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Joint Photographic Experts Group (JPEG)

JPEG ('Joint Photographic Experts Group') is a lossy compression method standardised by ISO. The formal name of the standard that most people refer to as ' JPEG ' is ISO/IEC IS 10918-1 | ITU-T Recommendation T.81pdf [1.01MB], as the document was published by both ISO through its national standards bodies, and CCITT, now called ITU-T.

JPEG stands for Joint Photographic Experts Group, the original name of the committee that wrote the standard.

Also, JPEG JFIF , which is what people generally mean when they refer to 'JPEG', the file format created by the Independent JPEG Group (IJG) for the transport of single JPEG-compressed images.

 

The JPEG committee has created many standards since it was created in 1986.  ISO had actually started to work on this 3 years earlier, in April 1983, in an attempt to find methods to add photo quality graphics to the text terminals of the time, but the 'Joint' that the 'J' in JPEG stands for refers to the merger of several groupings in an attempt to share and develop their experience.

The formal name of the standard that most people refer to as ' JPEG ' is ISO/IEC IS 10918-1 | ITU-T Recommendation T.81pdf [1.01MB], as the document was published by both ISO through its national standards bodies, and CCITT, now called ITU-T.  IS 10918 has actually 4 parts:

  • Part 1 - The basic JPEG standard, which defines many options and alternatives for the coding of still images of photographic quality.
  • Part 2 - Which sets rules and checks for making sure software conforms to Part 1.
  • Part 3 - Set up to add a set of extensions to improve the standard, including the SPIFF file format.
  • Part 4 - Defines methods for registering some of the parameters used to extend JPEG.

As well as the standard was created, nearly all of its real world applications require a file format, and example reference software to help implementors.  These functions were added by others - the file format was created originally by Eric Hamilton, the then convenor of JPEG as part of his work at C-Cube Microsystems, and was placed by them into the public domain under the name JFIF (version 1.02pdf [23KB]).  In the mean time, the JFIF format has been superseded by a new file format called SPIFF (the Still Picture Interchange File Formatpdf 105KB) which was completed in 1996. SPIFF is backwards compatible with JFIF.

Probably the largest and most important contribution however was the work of the Independent JPEG Group (IJG).  Their Open Source software implementation, as well as being one of the major Open Source packages was key to the success of the JPEG standard and was incorporated by many companies into a variety of products such as image editors and Internet browsers. 

 

JPEG is designed for compressing either full-color or gray-scale images of natural, real-world scenes. It works well on photographs, naturalistic artwork, and similar material; not so well on lettering, simple cartoons, or line drawings. JPEG handles only still images, but there is a related standard called MPEG for motion pictures.

JPEG is "lossy" meaning that the decompressed image isn't quite the same as the one you started with. There are lossless image compression algorithms, but JPEG achieves much greater compression than is possible with lossless methods.

JPEG is designed to exploit known limitations of the human eye, notably the fact that small color changes are perceived less accurately than small changes in brightness. Thus, JPEG is intended for compressing images that will be looked at by humans.

A useful property of JPEG is that the degree of lossiness can be varied by adjusting compression parameters. This means that the image maker can trade off file size against output image quality.  You can make ‘extremely’ small files if you don't mind poor quality; this is useful for applications such as indexing image archives.  Conversely, if you aren't happy with the output quality at the default compression setting, you can jack up the quality until you are satisfied, and accept lesser compression.

After creating the JPEG standard, the committee started to look at some of the criticisms of the existing standard.  High amongst these was the poor quality (and lack of integration) of lossless coding in the standard.  With this in mind, the committee developed the JPEG-LS standard - ISO/IEC IS 14495-1 | ITU-T Recommendation T.87pdf [353KB].

Why use JPEG?

There are two good reasons: to make your image files smaller, and to store 24-bit-per-pixel color  data  instead of 8-bit-per-pixel data.

Making image files smaller is a win for transmitting files across networks and for archiving libraries of images. JPEG can easily provide 20:1 compression of full-color data.  If you are comparing GIF and JPEG, the size ratio is usually more like 4.

JPEG is essentially a time/space tradeoff: you give up some time (to code/decode) in order to store or transmit an image more quickly or cheaply. The time savings from transferring a shorter file can be greater than the time needed to decompress the file.

