| 1. |
- Sjöström, Mårten, et al.
(författare)
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Identifiability Analysis of Prandtl-Ishilinskii Hysteresis Model with Saturation
- 2008
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Ingår i: Physica. B, Condensed matter. - : Elsevier BV. - 0921-4526 .- 1873-2135. ; 403:2-3, s. 266-270
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Tidskriftsartikel (refereegranskat)abstract
- A new class of Preisach operators based on play operators with an inverse in a closed form and allowing for saturation has recently been proposed. Its existence criteria and identification procedure were considered in earlier articles. The present paper analyses the identification procedure with respect to the sensitivity to underlying functions (i.e. intrinsic behaviour of the hysteretic system), to spline approximation, and to the least square error estimation procedure. The analysis shows that model errors are significantly influenced by large derivatives of the underlying functions. Spline approximations have generally little effect on model errors. In particular, an upper bound of for the relative parameter error due to measurement discrepancies has been derived for the least square error problem. The bound increases, the closer to saturation data are measured.
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| 2. |
- Sjöström, Mårten, et al.
(författare)
-
Identifiability Analysis of Prandtl-Ishilinskii Hysteresis Model with Saturation - Derivations
- 2007
-
Rapport (övrigt vetenskapligt/konstnärligt)abstract
- A new class of Preisach operators based on play operators with an inverse in a closed form and allowing for saturation has recently been proposed. Its existence criteria and identification procedure were considered in earlier articles. The present paper analyses the identification procedure with respect to the sensitivity to underlying functions (i.e. intrinsic behaviour of the hysteretic system), to spline approximation, and to the least square error estimation procedure. The analysis show that model errors are significantly influenced by large derivatives of the underlying functions. Spline approximations have generally little effect on model errors. In particular, an upper bound of for the relative parameter error due to measurement discrepancies has been derived for the least square error problem. The bound increases, the closer to saturation data are measured.
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| 3. |
- Ahmad, Waqas, et al.
(författare)
-
Compression scheme for sparsely sampled light field data based on pseudo multi-view sequences
- 2018
-
Ingår i: OPTICS, PHOTONICS, AND DIGITAL TECHNOLOGIES FOR IMAGING APPLICATIONS V Proceedings of SPIE - The International Society for Optical Engineering. - : SPIE - International Society for Optical Engineering.
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Konferensbidrag (refereegranskat)abstract
- With the advent of light field acquisition technologies, the captured information of the scene is enriched by having both angular and spatial information. The captured information provides additional capabilities in the post processing stage, e.g. refocusing, 3D scene reconstruction, synthetic aperture etc. Light field capturing devices are classified in two categories. In the first category, a single plenoptic camera is used to capture a densely sampled light field, and in second category, multiple traditional cameras are used to capture a sparsely sampled light field. In both cases, the size of captured data increases with the additional angular information. The recent call for proposal related to compression of light field data by JPEG, also called “JPEG Pleno”, reflects the need of a new and efficient light field compression solution. In this paper, we propose a compression solution for sparsely sampled light field data. In a multi-camera system, each view depicts the scene from a single perspective. We propose to interpret each single view as a frame of pseudo video sequence. In this way, complete MxN views of multi-camera system are treated as M pseudo video sequences, where each pseudo video sequence contains N frames. The central pseudo video sequence is taken as base View and first frame in all the pseudo video sequences is taken as base Picture Order Count (POC). The frame contained in base view and base POC is labeled as base frame. The remaining frames are divided into three predictor levels. Frames placed in each successive level can take prediction from previously encoded frames. However, the frames assigned with last prediction level are not used for prediction of other frames. Moreover, the rate-allocation for each frame is performed by taking into account its predictor level, its frame distance and view wise decoding distance relative to the base frame. The multi-view extension of high efficiency video coding (MV-HEVC) is used to compress the pseudo multi-view sequences. The MV-HEVC compression standard enables the frames to take prediction in both direction (horizontal and vertical d), and MV-HEVC parameters are used to implement the proposed 2D prediction and rate allocation scheme. A subset of four light field images from Stanford dataset are compressed, using the proposed compression scheme on four bitrates in order to cover the low to high bit-rates scenarios. The comparison is made with state-of-art reference encoder HEVC and its real-time implementation X265. The 17x17 grid is converted into a single pseudo sequence of 289 frames by following the order explained in JPEG Pleno call for proposal and given as input to the both reference schemes. The rate distortion analysis shows that the proposed compression scheme outperforms both reference schemes in all tested bitrate scenarios for all test images. The average BD-PSNR gain is 1.36 dB over HEVC and 2.15 dB over X265.
