Run-length encoding

Run-length encoding is a very simple form of data compression in which runs of data (that is, sequences in which the same data value occurs in many consecutive data elements) are stored as a single data value and count, rather than as the original run. This is most useful on data that contains many such runs; for example, simple graphic images such as icons and line drawings.

For example, consider a screen containing plain black text on a solid white background. There will be many long runs of white pixels in the blank space, and many short runs of black pixels within the text. Let us take a hypothetical single scan line[?], with B representing a black pixel and W representing white:

 WWWWWWWWWWWWBWWWWWWWWWWWWBBBWWWWWWWWWWWWWWWWWWWWWWWWBWWWWWWWWWWWWWW

If we apply a simple run-length code to the above hypothetical scan line, we get the following:

 12WB12W3B24WB14W

Interpret this as twelve W's, one B, twelve W's, three B's, etc. The run-length code represents the original 67 characters in only 16. Of course, the actual format used for the storage of images is generally binary rather than ASCII characters like this, but the principle remains the same. Even binary data files can be compressed with this method; file format specifications often dictate repeated bytes in files as padding space. However, newer compression systems often use deflation or other LZ77-based algorithms, which can take advantage of runs of strings of characters (such as BWWBWWBWWBWW).

Common formats for run-length encoded data include PackBits and PCX.

Run-length encoding performs lossless data compression and is well suited to palette-based iconic images. It does not work well at all on continuous-tone images such as photographs, although JPEG uses it quite effectively on the coefficients that remain after transforming and quantizing image blocks.