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The quality of coded images is often assessed by a subjective test. Usually the viewers get as much time as they need to find a stable result. In video sequences however, the viewer has to judge the quality in a shorter time that is defined by the changing content or a following scene cut. Therefore it is desirable to know the influence of a shorter presentation time on the perceptibility of distortions. In this paper we present the results of a suitable subjective test on coded still images. The images were presented for six different durations, ranging from 200 ms to 3 s. Special care was taken to avoid the memorization effect usually present after short presentations. The results show that the viewers tend to avoid extreme votings at short durations. The variance of the votings is also discussed in detail. Based on the result of the voting for the longest presentation time, we propose a prediction model for the voting of the shorter durations using a logistic curve fit. This presentation time model (PTM) is presented and analysed in detail.

In this contribution we propose a new spatio-temporal fading scheme for block loss recovery in block-based video decoding systems. Based on a boundary error criterion obtained from temporal error concealment, either spatial, temporal, or facing of both methods is used for recovering lost image samples in one macroblock. The weights for fading are interpolated from the boundary error. A weighted absolute difference between well received macroblock boundary samples from the current frame and motion compensated macroblock boundary samples from the previous frame represents the boundary error. It is shown, that in case of transmission errors this method can successfully be used for block loss recovery.

The frequency selective extrapolation extends an image signal beyond a limited number of known samples. This problem arises in image and video communication in error prone environments where transmission errors may lead to data losses. In order to estimate the lost image areas, the missing pixels are extrapolated from the available correctly received surrounding area which is approximated by a weighted linear combination of basis functions. In this contribution, we integrate the frequency selective extrapolation into the H.264/AVC coder as spatial concealment method. The decoder reference software uses spatial concealment only for I frames. Therefore, we investigate the performance of our concealment scheme for I frames and its impact on following P frames caused by error propagation due to predictive coding. Further, we compare the performance for coded video sequences in TV quality against the non-normative concealment feature of the decoder reference software. The investigations are done for slice patterns causing chequerboard and raster scan losses enabled by flexible macroblock ordering (FMO).

The presented work addresses the reduction of computational complexity for transcoding of interframes from H.264 to H.263 baseline profiles maintaining the quality of a full search approach. This scenario aims to achieve fast backward compatible interoperability inbetween new and existing video coding platforms, e.g. between DVB-H and UMTS. By exploiting side information of the H.264 input bitstream the encoding complexity of the motion estimation is strongly reduced. Due to the possibility to divide a macroblock (MB) into partitions with different motion vectors (MV), one single MV has to be selected for H.263. It will be shown, that this vector is suboptimal for all sequences, even if all existing MVs of a MB of H.264 are compared as candidate. Also motion vector refinement with a fixed ?-pel refinement window as used by transcoders throughout the literature is not sufficient for scenes with fast movement. We propose an algorithm for selecting a suitable vector candidate from the input bitstream and this MV is then refined using an adaptive window. Using this technique, the complexity is still low at nearly optimum rate-distortion results compared to an exhaustive full-search approach.