Compressing bitmaps with RLE

5 minAS Level1.3.3

AI-drafted, independently checked · not yet teacher-reviewed

By the end of this lesson

By the end you can compress a bitmap image with RLE, calculate its compressed size, and explain which kinds of image RLE suits.

Pixels in a queue

A bitmap stores a colour code for every pixel, row by row, left to right. Line those codes up and neighbouring pixels of the same colour form runs, exactly like the AAAAs in text.

RLE stores each run as a pair: (number of pixels, colour code).

R is a red pixel, W a white one. Each row becomes a short list of runs.

Worked example: The heart in numbers

The 12 × 8 heart uses 8-bit colour. Each RLE run is stored as a 1-byte count and a 1-byte colour code, and runs stop at the end of each row. Compare the uncompressed and compressed sizes.

Have a go first?

Quick check

Fill in the blanksQuestion 1 · Recall

One-line answerQuestion 2 · Recall

CalculateQuestion 3 · Apply

Each letter below is one pixel (G = green, Y = yellow) of an image with 8-bit colour.

GGGGGGGGGG
GGGYYYYGGG
GGGYYYYGGG
GGGGGGGGGG

Each run is stored as a 1-byte count and a 1-byte colour code, and runs stop at the end of each row. How many bytes does the RLE version take?

bytes

Answer the 3 questions above to continue.