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  • 1.
    Conti, Caroline
    et al.
    University of Lisbon, Portugal.
    Soares, Luis Ducla
    University of Lisbon, Portugal.
    Nunes, Paulo
    University of Lisbon, Portugal.
    Perra, Cristian
    University of Cagliari, Italy.
    Assunção, Pedro Amado
    Institute de Telecomunicacoes and Politecenico de Leiria, Portugal.
    Sjöström, Mårten
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Information Systems and Technology.
    Li, Yun
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Information Systems and Technology.
    Olsson, Roger
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Information Systems and Technology.
    Jennehag, Ulf
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Information Systems and Technology.
    Light Field Image Compression2018In: 3D Visual Content Creation, Coding and Delivery / [ed] Assunção, Pedro Amado, Gotchev, Atanas, Cham: Springer, 2018, p. 143-176Chapter in book (Refereed)
  • 2.
    Li, Yun
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Information and Communication systems.
    Coding of three-dimensional video content: Depth image coding by diffusion2013Licentiate thesis, comprehensive summary (Other academic)
    Abstract [en]

    Three-dimensional (3D) movies in theaters have become a massive commercial success during recent years, and it is likely that, with the advancement of display technologies and the production of 3D contents, TV broadcasting in 3D will play an important role in home entertainments in the not too distant future. 3D video contents contain at least two views from different perspectives for the left and the right eye of viewers. The amount of coded information is doubled if these views are encoded separately. Moreover, for multi-view displays (i.e. different perspectives of a scene in 3D are presented to the viewer at the same time through different angles), either video streams of all the required views must be transmitted to the receiver, or the displays must synthesize the missing views with a subset of the views. The latter approach has been widely proposed to reduce the amount of data being transmitted. The virtual views can be synthesized by the Depth Image Based Rendering (DIBR) approach from textures and associated depth images. However it is still the case that the amount of information for the textures plus the depths presents a significant challenge for the network transmission capacity. An efficient compression will, therefore, increase the availability of content access and provide a better video quality under the same network capacity constraints.

    In this thesis, the compression of depth images is addressed. These depth images can be assumed as being piece-wise smooth. Starting from the properties of depth images, a novel depth image model based on edges and sparse samples is presented, which may also be utilized for depth image post-processing. Based on this model, a depth image coding scheme that explicitly encodes the locations of depth edges is proposed, and the coding scheme has a scalable structure. Furthermore, a compression scheme for block-based 3D-HEVC is also devised, in which diffusion is used for intra prediction. In addition to the proposed schemes, the thesis illustrates several evaluation methodologies, especially, the subjective test of the stimulus-comparison method. It is suitable for evaluating the quality of two impaired images, as the objective metrics are inaccurate with respect to synthesized views.

    The MPEG test sequences were used for the evaluation. The results showed that virtual views synthesized from post-processed depth images by using the proposed model are better than those synthesized from original depth images. More importantly, the proposed coding schemes using such a model produced better synthesized views than the state of the art schemes. As a result, the outcome of the thesis can lead to a better quality of 3DTV experience.

  • 3.
    Li, Yun
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Information and Communication systems.
    Coding of Three-dimensional Video Content: Diffusion-based Coding of Depth Images and Displacement Intra-Coding of Plenoptic Contents2015Doctoral thesis, monograph (Other academic)
    Abstract [en]

