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1CP2-P-9.2 Bitmap file size and quality trade-offs

Part 2 of 6 · 1CP2-P-9 · Bitmap, sound and compression

Uncompressed bitmap size in bits is width × height × colour depth. Width and height are measured in pixels; colour depth is bits per pixel, so pixel units cancel and leave bits.

Students will:

  • construct the bitmap file-size expression
  • calculate in bits, bytes and binary storage units
  • rearrange the expression to find a missing dimension
  • explain quality and storage trade-offs precisely

Inside: 6 explanation cells, 1 fill-in-the-blanks cell, 2 multiple-choice questions and 3 written answers. 13 marks, about 45 minutes.

Series: 1CP2-P-9 · Bitmap, sound and compression, part 2 of 6.

Shared by Coding PathwayVerified teacher

  • 12 cells
  • About 45 minutes
  • CC BY-SA 4.0
  • Shared 17 Aug 2026

Preview

The whole resource, exactly as a class sees it. Answers and marking are held back.

Bitmap file size and quality trade-offs

Uncompressed bitmap size in bits is width × height × colour depth. Width and height are measured in pixels; colour depth is bits per pixel, so pixel units cancel and leave bits.

1. Build the representation

Bitmap pixels, coordinates and colour codes4 × 3 = 12 pixelshighlighted coordinate: (2, 1)example colour code: 1013-bit depth → 2³ = 8 colours

Convert after finding bits: divide by 8 for bytes, then by 1024 for KiB. To find a missing width, rearrange: width = bits ÷ (height × colour depth). This formula does not predict a compressed size unless a compression relationship is supplied.

Fill in the blanks3 marks
Uncompressed image bits = width × gap 1 × colour gap 2. Divide bits by gap 3 to obtain bytes.
  • 8
  • depth
  • height
Multiple choice1 mark

A 32 × 16 image uses 4-bit colour. Which expression gives its size in bits?

  • A32 + 16 + 4
  • B32 × 16 × 4
  • C32 × 16 ÷ 4
  • D32 × 4

2. Worked method

For 64 × 32 pixels at 4 bits per pixel: 64 × 32 × 4 = 8192 bits; 8192 ÷ 8 = 1024 bytes; 1024 ÷ 1024 = 1 KiB. Keep a unit at every stage.

Written answer2 marks

Calculate the uncompressed size in bits of a 20 × 10 bitmap at 4-bit colour depth.

Show the multiplication and unit.

Students type their answer here.

Written answer3 marks

Convert 8192 bits into bytes and KiB.

Use 8 bits per byte and 1024 bytes per KiB.

Students type their answer here.

3. Apply and check

Write the formula or reasoning step before substituting values. Keep the representation, the measurement and its units distinct.

Written answer3 marks

An uncompressed image is 4096 bits, 16 pixels high and uses 4-bit colour. Calculate its width.

Rearrange before substituting.

Students type their answer here.

Multiple choice1 mark

What happens if dimensions stay fixed and colour depth doubles?

  • AUncompressed bit size doubles
  • BPixel count doubles
  • CThe image duration doubles
  • DSize becomes zero

Examination method

Show relevant working, label units and answer the command word. A number without its meaning or unit may not demonstrate the required method.

Route forward

Next you will build the equivalent information model for sampled sound.