Wavelet compression of digital holograms: Towards a view-dependent framework

International audienceAn analysis and discussion on the relevance of various wavelet schemes for hologram compression and reconstruction when the rendering configuration makes it possible to exploit selective refinements to perform a viewpoint-based degraded reconstruction. It is observed that Gabor wavelet bases have better time-frequency localization as compared to Fresnelet bases and hence are well suited for view-dependent compression techniques for hologram reconstruction

A content based method for perceptually driven joint color/depth compression

International audienceMulti-view Video plus Depth (MVD) data refer to a set of conventional color video sequences and an associated set of depth video sequences, all acquired at slightly different viewpoints. This huge amount of data necessitates a reliable compression method. However, there is no standardized compression method for MVD sequences. H.264/MVC compression method, which was standardized for Multi-View-Video representation (MVV), has been the subject of many adaptations to MVD. However, it has been shown that MVC is not well adapted to encode multi-view depth data. We propose a novel option as for compression of MVD data. Its main purpose is to preserve joint color/depth consistency. The originality of the proposed method relies on the use of the decoded color data as a prior for the associated depth compression. This is meant to ensure consistency in both types of data after decoding. Our strategy is motivated by previous studies of artifacts occurring in synthesized views: most annoying distortions are located around strong depth discontinuities and these distortions are due to misalignment of depth and color edges in decoded images. Thus the method is meant to preserve edges and to ensure consistent localization of color edges and depth edges. To ensure compatibility, colored sequences are encoded with H.264. Depth maps compression is based on a 2D still image codec, namely LAR (Locally adapted Resolution). It consists in a quad-tree representation of the images. The quad-tree representation contributes in the preservation of edges in both color and depth data. The adopted strategy is meant to be more perceptually driven than state-of-the-art methods. The proposed approach is compared to H.264 encoding of depth images. Objective metrics scores are similar with H.264 and with the proposed method, and visual quality of synthesized views is improved with the proposed approach

Morlet Wavelet transformed holograms for numerical adaptive view-based reconstruction

International audienceWe provide an efficient method of using Morlet wavelets for transforming a hologram and reconstructing parts of a scene based on the position of viewer by using a sparse set of Morlet transformed coefficients. We provide a design of a Morlet wavelet and explain an efficient discretization method for the application of view-dependent representation systems. Results are provided based on the numerical reconstruction, and it is shown that view- dependent representation along with Morlet wavelets form a good starting step for compressing holographic data for next generation 3DTV applications

Incremental-LDI for Multi-View Coding

International audienceThis paper describes an Incremental algorithm for Layer Depth Image construction (I-LDI) from multi-view plus depth data sets. A solution to sampling artifacts is proposed, based on pixel interpolation (inpainting) restricted to isolated unknown pixels. A solution to ghosting artifacts is also proposed, based on a depth discontinuity detection, followed by a local foreground / background classification. We propose a formulation of warping equations which reduces time consumption, specifically for LDI warping. Tests on Breakdancers and Ballet MVD data sets show that extra layers in I-LDI contain only 10% of first layer pixels, compared to 50% for LDI. I-LDI Layers are also more compact, with a less spread pixel distribution, and thus easier to compress than LDI Visual rendering is of similar quality with I-LDI and LDI

Focus on visual rendering quality through content-based depth map coding

International audienceMulti-view video plus depth (MVD) data is a set of multiple sequences capturing the same scene at different viewpoints, with their associated per-pixel depth value. Overcoming this large amount of data requires an effective coding framework. Yet, a simple but essential question refers to the means assessing the proposed coding methods. While the challenge in compression is the optimization of the rate-distortion ratio, a widely used objective metric to evaluate the distortion is the Peak-Signal-to-Noise-Ratio (PSNR), because of its simplicity and mathematically easiness to deal with such purposes. This paper points out the problem of reliability, concerning this metric, when estimating 3D video codec performances. We investigated the visual performances of two methods, namely H.264/MVC and Locally Adaptive Resolution (LAR) method, by encoding depth maps and reconstructing existing views from those degraded depth images. The experiments revealed that lower coding efficiency, in terms of PSNR, does not imply a lower rendering visual quality and that LAR method preserves the depth map properties correctly

