An overview of compression methods
August 9, 2001
Lossy compression schemes:
Transform based compression methods
- JPEG
- Good for all types of shaded images - especially
photographic type. Compression factors with good
results depend on the type and size of the image.
A 1 MB image will in general compress a factor of
20 with little or no visual loss.
- Wavelets
- Similar but gives better quality at very high
compression ratios and avoids one of the JPEG
artifacts at high compression rates, namely
blockiness. Wavelets are used in JPEG2000.
- Fractals
- A totally different compression method. Good for
high compression rates when the decompressed
result does not have to be identical to the
original image, very good for clouds, trees and
other 'textures'.
Lossless compression schemes
- JPEG
- Does not give optimal compression factors (1.5
to 2) but supports 16 bit which is important in
medical applications.
- JPEG LS
- An improved JPEG lossless compression but not
yet very widespread.
- LZW
- Good for all types of data. Compresses normally
a factor of 2 but can for certain data types give
very good results. The method is patented by
UNISYS.
- Packbits
- Compresses long sequences of identical bytes
very well (up to a factor of 100). Never expands
more than about 1%. Unmodified Packbits is lousy
for RGB data (each pixel in a seemingly uniform
red surface is represented with red, green and
blue pixels). Packbits is generally good for
palette images and line drawings.
- Runlength encoding
- Not in any way superior to packbits. May expand
up to a factor of 3.
- CCITT Group 3 and 4 (FAX)
- Comparable to packbits with line drawings and
black-on-white text. Expands dramatically in case
of dithered data.(same compression used in Fax
group 3 and 4 because the methods are developed
for fax standards)
General Compression methods
- Compress
- Similar to LZW
- GZIP
- Similar to LZW but better
- B2Z
- Better yet but not very widespread
Other PC compression methods, ZIP, ARC, PKARC
are like GZIP hybrid methods that analyzes the data in
a file or image and select the best method for
compression among a set of methods. These methods
typically compress a factor of 2-4.
Compression in AGI Products
JPEG PRO is is a
comprehensive, high performance software
implementation of the JPEG standard for image
compression. It supports all lossy modes as well as
lossless. It is available for application builders as
a complete toolkit for easy integration into
applications.
EasyCopy has
implemented the compression schemes supported by the
various supported file
formats.
Furthermore EasyCopy supports import of files
compressed by other methods via filters. Some formats
have been integrated in the EasyCopy distribution.
Compress is expected to be part of any UNIX
operating system variants and GZIP is found on most
UNIX workstations. If support is available on the
system EasyCopy will automatically use it for import
and view of files compressed with these methods.
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