Using it
Type or paste text. Each run of the same character becomes its length and the character: WWWWBBB is 4W3B. The rows compare the length before and after. Swap the boxes to expand a code such as 12W1B12W3B back into text.
This form writes the count every time, even for a run of 1, so it can be decoded without any guessing. It can't take text with digits in, because a digit would be read as part of a count.
When it helps
Run-length encoding is the simplest compression there is, and it only pays off when the data has long runs: rows of one colour in a simple image, the white space on a faxed page. Ordinary writing rarely has two of the same letter in a row, so coding it this way usually doubles its length. Try a sentence and see.
Formats that use it include PackBits, Apple's scheme for MacPaint, and the TGA and PCX image formats. Fax machines combine it with Huffman coding, giving short codes to the run lengths that turn up most.
Questions
What is AAABBC in run-length encoding?
3A2B1C.
Is RLE lossless?
Yes. Decoding gives back exactly what went in.
Why did my text get longer?
Each single character became two: a 1 and the character. RLE only shrinks data that repeats in runs.
Sources
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