Hcompress Image
Compression Software

Hcompress is the image compression package written by Richard L. White for use at the Space Telescope Science Institute. Hcompress was used to compress the STScI Digitized Sky Survey and has also been used to compress the preview images in the Hubble Data Archive. Briefly, the method used is:

a wavelet transform called the H-transform (a Haar transform generalized to two dimensions), followed by
quantization that discards noise in the image while retaining the signal on all scales, followed by
quadtree coding of the quantized coefficients.

The technique gives very good compression for astronomical images and is fast, requiring about 1 second for compression or decompression of a 512x512 image on a Sun SPARCstation 2. The calculations are carried out using integer arithmetic and are entirely reversible. Consequently, the program can be used for either lossy or lossless compression, with no special approach needed for the lossless case (e.g. there is no need for a file of residuals.) Documentation on the use of the programs and on the compression method is included in the doc subdirectory.

These programs can compress 2-byte integer images in a small variety of input formats (including FITS format, the most widely used format for astronomical images.) The resulting compressed image file is machine-independent and may be transferred between computers with binary FTP. The source code is reasonably machine-independent, although it has not been tested on a wide range of machines. Installation instructions for Unix and VMS (VAX) machines are given in the README files in the appropriate subdirectories. I would be very interested to hear of any attempts to install this code on other kinds of machines (e.g. MS-DOS, Macs.)

This directory has two files that contain the hcompress image compression package:

hcompress.tar.Z - This is a compressed tar file, which must be transferred using BINARY mode in FTP. You can extract the files on a Unix system by doing zcat hcompress.tar.Z | tar xvf This will create a directory called ./hcompress (with some subdirectories.) The hcompress/aareadme file gives installation information.
hcompress.bck - This is a VMS backup file, which must be transferred using BINARY mode in FTP. When retrieved via anonymous FTP, this file is created on your system with an incorrect blocksize. To fix this, you must get 2 additional files: BACKUP.COM (using ASCII mode) and MODIFY.EXE (using BINARY mode). Then you can extract the files on a VMS system by doing @BACKUP This will create a directory called [.HCOMPRESS] (with some subdirectories.) The [.HCOMPRESS]AAREADME file gives installation information.

This version (28 February 1994) corrects one bug in the decompression from the previous version (20 April 1992). The effect of this bug was only noticeable for lossless compression of odd-sized images (images with dimensions that are not a power of 2.) The compressed files in that case were correct, but the decompressed images could have pixel values that differed by +/- 1 count compared to the original. If you have any old compressed files, they will decompress properly using this new version of the software. Only hinv.c has changed from the previous version. I know of no other bugs that have been discovered (but would welcome reports of such.)

Get Unix Compressed Tar File

File hcompress.tar.Z File hcompress.tar.gz (downloaded on 8/15/2006). See editor's note for compiling with gcc.

Get VMS Files

[Link] File hcompress.bck
[Link] File backup.com
[Link] File modify.exe

STScI Home Page | Software

Richard L. White, rlw@stsci.edu
1997 April 23

Hcompress Image Compression Software

Get Unix Compressed Tar File

Get VMS Files

Hcompress Image
Compression Software