Improvement of Laplacian Model

For higher coding efficiency and better subjective quality, our CABIC and SBR incorporates an estimated Laplacian model for parameter estimation. Particularly, the assumption of inde-pendent and identical distribution for co-located coefficients is used to simplify the derivation.

However, as it has been proved by our CABIC, co-located coefficients actually exhibit corre-lations. Therefore, the accuracy of Laplacian model may be further improved by a presumed random process such as Markov process. Nevertheless, it is still an open problem whether us-ing a more accurate model will actually improve the performance. One should note that usus-ing a model of higher order may suffer from inaccurate parameter estimation due to a limited number of observations. Thus, there are still many problems to be further investigated.

6.3.3 Error Resilience

In addition to coding efficiency and subjective quality, another important issue that is not thor-oughly discussed in this thesis is error resilience. Due to the data dependency from the context models and the nature of arithmetic coding, the proposed CABIC and SBR are less robust to transmission errors. For error resilience, one of the possible solutions is to partition the enhancement-layer frames into multiple slices as the flexible macroblock ordering (FMO) in H.264 [31]. However, penalty on coding efficiency is expected, as reported in Section 4.8.

Thus, how to provide the optimal trade-off between coding efficiency and error resilience still leaves lots of spaces for future research activities.

6.3.4 Applications in Scalable Video Coding Standard

Although the proposed EMFGS and CABIC schemes are specifically designed for MPEG-4 FGS [15], in Chapter 5, we also illustrate their applications in the upcoming standard [23] for scalable video coding (SVC).

In Chapter 5, we point out that the anchor frames and their enhancement-layer frames are coded in a way similar to that used by MPEG-4 FGS [15]. In Chapter 3, we have shown that

simply using the base layer for prediction causes poor coding efficiency. In the SVC [23], the penalty would become more apparent in the low-delay applications in which the anchor frames are coded more frequently. To resolve this issue, we propose to use the prediction scheme of EMFGS for improving the coding efficiency of anchor frames. Preliminary experiments have shown very promising results for such an idea. On the average, 1∼2dB gain can be observed when the mode-adaptive prediction is applied.

In addition, we also demonstrate that the idea of SBR can be extended for coding the FGS layers. Currently, the FGS layers are coded by a cyclical block coding scheme [26]. Each block is equally coded with one symbol in a cycle. Through the concept of SBR, we propose a prioritized block coding to have the symbols with better rate-distortion performance be coded with higher priority. Also, by using explicit syntax for the priority information, the prioritized block coding can serve the functionality of region-of-interest. Preliminary experiments also show very good results for these techniques. Further research activities have been initiated to explore how to modify and optimize these techniques for the SVC standard [23]. Still, there are many open problems for further research activities.

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APPENDIX A