Colour depth

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Quick Questions

Shade in the A or B circle for each question.

1. What does colour depth measure?

A The number of bits used for each pixel's colour

B The number of pixels in the image

2. How many colours can a 1-bit colour depth give?

A 2 colours (e.g. black and white)

B 1 colour

3. An image uses 8 bits per pixel. How many different colours is that?

A 8 colours

B 256 colours

4. 24-bit true colour is made from:

A Three 8-bit values (red, green, blue)

B One 24-bit grey value

5. Which change increases the colour depth of an image?

A Adding more pixels to the image

B Using more bits for each pixel

Match the Pairs

Draw a line connecting each item on the left to its match on the right.

Left column		Right column
1colour depth = 1 bit		AColour depth is not the number of pixels. 800 x 600 is the image resolution. Colour depth is the bits used per pixel.
2colour depth = 24 bits		B4 bits per pixel = 2^4 = 16 colours
3Colour depth counts the bits used for each pixel, so a depth of 8 bits gives 2^8 different colours		C8 bits per pixel = 2^8 = 256 colours
4Colour depth is bits per pixel, not the number of pixels, so a 4-bit pixel can show 16 colours		D1 bit has two values (0 and 1), so it gives 2 colours, not 1. The number of colours is 2^bits.
5colour depth = 800 x 600 pixels		EEach pixel is stored with 1 bit, so it can only be one of 2 colours, such as black or white
61 bit per pixel = 1 colour		FEach pixel uses 24 bits, made of three 8-bit values for red, green and blue, giving true colour

Definitions

colour depth

Fill in the Blanks

Complete each line of code by filling in the missing parts.

depth = _ # black and white

colours = 2 ** _ # 256 colours

trueColour = 3 * _

colours = _ ** depth

bits = pixels _ depth

Spot the Error

Shade in the letter of the line that contains an error.

Question 1

A colourDepth ← 8

B colours ← 2 * colourDepth

C OUTPUT colours

Question 2

A red ← 8

B green ← 8

C blue ← 8

D colourDepth ← red * green * blue

E OUTPUT colourDepth

Question 3

A bitsPerPixel ← 1

B colours ← 2 ^ bitsPerPixel

C IF colours = 1 THEN

D OUTPUT "Black and white"

E ENDIF

Question 4

A pixels ← 100

B bitsPerPixel ← 256

C totalBits ← pixels * bitsPerPixel

Question 5

A colourDepth ← 24

B channels ← 3

C bitsEach ← colourDepth * channels

D OUTPUT bitsEach

Answer Key

Colour depth

Section A: Quick Questions

A — The number of bits used for each pixel's colour

A — 2 colours (e.g. black and white)

B — 256 colours

A — Three 8-bit values (red, green, blue)

B — Using more bits for each pixel

Section B: Match the Pairs

colour depth = 1 bit → Each pixel is stored with 1 bit, so it can only be one of 2 colours, such as black or white

colour depth = 24 bits → Each pixel uses 24 bits, made of three 8-bit values for red, green and blue, giving true colour

Colour depth counts the bits used for each pixel, so a depth of 8 bits gives 2^8 different colours → 8 bits per pixel = 2^8 = 256 colours

Colour depth is bits per pixel, not the number of pixels, so a 4-bit pixel can show 16 colours → 4 bits per pixel = 2^4 = 16 colours

colour depth = 800 x 600 pixels → Colour depth is not the number of pixels. 800 x 600 is the image resolution. Colour depth is the bits used per pixel.

1 bit per pixel = 1 colour → 1 bit has two values (0 and 1), so it gives 2 colours, not 1. The number of colours is 2^bits.

Section C: Definitions

colour depth — The number of bits used to store the colour of each pixel. More bits per pixel means more possible colours, such as 1 bit for 2 colours or 24 bits for true colour.

Section D: Fill in the Blanks

depth = 1 # black and white

colours = 2 ** 8 # 256 colours

trueColour = 3 * 8

colours = 2 ** depth

bits = pixels * depth

Section E: Spot the Error

B — The number of colours is 2 raised to the power of the colour depth, so it should be 2 ^ colourDepth. Multiplying gives 16 instead of 256.

D — 24-bit true colour is three 8-bit colours added together (8 + 8 + 8 = 24 bits per pixel). Multiplying gives 512, which is wrong.

C — 1 bit per pixel gives 2 colours (black and white), so the test should be colours = 2. A single colour would need 0 bits.

B — Colour depth is the number of bits per pixel, not the number of colours. 256 colours need 8 bits, so it should be bitsPerPixel ← 8.

C — 24 bits are shared between the red, green and blue channels, so divide by 3 to get 8 bits each. Multiplying gives 72, which is wrong.