Up until the 1980'ies regional and national telephone switches operated by large telephone companies were analogue. Managing the telephone lines of the subscribers
required screwdrives and/or soldering irons. At that time Integrated Services Digital Network (ISDN) was about to be introduced in connection with digital switches
that would make the management much easier. This new technology did however require massive investments in the communication infrastructure everywhere.
How could such investments be financed? It would be hard to ask telephone customers to pay more for the same speech quality they had known since 1870'ies. The incentive
for customers to reach deeper into their pockets was thus thought to be value added services on top of just telephone calls. One such service could be transmission of
high quality photographic images. Very crude images was already in use in the form of teledata, but never really caught on.
At the time a high quality photographic image was considered to be
a color still image with the same resolution as one frame on a color TV set:
720 pixels across on 576 lines with 16 bits pr. pixel. The 16 bits per pixel were distributed with 8 bits for the luminance known from black and white TV and 8 bits
for each of the two chrominance (color) components. The 16 bits for color were however shared by two neighbouring pixels, yielding on average 8 bits of color per pixel.
In total one such image consisting of 414720 pixels therefore contained 829440 bytes of data.
Subscribers to an ISDN telephone line would get two data channels of 64 kbits/second and one signalling channel of 16 kbits/second. Only the data channels could be used
for transmitting digital sound and photographic images. Using one of these channels to download an image with the resolution described above would take more than 100 seconds.
If you would stop talking during download both channels could in principle be used, bringing the download time down to just under one minute. That was not considered terribly
It was clear that for such a service to be a success, the download time should be brought down to just a few seconds.This would however require that the amount of data in the images
should be reduced by a factor of 20 or more. In other words - the images should be compressed. Furthermore it should be possible to decompress the datastream in real time
arriving over an ISDN line.
A number of large tele communication companies across Europe decided to cooperate to develop methods of image compression to do the job. This joint research project
was in part financed by the Commision of the European Communities, a precursor of the present European Union, as part of their
European Strategic Program on Research in Information Technology (ESPRIT). The project - number 563 under ESPRIT - was named Photovideotex Image Compression Algorithm,
or PICA for short.
Research on image compression had been done earlier but without much success. Different approaches had been investigated - e.g. vector quantization and transform coding using
various transforms. At the onset of the project various methods were distributed between the PICA partners for possible refinement.
It quickly became clear however that transform coding looked most promissing.
The optimal transform is the Karhounen-Loeve Transform (KLT). The KLT is however computational intensive, far more than could realistically be used with the computers
available in the late 1980'ies. Various other transforms were investigated, but the Discrete Cosine Transform (DCT) was clearly the one closest to the KLT in concentrating
the image energy.
The DCT converts pixel values to amplitudes of two-dimensional cosine functions. Instead of storing and transmitting the pixel values, the amplitudes are stored and transmitted.
Whereas all pixels are equally important, that is not the case for the amplitudes. The low frequency cosine functions are by far the most important whereas the high frequency
functions represent details in the image that are much harder for the human eye to distinguish. Very often the amplitudes of these high frequency functions can be discarded
altogether without ruining the visual apprearance of the image. In some images the details are however so prominent that they must be retained. The way to decide which
amplitudes to retain and which to throw away is by the process of quantization. After quantization an all important entropy coding of the quantized amplitudes takes place
to reduce the amount of data to be stored and transmitted.
The three step compression process DCT=>Quantization=>Entropy coding ensures that every image going through the process comes out with the same quality.
Simple images without many details are compressed a lot, while images with lots of visual important details are less compressed. The algorithm therefore adapts to the
image in question and was therefore named Adaptive Discrete Cosine Transform, or ADCT for short.
While the PICA project was working on image compression primarily for telecommunication purposes, ISO and CCITT became interested in standardizing image compression in
general. Research labs from around the world were invited to present their algorithms to ISO/CCITT. Ten image compression algorithms were presented including the ADCT
from Europe. In a selection meeting in June 1987 in Copenhagen the three most promising algorithms were selected for further improvement:
The Adaptive Binary Arithmetic Coding (ABAC) from IBM, The Block Separated Progressive Coding (BSPC) from Japan, and the ADCT from Europe.
During the next six months the teams behind the three algorithms worked very hard to improve their methods. Five standard images were distributed to the teams
and it was decided that each team should compress these images from 16 bits per pixel (bpp) down to 2.25 bpp, 0.75 bpp, 0.25 bpp and 0.08 bpp. To make sure that the algorithms
were not tuned to the specific standard images, computer code with the algorithms was sent to ISO before receiving the images.
Not only should the teams compress the images to the decided size, they should also demonstrate that the compressed datastreams could be decompressed in real time arriving
over a 64 kbit/sec communication line on a 25 Mhz IBM-PC.
In a final selection meeting in Copenhagen in January 1988, subjective blind testing of the standard images compressed at the different compression levels took place.
Blind test and final results, January 1988, Copenhagen
It turned out that the ADCT performed best on all compression levels with excellent quality at 0.75 bpp (less than 5% of the original amount of data)
and indistinguishable from the original at 2.25 bpp.
Furthermore the ADCT was the only algorithm shown to be decompressible in real time as required. It was therefore decided that the Adaptive Discrete Cosine Transform algorithm should
form the basis for the coming international standard for image compression - JPEG.
Following the meeting in January 1988 a lot of work took place in defining and developing in detail other modes of JPEG:
During this work the standard was drafted and sent out for testing - was it implementable and unequivocable?
