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.)
