[Jan 21] New ICASSP paper accepted

Title: Rate-distortion optimized motion estimation for on-the-sphere compression of 360 videos

Authors: Alban Marie, Navid Mahmoudian Bidgoli, Thomas Maugey, Aline Roumy

Abstract: On-the-sphere compression of omnidirectional videos is a very promising approach. First, it saves computational complexity as it avoids to project the sphere onto a 2D map, as classically done. Second, and more importantly, it allows to achieve a better rate-distortion tradeoff, since neither the visual data nor its domain of definition are distorted. In this paper, the on-the-sphere compression for omnidirectional still images is extended to videos. We first propose a complete review of existing spherical motion models. Then we propose a new one called tangent-linear+t. We finally propose a rate-distortion optimized algorithm to locally choose the best motion model for efficient motion estimation/compensation. For that purpose, we additionally propose a finer search pattern, called spherical-uniform, for the motion parameters, which leads to a more accurate block prediction. The novel algorithm leads to rate-distortion gains compared to methods based on a unique motion model.


[Jan 21] New IEEE Com letter accepted

Title: Bit-Plane Coding in Extractable Source Coding: optimality, modeling, and application to 360° data

Authors: Fangping Ye, Navid Mahmoudian Bidgoli, Elsa Dupraz, Aline Roumy, Karine Amis, Thomas Maugey

In extractable source coding, multiple correlated sources are jointly compressed but can be individually accessed in the compressed domain. Performance is measured in terms of storage and transmission rates. This problem has multiple applications in interactive video compression such as Free Viewpoint Television or navigation in 360° videos. In this paper, we analyze and improve a practical coding scheme. We consider a binarized coding scheme, which insures a low decoding complexity. First, we show that binarization does not impact the transmission rate but only slightly the storage with respect to a symbol based approach. Second, we propose a Q-ary symmetric model to represent the pairwise joint distribution of the sources instead of the widely used Laplacian model. Third, we introduce a novel pre-estimation strategy, which allows to infer the symbols of some bit planes without any additional data and therefore permits to reduce the storage and transmission rates. In the context of 360° images, the proposed scheme allows to save 14\% and 34\% bitrate in storage and transmission rates respectively.


[Jun 20] New journal article accepted in IEEE TMM

Title: Fine granularity access in interactive compression of 360-degree images based on rate adaptive channel codes

Authors: N. Mahmoudian-Bidgoli, T. Maugey, A. Roumy

Abstract: In this paper, we propose a new interactive compression scheme for omnidirectional images. This requires two characteristics: efficient compression of data, to lower the storage cost, and random access ability to extract part of the compressed stream requested by the user (for reducing the transmission rate). For efficient compression, data needs to be predicted by a series of references that have been pre-defined and compressed. This contrasts with the spirit of random accessibility. We propose a solution for this problem based on incremental codes implemented by rate adaptive channel codes. This scheme encodes the image while adapting to any user request and leads to an efficient coding that is flexible in extracting data depending on the available information at the decoder. Therefore, only the information that is needed to be displayed at the users side is transmitted during the user’s request as if the request was already known at the encoder. The experimental results demonstrate that our coder obtains better transmission rate than the state-of-the-art tile-based methods at a small cost in storage. Moreover, the transmission rate grows gradually with the size of the request and avoids a staircase effect, which shows the perfect suitability of our coder for interactive transmission.

[Jun 20] New journal article accepted in IEEE T-Com

Title: Optimal reference selection for random access in predictive coding schemes

Authors: M. Q. Pham, A. Roumy, T. Maugey, E. Dupraz, M. Kieffer

Abstract: Data acquired over long periods of time like High Definition (HD) videos or records from a sensor over long time intervals, have to be efficiently compressed, to reduce their size. The compression has also to allow efficient access to random parts of the data upon request from the users. Efficient compression is usually achieved with prediction between data points at successive time instants. However, this creates dependencies between the compressed representations, which is contrary to the idea of random access. Prediction methods rely in particular on reference data points, used to predict other data points, and the placement of these references balances compression efficiency and random access. Existing solutions to position the references use ad hoc methods. In this paper, we study this joint problem of compression efficiency and random access. We introduce the storage cost as a measure of the compression efficiency and the transmission cost for the random access ability. We show that the reference placement problem that trades off storage with transmission cost is an integer linear programming problem, that can be solved by standard optimizer. Moreover, we show that the classical periodic placement of the references is optimal, when the encoding costs of each data point are equal and when requests of successive data points are made. In this particular case, a closed form expression of the optimal period is derived. Finally, the optimal proposed placement strategy is compared with an ad hoc method, where the references correspond to sources where the prediction does not help reducing significantly the encoding cost. The optimal proposed algorithm shows a bit saving of -20% with respect to the ad hoc method.

