle picture element — the smallest unit of a bitmapEach pixel stores one colour value

Colour Depth

Bits per pixelColours possibleExample
12Black and white
8256Greyscale or limited palette
2416,777,216True colour (8 bits each: R, G, B)
3216,777,216 + alphaTrue colour + transparency channel

Calculating File Size

Formula: File size (bits) = width (px) × height (px) × colour depth (bits per pixel)

Convert to bytes by dividing by 8; to KB by dividing by 8,000 (SI) or 8,192 (binary).

Worked example

Image: 800 × 600 pixels, 24-bit colour depth (uncompressed)

  • Total bits = 800 × 600 × 24 = 11,520,000 bits
  • Total bytes = 11,520,000 ÷ 8 = 1,440,000 bytes
  • In MB = 1,440,000 ÷ 1,000,000 = 1.44 MB

Metadata

Bitmap files include a header (metadata) storing: image width, height, colour depth, file format, compression type. This is additional storage overhead on top of the pixel data.

Factors Affecting Image Quality and File Size

  • Higher resolution → better quality → larger file
  • Greater colour depth → more colours → larger file
  • Compression (lossy or lossless) reduces file size at the cost of quality or complexity

Vector Graphics vs Bitmap

AspectBitmapVector
StoragePixel gridMathematical equations/paths
ScalingPixelates when enlargedScales perfectly at any size
File sizeLarge (complex images)Small for diagrams/logos
Best forPhotographs, natural imagesLogos, icons, diagrams
Exam tip: Always know the file size formula: width × height × colour depth (bits) ÷ 8 = bytes. Know that more pixels and more bits per pixel increase quality AND file size. 24-bit colour = true colour (2²⁴ colours). Compare bitmap vs vector: vector scales perfectly because it stores equations, not pixels. Metadata adds overhead. AQA often asks you to calculate uncompressed file size.
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Worksheet — 4.5.6a Representing Images

8 questions · instantly marked · AQA 7517 standard

Q1Define the terms 'pixel', 'resolution', and 'colour depth' in the context of bitmap images.[3]
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Pixel: the smallest unit of a bitmap image — a single picture element storing one colour value [1]; resolution: the number of pixels in the image (width × height), e.g. 1920×1080 [1]; colour depth: the number of bits used to represent each pixel's colour — more bits = more possible colours [1].
Q2Calculate the uncompressed file size in bytes of a 1280×720 pixel image with a 16-bit colour depth. Show all working.[3]
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Total pixels = 1280 × 720 = 921,600 [1]; total bits = 921,600 × 16 = 14,745,600 bits [1]; total bytes = 14,745,600 ÷ 8 = 1,843,200 bytes (≈ 1.84 MB) [1].
Q3How many colours can be represented with a 24-bit colour depth? How are the bits typically divided in true colour systems?[2]
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2²⁴ = 16,777,216 colours [1]; divided as 8 bits red + 8 bits green + 8 bits blue (RGB) [1].
Q4A photographer doubles the resolution of an image (both width and height). By what factor does the uncompressed file size increase? Explain.[2]
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File size increases by a factor of 4 [1]; because doubling width AND height multiplies pixel count by 2×2=4; with colour depth unchanged, total bits quadruple [1].
Q5Explain why a bitmap image becomes pixelated when enlarged significantly.[2]
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A bitmap stores a fixed grid of pixels — no additional detail is stored [1]; when enlarged, each pixel is stretched over more screen area, making the grid of squares visible as pixelation [1].
Q6State two differences between bitmap and vector graphics.[2]
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Any two from: bitmap stores pixel grid / vector stores mathematical equations; bitmap pixelates when scaled / vector scales perfectly at any size; bitmap is better for photographs / vector is better for logos/diagrams; bitmap file size proportional to resolution / vector file size independent of output size [1 each].
Q7What is metadata in a bitmap file? Give two examples of data stored in the metadata.[2]
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Metadata is header data stored alongside the pixel data describing properties of the image [1]; examples: image width/height, colour depth, file format, compression method, creation date (any two) [1].
Q8A 1-bit colour depth image is 400×300 pixels. Calculate its size in bytes. What does 1-bit colour represent?[3]
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Total bits = 400 × 300 × 1 = 120,000 bits [1]; ÷ 8 = 15,000 bytes [1]; 1-bit colour = 2 possible values (0 or 1) = black and white only [1].
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Mini Test — Representing Images

10 questions · 10 minutes

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