    In recent years, the three-dimensional (3D) movie industry has reaped massive commercial success in the theaters. With the advancement of display technologies, more experienced capturing and generation of 3D contents, TV broadcasting, movies, and games in 3D have entered home entertainment, and it is likely that 3D applications will play an important role in many aspects of people's life in a not distant future. 3D video contents contain at least two views from different perspectives for the left and the right eye of viewers. The amount of coded information is doubled if these views are encoded separately. Moreover, for multi-view displays (i.e. different perspectives of a scene in 3D are presented to the viewer at the same time through different angles), either video streams of all the required views must be transmitted to the receiver, or the displays must synthesize the missing views with a subset of the views. The latter approach has been widely proposed to reduce the amount of data being transmitted and make data adjustable to 3D-displays. The virtual views can be synthesized by the Depth Image Based Rendering (DIBR) approach from textures and associated depth images. However, it is still the case that the amount of information for the textures plus the depths presents a significant challenge for the network transmission capacity. Compression techniques are vital to facilitate the transmission. In addition to multi-view and multi-view plus depth for reproducing 3D, light field techniques have recently become a hot topic. The light field capturing aims at acquiring not only spatial but also angular information of a view, and an ideal light field rendering device should be such that the viewers would perceive it as looking through a window. Thus, the light field techniques are a step forward to provide us with a more authentic perception of 3D. Among many light field capturing approaches, focused plenoptic capturing is a solution that utilize microlens arrays. The plenoptic cameras are also portable and commercially available. Multi-view and refocusing can be obtained during post-production from these cameras. However, the captured plenoptic images are of a large size and contain significant amount of a redundant information. An efficient compression of the above mentioned contents will, therefore, increase the availability of content access and provide a better quality experience under the same network capacity constraints. In this thesis, the compression of depth images and of plenoptic contents captured by focused plenoptic cameras are addressed. The depth images can be assumed to be piece-wise smooth. Starting from the properties of depth images, a novel depth image model based on edges and sparse samples is presented, which may also be utilized for depth image post-processing. Based on this model, a depth image coding scheme that explicitly encodes the locations of depth edges is proposed, and the coding scheme has a scalable structure. Furthermore, a compression scheme for block-based 3D-HEVC is also devised, in which diffusion is used for intra prediction. In addition to the proposed schemes, the thesis illustrates several evaluation methodologies, especially the subjective test of the stimulus-comparison method. This is suitable for evaluating the quality of two impaired images, as the objective metrics are inaccurate with respect to synthesized views. For the compression of plenoptic contents, displacement intra prediction with more than one hypothesis is applied and implemented in the HEVC for an efficient prediction. In addition, a scalable coding approach utilizing a sparse set and disparities is introduced for the coding of focused plenoptic images. The MPEG test sequences were used for the evaluation of the proposed depth image compression, and public available plenoptic image and video contents were applied to the assessment of the proposed plenoptic compression. For depth image coding, the results showed that virtual views synthesized from post-processed depth images by using the proposed model are better than those synthesized from original depth images. More importantly, the proposed coding schemes using such a model produced better synthesized views than the state of the art schemes. For the plenoptic contents, the proposed scheme achieved an efficient prediction and reduced the bit rate significantly while providing coding and rendering scalability. As a result, the outcome of the thesis can lead to improving quality of the 3DTV experience and facilitate the development of 3D applications in general.

  • 4.
    Li, Yun
    et al.
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Information and Communication systems.
    Olsson, Roger
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Information and Communication systems.
    Sjöström, Mårten
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Information and Communication systems.
    Compression of Unfocused Plenoptic Images using a Displacement Intra prediction2016In: 2016 IEEE International Conference on Multimedia and Expo Workshop, ICMEW 2016, IEEE Signal Processing Society, 2016, article id 7574673Conference paper (Refereed)
    Abstract [en]

    Plenoptic images are one type of light field contents produced by using a combination of a conventional camera and an additional optical component in the form of microlens arrays, which are positioned in front of the image sensor surface. This camera setup can capture a sub-sampling of the light field with high spatial fidelity over a small range, and with a more coarsely sampled angle range. The earliest applications that leverage on the plenoptic image content is image refocusing, non-linear distribution of out-of-focus areas, SNR vs. resolution trade-offs, and 3D-image creation. All functionalities are provided by using post-processing methods. In this work, we evaluate a compression method that we previously proposed for a different type of plenoptic image (focused or plenoptic camera 2.0 contents) than the unfocused or plenoptic camera 1.0 that is used in this Grand Challenge. The method is an extension of the state-of-the-art video compression standard HEVC where we have brought the capability of bi-directional inter-frame prediction into the spatial prediction. The method is evaluated according to the scheme set out by the Grand Challenge, and the results show a high compression efficiency compared with JPEG, i.e., up to 6 dB improvements for the tested images.

  • 5.
    Li, Yun
    et al.
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Information Technology and Media.
    Sjöström, Mårten
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Information Technology and Media.
    Jennehag, Ulf
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Information Technology and Media.
    Olsson, Roger
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Information Technology and Media.
    A Scalable Coding Approach for High Quality Depth Image Compression2012In: 3DTV-Conference, IEEE conference proceedings, 2012, p. Art. no. 6365469-Conference paper (Refereed)
    Abstract [en]

    The distortion by using traditional video encoders (e.g. H.264) on the depth discontinuity can introduce disturbing effects on the synthesized view. The proposed scheme aims at preserving the most significantdepth transition for a better view synthesis. Furthermore, it has a scalable structure. The scheme extracts edge contours from a depth image and represents them by chain code. The chain code and the sampleddepth values on each side of the edge contour are encoded by differential and arithmetic coding. The depthimage is reconstructed by diffusion of edge samples and uniform sub-samples from the low quality depthimage. At low bit rates, the proposed scheme outperforms HEVC intra at the edges in the synthesized views, which correspond to the significant discontinuities in the depth image. The overall quality is also better with the proposed scheme at low bit rates for contents with distinct depth transition. © 2012 IEEE.