Finally in 1994 the standard was officially released:
Extended Sequential Mode:
Choice between Huffman coding and arithmetic coding
12 bits per pixel per color channel
Build up of image quality in steps using spectral selection and/or bit slicing
Build up of image resolution
Image pixelwise identical to the original as opposed to the other lossy modes of JPEG
Information technology — Digital compression and coding of continuous-tone still images: Requirements and guidelines
We have seen literally thousands of standards: bad ones, good ones, but only a very few what one may call “Standardization Miracle” –
or HTML belong to that category. But the first JPEG Standard (informally JPEG-1 or formally ITU-T T.81¦ISO/IEC 10918-1) belongs to those very few as well.
- The JPEG-1 Standard was developed – no surprise – by JPEG or with full name the “JOINT PHOTOGRAPHIC EXPERTS GROUP”.
JPEG was formally never an ISO, ITU or IEC subgroup or ad-hoc group, but a body of its own, with own membership and management, policies, documentation etc. But it is true it had two “parents” when it was created, members (not the SDO itself) from ITU (more precisely CCITT SGVIII) and ISO TC97/SC2/WG8 in their individual capacity. These brilliant experts created JPEG in November of 1986 with the goal and vision to develop jointly a new generation of photographic image compression and coding standard. All these individuals came from companies of the ICT industry, from manufacturers and from telecom service providers. The reason for creating an independent expert group for this purpose and e.g. not a formal joint ITU/ISO group was that in 1986 simply no formal ITU/ISO policies and procedures existed for joint work and for joint text standards. Such policy was developed later between 1989 and 1993 by a joint ISO/IEC/ITU group and was ratified by the ITU WTSA in 1992 and by the ISO/IEC JTC1 meeting in their March 1993 Berlin meeting. After that time the joint work of the “parents” continued but it was not JPEG (i.e. the Original JPEG Engineering Team) anymore, but ITU-T SG8 and ISO/IEC JTC1 SC29 working on common text still picture coding standard. Between 1993 March and sometimes in 1997 the joint work and common text was developed by the so called “Collaborative Interchange” which was then replaced in 1997 by a “Collaborative Team”. This constellation remained unchanged until 2000, when ITU-T SG8 was discontinued and some of its worked including media coding was merged into ITU-T SG16, the “Multimedia SG” of ITU-T.
This it is important to know that the “miracle part” of JPEG-1 standard (between 1986 and 1992 when the standard was first formally approved by the CCITT) the technical development work was done by the “Original JPEG Team”. The two “parents” (CCITT SGVIII and ISO TC97/SC2) provided important requirements and advises for the direction of the technical developments) and after the technical work by JPEG was completed the JPEG specification was picked up by them to edit and approve the final JPEG-1 standard.
- Thus, JPEG was an informal group of about 15-20 individual experts from ICT companies and telecom service operators who also contributed to ITU/ISO directly via so called “National Bodies” or “ITU members”. This was also important for the success of the project. Most of us have served at the same time in parallel several related groups.. Basically the same people with many different hats and roles…but serving the same purpose: developing, editing and approving the JPEG standard.
- JPEG had its own structure (incl. management), procedures, policies and documentation – that JPEG themselves had just invented. Good engineers are even talented in that. Just one example: the JPEG Patent Policy required a “Royalty Free” baseline standard in contrast to ITU/ISO/IEC which cannot ensure that requirement (only FRAND, but that was not enough for JPEG-1). This was in favor of Open Source implementations (IJG) and fast market breakthrough. But many other things were also different.
- JPEG was an excellent Engineering Team with brilliant image compression-, image communication- and storage experts.
- Based on standardization- and design-requirements from ITU and ISO – in 4 years between 1986 and 1990 – JPEG tested and selected many still image compression methods and designed the well-known DCT based JPEG-1 Standard. The final JPEG-1 specification was released in August 1990, which was submitted to ITU and ISO/IEC JTC1 for parallel approval. The ITU approval took 2 years, the JTC1 approval took over 3 years for the common text JPEG-1 standard. In the end both approved standards shared the same text, except for some informal parts, like the Introduction.
- Key to success of JPEG-1 was that the final JPEG specification was early on picked-up by significant user communities (like the IJG for Open Source JPEG implementation) and the emerging Web Browser community. These were in 1991-1992 the first “JPEG killer applications” even before the formal ITU and ISO/IEC approval of the standard was completed.
Then a series of further “killer applications” followed – most of them were not imagined by the Original JPEG Team at design time:
- Digital photography about 1996-2000
- Photography with mobile phones about 2000
- Apple iPhone / Smart Phone in 2007
- And so on….
- As example just a figure: Today about 5 billion smart phones take in a year about 1 Trillion photos.
- Today JPEG-1 has an absolute dominant market share of still images taken since the beginning of photography 150 years ago, and I am sure that therefore it will never go away. JPEG-1 has become – what I call – a “Human Heritage” standard.
- This is the story of the JPEG-1 “Standardization Miracle”.
- As pointed out the Original JPEG Team ended in 1993 when ITU and JTC1 have approved the policy for common standardization work between the two organizations. Today you can trace in both organizations some of the old JPEG elements (like “toolbox type standards”, testing and selecting several methods for a standard etc.) but some of the key components (like JPEG patent policy) are not there anymore. · Nevertheless, we all hope that the “JPEG-1 miracle” will be repeated many times by many new JPEG standards.