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de Oliveira Pegado de Noronha E Távora, Luís Miguel
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- Lossless light-field compression using reversible colour transformationsPublication . Santos, Joao M.; Pedro A. A. Assuncao; Cruz, Luis A. da Silva; Luís Távora; Fonseca-Pinto, Rui; Sergio M. M. FariaRecent advances in Light Field acquisition and rendering are pushing research efforts towards increasingly efficient methods to encode this particular type of data. Light Field image compression is of the utmost importance, not only due to the large amount of data required for its representation but also due to quality requirements of many applications and computational photography methods. This paper presents a research study about the impact of reversible colour transformations and alternative data arrangements in Light Field lossless coding. The experimental results indicate that the RCT reversible transform consistently achieves the highest compression performance across all data arrangements and lossless encoders. In particular, the best results are obtained with MRP when encoding the stack of sub-aperture images using a spiral scan order, achieving 6.41 bpp, on average.
- Complexity Estimation for Load Balancing of 360-Degree Intra Versatile Video CodingPublication . Filipe, Jose N.; Monteiro Carreira, João Filipe; Tavora, Luis M. N.; Faria, Sergio; Navarro, Antonio; Assuncao, Pedro A. A.The ever increasing demand for image and video content poses new requirements to support higher resolutions and richer representation formats, creating new challenges in coding algorithms. The forthcoming Versatile Video Coding (VVC) standard aims to increase the coding efficiency of existing algorithms and it is particularly suitable for Ultra-High Definition (UHD) resolutions and 360° video. However, since coding efficiency gains are obtained at the cost of increased complexity, fast computational approaches are needed to cope with realtime requirements, such as parallel processing. Thus, this work presents a contribution towards efficient parallel encoding of 360° video, based on coding complexity estimation and nonuniform data-level splitting (slice-based) for load balancing across multiple processors. A machine learning approach is proposed to estimate the complexity of intra coding VVC, using uncorrelated features, obtained through Principal Component Analysis (PCA) and Extremely Randomised Trees (ERT). Then, a complexity-balanced slice partition is devised, taking advantage of the clustered complexity inherent to Equirectangular Projection (ERP). It is shown that coding complexity is estimated with an accuracy of 92.25%, and the encoding time is reduced by 8.50%, when compared to the case where the 360° frames are evenly split.
- Robust Depth Estimation From Multi-Focus Plenoptic ImagesPublication . Cunha, Francisco; Thomaz, Lucas; Tavora, Luis M. N.; Assunção, Pedro A. A.; Fonseca-Pinto, Rui; Faria, Sérgio M. M.This paper describes a robust depth estimation algorithm for multi-focus plenoptic images. The main feature of the proposed method consists of a hybrid template matching scheme built-upon intensity and local phase information, which adapts to the blurriness of neighbouring lenslet microimages. By reducing the impact of defocusblur on the template matching accuracy, the proposed method efficiently handles the varying triangulation baseline over the depth-of-field, thus discarding the need for scene-related information such as the expected range of disparities. Experimental results demonstrate the robustness of the proposed method over the most used commercially available depth estimation algorithm, achieving a reduction of 73% on the depth estimation error.
- Using local binary patterns in speckle image analysisPublication . Bento, L.; de Oliveira Pegado de Noronha E Távora, Luís Miguel; Assunção, Pedro; Faria, Sergio; Fonseca-Pinto, RuiFirstly described by Newton in the 17th century, speckle is an optical phenomenon which can be translated into image patterns produced by wave interferences of diffused reflections. In fact, the speckle pattern is generated by the multiple interference phenomena that occur when a rough surface is illuminated with a coherent source of light, producing randomly distributed reflected waves of the same frequency but different phases and amplitudes. Although it has been known for a long time, capturing video sequences of speckle patterns was dependent on recent technological developments, in particular, related to laser technology and microsensors. The speckle acquisition setup comprises a light source, usually a laser, an optical beam expander and a CCD camera. The generated interference patterns are captured in series of video sequences, to further be processed. In previous works, several image processing algorithms have been applied to analyze video frames of speckle, aimed to capture the evolution patterns in dynamic processes. However, due to the typical high frequencies of the changing patterns, classical texture algorithms mostly fail this goal. In this work, speckle dynamics are evaluated using Local Binary Patterns (LBP) jointly with some of its main variants and a newly proposed algorithm, in a reactive hyperemia controlled test. The proposed methodology goes beyond the traditional implementations of LBPs, by considering an additional Gaussian filtering, a methodology thus coined as LBPg. The results, on one hand, confirm that the classical formulations of LBP are not sensitive to changes in the simulated patterns but, on the other hand, demonstrate that the newly proposed LBP-adapted algorithm successfully identify the dynamics of the processes under study.
- Evaluation of cutaneous microcirculation patterns by laser speckle imagingPublication . Bento, L.; de Oliveira Pegado de Noronha E Távora, Luís Miguel; Assunção, Pedro; Faria, Sergio; Fonseca-Pinto, RuiAtherosclerosis is a chronic systemic process affecting distal circulation, a condition known as Peripheral Artery Disease (PAD). Epidemiological and clinical studies indicate a strong association between PAD and death due to cardiovascular diseases, thus the early detection of DAP by measuring perfusion levels of distal body regions is critical to prevent ischemic events. This work proposes a new methodology for the evaluation of local cutaneous perfusion through laser speckle video processing. The speckle pattern generated by a laser beam projected onto the skin surface after being expanded by an optical setup is captured by a CCD video camera. To evaluate the particular characteristics of the speckle pattern, a video processing algorithm based on an adaptive Local Binary Pattern approach, highlighted by a local Gaussian filtering scheme, the LBPg, was developed. In order to test this new method of video speckle analysis, different patterns of microcirculation were evaluated in skin regions with different textures. The experimental results were compared with others obtained in two clinical conditions associated with PAD (i.e., Deep Vein Thrombosis and Diabetic foot). The results show that the proposed approach is sensitive to the change in perfusion levels (even in cases with reduced perfusion variations), thus indicating that the use of laser speckle technology, jointly with LBPg, is a promising noninvasive, low cost and sensitive method for the early detection of PAD-related diseases.