  • 6.
    Li, Yun
    et al.
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Information and Communication systems.
    Sjöström, Mårten
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Information and Communication systems.
    Jennehag, Ulf
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Information and Communication systems.
    Olsson, Roger
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Information and Communication systems.
    Depth Image Post-processing Method by Diffusion2013In: Proceedings of SPIE-The International Society for Optical Engineering: 3D Image Processing (3DIP) and Applications, SPIE - International Society for Optical Engineering, 2013, p. Art. no. 865003-Conference paper (Refereed)
    Abstract [en]

    Multi-view three-dimensional television relies on view synthesis to reduce the number of views being transmitted.  Arbitrary views can be synthesized by utilizing corresponding depth images with textures. The depth images obtained from stereo pairs or range cameras may contain erroneous values, which entail artifacts in a rendered view. Post-processing of the data may then be utilized to enhance the depth image with the purpose to reach a better quality of synthesized views. We propose a Partial Differential Equation (PDE)-based interpolation method for a reconstruction of the smooth areas in depth images, while preserving significant edges. We modeled the depth image by adjusting thresholds for edge detection and a uniform sparse sampling factor followed by the second order PDE interpolation. The objective results show that a depth image processed by the proposed method can achieve a better quality of synthesized views than the original depth image. Visual inspection confirmed the results.

  • 7.
    Li, Yun
    et al.
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Information and Communication systems.
    Sjöström, Mårten
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Information and Communication systems.
    Jennehag, Ulf
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Information and Communication systems.
    Olsson, Roger
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Information and Communication systems.
    Depth Map Compression with Diffusion Modes in 3D-HEVC2013In: MMEDIA 2013 - 5th International Conferences on Advances in Multimedia / [ed] Philip Davies, David Newell, International Academy, Research and Industry Association (IARIA), 2013, p. 125-129Conference paper (Refereed)
    Abstract [en]

    For three-dimensional television, multiple views can be generated by using the Multi-view Video plus Depth (MVD) format. The depth maps of this format can be compressed efficiently by the 3D extension of High Efficiency Video Coding (3D-HEVC), which has explored the correlations between its two components, texture and associated depth map. In this paper, we introduce two modes for depth map coding into HEVC, where the modes use diffusion. The framework for inter-component prediction of Depth Modeling Modes (DMM) is utilized for the proposed modes. They detect edges from textures and then diffuse an entire block from known adjacent blocks by using Laplace equation constrained by the detected edges. The experimental results show that depth maps can be compressed more efficiently with the proposed diffusion modes, where the bit rate saving can reach 1.25 percentage of the total depth bit rate with a constant quality of synthesized views.

  • 8.
    Li, Yun
    et al.
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Information and Communication systems.
    Sjöström, Mårten
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Information and Communication systems.
    Jennehag, Ulf
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Information and Communication systems.
    Olsson, Roger
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Information and Communication systems.
    Tourancheau, Sylvain
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Information and Communication systems.
    Subjective Evaluation of an Edge-based Depth Image Compression Scheme2013In: Proceedings of SPIE - The International Society for Optical Engineering: Stereoscopic Displays and Applications XXIV, SPIE - International Society for Optical Engineering, 2013, p. Art. no. 86480D-Conference paper (Refereed)
    Abstract [en]

    Multi-view three-dimensional television requires many views, which may be synthesized from two-dimensional images with accompanying pixel-wise depth information. This depth image, which typically consists of smooth areas and sharp transitions at object borders, must be consistent with the acquired scene in order for synthesized views to be of good quality. We have previously proposed a depth image coding scheme that preserves significant edges and encodes smooth areas between these. An objective evaluation considering the structural similarity (SSIM) index for synthesized views demonstrated an advantage to the proposed scheme over the High Efficiency Video Coding (HEVC) intra mode in certain cases. However, there were some discrepancies between the outcomes from the objective evaluation and from our visual inspection, which motivated this study of subjective tests. The test was conducted according to ITU-R BT.500-13 recommendation with Stimulus-comparison methods. The results from the subjective test showed that the proposed scheme performs slightly better than HEVC with statistical significance at majority of the tested bit rates for the given contents.

  • 9.
    Li, Yun
    et al.
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Information and Communication systems.
    Sjöström, Mårten
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Information and Communication systems.
    Olsson, Roger
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Information and Communication systems.
    Coding of plenoptic images by using a sparse set and disparities2015In: Proceedings - IEEE International Conference on Multimedia and Expo, IEEE conference proceedings, 2015, p. -Art. no. 7177510Conference paper (Refereed)
    Abstract [en]

    A focused plenoptic camera not only captures the spatial information of a scene but also the angular information. The capturing results in a plenoptic image consisting of multiple microlens images and with a large resolution. In addition, the microlens images are similar to their neighbors. Therefore, an efficient compression method that utilizes this pattern of similarity can reduce coding bit rate and further facilitate the usage of the images. In this paper, we propose an approach for coding of focused plenoptic images by using a representation, which consists of a sparse plenoptic image set and disparities. Based on this representation, a reconstruction method by using interpolation and inpainting is devised to reconstruct the original plenoptic image. As a consequence, instead of coding the original image directly, we encode the sparse image set plus the disparity maps and use the reconstructed image as a prediction reference to encode the original image. The results show that the proposed scheme performs better than HEVC intra with more than 5 dB PSNR or over 60 percent bit rate reduction.

