← ACARA Digital Technologies Years 7–8

24-bit colour: three bytes per pixel

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.

Part of: ACARA Digital Technologies Years 7–8 — Representing Text, Images and Sound — Module 39

Understanding 24-bit colour: three bytes per pixel in ACARA Digital Technologies Years 7–8

What it does: A byte is 8 bits, so dividing 24 bits by 8 shows each pixel needs 3 bytes, one per colour channel. The 24 bits are split into three equal parts: 8 for red, 8 for green and 8 for blue. Each of the three channels has 256 levels, and every combination is a different colour, so you multiply. That gives about 16.7 million colours. 256 x 256 x 256 = 16,777,216. Adding 256 three times only gives 768, which is not how combinations work.

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

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Learning
Duo
How many bits does one pixel use in 24-bit RGB colour?
24 bits (3 x 8)8 bits
Matches
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.
Applying
Meanings
Term
24-bit colour
Meaning
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.
Practice
bitsPerPixel ← 3 * 8
Testing
Gaps
bitsPerPixel = 3 *
Check Answer
Spots
1r = 8
2g = 6

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Quick Questions

1. How many bits does one pixel use in 24-bit RGB colour?
A. 8 bits
B. 24 bits (3 x 8) ✓
Red, green and blue each get 8 bits. 3 x 8 = 24 bits for every pixel.
2. How many bytes does one 24-bit colour pixel take up?
A. 24 bytes
B. 3 bytes ✓
A byte is 8 bits, so 24 bits is 24 / 8 = 3 bytes, one byte each for red, green and blue.
3. In 24-bit colour, how many bits are used for the red part of a pixel?
A. 8 bits ✓
B. 24 bits
The 24 bits are split into three equal parts: 8 for red, 8 for green and 8 for blue.
4. How many different levels can the green part of a 24-bit pixel have?
A. 256 (0 to 255) ✓
B. 24
8 bits can store 256 values, from 0 to 255. Each colour channel works this way.
5. Which calculation gives the number of possible colours in 24-bit colour?
A. 256 x 256 x 256 ✓
B. 256 + 256 + 256
Each of the three channels has 256 levels, and every combination is a different colour, so you multiply. That gives about 16.7 million colours.

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.

Key Term

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.

Fill in the Blanks

1. bitsPerPixel = 3 * 8
Hint: Type the number of bits used for each of the red, green and blue channels.
2. colours = 256 * 256 * 256
Hint: Type how many different values the blue channel can have.
3. red = 8 bits green = 8 bits blue = 8 bits
Hint: Type the number of bits the blue channel uses, the same as red and green.
4. 24 bits per pixel = 3 bytes per pixel
Hint: Type how many bytes make up 24 bits, remembering a byte is 8 bits.
5. colours = 2 ^ 24
Hint: Type the total number of bits per pixel, used as the power of 2.

Spot the Error

Question 1
A r = 8
B g = 6
C b = 8
D total = r + g + b
Line 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.
Question 2
A bits = 8
B channels = 3
C total = bits + channels
D print(total)
Line 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.
Question 3
A bytes = 3
B bitsInByte = 8
C bits = bytes + bitsInByte
D print(bits)
Line C — Each byte holds 8 bits, so the number of bits is bytes * bitsInByte = 3 * 8 = 24. Adding gives 11.
Question 4
A bits = 8
B levels = 2 * bits
C print(levels)
Line 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.
Question 5
A colours = 256 + 256 + 256
B print(colours)
C # about 16.7 million
Line 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.

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