B. Multimedia Delivery: Improving Resilience to Transmission Errors
Packets in a video bitstream contain data with different levels of importance from the visual information point of view. This results in unequal amounts of perceived image quality degradation when these packages are lost. Quality assessment experiments with observers have demonstrated that the effect of a lost packet depends on the spatio– temporal location of the visual information coded in the packet. Perceived quality degradation is the lowest when the loss affects regions of ‘‘noninterest’’ [52]–[54]. Visualattention-based error resilience or RoI-based channel coding methods are consequently good candidates to attenuate the perceptual quality loss resulting from packet loss. In the context of highly prediction-based coding technologies such as H.264/AVC, for good compression performance, there is a high dependency between many parts of the coded video sequence. However, this dependency comes with the drawback of allowing a spatio– temporal propagation of the error resulting from a packet loss. RoI-based coding should also consider attenuating the effect of this spatio–temporal dependency when important parts of the bitstream are lost. As part of the H.264/AVC video coding standard, error resilience features such as flexible macroblock ordering (FMO) and data partitioning (DP) can be exploited to improve resilience of salient regions of video content. DP partitions code slice into three separate network abstract layer (NAL) units, containing each different part of the slice. FMO allows the ordering of macroblocks in slices according to a predefined map rather than using the usual raster scan order. Coupled