  • 10.
    Li, Yun
    et al.
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Information and Communication systems.
    Sjöström, Mårten
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Information and Communication systems.
    Olsson, Roger
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Information and Communication systems.
    Jennehag, Ulf
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Information and Communication systems.
    Coding of focused plenoptic contents by displacement intra prediction2016In: IEEE transactions on circuits and systems for video technology (Print), ISSN 1051-8215, E-ISSN 1558-2205, Vol. 26, no 7, p. 1308-1319, article id 7137669Article in journal (Refereed)
    Abstract [en]

    A light field is commonly described by a two-plane representation with four dimensions. Refocused three-dimensional contents can be rendered from light field images. A method for capturing these images is by using cameras with microlens arrays. A dense sampling of the light field results in large amounts of redundant data. Therefore, an efficient compression is vital for a practical use of these data. In this paper, we propose a displacement intra prediction scheme with a maximum of two hypotheses for the compression of plenoptic contents from focused plenoptic cameras. The proposed scheme is further implemented into HEVC. The work is aiming at coding plenoptic captured contents efficiently without knowing underlying camera geometries. In addition, the theoretical analysis of the displacement intra prediction for plenoptic images is explained; the relationship between the compressed captured images and their rendered quality is also analyzed. Evaluation results show that plenoptic contents can be efficiently compressed by the proposed scheme. Bit rate reduction up to 60 percent over HEVC is obtained for plenoptic images, and more than 30 percent is achieved for the tested video sequences.

  • 11.
    Li, Yun
    et al.
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Information and Communication systems.
    Sjöström, Mårten
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Information and Communication systems.
    Olsson, Roger
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Information and Communication systems.
    Jennehag, Ulf
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Information and Communication systems.
    Efficient Intra Prediction Scheme For Light Field Image Compression2014In: ICASSP, IEEE International Conference on Acoustics, Speech and Signal Processing - Proceedings, IEEE conference proceedings, 2014, p. Art. no. 6853654-Conference paper (Refereed)
    Abstract [en]

    Interactive photo-realistic graphics can be rendered by using light field datasets. One way of capturing the dataset is by using light field cameras with microlens arrays. The captured images contain repetitive patterns resulted from adjacent mi-crolenses. These images don't resemble the appearance of a natural scene. This dissimilarity leads to problems in light field image compression by using traditional image and video encoders, which are optimized for natural images and video sequences. In this paper, we introduce the full inter-prediction scheme in HEVC into intra-prediction for the compression of light field images. The proposed scheme is capable of performing both unidirectional and bi-directional prediction within an image. The evaluation results show that above 3 dB quality improvements or above 50 percent bit-rate saving can be achieved in terms of BD-PSNR for the proposed scheme compared to the original HEVC intra-prediction for light field images.

  • 12.
    Li, Yun
    et al.
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Information and Communication systems.
    Sjöström, Mårten
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Information and Communication systems.
    Olsson, Roger
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Information and Communication systems.
    Jennehag, Ulf
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Information and Communication systems.
    Scalable coding of plenoptic images by using a sparse set and disparities2016In: IEEE Transactions on Image Processing, ISSN 1057-7149, E-ISSN 1941-0042, Vol. 25, no 1, p. 80-91, article id 7321029Article in journal (Refereed)
    Abstract [en]

    One of the light field capturing techniques is the focused plenoptic capturing. By placing a microlens array in front of the photosensor, the focused plenoptic cameras capture both spatial and angular information of a scene in each microlens image and across microlens images. The capturing results in significant amount of redundant information, and the captured image is usually of a large resolution. A coding scheme that removes the redundancy before coding can be of advantage for efficient compression, transmission and rendering. In this paper, we propose a lossy coding scheme to efficiently represent plenoptic images. The format contains a sparse image set and its associated disparities. The reconstruction is performed by disparity-based interpolation and inpainting, and the reconstructed image is later employed as a prediction reference for the coding of the full plenoptic image. As an outcome of the representation, the proposed scheme inherits a scalable structure with three layers.The results show that plenoptic images are compressed efficiently with over 60 percent bit rate reduction compared to HEVC intra, and with over 20 percent compared to HEVC block copying mode.

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