IJG is back

When FFmpeg released version 0.5 earlier this year, nearly five years had passed since the previous release, during which time the project had attracted frequent criticism for the lack of regular releases. There exists, however, a project whose release interval dwarves that of FFmpeg. I speak of the Independent JPEG Group’s libjpeg, version 7 of which was recently released after 11 years of silence.

So what have they been doing during the last 11 years? Not a lot, it seems. The only change log entry I find noteworthy is the addition of arithmetic entropy coding, previously omitted due to patent concerns. Contrast this with the TO DO note from the previous release:

The major thrust for v7 will probably be improvement of visual quality. The current method for scaling the quantization tables is known not to be very good at low Q values.  We also intend to investigate block boundary smoothing, “poor man’s variable quantization”, and other means of improving quality-vs-file-size performance without sacrificing compatibility.

In future versions, we are considering supporting some of the upcoming JPEG Part 3 extensions — principally, variable quantization and the SPIFF file format.

As always, speeding things up is of great interest.

Eleven years is of course plenty of time for the developers to change their minds, or perhaps even lose them. The TO DO note in version 7 reads thus:

v7 is basically just a necessary interim release, paving the way for a major breakthrough in image coding technology with the next v8 package which is scheduled for release in the year 2010.

Immediately preceding this note is a rant against rival image compression formats:

The ISO JPEG standards committee actually promotes different formats like JPEG-2000 or JPEG-XR which are incompatible with original DCT-based JPEG and which are based on faulty technologies. IJG therefore does not and will not support such momentary mistakes (see REFERENCES). We have little or no sympathy for the promotion of these formats. Indeed, one of the original reasons for developing this free software was to help force convergence on common, interoperable format standards for JPEG files. Don’t use an incompatible file format!

Evidently, the authors are of the opinion that JPEG is the one, true image coding format. Under this premise, what could the “major breakthrough” possibly be all about? The REFERENCES section provides a hint:

The best currently available description of JPEG is the textbook “JPEG Still Image Data Compression Standard” by William B. Pennebaker and Joan L. Mitchell, published by Van Nostrand Reinhold, 1993, ISBN 0-442-01272-1. Price US$59.95, 638 pp. The book includes the complete text of the ISO JPEG standards (DIS 10918-1 and draft DIS 10918-2).

Although this is by far the most detailed and comprehensive exposition of JPEG publicly available, we point out that it is still missing an explanation of the most essential properties and algorithms of the underlying DCT technology.

If you think that you know about DCT-based JPEG after reading this book, then you are in delusion. The real fundamentals and corresponding potential of DCT-based JPEG are not publicly known so far, and that is the reason for all the mistaken developments taking place in the image coding domain.

Again, a comparison with the corresponding paragraph from the previous release is revealing:

The best full description of JPEG is the textbook “JPEG Still Image Data Compression Standard” by William B. Pennebaker and Joan L. Mitchell, published by Van Nostrand Reinhold, 1993, ISBN 0-442-01272-1. Price US$59.95, 638 pp. The book includes the complete text of the ISO JPEG standards (DIS 10918-1 and draft DIS 10918-2). This is by far the most complete exposition of JPEG in existence, and we highly recommend it.

Even if the IJG were to be in possession of the powerful secret they allude to, how they would make use of it while staying compatible with the JPEG specification is a mystery to me. Short of using a time machine to travel back in time and alter the original text, nothing springs to mind, although that would of course qualify as a “major breakthrough.” Next year we will know. Until then, I call vapour-ware.