24-bit colour: three bytes per pixel

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Quick Questions

Shade in the A or B circle for each question.

1. How many bits does one pixel use in 24-bit RGB colour?

A 8 bits

B 24 bits (3 x 8)

2. How many bytes does one 24-bit colour pixel take up?

A 24 bytes

B 3 bytes

3. In 24-bit colour, how many bits are used for the red part of a pixel?

A 8 bits

B 24 bits

4. How many different levels can the green part of a 24-bit pixel have?

A 256 (0 to 255)

B 24

5. Which calculation gives the number of possible colours in 24-bit colour?

A 256 x 256 x 256

B 256 + 256 + 256

Match the Pairs

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

Left column		Right column
13 x 8 = 24		AThe number of different colours a 24-bit pixel can show, about 16.7 million
2256 x 256 x 256		BThree colour channels (red, green, blue) with 8 bits each gives 24 bits for every pixel
3A 24-bit pixel uses 3 bytes, because each channel (red, green, blue) takes 1 byte of 8 bits		C24 bits / 8 = 3 bytes
4The colour white in 24-bit colour uses the largest value in every channel, written as three bytes		D3 x 8 equals 24, not 32. A 24-bit pixel has three channels of 8 bits each.
53 x 8 = 32 bits per pixel		EThe channel values must be multiplied, not added. Adding gives only 768.
6256 + 256 + 256 = 16.7 million colours		F11111111 11111111 11111111

Definitions

24-bit colour

Fill in the Blanks

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

bitsPerPixel = 3 * _

colours = 256 * 256 * ___

red = 8 bits green = 8 bits blue = _ bits

24 bits per pixel = _ bytes per pixel

colours = 2 ^ __

Spot the Error

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

Question 1

A r = 8

B g = 6

C b = 8

D total = r + g + b

Question 2

A bits = 8

B channels = 3

C total = bits + channels

D print(total)

Question 3

A bytes = 3

B bitsInByte = 8

C bits = bytes + bitsInByte

D print(bits)

Question 4

A bits = 8

B levels = 2 * bits

C print(levels)

Question 5

A colours = 256 + 256 + 256

B print(colours)

C # about 16.7 million

Answer Key

24-bit colour: three bytes per pixel

Section A: Quick Questions

B — 24 bits (3 x 8)

B — 3 bytes

A — 8 bits

A — 256 (0 to 255)

A — 256 x 256 x 256

Section B: Match the Pairs

3 x 8 = 24 → Three colour channels (red, green, blue) with 8 bits each gives 24 bits for every pixel

256 x 256 x 256 → The number of different colours a 24-bit pixel can show, about 16.7 million

A 24-bit pixel uses 3 bytes, because each channel (red, green, blue) takes 1 byte of 8 bits → 24 bits / 8 = 3 bytes

The colour white in 24-bit colour uses the largest value in every channel, written as three bytes → 11111111 11111111 11111111

3 x 8 = 32 bits per pixel → 3 x 8 equals 24, not 32. A 24-bit pixel has three channels of 8 bits each.

256 + 256 + 256 = 16.7 million colours → The channel values must be multiplied, not added. Adding gives only 768.

Section C: Definitions

24-bit colour — Each pixel uses 8 bits for red, 8 for green and 8 for blue. That is 3 bytes (24 bits) per pixel, giving 256 x 256 x 256 = about 16.7 million possible colours.

Section D: Fill in the Blanks

bitsPerPixel = 3 * 8

colours = 256 * 256 * 256

red = 8 bits
green = 8 bits
blue = 8 bits

24 bits per pixel = 3 bytes per pixel

colours = 2 ^ 24

Section E: Spot the Error

B — In 24-bit colour each of red, green and blue uses 8 bits. Green should be g = 8, so the total is 8 + 8 + 8 = 24 bits.

C — The 8 bits are used for each of the 3 channels, so you multiply: 8 * 3 = 24 bits. Adding gives 11, which is wrong.

C — Each byte holds 8 bits, so the number of bits is bytes * bitsInByte = 3 * 8 = 24. Adding gives 11.

B — The number of values 8 bits can store is 2 to the power of 8, written 2 ** bits. This gives 256 levels per colour. Multiplying gives only 16.

A — Each pixel has 256 choices for red, for green and for blue. The choices are multiplied: 256 * 256 * 256 = about 16.7 million colours. Adding gives only 768.