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Representing Text, Sound, Images and Storage: Question 2

Syllabus 1.2, 1.3

Structured 7 marks

A weather-monitoring drone has a camera that captures still images of cloud cover. Each image is stored with a resolution of 480 pixels by 320 pixels and a colour depth of 24 bits.

(a) State what is meant by the resolution of an image. [1]

(b) State what is meant by the colour depth of an image. [1]

(c) Calculate the file size of one image, in bytes. Show all your working. [3]

(d) Give the file size of one image in kibibytes (KiB). [2]

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Worked solution

Part (a): Resolution

Resolution is the number of pixels that make up an image, usually described as the width in pixels multiplied by the height in pixels.

Part (b): Colour depth

Colour depth is the number of bits used to represent the colour of a single pixel. A higher colour depth allows more possible colours to be shown for each pixel.

Part (c): Calculating the file size in bytes

Every pixel in the image needs colour depth bits to store its colour, so:

file size (bits) = number of pixels x colour depth

First find the number of pixels:

480 x 320 = 153,600 pixels

Then multiply by the colour depth (24 bits per pixel):

153,600 x 24 = 3,686,400 bits

Convert bits to bytes by dividing by 8:

3,686,400 / 8 = 460,800 bytes

So one image is 460,800 bytes.

Part (d): Converting to kibibytes (KiB)

A kibibyte is 1024 bytes, so divide the number of bytes by 1024 (not 1000):

460,800 / 1024 = 450

So one image is 450 KiB.

Final answers

  • (a) Resolution = the number of pixels that make up the image.
  • (b) Colour depth = the number of bits used to represent each pixel’s colour.
  • (c) File size = 460,800 bytes
  • (d) File size = 450 KiB