1999.02.02 17:07 "Re: Basic question regarding PackBits...", by Helge Blischke

Here is the relevant section from the TIFF 6.0 spec. Note that the runs DO break at the end of a line. You do NOT compress across lines as if the strip is one homogenous buffer. [...] [...] Here are some additional rules:

. Pack each row separately. Do not compress across row boundaries.

[...] When PackBits data is decompressed, the result should be interpreted as per compression type 1 (no compression).

As each scanline in an uncompressed image starts at a byte boundary ((ImageWidth+7)/8), it actually does not matter if the PackBits compression compressed across lines, though any TIFF writer should follow this recommendation, as TIFF readers might depend on this recommendation.

What I meant by strip/tile unit is, be to precise, that whatever compression algorithm is used, the compression _must not_ cross tile or strip boundaries.

Helge

--
H.Blischke@srz-berlin.de
H.Blischke@srz-berlin.com
H.Blischke@acm.org