COMIS project

Part of the project is the downloading of the decoder software and the real transit of the video bitstream over ISDN.

• Benoît Macq, responsible;
• Thierry Delmot, Ir. Belgacom;
• Joan Maria Mas Ribés, Ir. Belgacom;
• Christophe De Vleeschouwer, PhD student;
• Xavier Marichal, PhD student.
• Frédéric Oger, Developper.

Multigrid scheme

Image decomposition using binary trees

Initially developed by M.P. Queluz, B. Simon and B. Macq in the context of the European COST 211ter research group, the UCL proposed a new approach for VLBR video coding in which spatial information and temporal changes are encoded using similar tools. In this spatio-temporal integrated approach, adaptive multigrids are used for both predicted images (inter-images) and intra-images. The intra-images are encoded using tree segmentation and transcripted on an adaptive sampling grid (with variable sampling mesh), as depicted on the following figures.

Figure 1

Figure 1 demonstrates how an image can be described using a binary segmentation tree, while figure 2 shows the result of the algorithm on an orignal image(left), with the reconstructed image (center) and the associated multigrid (right).

Figure 2

The inter-images are mainly encoded by a reliable motion field analysis and transcribed in a way similar to the intra transcription. The algorithm received thus the name of COMIS (Coding On Multigrids Image Sequences) and was essentially a spatio-temporal block-based coding technique.

Reference:

MACQ, B. AND QUELUZ, M.P. AND SIMON, B.
Very Low Bit-Rate Image Coding on Adaptive Multigrids
Signal Processing: Image Communication, Vol. 7, Number 4-6, November 1995, pp. 313-331

Object-oriented scheme

Introducing object representations

However, one wanted to reach very-low bit-rate (under 64 kbits/s) and add some segmentation-oriented features in order to make the scheme open to interactivity, content-based manipulation and editing, scalability,...

Indeed the only scalabilty that was possible with the first version of COMIS is depicted on figure 3 (left: original, center: coarse multigrid, right: first step towards a new scheme).

Figure 3

A reorganization of the scheme towards an improved coding efficiency and object-oriented functionnalities has thus been performed, as depicted on figure 4: a region model box is present in both the encoder and the decoder sides, and allow the scheme to deal with objects or regions as abstract entities.

Figure 4

Image analysis

On the basis of the object interpretation of the scene, some "clever" functionnalities can be added as the one of figure 5, that allows the codec to achieve better subjective quality, scalibility and reduced bit-rate. The corresponding intermediate results of figure 5 are depicted on figure 6 where one can notice the improvement of image E with regards to figure 3.

Figure 5

Figure 6

References

• MARICHAL, X.
  An original approach towards very-low bit-rate image coding
  revue HF, Number 4, December 1995, pp. 29-46
• MARICHAL, X. AND DE VLEESCHOUWER, C. AND DELMOT, T. and MACQ, B.
  Object based coding through multigrid representation
  SPIE Electronic Imaging: science and technology, San Jose, January 1996, pp.

For more information on our researches in Image Analysis and Compression, please contact Benoît Macq

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