[Feb 19] New journal article accepted in IEEE TSIPN

Title: Incremental coding for extractable compression in
the context of Massive Random Access

Autors: Thomas Maugey, Aline Roumy, Elsa Dupraz, Michel Kieffer

Abstract: in this paper, we study the problem of source coding with Massive Random Access (MRA). A set of correlated sources is encoded once for all and stored on a server while a large number of clients access various subsets of these sources. Due to the number of concurrent requests, the server is only able to extract a bitstream from the stored data: no re-encoding can be performed before the transmission of the data requested by the clients.
First, we formally define the MRA framework and propose to model the constraints on the way subsets of sources may be accessed by a navigation graph. We introduce both storage and transmission costs to characterize the performance of MRA. We then propose an Incremental coding Based Extractable Compression (IBEC) scheme. We first show that this scheme is optimal in terms of achievable storage and transmission costs. Second, we propose a practical implementation of our IBEC scheme based on rate-compatible LDPC codes. Experimental results show that our IBEC scheme can almost reach the same transmission costs as in traditional point-to-point source coding schemes, while having a reasonable overhead in terms of storage cost.

[Sep 19] New journal article accepted in Physical Communication

Title: Rate-Storage Regions for Extractable Source Coding with Side Information

Authors: E. Dupraz, T. Maugey, A. Roumy, M. Kieffer

Abstract:This papers considers the coding of a source with decoders, each having access to a different side information . We define a new source coding problem called Extractable Source Coding with Side Information (ESC-SI). In this problem, the server stores one single coded description of the source, from which descriptions can be extracted without re-encoding, depending on the side information available at the decoder. We want to minimize both the storage rate of the source on the server, and the transmission rates from the server to the decoders. We provide the achievable storage-transmission rate regions for lossless source coding of general, non i.i.d., non-ergodic sources, and the achievable storage-transmission rate–distortion regions for lossy source coding for non i.i.d. Gaussian sources. The regions obtained for such general source models provide insightful design guidelines for practical applications.

[Aug 19] three papers accepted at PCS 2019

The three following papers are accepted and will be presented at the Picture Coding Symposium (PCS) 2019, in China:

N. Mahmoudian Bidgoli, T. Maugey , A. Roumy, Intra-coding of 360-degree images on the sphere
Picture Coding Symposium (PCS), Ningbo, China, Nov. 2019

N. Mahmoudian Bidgoli, T. Maugey, A. Roumy, F. Nasiri and F. Payan, A geometry-aware compression of 3D mesh texture with random access
Picture Coding Symposium (PCS), Ningbo, China, Nov. 2019

P. Garus, J. Jung, T. Maugey and C. Guillemot, Bypassing Depth Maps Transmission For Immersive Video Coding
Picture Coding Symposium (PCS), Ningbo, China, Nov. 2019

[June 19] New journal article accepted in IEEE TIP

Title: Geometry-Aware Graph Transforms for Light Field Compact Representation

Authors: Mira Rizkallah, Xin Su, Thomas Maugey, Christine Guillemot

Abstract: The paper addresses the problem of energy compaction of dense 4D light fields by designing geometry-aware local graph-based transforms. Local graphs are constructed on super-rays that can be seen as a grouping of spatially and geometry-dependent angularly correlated pixels. Both non separable and separable transforms are considered. Despite the local support of limited size defined by the super-rays, the Laplacian matrix of the non separable graph remains of high dimension and its diagonalization to compute the transform eigen vectors remains computationally expensive. To solve this problem, we then perform the local spatio-angular transform in a separable manner. We show that when the shape of corresponding super-pixels in the different views is not isometric, the basis
functions of the spatial transforms are not coherent, resulting in decreased correlation between spatial transform coefficients. We hence propose a novel transform optimization method that aims at preserving angular correlation even when the shapes of the super-pixels are not isometric. Experimental results show the benefit of the approach in terms of energy compaction. A coding scheme is also described to assess the rate-distortion perfomances of the proposed transforms and is compared to state of the art
encoders namely HEVC-lozenge [1], JPEG pleno 1.1 [2], HEVC- pseudo [3] and HLRA [4] .

[May 2019] One paper accepted in ICIP

Title: Evaluation framework for 360-degree visual content compression with user-dependent transmission

Authors: Navid MAHMOUDIAN BIDGOLI, Thomas MAUGEY, Aline ROUMY

Abstract: Immersive visual experience can be obtained by allowing the user to navigate in a 360-degree visual content. These contents are stored in high resolution and need a lot of space on the server to store them. The transmission depends on the user’s request and only the spatial region which is requested by the user is transmitted to avoid wasting network bandwidth. Therefore, storage and transmission rates are both critical.
%The former is important to reduce the space for storage on the server and the latter reduces the bitrate for the available network bandwidth.
Splitting the rates into storage and transmission has not been formally considered in the literature for evaluating 360-degree content compression algorithms. In this paper, we propose a framework to evaluate the coding efficiency of 360-degree content while discriminating between storage and transmission rate and taking into account user dependency. This brings the flexibility to compare different coding methods based on the storage capacity on the server and network bandwidth of users.