Representation and coding of 3D video data

Livrable D4.1 du projet ANR PERSEECe rapport a été réalisé dans le cadre du projet ANR PERSEE (n° ANR-09-BLAN-0170). Exactement il correspond au livrable D4.1 du projet

ConvNeXt-ChARM: ConvNeXt-based Transform for Efficient Neural Image Compression

Over the last few years, neural image compression has gained wide attention from research and industry, yielding promising end-to-end deep neural codecs outperforming their conventional counterparts in rate-distortion performance. Despite significant advancement, current methods, including attention-based transform coding, still need to be improved in reducing the coding rate while preserving the reconstruction fidelity, especially in non-homogeneous textured image areas. Those models also require more parameters and a higher decoding time. To tackle the above challenges, we propose ConvNeXt-ChARM, an efficient ConvNeXt-based transform coding framework, paired with a compute-efficient channel-wise auto-regressive prior to capturing both global and local contexts from the hyper and quantized latent representations. The proposed architecture can be optimized end-to-end to fully exploit the context information and extract compact latent representation while reconstructing higher-quality images. Experimental results on four widely-used datasets showed that ConvNeXt-ChARM brings consistent and significant BD-rate (PSNR) reductions estimated on average to 5.24% and 1.22% over the versatile video coding (VVC) reference encoder (VTM-18.0) and the state-of-the-art learned image compression method SwinT-ChARM, respectively. Moreover, we provide model scaling studies to verify the computational efficiency of our approach and conduct several objective and subjective analyses to bring to the fore the performance gap between the next generation ConvNet, namely ConvNeXt, and Swin Transformer.Comment: arXiv admin note: substantial text overlap with arXiv:2307.02273. text overlap with arXiv:2307.0609

Objective View Synthesis Quality Assessment

International audienceView synthesis brings geometric distortions which are not handled efficiently by existing image quality assessment metrics. Despite the widespread of 3-D technology and notably 3D television (3DTV) and free-viewpoints television (FTV), the field of view synthesis quality assessment has not yet been widely investigated and new quality metrics are required. In this study, we propose a new full-reference objective quality assessment metric: the View Synthesis Quality Assessment (VSQA) metric. Our method is dedicated to artifacts detection in synthesized view-points and aims to handle areas where disparity estimation may fail: thin objects, object borders, transparency, variations of illumination or color differences between left and right views, periodic objects... The key feature of the proposed method is the use of three visibility maps which characterize complexity in terms of textures, diversity of gradient orientations and presence of high contrast. Moreover, the VSQA metric can be defined as an extension of any existing 2D image quality assessment metric. Experimental tests have shown the effectiveness of the proposed method

BIT-RATE ALLOCATION BETWEEN TEXTURE AND DEPTH: INFLUENCE OF DATA SEQUENCE CHARACTERISTICS

International audienceThis paper questions the existence of factors influencing the quality of the synthesized views, in the context of multi-view video plus depth coding (MVC). The issue of bit-rate allocation between texture and depth data in MVC is still open, despite the many efforts already raised for the development of optimization techniques. The originality of this study lies in the investigation of direct relationships between the best bit-rate allocation, in terms of objective quality of synthesized views, and the sequence characteristics (entropy of depth maps, depth complexity and camera baseline distance, background/foreground contrast areas). The results confirm our assumptions regarding the impact of the sequence features on the bit-rate allocation. The results and the limitations of the study are also discussed

Génération Numérique d'Hologrammes : État de l'Art

National audienceThis paper reviews Computer Generated Holography techniques applied to 3D video. Using these methods, it is possible to acquire holograms of synthetic or existing scenes without physical interference between light waves. Most limitations characterizing conventional holography, namely the need for a powerful, highly coherent laser and extreme stability of the optical system are thus avoided.Cet article présente un état de l'art des méthodes de génération numérique d'hologrammes appliquées à la vidéo 3D. Les méthodes présentées permettent de générer l'hologramme d'une scène synthétique ou réelle sans passer par le processus physique réel d'interférence entre deux ondes lumineuses. Les principales limitations liées à l'holographie conventionnelle, qui sont (1) la nécessité d'utiliser une source laser cohérente et (2) l'obligation d'avoir un système optique extrêmement stable, peuvent ainsi être évitées