Coding Standards [GW 6.6]

• A. K. Jain, Image Data Compression: A Review, Proc. IEEE, Vol 69, pp. 349-389, 1981.
• A. Skodras, C. Christopoulos, and T. Ebrahimi}, The JPEG2000 Still Image Standard, IEEE Signal Processing Magazine, Vol 18, No. 5, pp. 36-58, Sept 2001.
• Image Processing Links on Subject homepage

Main bodies for developing standards

• ISO: International Standardization Organization

• CCITT (ITU): Consultative Committee of the International Telephone and Telegraph

Issues of Interest

• basic compression using 1D-RLC

• advanced technique using 2D-RLC (very similar to RAC)

• adaptive techniques to handle progressive transmission and halftone images

Continuous tone image compression (lossy compression based on DCT)

• still frame/video (enhancement using wavelet transform)

• monochrome/colour compression

• adaptive quantization

• alternative scan order to form 1D array from the 2D DCT transform coefficients (for MPEGII)

• progressive transmission/multi-resolution

• motion compensation for video compression

Some Important Standards

• CCITT develop 4 groups of standards for bilevel image compression (now widely used in FAX transmission)

  The first two groups are now obsolete. Group 3 is based on 1D-RLC and can reach compression ratio of 10:1. Group 4 is based on a technique very similar to RAC (see Fig 6.37, 6.38) and can reach CR of 25:1.

  These standards were developed using 8 test documents for evaluation purposes. These documents included typed and handwritten text in several languages as well as a few line drawings. Thus documents very different from these 8 test documents may cause bit expansion!

• ISO & CCITT formed a committee called JBIG (Joint Bilevel Imaging Group) to develop the next generation of standards. In particular, to address the issues of progressive transmission and the handling of halftone images, which currently causes bit expansion.

• ISO & CCITT formed the Joint Photographic Experts Group (JPEG) to develop an international standard for continuous tone images. It selected DCT as the baseline coding scheme in 1988 and developed the JPEG standard in 1992.

  The JPEG baseline coding is essentially a DCT based, threshold coding technique described earlier, and is the one of the most popular and comprehensive standard for continuous-tone still images. However, due to the continual expansion of multi-media and internet applications, JPEG decided in 1997 to develop a new standard based on Wavelet Transform: the JPEG2000 standard which provides better compression performance as well as a set of new and powerful features for high-end applications.

• CCITT developed H.261, the only formally adopted standard for sequential frame imagery currently used for video-conferencing

• ISO & CCITT formed the Motion Picture Experts Group (MPEG) which developed the following standards: MPEGI (for VCD), MPEGII (for DVD and Digital TV), and MPEG-4 (for multimedia for the web and mobility). Current work in progress are MPEG-7 (for Multimedia Content Description Interface, scheduled for July 2001) and the new standard MPEG-21 for Multimedia Framework.

  The main difference between MPEGI and MPEGII is that the latter supports advanced algorithms to handle interlaced frames and alternative scan order. MPEGII also supports a new image format SIF: 360-by-240-by-30 frames per sec in addition to CCIR601: 720-by-480-by-30 frames per sec or 720-by-576-by-25 frames per sec

Brief introduction to video compression

• MPEG I/MPEG II standards

• CR up to around 100

• 6-layer structure
  • Sequence Layer
  • GOP (group of pictures of around 0.5 sec)
  • Picture Layer
  • Slice Layer
  • Macro-block Layer (16-by-16 blocks)
  • Block-Layer (8-by-8 blocks)

• Block Layer: essentially the JPEG baseline compression CR around 25

• Macro-Block Layer: motion compensation for macro-blocks
  • predict motion of next picture based on previous picture
  • motion vector are transmitted as well as the prediction error which can be quantized and Run Length Coded (cf predictive coding)

• Picture Layer: transmit reference picture every 15 frames to minimize error-build up