Encoding a picture with a colour key

Name:

Date:

Class:

Quick Questions

Shade in the A or B circle for each question.

1. You have a colour key: 0 = white, 1 = red, 2 = blue. A pixel is red. Which number do you write in the grid?

A 2

B 1

2. Which tells you how to encode a pixel picture with a colour key?

A Write the colour number for each pixel in a grid, row by row

B Write the colour names in a list in any order

3. Why do you give your partner the colour key along with your number grid?

A So they know which colour each number stands for

B So they can see how many pixels are in the picture

4. Your pixel art on squared paper has 4 rows. How many rows will your number grid have?

A 1 row

B 4 rows

5. Key: 0 = white, 1 = green. The top row of squares is white, green, green, white. Which row of numbers is correct?

A 0 1 1 0

B 1 0 0 1

Match the Pairs

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

Left column		Right column
11 = red, 2 = blue, 3 = green		ATwo colours share the same number, so a partner cannot tell if a 1 means red or blue. Each colour needs its own number.
22 2 1 3		BThe grid row has only two numbers for three pixels. Every pixel needs its own number, so the row should be 1 2 1.
3To encode a pixel, look up its colour in the key and write that colour's number		C1 2 1 2 1 2 1 2 1
4Write the numbers row by row, so the grid of numbers has the same shape as the picture		DKey: 1 = yellow. Pixel is yellow, so write 1
5Key: 1 = red, 1 = blue		EOne row of a number grid: the colour numbers of four pixels, read from left to right
6Picture row: red, blue, red Grid row: 1 2		FA colour key that says which number stands for which colour

Definitions

colour key

Fill in the Blanks

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

Key: 1 = red, 2 = blue Pixel colour: blue Pixel number: _

Key: 0 = white, 1 = black, 2 = grey Row 1: black white grey Row 1: 1 0 _

___: 1 = red, 2 = blue, 3 = green

Key: 1 = red, 2 = blue Row 1: 2 1 1 Row 1 colours: blue red ___

Pixel art: 4 across, 3 down Write the numbers row by row Each row has _ numbers

Spot the Error

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

Question 1

A key: 0=white 1=red 2=blue

B pixel 1 is red, so write 2

C pixel 2 is white, so write 0

D pixel 3 is blue, so write 2

Question 2

A key: 0=black 1=white

B row 1: black white black

C row 1 code: 1 0 1

Question 3

A key: 1=red 2=green 3=blue

B row 1: green blue red = 2 1 3

C row 2: red red blue = 1 1 3

Question 4

A key: 0=white 1=blue

B row 1: 0 1 0

C row 2: 1 1 1

D row 3: 0 3 0

Question 5

A FOR each row of the picture

B FOR each pixel in the row

C WRITE its colour name

D START a new line

Answer Key

Encoding a picture with a colour key

Section A: Quick Questions

B — 1

A — Write the colour number for each pixel in a grid, row by row

A — So they know which colour each number stands for

B — 4 rows

A — 0 1 1 0

Section B: Match the Pairs

1 = red, 2 = blue, 3 = green → A colour key that says which number stands for which colour

2 2 1 3 → One row of a number grid: the colour numbers of four pixels, read from left to right

To encode a pixel, look up its colour in the key and write that colour's number → Key: 1 = yellow. Pixel is yellow, so write 1

Write the numbers row by row, so the grid of numbers has the same shape as the picture → 1 2 1 2 1 2 1 2 1

Key: 1 = red, 1 = blue → Two colours share the same number, so a partner cannot tell if a 1 means red or blue. Each colour needs its own number.

Picture row: red, blue, red Grid row: 1 2 → The grid row has only two numbers for three pixels. Every pixel needs its own number, so the row should be 1 2 1.

Section C: Definitions

colour key — A list that says which number stands for which colour. To encode a picture, write each pixel's colour number in a grid, row by row, for a partner to decode.

Section D: Fill in the Blanks

Key: 1 = red, 2 = blue
Pixel colour: blue
Pixel number: 2

Key: 0 = white, 1 = black, 2 = grey
Row 1: black white grey
Row 1: 1 0 2

Key: 1 = red, 2 = blue, 3 = green

Key: 1 = red, 2 = blue
Row 1: 2 1 1
Row 1 colours: blue red red

Pixel art: 4 across, 3 down
Write the numbers row by row
Each row has 4 numbers

Section E: Spot the Error

B — The colour key says 1 stands for red and 2 stands for blue. A red pixel must be written as 1, not 2.

A — The key is swapped. The code 1 0 1 for black white black only works if 1=black and 0=white, so the key must be 0=white 1=black.

B — Green is 2, blue is 3 and red is 1 in the key. So green blue red must be written 2 3 1, not 2 1 3.

D — The key only has the numbers 0 and 1. The number 3 is not in the key, so no colour matches it. Row 3 should be 0 1 0.

C — An encoded grid holds numbers, not words. Use the colour key to write the pixel's colour number.