| Summary: | Compressed image and video bit streams are very sensitive to channel errors and
may be altered or lost during transmission. Error concealment by post-processing
intends to reconstruct lost visual information by exploiting the correlation between
the image/video data. The applications of concealment of errors in coded visual
information include visual communication over unreliable channels such as wireless
networks and the Internet.
For most types of encoders and input data, coded visual information consists
of a collection of coded texture (DCT coefficients), shape and motion information.
In this thesis we present concealment methods for errors in texture and shape information,
and address the concealment of errors in motion data in conjunction with its
corresponding texture or shape information. The method developed for concealment
of errors in coded texture involves compensation of the effects of the missing data
on the rest of the texture information and then using, a deterministic or a statistical
algorithm for the restoration of missing information. The deterministic algorithm
achieves a good performance level in the reconstruction of edges. The statistical
algorithm which is based on maximum a posteriori (MAP) estimation, employs an
adaptive Markov random field (MRF) as the image a-priori model. The adaptation
enables the estimation procedure to incorporate more information without a dramatic
increase in computational complexity. MAP estimation is also employed for the reconstruction
of missing shape data. Although it uses an adaptive MRF, the estimator
is different in the sense that it is designed for binary shape information.
In the second part of the thesis, we evaluate the performance of the developed
concealment methods for three different types of coded visual data: baseline JPEG
coded still images, H.263 coded video and MPEG-4 coded video. Our experimental
results demonstrate that the methods presented in this thesis achieve consistently
good computation-performance tradeoffs, making them very beneficial for real time
communication over error prone networks. In fact, the proposed error concealment
methods can lead to acceptable visual quality at loss rates as high as 20%.
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