Run-length encoding
From Just Solve the File Format Problem
Run-length encoding (RLE) is a simple kind of data compression, in which a run of consecutive equal data values is replaced by a code indicating the run length and the data value.
RLE can be effective for some types of data, or as part of a more sophisticated algorithm, but by itself it is a poor general purpose compression algorithm.
RLE is used in countless raster graphics formats.
