JPEG and JPEG2000, photorealism over the Web

The second type of graphic formats is the Joint Photographic Experts Group (JPEG). This format always works at a colour depth of 32 bit (thousands of millions colours, since the 32nd power of two is 4'294'967'296), whatever the number of colours of the source image will be, and to maintain the file sizes into reasonable limits it uses a 'destructive' compression algorithm (lossy), called DCT (Discrete Cosine Transformation), which involves a loss of quality over the original: the visible effect of the DCT is the sadly known creation of square blocks, particularly visible and fastidious in the flat parts as, typically, skies and backgrounds.

The method of compression (and of destruction) is irreversible: after saving an image into JPEG we can never come back to the original quality. The logic consequence is that the saving of an image upon which we are working should NEVER be done in JPEG, but in a format not destructive like, for example, the TIFF.

JPEG permits to choose the compression ratio, the more the dimensions of the file are, the less the quality will be. Almost every graphics package has its own 'scale' to represent the compression, so it is impossible to establish a rule: the one thing to do is to open your TIFF in your preferred software and try to save it with different ratios of quality and compression, then decide what is the file which presents the best compromise.

This format, even though being extremely interesting, starts to get old and to show its limits. As a logic consequence, very soon we'll have on our monitors the new version, called (gee, what a fervid imagination!) JPEG2000.

If the characteristics of the JPEG will remain mostly the same, it will dramatically change the compression algorithm. From the blocks system of the DCT (with the related problems stated above) the format shifts to the Wavelet Compression, based on a stream of data which will bring at a progressive increasing of the resolution; the stream will be interrupted considering the level of quality desired by the user, with an amazing optimisation of the level of compression. Practically our browser will stop once arrived at 96 dpi, but you could ask for a better quality resolution, such as 300 dpi for a fair-quality print or 600 dpi for a semi-pro print.

Among the other characteristics provided, the capacity of storing the crypted information on the copyright, description and other chromatic spaces (such as CYMK), hardware and software profiles to optimise the use of the file in different environments (browser, digital cameras, etc...) and so on.

For more on the subject, try the homepage of the JPEG2000 committee and the great article from Anthony Celeste, appeared on Corel Designer.com: The Future Of Web Design - Part I.

Anyway keep in mind that still for a bunch of time (at least until next year) this will all be hypothesis, since this format will become a standard in 2001 (from which the film '2001: Odyssey in the chromatic space' ;-)

go on...

1. Presentation vs. content: Graphic Formats
2. GIF, this well-known
3. JPEG and JPEG2000, photorealism over the Web
4. GIF vs. JPG