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AQA GCSE CS · 3.3.5

Representing Images

Section A: 10 multiple choice questions, 10 marks. Section B: 4 written questions, 11 marks. Total 21 marks.

Name

Class

Date

Mark  / 21

Section A

Circle the letter of the correct answer. One mark each.

  1. 1What does colour depth control?1
    1. AThe number of different colours each pixel can be
    2. BThe number of pixels in the image
    3. CThe file format used
    4. DThe physical size of the image on screen
  2. 2How many colours can be represented with a colour depth of 4 bits?1
    1. A4
    2. B8
    3. C16
    4. D256
  3. 3An image is 400 by 300 pixels with a colour depth of 8 bits. What is its size in bits?1
    1. A120000
    2. B480000
    3. C960000
    4. D1200000
  4. 4Why does an image file need to store its width as metadata?1
    1. AIt is required by law
    2. BWithout it, software cannot know where each row of pixels ends
    3. CIt stores the colour of the first pixel
    4. DIt makes the file smaller
  5. 5What happens to the file size if the colour depth is increased from 8 bits to 16 bits?1
    1. AIt quadruples
    2. BIt increases by 8 bytes
    3. CIt stays the same
    4. DIt doubles
  6. 6An image of 2 megabytes is described as 1000 by 500 pixels. What is the colour depth, using 1 kB as 1000 bytes?1
    1. A16 bits
    2. B24 bits
    3. C8 bits
    4. D32 bits
  7. 7What is a pixel?1
    1. AThe smallest individual element of a bitmap image, holding one colour value
    2. BA unit of file size
    3. CThe number of colours available
    4. DA type of metadata
  8. 8Doubling both the width and the height of an image has what effect on its file size?1
    1. AIt doubles
    2. BIt stays the same
    3. CIt becomes four times larger
    4. DIt becomes eight times larger
  9. 9Which of these is metadata rather than image data?1
    1. AThe colour of the top left pixel
    2. BThe list of colour values
    3. CThe blue value of pixel 200
    4. DThe date the photograph was taken
  10. 10Why can sharing a photograph raise a privacy concern even if the picture itself is harmless?1
    1. AThe file size shows where the photograph was stored
    2. BThe metadata may include the exact location and time it was taken
    3. CPixels can be decoded into text
    4. DThe colour depth reveals the camera model

Section B

Answer in the space provided. The number of marks tells you how many separate points to make.

  1. 1StateState what is meant by the resolution of an image.1
  2. 2CalculateAn image is 640 pixels wide, 480 pixels high and uses a colour depth of 24 bits. Calculate the file size in kilobytes, using 1 kilobyte as 1000 bytes. Show your working.4
  3. 3ExplainExplain why increasing the colour depth of an image increases its file size.3
  4. 4ExplainExplain why metadata must be stored with an image file.3

Mark scheme

Section A

1A2C3C4B5D6D7A8C9D10B

Section B

1 State what is meant by the resolution of an image. 1

One mark for each of the following, to a maximum of 1.

  • The number of pixels in the image, given as width by height

Model answer. The resolution of an image is the number of pixels it contains, expressed as the width in pixels multiplied by the height in pixels.

2 An image is 640 pixels wide, 480 pixels high and uses a colour depth of 24 bits. Calculate the file size in kilobytes, using 1 kilobyte as 1000 bytes. Show your working. 4

One mark for each of the following, to a maximum of 4.

  • Multiplies width by height to get the number of pixels
  • Multiplies by the colour depth to get the size in bits
  • Divides by 8 to get bytes
  • Divides by 1000 to give 921.6 kB

Model answer. The number of pixels is 640 multiplied by 480, which is 307200. Each pixel needs 24 bits, so the image data is 307200 multiplied by 24, which is 7372800 bits. Dividing by 8 gives 921600 bytes, and dividing by 1000 gives 921.6 kilobytes.

3 Explain why increasing the colour depth of an image increases its file size. 3

One mark for each of the following, to a maximum of 3.

  • Colour depth is the number of bits stored for each pixel
  • Increasing it adds bits to every pixel in the image, not just to some
  • So the total number of bits, and therefore the file size, increases in proportion

Model answer. Colour depth is the number of bits used to store the colour of each individual pixel. Increasing the colour depth therefore adds those extra bits to every single pixel in the image rather than to the file as a whole, so an image containing two million pixels gains two million extra bits for each extra bit of colour depth. Since the file size is the number of pixels multiplied by the colour depth, raising the depth raises the size in direct proportion.

4 Explain why metadata must be stored with an image file. 3

One mark for each of the following, to a maximum of 3.

  • The pixel data on its own is just a sequence of values
  • The width and height tell software how the pixels are arranged into a grid
  • The colour depth tells software how many bits belong to each pixel

Model answer. Stored on its own, the pixel data is only a long sequence of binary values with nothing to say how they should be arranged. The metadata supplies that missing information. The width tells the software where one row of pixels ends and the next begins, which is what turns the sequence back into a rectangle, and the height confirms how many rows there should be. The colour depth tells the software how many bits belong to each pixel, without which it cannot divide the sequence into pixels at all. Without metadata the same file could be read as a completely different picture.