PackBits

Apple introduced the PackBits format for run-length encoding of data with the release of MacPaint on the Macintosh computer. This compression scheme is one of the types of compression that can be used in TIFF-files.

A PackBits data stream consists of packets of one byte of header followed by data. The header is a signed byte; the data can be signed, unsigned, or packed (such as MacPaint pixels).

Header byte	Data
0 to 127	1 + n literal bytes of data
0 to -127	One byte of data, repeated 1 - n times in the decompressed output
-128	No operation

Note that interpreting 0 as positive or negative makes no difference in the output. Runs of two bytes adjacent to non-runs are typically written as literal data.