Compression(3) User Contributed Perl Documentation Compression(3)NAMEPDL::Compression - compression utilities
DESCRIPTION
These routines generally accept some data as a PDL and compress it into
a smaller PDL. Algorithms typically work on a single dimension and
thread over other dimensions, producing a threaded table of compressed
values if more than one dimension is fed in.
The Rice algorithm, in particular, is designed to be identical to the
RICE_1 algorithm used in internal FITS-file compression (see
PDL::IO::FITS).
SYNOPSIS
use PDL::Compression
($b,$asize) = $a->rice_compress();
$c = $b->rice_expand($asize);
FUNCTIONSMETHODS
rice_compress
Signature: (in(n); [o]out(m); int[o]len(); lbuf(n); int blocksize)
Squishes an input PDL along the 0 dimension by Rice compression. In
scalar context, you get back only the compressed PDL; in list context,
you also get back ancillary information that is required to uncompress
the data with rice_uncompress.
Multidimensional data are threaded over - each row is compressed
separately, and the returned PDL is squished to the maximum compressed
size of any row. If any of the streams could not be compressed (the
algorithm produced longer output), the corresponding length is set to
-1 and the row is treated as if it had length 0.
Rice compression only works on integer data types -- if you have
floating point data you must first quantize them.
The underlying algorithm is identical to the Rice compressor used in
CFITSIO (and is used by PDL::IO::FITS to load and save compressed FITS
images).
The optional blocksize indicates how many samples are to be compressed
as a unit; it defaults to 32.
How it works:
Rice compression is a subset of Golomb compression, and works on data
sets where variation between adjacent samples is typically small
compared to the dynamic range of each sample. In this implementation
(originally written by Richard White and contributed to CFITSIO in
1999), the data are divided into blocks of samples (by default 32
samples per block). Each block has a running difference applied, and
the difference is bit-folded to make it positive definite. High order
bits of the difference stream are discarded, and replaced with a unary
representation; low order bits are preserved. Unary representation is
very efficient for small numbers, but large jumps could give rise to
ludicrously large bins in a plain Golomb code; such large jumps ("high
entropy" samples) are simply recorded directly in the output stream.
Working on astronomical or solar image data, typical compression ratios
of 2-3 are achieved.
$out = $pdl->rice_compress($blocksize);
($out, $len, $blocksize, $dim0) = $pdl->rice_compress;
$new = $out->rice_expand;
rice_compress ignores the bad-value flag of the input piddles. It will
set the bad-value flag of all output piddles if the flag is set for any
of the input piddles.
rice_expand
Signature: (in(n); [o]out(m); lbuf(n); int blocksize)
Unsquishes a PDL that has been squished by rice_expand.
($out, $len, $blocksize, $dim0) = $pdl->rice_compress;
$copy = $out->rice_expand($dim0, $blocksize);
rice_expand ignores the bad-value flag of the input piddles. It will
set the bad-value flag of all output piddles if the flag is set for any
of the input piddles.
AUTHORS
Copyright (C) 2010 Craig DeForest. All rights reserved. There is no
warranty. You are allowed to redistribute this software / documentation
under certain conditions. For details, see the file COPYING in the PDL
distribution. If this file is separated from the PDL distribution, the
copyright notice should be included in the file.
The Rice compression library is derived from the similar library in the
CFITSIO 3.24 release, and is licensed under yet more more lenient terms
than PDL itself; that notice is present in the file "ricecomp.c".
BUGS
· Currently headers are ignored.
· Currently there is only one compression algorithm.
TODO
· Add object encapsulation
· Add test suite
perl v5.18.1 2014-01-17 Compression(3)
