[fitsbits] BZIP2 FITS image compression feasibility study
William Pence
http://fits.gsfc.nasa.gov/registry/tilecompression.html) currently
supports 3 general-purpose image compression algorithms: Rice,
Hcompress, and GZIP. Recently, I did some comparison tests using the
BZIP2 compression algorithm to see if it offers any unique advantages
that would justify adding it as one of the supported algorithms.
A link to the complete feasibility report is available from the fpack
web page at http://heasarc.gsfc.nasa.gov/fitsio/fpack/ and the main
results are as follows:
- The Rice algorithm generally provides better compression than BZIP2
when the image is compressed using the default row-by-row tiling
pattern. BZIP2 only produces (slightly) better compression than Rice or
Hcompress when using much larger sized tiles (e.g., compressing the
whole image as a single tile), but this can seriously degrade the I/O
performance for software applications that read the compressed image on
a row by row basis.
- Compressing an image using BZIP2 typically takes 15 to 30 times more
CPU time than the Rice algorithm, depending on how much noise is in the
image.
- Similarly, BZIP2 takes 4 to 6 times more CPU time than Rice to
uncompress the image.
Thus, based on these tests, there appears to be little justification for
adding support for BZIP2 within the FITS tiled-image compression
convention.
-Bill Pence
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____________________________________________________________________
Dr. William Pence Willia...@nasa.gov
NASA/GSFC Code 662 HEASARC +1-301-286-4599 (voice)
Greenbelt MD 20771 +1-301-286-1684 (fax)