Percorrer por autor "Assunção, P."
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- Adaptive coding tree for complexity control of high efficiency video encodersPublication . Correa, G.; Assunção, P.; Silva Cruz, L. A. da; Agostini, L.The emerging HEVC standard introduces several techniques which increase compression efficiency in comparison to its predecessors. However, such advances are accompanied by increases in computational complexity, limiting the encoder use in computational or power-constrained devices. This paper proposes a novel complexity control method for the future HEVC encoders based on a dynamic adjustment of the newly proposed coding tree structures. The relationship between coding tree depths and the encoding complexity is explored to selectively constrain encoding possibilities in order to not exceed a predefined complexity target. Experimental results show that the encoder computational complexity can be downscaled to 60% with a bit rate increase under 3.5% and a PSNR decrease under 0.1 dB.
- Efficient MV prediction for zonal search in video transcodingPublication . Marcelino, S.; Faria, S.; Assunção, P.; Moiron, S.; Ghanbari, M.This paper proposes a method to efficiently find motion vector predictions for zonal search motion re-estimation in fast video transcoders. The motion information extracted from the incoming video stream is processed to generate accurate motion vector predictions for transcoding with reduced complexity. Our results demonstrate that motion vector predictions computed by the proposed method outperform those generated by the highly efficient EPZS (Enhanced Predictive Zonal Search) algorithm in H.264/AVC transcoders. The computational complexity is reduced up to 59.6% at negligible cost in R-D performance. The proposed method can be useful in multimedia systems and applications using any type of transcoder, such as transrating and/or spatial resolution downsizing.
- Enhanced H.264/AVC video streaming using network-adaptive multiple description codingPublication . Correia, P.; Assunção, P.; Silva, V.This paper presents a network-adaptive multiple description coding (MDC) method for enhanced video streaming over multipath channels. The novel feature of the proposed MDC network adaptation scheme is the generation of a controlled amount of side information to compensate for drift distortion due to packet loss. This yields higher robustness of multiple description (MD) H.264/AVC streams than single description (SD) ones. Multiple Description Scalar Quantisation (MDSQ) is used to split an SD video stream in two complementary streams (two descriptions) for transmission over a multipath channel. The simulation results show that significantly better quality is achieved by using the proposed scheme instead of either SDC streams or classic SDC-MDSQ network adaptation without side information.
- Frame loss concealment for 3D video decoders based on disparity-compensated motion fieldPublication . Carreira, J.; Assunção, P.; Rodrigues, N.; Faria, S.This paper addresses the problem of frame loss concealment in 3D video decoders capable of handling stereoscopic views compliant with H.264/MVC. A joint motion-disparity compensation method is proposed to fully recover an estimated motion field for the lost frame in stereoscopic video, by using a combination of inter-view disparity-compensated motion vectors and intra-view motion extrapolation. The overall motion field is estimated from both the motion information of the co-located view, through disparity compensation, and neighboring frames of the same view, through motion vector extrapolation. The results show that the proposed method outperforms currently used methods such as frame-copy and motion-copy. In video sequences, average PSNR gains up to 1.6 dB are obtained over motion-copy, while for individual frames a maximum of 5 dB is achieved.
- Quality evaluation of depth map error concealment using a perceptually-aware objective metricPublication . Marcelino, S.; Faria, S.; Pepion, R.; Le Callet, P.; Soares, S.; Assunção, P.This paper presents a quality evaluation study on the performance of error concealment methods for depth maps used in multiview video-plus-depth (MVD). The research deals with the problem of decoding corrupted depth maps received from error-prone networks, where the quality of the reconstructed depth data is not always directly related to the quality of the virtual views synthesised by those maps. Even after error concealment such distortions are not particularly perceived as other known types, such as coding distortion. Thus, traditional quality metrics are not adequate to capture all the relevant features. In this work, the performance of two error concealment methods for depth maps is evaluated using a perceptually-aware objective metric. This metric is validated through subjective assessment of virtual views synthesised with concealed depth maps. Each subjective test is performed by comparing the relative quality between between two synthesised images using different error concealment methods. The perceptual impact of reconstruction in corrupted depth of MVD is evaluated under various loss rates, using several colour images and depth maps encoded at multiple quantisation steps. The achieved results reveal that the proposed objective quality metric is mostly inline with user preferences, in respect to the relative performance of each error concealment method.
- Reference picture selection using checkerboard pattern for resilient video codingPublication . Carreira, J.; Assunção, P.; Faria, S.; Ekmekcioglu, E.; Kondoz, A.; Lim, H.The improved compression efficiency achieved by the High Efficiency Video Coding (HEVC) standard has the counter-effect of decreasing error resilience in transmission over error-prone channels. To increase the error resilience of HEVC streams, this paper proposes a checkerboard reference picture selection method in order to reduce the prediction mismatch at the decoder in case of frame losses. The proposed approach not only allows to reduce the error propagation at the decoder, but also enhances the quality of reconstructed frames by selectively constraining the choice of reference pictures used for temporal prediction. The underlying approach is to increase the amount of accurate temporal information at the decoder when transmission errors occur, to improve the video quality by using an efficient combination of diverse motion fields. The proposed method compensates for the small loss of coding efficiency at frame loss rates as low as 3%. For a single frame-loss event the proposed method can achieve up to 2 dB of gain in the affected frames and an average quality gain of 0.84 dB for different error prone conditions.
- Subjective quality factors in packet 3D videoPublication . Pinto, L.; Carreira, J.; Faria, S.; Rodrigues, N.; Assunção, P.; Carreira, J.This paper presents an experimental study on subjective quality of 3D video under packet loss conditions. A priority network is assumed, such that the base view of a stereoscopic video stream is delivered through a guaranteed channel and the auxiliary view is subject to packet losses over a low priority channel. Due to the packetisation scheme, one frame is lost whenever a packet is lost. Three frame concealment methods are proposed to deal with frame losses and seven different 3DTV sequences are used. The subjective evaluation study was carried out for different packet loss ratios and concealment methods, using shutter glasses with synchronized display. The experimental results show that sequence disparity, type of loss concealment and packet loss pattern should be jointly considered as relevant quality factors in 3D video over packet networks.