This brief technical appendix details the compression routine implemented in the functions TIFFPACK and TIFFUNPACK. The TIFF (Tagged Image File Format) compression is a variant of Run Length Encoding (RLE) and allows to encode a sequence of bytes according to these rules:

• a control byte is followed by a sequence of data bytes (up to 127);
• the control byte tells if the following byte must be repeated a number of times (up to 127) or must be represented literally;
• the sign of the control byte tells if the following byte must be repeated (negative, 2-complement) or if it is followed by a sequence of bytes to be interpreted literally (positive);
• if the control byte is non-negative, the following data length is given by the value of the control byte increased by 1.

This compression scheme is good to compress byte sequences in which repeated bytes are very common.