Percorrer por autor "Navarro, António"
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- Retargeting 4K Video for Mobile Access Using Visual Attention and Temporal StabilizationPublication . Kumar, Rohit; Ferreira, Lino; Assunção, Pedro; Navarro, AntónioThis paper presents different methods of retargeting 4K resolution videos for the smaller resolutions of mobile devices. Several methods are investigated using image cropping based on visual attention maps with removal of temporal jitter. Visual comparison between video sequences obtained from the retargeting methods is presented and evaluated through subjective testing. Based on subjective scores, the proposed algorithm shows the best performance for 480p target resolution.
- Retargeting UHD 4k Video for SmartphonesPublication . Kumar, Rohit; Navarro, António; Assunção, Pedro; Ferreira, LinoThis paper evaluates a new video-retargeting algorithm jointly with two others previously published in the literature. Four Ultra High Definition (UHD) 4K resolution video sequences were used for subjective assessment. Subjective testing results show that the proposed algorithm yields the best performance when UHD 4K video is retargeted to 1920 × 1080 resolution. The main application of these retargeting algorithms is to increase the quality of experience (QoE) provided to consumers when playing UHD videos on small screen devices like smartphones.
- Versatile Video Coding of 360-Degree Video using Frame-Based FoV and Visual AttentionPublication . Carreira, J.; Faria, Sérgio M. M. de; Távora, L. M. N.; Navarro, António; Assunção, Pedro A. A.High quality omnidirectional video requires ultra high resolution formats encoded with very high bit rates to guarantee acceptable QoE in video delivery services. Since in general the full FoV, i.e., 360º, is not required at once by users, rather than agnostic encoding of the whole 360º video, this work proposes a flexible coding approach where the full FoV is mapped into video frames and efficiently encoded using intra-FoV prediction. By avoiding inter-FoV prediction, the proposed approach enables independent decoding of one or more FoVs extracted from a single compressed stream containing the full FoV video. To achieve improved quality in those FoVs which attract more visual attention, non-uniform coding is proposed for the new Versatile Video Coding standard (VVC), using perceptually-driven quantisation for each FoV. This strategy, makes use of visual attention maps to decrease the overall bit rate without compromising the quality of the most relevant regions. The simulation results show that the proposed coding mechanism achieves consistent quality gains in the relevant FoV without significant losses in the remaining ones. In comparison with the reference VVC, the proposed method is able to achieve average quality gains up to 1.56 dB and to efficiently adapt the coding parameters to the visual attention information.
- Video transcoding from H.264/AVC to MPEG-2 with reduced computational complexityPublication . Moiron, Sandro; Faria, Sergio; Navarro, António; Silva, Vitor; Assunção, PedroThis paper addresses video transcoding from H.264/AVC into MPEG-2 with reduced complexity and high rate-distortion efficiency. While the overall concept is based on a cascaded decoder-encoder, the novel adaptation methods developed in this work have the advantage of providing very good performance in H.264/AVC to MPEG-2 transcoding. The proposed approach exploits the similarities between the coding tools used in both standards, with the objective of obtaining a computationally efficient transcoder without penalising the signal quality. Fast and efficient methods are devised for conversion of macroblock coding modes and translation of motion information in order to compute the MPEG-2 coding format with a reduced number of operations, by reusing the corresponding data embedded in the incoming H.264/AVC coded stream. In comparison with a cascaded decoder-encoder, the fast transcoder achieves computational complexity savings up to 60% with slightly better peak signal-to-noise ratio (PSNR) at the same bitrate.