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1CP2-P-9.1 Bitmap images pixels resolution and colour depth

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

A bitmap stores an image as a rectangular grid of pixels. Each pixel position stores a binary colour code. Width × height gives the number of pixels; it does not give file size.

Students will:

  • explain how pixels and colour codes form a bitmap
  • calculate pixel counts and available colours
  • distinguish image dimensions from storage size
  • encode and decode a small monochrome bitmap

Inside: 7 explanation cells, 1 fill-in-the-blanks cell, 2 multiple-choice questions and 4 written answers. 15 marks, about 45 minutes.

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

Shared by Coding PathwayVerified teacher

  • 14 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 images pixels resolution and colour depth

A bitmap stores an image as a rectangular grid of pixels. Each pixel position stores a binary colour code. Width × height gives the number of pixels; it does not give file size.

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

Resolution describes pixel dimensions or pixel density in context. Colour depth is bits per pixel. With n bits, each pixel can represent 2^n colours. Changing colour depth changes available colours, not the number of pixels.

Fill in the blanks3 marks
A bitmap is a grid of gap 1. Width × height gives the gap 2 count. Bits used for each pixel are the colour gap 3.
  • depth
  • pixel
  • pixels
Multiple choice1 mark

Which statement is accurate?

  • AOne pixel always uses one byte
  • BColour depth changes how many pixels exist
  • C4-bit colour depth provides 16 possible codes per pixel
  • DWidth × height is file size in bits

2. Worked method

A 6 × 4 bitmap has 24 pixels. At 3-bit colour depth, each pixel selects one of 8 possible colours. Pixel count and colour choices are separate properties.

Written answer2 marks

Calculate the number of pixels in a 12 × 8 bitmap.

Give the value and unit.

Students type their answer here.

Written answer2 marks

A bitmap uses 5-bit colour depth. Calculate the number of colours available.

Use 2^n.

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

Explain one effect of increasing colour depth from 2 bits to 4 bits per pixel.

Link available patterns to image appearance.

Students type their answer here.

Multiple choice1 mark

What does increasing resolution change most directly?

  • AThe number of pixels used to represent the image
  • BThe number of seconds in audio
  • CThe program licence
  • DThe CPU instruction set

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.

4. Encode and decode a tiny monochrome bitmap

For a one-bit image, agree that 0 means white and 1 means black. Read rows from left to right and top to bottom. The three rows 010, 111, 010 therefore show a black centre column with a complete black middle row.

Written answer3 marks

Using 0 for white and 1 for black, encode a 2 × 2 image whose top row is black, white and whose bottom row is white, black.

Write the two binary rows in order.

Students type their answer here.

Route forward

Next you will combine pixel dimensions and colour depth to calculate uncompressed image size.