Run-length encoding of a pixel image

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Quick Questions

Shade in the A or B circle for each question.

1. A row reads 0000011100 (0 = white, 1 = black). Which run-length code starts with white?

A 3,5,2

B 5,3,2

2. A row reads 1100000. The code always starts with white. Which code is correct?

A 0,2,5

B 2,5

3. Using white first (0 = white, 1 = black), what row do the counts 4,2,1 rebuild?

A 0000110

B 1111001

4. How are the counts in a run-length code for a pixel row arranged?

A They all count the black pixels only

B They alternate between white runs and black runs

5. An all-black row of 4 pixels (1111) is encoded, starting with white. What is the code?

A 0,4

B 4

Match the Pairs

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

Left column		Right column
15,3,2		AThe last run of 2 white pixels is missing. Every run must be counted, so it should be 3,2,2.
20,4,2		BThe row starts with black, but the first count always means white pixels. A 0 is needed first, so it should be 0,2,2.
3Encoding a row of 4 white pixels followed by 6 black pixels		C001000
4Rebuilding a row from the counts 2,1,3 (white first, then alternating, 0 = white, 1 = black)		D4,6
5Row 1100 encoded as 2,2		E5 white pixels, then 3 black, then 2 white: the row 0000011100
6Row 0001100 encoded as 3,2		FThe row starts with black: 0 white pixels, then 4 black, then 2 white (111100)

Definitions

run-length encoding

Fill in the Blanks

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

Row: 0000011100 Runs: _,3,2

Row: 1110000 Runs: _,3,4

Runs: 2,3,1 Row: 00___0

Row: 00110000 Runs: 2,2,4 Width: _

Runs alternate: white, _____, white

Spot the Error

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

Question 1

A Pixels: 0 = white, 1 = black

B Row: 0000011100

C Runs: 5,2,3

Question 2

A Row: 1110000

B Runs: 3,4

C Rebuild: 1110000

Question 3

A Image row 1: 00111

B Runs: 3,2

C Total pixels: 5

Question 4

A Row: 000110

B Runs: 3,2,2

C Total: 3+2+2 = 7 pixels

Question 5

A Rule: first run is black

B Row: 1100

C Runs: 0,2,2

Answer Key

Run-length encoding of a pixel image

Section A: Quick Questions

B — 5,3,2

A — 0,2,5

A — 0000110

B — They alternate between white runs and black runs

A — 0,4

Section B: Match the Pairs

5,3,2 → 5 white pixels, then 3 black, then 2 white: the row 0000011100

0,4,2 → The row starts with black: 0 white pixels, then 4 black, then 2 white (111100)

Encoding a row of 4 white pixels followed by 6 black pixels → 4,6

Rebuilding a row from the counts 2,1,3 (white first, then alternating, 0 = white, 1 = black) → 001000

Row 1100 encoded as 2,2 → The row starts with black, but the first count always means white pixels. A 0 is needed first, so it should be 0,2,2.

Row 0001100 encoded as 3,2 → The last run of 2 white pixels is missing. Every run must be counted, so it should be 3,2,2.

Section C: Definitions

run-length encoding — Compress a black-and-white image by writing each row as counts of same-coloured pixels in a run, starting with white. Row 00000 111 00 becomes 5,3,2. Counts can rebuild the grid.

Section D: Fill in the Blanks

Row: 0000011100
Runs: 5,3,2

Row: 1110000
Runs: 0,3,4

Runs: 2,3,1
Row: 001110

Row: 00110000
Runs: 2,2,4
Width: 8

Runs alternate: white, black, white

Section E: Spot the Error

C — The row 0000011100 has 5 white, then 3 black, then 2 white pixels. The runs should be 5,3,2, not 5,2,3.

B — The row starts with black pixels, but the first count always means white. It needs a 0 at the start, so the runs are 0,3,4.

B — The row has 2 white pixels first, then 3 black pixels. The runs should be 2,3. The counts are in the wrong order.

A — The runs 3,2,2 add up to 7 pixels, so the row must have 7 pixels. The row should be 0001100. 000110 has only 6 pixels.

A — The convention is that the first count is always the white run. That is why a row starting with black begins with 0, as in 0,2,2. The rule should say white.