- Scalable Coding of 360-degree Video for Streaming Adaptation at 5G Network EdgesPublication . Carreira, J.; Faria, Sergio M. M. de; Tavora, Luis M. N.; Navarro, Antonio; Assuncao, Pedro A. A.The huge amount of data that is necessary to capture the full field-of-view (FoV) in omnidirectional video, i.e., 360°, imposes the use of highly efficient compressed formats as well as adaptive broadcast and streaming mechanisms, such as those foreseen for 5G networks. To cope with the demanding requirements of 360° video streaming over 5G networks, this work proposes a scalable 360° video coding architecture, by enabling adaptation through the Multi-Access Edge Computing (MEC) server in two different domains of the spherical visual content, namely spatial resolution and FoV. In the proposed architecture two-layers are encoded from the input 360° video content: (i) the base-layer (BL), encoding each 360° image as a whole, at a lower spatial resolution; (ii) the enhancement-layer (EL) encoding each spherical image as a set of multiple FoVs with higher spatial resolution. Such arrangement enables flexible stream adaptation for the smart decision algorithms to be implemented at the MEC server, enabling significant reduction of the overall bit rate through the radio interface. The simulation results show that the proposed scalable coding scheme allows a great deal of bit rate savings across the 5G network, achieving 36% of bit rate saving, on average, for a 90° FoV in comparison with conventional single-layer coding.
- 4D Light Field Disparity Map estimation using Krawtchouk PolynomialsPublication . Lourenco, Rui; Rivero-Castillo, Daniel; Thomaz, Lucas A.; Assuncao, Pedro A. A.; Tavora, Luis M. N.; Faria, Sergio M. M. deThis work presents an improved method to estimate disparity maps obtained from light field cameras using a novel edge detection algorithm based on Krawtchouk polynomials. The proposed method takes advantage of these polynomials to determine gradient information and find the edges based on automatically estimated weak and strong thresholds. The calculated edges in the gray scale epipolar plane image representation of a light field are then used to improve the accuracy of object boundaries in the the disparity map. The proposed method achieves better results when compared to other edge detection algorithms, both in terms of objective and subjective quality, specifically by reducing the mean squared error and the artifacts in the object boundaries. Furthermore, on average, the proposed method outperforms the state-of-the-art depth estimation algorithms, in terms of the objective quality of the final disparity map, namely for the commonly used HCI dataset.
- Disparity compensation of light fields for improved efficiency in 4D transform-based encodersPublication . Santos, Joao M.; Thomaz, Lucas A.; Assuncao, Pedro A. A.; Cruz, Luis A. da Silva; Tavora, Luis M. N.; Faria, Sergio M. M. deEfficient light field en coders take advantage of the inherent 4D data structures to achieve high compression performance. This is accomplished by exploiting the redundancy of co-located pixels in different sub-aperture images (SAIs) through prediction and/or transform schemes to find a m ore compact representation of the signal. However, in image regions with higher disparity between SAIs, such scheme's performance tends to decrease, thus reducing the compression efficiency. This paper introduces a reversible pre-processing algorithm for disparity compensation that operates on the SAI domain of light field data. The proposed method contributes to improve the transform efficiency of the encoder, since the disparity-compensated data presents higher correlation between co-located image blocks. The experimental results show significant improvements in the compression performance of 4D light fields, achieving Bjontegaard delta rate gains of about 44% on average for MuLE codec using the 4D discrete cosine transform, when encoding High Density Camera Arrays (HDCA) light field images.
- Versatile Video Coding Of 360° Video Using Adaptive Resolution ChangePublication . Carreira, J.; Faria, Sergio M. M. de; Tavora, Luis M. N.; Navarro, Antonio; Assuncao, Pedro A. A.Encoding 360° video with ultra high spatial resolution requires high bitrates to guarantee acceptable QoE in video delivery services. However, since in general the full Field-of-View (FoV), i.e., 360°, is not required at once, a great deal of bandwidth can be saved if only a limited FoV is delivered, according to content relevancy or/and user demand. This work addresses this problem using the concept of Adaptive Resolution Change (ARC) defined in the forthcoming Versatile Video Coding (VVC) standard, by dynamically mapping the full FoV into multiple video frames with different spatial resolutions. Those FoVs attracting more visual attention are encoded with higher resolution while the others are encoded with lower resolution, thus without compromising the visual quality and resolution of the most relevant regions. The simulation results show that the proposed adaptive coding scheme is able to deliver high quality video for the most relevant FoV at any time instant, achieving a maximum bitrate reduction of 37.2%.
- Measuring the mobilities of the ions formed in P-10 mixturesPublication . Neves, P. N. B.; Trindade, A. M. F.; Barata, J. A. S.; de Oliveira Pegado de Noronha E Távora, Luís Miguel; Conde, C. A. N.We measured the reduced mobilities (k0 in cm2V -1s-1) of the ions that are formed in P-10 mixtures under different pressures and various reduced electric fields. Two types of ions were identified. We then used the extrapolated values of k0 when E/N→0 (2.41 and 2.76 cm2V-1s-1), the Langevin formula and the Blanc's law, to calculate the masses of these ions. Calculations indicate that the two ions have masses of 41 a.u. and 25 a.u., respectively. We propose hypothesis to explain the nature and origin of those ions.