The ZLIB_COMPRESS function compresses a given input array using the DEFLATE compression algorithm, and returns a byte array containing the compressed data, along with a header and trailer describing the compressed data. The compression is done using the ZLIB library, written by Jean-Loup Gailly and Mark Adler.
The compressed data can be uncompressed using the ZLIB_UNCOMPRESS function.
Here, we compress and uncompress a float array:
data = FINDGEN(100, 200)
zdata = ZLIB_COMPRESS(data)
dataout = ZLIB_UNCOMPRESS(zdata, TYPE=4, DIMENSIONS=[100, 200])
PRINT, ARRAY_EQUAL(dataout, data)
ZDATA BYTE = Array
Since the float array has a total of 80,000 bytes, the compression space savings is 71%.
Result = ZLIB_COMPRESS( Array [, /GZIP_HEADER]
[, LEVEL=value] [, /NO_HEADER]
Set this argument to an array of any IDL numeric type.
Set this keyword to output the results using a GZIP-style header and trailer, instead of the default ZLIB header/trailer. In this case, the resulting array could be written out to a disk file which would be fully compatible with the GZIP file format.
Tip: Unlike the default ZLIB header/trailer, the GZIP trailer has a field containing the uncompressed data size. Therefore, when ZLIB_UNCOMPRESS is called, IDL will be able to immediately allocate an output array of the correct size, which can significantly reduce memory overhead.
Note: If you set /GZIP_HEADER in ZLIB_COMPRESS, you must set /GZIP_HEADER when calling ZLIB_UNCOMPRESS on the compressed stream.
Set this keyword to an integer in the range 0-9 giving the desired compression level. A value of LEVEL=0 gives no compression, while LEVEL=9 gives the highest compression. Higher values for LEVEL will result in better compression ratios but slower compress and uncompress speeds. The default is LEVEL=6, which is a good compromise between compression and speed. Note that for low compression levels (or highly-random data), the size of the compressed result may be larger than the original data.
Set this keyword to not include any header or trailer, instead of the default ZLIB header/trailer.
Note: If you set /NO_HEADER in ZLIB_COMPRESS, you must set /NO_HEADER when calling ZLIB_UNCOMPRESS on the compressed stream.
FILE_GZIP, FILE_GUNZIP, FILE_TAR, FILE_UNTAR, FILE_ZIP, FILE_UNZIP, OPEN, ZLIB_UNCOMPRESS