Chapter 5: Coding of Still Pictures (JPEG and JPEG2000)

Overview

In the mid 1980s joint work by the members of the ITU-T (International Telecommunication Union) and the ISO (International Standards Organisation) led to standardisation for the compression of grey scale and colour still images [1]. This effort was then known as JPEG: the Joint Photographic Experts Group. As is apparent, the word joint refers to the collaboration between the ITU-T and ISO. The JPEG encoder is capable of coding full colour images at an average compression ratio of 15:1 for subjectively transparent quality [2]. Its design meets special constraints, which make the standard very flexible. For example, the JPEG encoder is parametrisable, so that the desired compression/quality trade-offs can be determined based on the application or the wishes of the user [3].

JPEG can also be used in coding of video, on the basis that video is a succession of still images. In this case the process is called motion JPEG. Currently, motion JPEG has found numerous applications, the most notable one being video coding for transmission over packet networks with unspecified bandwidth or bit rates (UBR). A good example of UBR networks is the Internet where, due to unpredictability of the network load, congestion may last for a significant amount of time. Since in motion JPEG each frame is independently coded, it is an ideal encoder of video for such a hostile environment.

Another application of motion JPEG is video compression for recording on magnetic tapes, where again the independent coding of pictures increases the flexibility of the encoder for recording requirements, such as editing, pause, fast forward, fast rewind etc. Also, such an encoder can be very resilient to loss of information, since the channel error will not propagate through the image sequence. However, since the coding of I-pictures in the MPEG-2 standard is similar to motion JPEG, normally video compression for recording purposes is carried out with the I-picture part of the MPEG-2 encoder. The I-pictures are those which are encoded without reference to previous or subsequent pictures. This will be explained in Chapter 7.

At the turn of the millennium, the JPEG committee decided to develop another standard for compression of still images, named the JPEG2000 standard [4]. This was in response to growing demands for multimedia, Internet and a variety of digital imagery applications. However, in terms of compression methodology these two standards are very different. Hence in order to discriminate between them, throughout the book, the original JPEG is called JPEG and the new one JPEG2000.