← ACARA Digital Technologies Years 5–6

Two bits give four shades: greyscale

Two bits per pixel make four codes, so each pixel can be one of four shades: 00 is black, 01 is dark grey, 10 is light grey and 11 is white.

Part of: ACARA Digital Technologies Years 5–6 — Representing Data with Numbers — Module 18, ACARA Digital Technologies Years 7–8 — Representing Text, Images and Sound — Module 29

Understanding Two bits give four shades: greyscale in ACARA Digital Technologies Years 5–6

What it does: Listing every two-digit combination of 0s and 1s gives exactly four codes, one for each shade from black to white. Each of the two bits can be 0 or 1, so two bits make 4 codes. One bit only makes 2. 00 is black. 11 is the code for white. In our code, 01 is dark grey and 10 is light grey. The bigger number is the lighter shade. Black is 00, white is 11 and light grey is 10, so the row is 00 11 10. One bit can only show two shades. To include greys we need two bits per pixel.

Common mistakes: Each pixel uses two bits, so bits should be 2. One bit only gives two codes (0 and 1), not the four codes needed for four shades. The code 01 is already dark grey. The code 10 should be light grey, so each of the four codes stands for a different shade.

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Free printable worksheet with full answer key — or add this module to a class and your students practise it interactively.

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This module's key terms are also in a printable crossword (Modules 59–71) and word search (Modules 44–71).

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Learning
Duo
How many different codes can two bits make?
2 codes: 0 and 14 codes: 00, 01, 10 and 11
Matches
01 = white
↓
01 is dark grey, not white. White is 11.
Applying
Meanings
Term
00 01 10 11
Meaning
Two bits per pixel make four codes, so each pixel can be one of four shades: 00 is black, 01 is dark grey, 10 is light grey and 11 is white.
Practice
00 01 10 11
Testing
Gaps
01 =
Check Answer
Spots
1bits ← 1
2codes ← 4

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Quick Questions

1. How many different codes can two bits make?
A. 4 codes: 00, 01, 10 and 11 ✓
B. 2 codes: 0 and 1
Each of the two bits can be 0 or 1, so two bits make 4 codes. One bit only makes 2.
2. In our greyscale code, which two-digit code means black?
A. 11
B. 00 ✓
00 is black. 11 is the code for white.
3. Which code stands for the darker grey?
A. 01 is dark grey ✓
B. 10 is dark grey
In our code, 01 is dark grey and 10 is light grey. The bigger number is the lighter shade.
4. How many shades can a pixel show if it uses two bits?
A. Four shades, from black to white ✓
B. Two shades, black and white only
Two bits give four codes, so each pixel can be black, dark grey, light grey or white.
5. A pixel row is encoded as 11 10 01 00. What shades does it show from left to right?
A. Black, dark grey, light grey, white
B. White, light grey, dark grey, black ✓
Decode one pair at a time: 11 is white, 10 is light grey, 01 is dark grey and 00 is black.

Match the Pairs

00 → The two-bit code for a black pixel
10 → The two-bit code for a light grey pixel
Two bits can make four different codes, so a pixel can be one of four shades → 00 01 10 11
Encoding a row of pixels: black, dark grey, light grey, white → 00 01 10 11
01 = white → 01 is dark grey, not white. White is 11.
Row 11 00 01 is read as 3 pixels, each using 1 bit → Each pixel uses 2 bits, so the row is read in pairs: 11, 00, 01 (white, black, dark grey). Each pair is one pixel, not each single digit.

Key Term

00 01 10 11
Two bits per pixel make four codes, so each pixel can be one of four shades: 00 is black, 01 is dark grey, 10 is light grey and 11 is white.

Fill in the Blanks

1. 00 = black 01 = dark grey
Hint: Type the shade that the code 01 stands for.
2. 10 = light grey 11 = white
Hint: Type the shade that the code 11 stands for.
3. Each pixel uses 2 bits, giving 4 shades
Hint: Type how many bits each pixel uses to get four shades.
4. Row: black, white, dark grey Code: 00 11 01
Hint: Type the two-digit code for the white pixel.
5. Code: 10 00 11 Pixels: light grey, black, white
Hint: Decode the first two-digit code into its shade.

Spot the Error

Question 1
A bits ← 1
B codes ← 4
C 00 is black
D 11 is white
Line A — Each pixel uses two bits, so bits should be 2. One bit only gives two codes (0 and 1), not the four codes needed for four shades.
Question 2
A 00 = black
B 01 = dark grey
C 10 = dark grey
D 11 = white
Line C — The code 01 is already dark grey. The code 10 should be light grey, so each of the four codes stands for a different shade.
Question 3
A pixels: black, light, white
B codes: 00 01 11
C OUTPUT codes
Line B — Light grey is 10, not 01. The code 01 is dark grey, so the middle code should be 10.
Question 4
A code ← 011
B IF code = 01 THEN
C shade ← dark grey
D OUTPUT shade
Line A — With two bits per pixel, a code has exactly two digits. It should be code ← 01, not a three-digit code.
Question 5
A black ← 00
B dark grey ← 01
C light grey ← 10
D white ← 111
Line D — Every greyscale code has two digits. White is 11, not 111.

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