Two bits give four shades: greyscale

Name:

Date:

Class:

Quick Questions

Shade in the A or B circle for each question.

1. How many different codes can two bits make?

A 4 codes: 00, 01, 10 and 11

B 2 codes: 0 and 1

2. In our greyscale code, which two-digit code means black?

A 11

B 00

3. Which code stands for the darker grey?

A 01 is dark grey

B 10 is dark grey

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

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

Match the Pairs

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

Left column		Right column
100		A00 01 10 11
210		B00 01 10 11
3Two bits can make four different codes, so a pixel can be one of four shades		CThe two-bit code for a light grey pixel
4Encoding a row of pixels: black, dark grey, light grey, white		DEach 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.
501 = white		EThe two-bit code for a black pixel
6Row 11 00 01 is read as 3 pixels, each using 1 bit		F01 is dark grey, not white. White is 11.

Definitions

00 01 10 11

Fill in the Blanks

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

00 = black 01 = _________

10 = light grey 11 = _____

Each pixel uses _ bits, giving 4 shades

Row: black, white, dark grey Code: 00 __ 01

Code: 10 00 11 Pixels: __________, black, white

Spot the Error

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

Question 1

A bits ← 1

B codes ← 4

C 00 is black

D 11 is white

Question 2

A 00 = black

B 01 = dark grey

C 10 = dark grey

D 11 = white

Question 3

A pixels: black, light, white

B codes: 00 01 11

C OUTPUT codes

Question 4

A code ← 011

B IF code = 01 THEN

C shade ← dark grey

D OUTPUT shade

Question 5

A black ← 00

B dark grey ← 01

C light grey ← 10

D white ← 111

Answer Key

Two bits give four shades: greyscale

Section A: Quick Questions

A — 4 codes: 00, 01, 10 and 11

B — 00

A — 01 is dark grey

A — Four shades, from black to white

B — White, light grey, dark grey, black

Section B: 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.

Section C: Definitions

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.

Section D: Fill in the Blanks

00 = black
01 = dark grey

10 = light grey
11 = white

Each pixel uses 2 bits, giving 4 shades

Row: black, white, dark grey
Code: 00 11 01

Code: 10 00 11
Pixels: light grey, black, white

Section E: Spot the Error

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.

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.

B — Light grey is 10, not 01. The code 01 is dark grey, so the middle code should be 10.

A — With two bits per pixel, a code has exactly two digits. It should be code ← 01, not a three-digit code.

D — Every greyscale code has two digits. White is 11, not 111.