The second fundamental advantage of JPEG is that it stores full color information: 24 bits/pixel (16 million colors).  GIF, the other image format widely used on the net, can only store 8 bits/pixel (256 or fewer colors).

Hence JPEG is considerably more appropriate than GIF for use as a World Wide Web standard photo format. The real disadvantage of lossy compression is that if you repeatedly compress and decompress an image, you lose a little more quality each time.

File Extension and others notes:

  • .JPG:...... JPEG image.
  • .JPE:....... JPEG image.
  • .JPEG:.... JPEG image.
  • .JP2:....... JPEG-2000 JP2 file format image format.
  • .JPC:....... JPEG-2000 code stream image format.

The MIME media type for JFIF is image/jpeg (defined in RFC 1341).

A Progressive JPEG is the JPEG equivalent of the interlaced GIF Graphics Interchange Format. It is an image created using the JPEG suite of compression algorithms that will "fade in" in successive waves of lines until the entire image has completely arrived. A progressive JPEG is a more appealing way to deliver an image at modem connection speeds. The user should see a gradual improvement of the quality of the image. This was quite useful in the past since many users (especially home users) were on dial-up access where the bandwidth is very limiting. Users with faster connections are not likely to notice the difference.

Web Browsers Support: All latest versions of browsers support JPEG format.

References & Background information:

Joint Photographic Experts Group (JPEG). The official site of the Joint Photographic Experts Group (JPEG), and Joint Bi-level Image Experts Group (JBIG).

ISO. International Organization for Standardization.

ITU-T. The ITU Telecommunication Standardization Sector is one of the three Sectors of the International Telecommunication Union (ITU).

Independent JPEG Group (IJG). IJG is an informal group that writes and distributes a widely used free library for JPEG image compression.

ISO/IEC IS 10918-1 | ITU-T Recommendation T.81pdf [1.01MB]. Information Technology - Digital compression and coding of continuous-tone still images: Requirements and  Guidelines. 

JPEG File Interchange Format Version 1.02pdf [17KB]. JPEG File Interchange Format is a minimal file format which enables JPEG bitstreams to be exchanged between a wide variety of platforms and applications. 

SPIFF (Still Picture Interchange File Format)pdf [105KB]. SPIFF specification. 

ISO/IEC IS 14495-1 | ITU-T Recommendation T.87pdf [353KB]. Lossless and near-lossless coding of continuous tone still images (JPEG-LS). 

ISO/IEC IS 15444 | ITU-T Recommendation T.800pdf [2.21KB]. Information Technology – JPEG 2000 Image Coding System. Last working draft before standardisation.

Adobe PHOTOSHOP. The official site of the last version of PHOTOSHOP.

CorelDRAW. The official site of the last version of CorelDRAW.

MS VISIO 2002 Standard Edition. The official site of VISIO 2002 SE.

 
 

Related tools and services

  • Recommended Authoring Graphics Tools:
    • Adobe Photoshop 7. (For discussion: Attention nowadays is Status EV)
    • CorelDraw 10. (For discussion: Attention nowadays is Class C)
  • Recommended Authoring Graphics Tools (for non-professional use):
    • Microsoft Visio 2002.
    • Graphical tools provided by the operating system or Office (Paint 5.1, Photoed 3.0).
 
 

Evolution of standard

JPEG 2000. Is a new image coding system that uses state-of-the-art compression techniques based on ‘wavelet technology’ (ISO/IEC IS 15444 | ITU-T T.800pdf [2.21MB]), and as well as being better at compressing images (up to 20 per cent plus), it can allow an image to be retained without any distortion or loss.

Wavelet technology  differs from the original JPEG strategy in the way it summarizes groups of pixels. One of the original JPEG compression algorithms, discrete cosine transformation, saved groups of eight pixels as individual blocks. Wavelet technology substitutes wavelets for blocks.

 
 

Training

Please consult SyslogRestricted area: This link points to internal pages and may not work if you are browsing as an external user., the training information system, which gives you access to the training catalogue, the training map and allows you to introduce your application for a training course.

 
 

Activities

  • How to decide the correct format for an Image.
 
 

Roles (concerned)