Percorrer por autor "Faria, S."
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- 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.
- 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.
- Predictive models for physical and mechanical properties of 316L stainless steel produced by selective laser meltingPublication . Miranda, G.; Faria, S.; Bartolomeu, F.; Pinto, Elodie; Madeira, S.; Mateus, Artur; P.Carreira; Alves, Nuno; Silva, F.S.; Carvalho, O.Selective Laser Melting (SLM) processing parameters are known to greatly influence 316L stainless steel final properties. A simple energy density calculation is insufficient for explaining mechanical and physical properties as well as microstructural characteristics, which are known to significantly influence these parts performance. In fact, parts produced by using different combinations of processing parameters, even presenting similar energy density, can display different properties. Thus, it is necessary to assess their influence as isolated parameters but also their interactions. This work presents a study on the influence of several SLM processing parameters (laser power, scanning speed and scanning spacing) on density, hardness and shear strength of 316L stainless steel. The influence of these processing parameters on the abovementioned properties is assessed by using statistical analysis. In order to find the significant main factors and their interactions, analysis of variance (ANOVA) is used. Furthermore, in order to assess the effect of the part building orientation, two different building strategies were tested. The influence of these processing parameters on shear strength, hardness and density were assessed for the two building strategies, thus resulting six different models that can be used as predictive design tools. The microstructures experimentally obtained were analyzed, discussed and correlated with the obtained models.
- Predictive models for physical and mechanical properties of Ti6Al4V produced by Selective Laser MeltingPublication . Bartolomeu, F.; Faria, S.; Carvalho, O.; Pinto, E.; Alves, Nuno; Silva, F.S.; Miranda, G.The properties of parts produced by Selective Laser Melting (SLM) are highly influenced by the processing parameters. The calculation of the energy density is not enough for justifying the mechanical and physical properties, and understand the microstructures of Ti6Al4V samples produced by SLM. In fact, samples produced with the same energy density, but with different processing parameters, present distinct properties. In this sense, it is necessary to assess the influence of processing parameters on SLM fabrication, as isolated parameters but also their interactions. This work presents a study on the influence of several SLM processing parameters (laser power, scan speed and scan spacing) on density, hardness and shear strength of Ti6Al4V samples. The influence of these processing parameters on the final properties of SLM parts was obtained by using statistical analysis. Additionality, with the drive of finding the significant main factors and their interactions, analysis of variance (ANOVA) was performed. The influence of the above mentioned processing parameters on shear strength, hardness and density resulted in three models that can be used as predictive design tools for Ti6Al4V parts produced by SLM technology. Moreover, a study on the microstructures of the Ti6Al4V samples was performed and correlated with the obtained models.
- 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.
- A study on the production of thin-walled Ti6Al4V parts by selective laser meltingPublication . Miranda, G.; Faria, S.; Bartolomeu, F.; Pinto, E.; Alves, N.; Peixinho, N.; Gasik, M.; Silva, F.S.Selective Laser Melting (SLM) is an extremely versatile technology especially suited for the manufacturing of thin-walled parts. Micro-sized parts are highly influenced and dependent on the SLM processing parameters; thus being indispensable to assess the influence of processing parameters on SLM fabrication, as isolated parameters but also their interactions. In this study, the influence of SLM laser power and scanning speed on Ti6Al4V micropillars and micro-plates thickness was assessed by applying response surface methodology (RSM). These analyses resulted in four models that exhibit complex correlations of SLM process parameters, with non-linear equations, having coefficients of determination that assess the quality of the models. These developed models are accurate tools that can be used to optimize the micro manufacture of Ti6Al4V thin-walled parts by SLM.
- Subjective assessment of frame loss concealment methods in 3D videoPublication . Carreira, J.; Pinto, L.; Rodrigues, N.; Faria, S.; Assuncao, P.This paper investigates the subjective impact resulting from different concealment methods for coping with lost frames in 3D video communication systems. It is assumed that a high priority channel is assigned to the main view and only the auxiliary view is subject to either transmission errors or packet loss, leading to missing frames at the decoder output. Three methods are used for frame concealment under different loss ratios. The results show that depth is well perceived by users and the subjective impact of frame loss not only depends on the concealment method but also exhibits high correlation with the disparity of the original sequence. It is also shown that under heavy loss conditions it is better to switch from 3D to 2D rather than presenting concealed 3D video to users.
- 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.