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 EasyCopy 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.