Apple introduced the PackBits format 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).
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.
External links
Apple webpage describing the PackBits format (http://developer.apple.com/technotes/tn/tn1023.html)