Run-length encoding of a row of coloured pixels

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Quick Questions

Shade in the A or B circle for each question.

1. A row of pixels is WWWBB (W = white, B = black). Which is its run-length encoding?

A 3W2B

B W3B2

2. A black and white row is BBBBWWB. Which encoding is correct?

A 4B2W1B

B 5B2W

3. Using the table R = red, G = green, B = blue, how is the row RRRGGB encoded?

A 3R2G1B

B 3R2G

4. How many pixels are in a row encoded as 5W3B?

A 2 pixels, because there are two pairs

B 8 pixels, because 5 + 3 = 8

5. Which row does the encoding 2R4G1R decode to?

A RRRRGGR

B RRGGGGR

Match the Pairs

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

Left column		Right column
13W2B		AThe final run of one red pixel is missing, so the row cannot be rebuilt correctly.
24R1G3R		BThree white pixels followed by two black pixels along the row (W = white, B = black)
3A black and white row with 5 white pixels then 3 black pixels is stored as one count and colour code per run (W = white, B = black)		C3R2G1Y
4Using the table R = red, G = green, Y = yellow, the row RRRGGY is encoded run by run, in order		D5W3B
5WWWWBB encoded as 4B2W		EThe colour codes are swapped. The four pixels are white, not black, and the two pixels are black, not white.
6RRGGGR encoded as 2R3G		FFour red pixels, one green pixel, then three red pixels (R = red, G = green)

Definitions

RLE pixel row

Fill in the Blanks

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

Row: WWWBB RLE: _W2B

Table: R=red, G=green, B=blue Row: RRGGGB RLE: 2R3_1B

Row 1: BBBW RLE: 3B1W Row 2: WWBB RLE: _W2B

Row: BBBBWWB RLE: 4B2W_B

Row: BBWWWWWB RLE: 2B5W1_

Spot the Error

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

Question 1

A Key: W=white, K=black

B Row: W W K K W W W W

C Runs: 3 W, 2 K, 4 W

D RLE: 3W2K4W

Question 2

A Key: R=red, G=green, B=blue

B Row: R R G G G G B

C Runs: 2 R, 3 G, 1 B

D RLE: 2R4G1B

Question 3

A Key: Y=yellow, K=black

B Row: Y Y Y K K Y

C Runs: 3 Y, 2 K, 1 Y

D RLE: Y3K2Y1

Question 4

A Row: R R R G G B

B Key: R=red, G=green, B=blue

C Runs: 4 R, 2 G, 1 B

D RLE: 4R2G1B

Question 5

A Key: W=white, K=black

B Row: K K K W W K

C RLE: 4K2W1K

D Total pixels: 4+2+1 = 7

Answer Key

Run-length encoding of a row of coloured pixels

Section A: Quick Questions

A — 3W2B

A — 4B2W1B

A — 3R2G1B

B — 8 pixels, because 5 + 3 = 8

B — RRGGGGR

Section B: Match the Pairs

3W2B → Three white pixels followed by two black pixels along the row (W = white, B = black)

4R1G3R → Four red pixels, one green pixel, then three red pixels (R = red, G = green)

A black and white row with 5 white pixels then 3 black pixels is stored as one count and colour code per run (W = white, B = black) → 5W3B

Using the table R = red, G = green, Y = yellow, the row RRRGGY is encoded run by run, in order → 3R2G1Y

WWWWBB encoded as 4B2W → The colour codes are swapped. The four pixels are white, not black, and the two pixels are black, not white.

RRGGGR encoded as 2R3G → The final run of one red pixel is missing, so the row cannot be rebuilt correctly.

Section C: Definitions

RLE pixel row — Run-length encoding of a pixel row stores each run of same-coloured pixels as a count then a colour code, in order along the row. E.g. WWWBB becomes 3W2B.

Section D: Fill in the Blanks

Row: WWWBB
RLE: 3W2B

Table: R=red, G=green, B=blue
Row: RRGGGB
RLE: 2R3G1B

Row 1: BBBW  RLE: 3B1W
Row 2: WWBB  RLE: 2W2B

Row: BBBBWWB
RLE: 4B2W1B

Row: BBWWWWWB
RLE: 2B5W1B

Section E: Spot the Error

B — The row should start with three white pixels (W W W), not two. The runs 3 W, 2 K, 4 W and the code 3W2K4W all match the row W W W K K W W W W.

C — The row has four green pixels in a row, so the green run count must be 4, not 3. It should read 'Runs: 2 R, 4 G, 1 B'.

D — Each run is written as the count first, then the colour code. The encoding should be 3Y2K1Y, not colour then count.

A — The row only has three red pixels here, but the runs and RLE say 4 R. The row should be R R R R G G B to match 4R2G1B.

B — The row should start with four black pixels and have seven pixels in total. K K K W W K has only 3 black pixels at the start and 6 pixels, so it does not match 4K2W1K.