Subject: Compression

From: thompson at stars.gsfc.nasa.gov (William Thompson, code 682.1, x2040)
Date: 4 Apr 92 22:56:00 GMT

Has anyone come to any conclusions about the FITS compression proposal by
Archie Warnock et al.?  I'm interested in the subject of data compression in
FITS files.

Warnock et al. propose to compress whole FITS header and data units.  The
advantage of this approach is that any FITS data structure can be compressed,
including ASCII and binary tables.  However, as I see it, this scheme also has
the following consequences:

	* Only lossless compression algorithms can be used, so that the
	  compressed header can be reconstructed correctly.

	* Information about the data in the compressed header should also be
	  reproduced in the compression extension header, so that the user can
	  understand what the file contains.  Since the same data is included
	  twice, some loss of compression efficiency is engendered.

	* Compression schemes that work well on the data, may be inefficient in
	  reducing the extension header.

Thus, I have the following questions for the FITS community:

	* Is there currently support within the FITS community for adopting the
	  Warnock et al. proposal as a standard?

	* Could the Warnock et al. proposal be changed to *not* compress the
	  header by making the optional keywords LBITPIX, etc. mandatory
	  instead, or would this make the proposal less flexible?

Bill Thompson