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| 4. |
- Ahmad, Waqas, et al.
(författare)
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Computationally Efficient Light Field Image Compression Using a Multiview HEVC Framework
- 2019
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Ingår i: IEEE Access. - 2169-3536. ; 7, s. 143002-143014
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Tidskriftsartikel (refereegranskat)abstract
- The acquisition of the spatial and angular information of a scene using light eld (LF) technologies supplement a wide range of post-processing applications, such as scene reconstruction, refocusing, virtual view synthesis, and so forth. The additional angular information possessed by LF data increases the size of the overall data captured while offering the same spatial resolution. The main contributor to the size of captured data (i.e., angular information) contains a high correlation that is exploited by state-of-the-art video encoders by treating the LF as a pseudo video sequence (PVS). The interpretation of LF as a single PVS restricts the encoding scheme to only utilize a single-dimensional angular correlation present in the LF data. In this paper, we present an LF compression framework that efciently exploits the spatial and angular correlation using a multiview extension of high-efciency video coding (MV-HEVC). The input LF views are converted into multiple PVSs and are organized hierarchically. The rate-allocation scheme takes into account the assigned organization of frames and distributes quality/bits among them accordingly. Subsequently, the reference picture selection scheme prioritizes the reference frames based on the assigned quality. The proposed compression scheme is evaluated by following the common test conditions set by JPEG Pleno. The proposed scheme performs 0.75 dB better compared to state-of-the-art compression schemes and 2.5 dB better compared to the x265-based JPEG Pleno anchor scheme. Moreover, an optimized motionsearch scheme is proposed in the framework that reduces the computational complexity (in terms of the sum of absolute difference [SAD] computations) of motion estimation by up to 87% with a negligible loss in visual quality (approximately 0.05 dB).
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| 5. |
- Ahmad, Waqas
(författare)
-
High Efficiency Light Field Image Compression : Hierarchical Bit Allocation and Shearlet-based View Interpolation
- 2021
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Over the years, the pursuit of capturing the precise visual information of a scenehas resulted in various enhancements in digital camera technology, such as highdynamic range, extended depth of field, and high resolution. However, traditionaldigital cameras only capture the spatial information of the scene and cannot pro-vide an immersive presentation of it. Light field (LF) capturing is a new-generationimaging technology that records the spatial and angular information of the scene. Inrecent years, LF imaging has become increasingly popular among the industry andresearch community mainly for two reasons: (1) the advancements made in optical and computational technology have facilitated the process of capturing and processing LF information and (2) LF data have the potential to offer various post-processing applications, such as refocusing at different depth planes, synthetic aperture, 3Dscene reconstruction, and novel view generation. Generally, LF-capturing devicesacquire large amounts of data, which poses a challenge for storage and transmissionresources. Off-the-shelf image and video compression schemes, built on assump-tions drawn from natural images and video, tend to exploit spatial and temporalcorrelations. However, 4D LF data inherit different properties, and hence there is aneed to advance the current compression methods to efficiently address the correla-tion present in LF data.In this thesis, compression of LF data captured using a plenoptic camera andmulti-camera system (MCS) is considered. Perspective views of a scene capturedfrom different positions are interpreted as a frame of multiple pseudo-video se-quences and given as an input to a multi-view extension of high-efficiency videocoding (MV-HEVC). A 2D prediction and hierarchical coding scheme is proposedin MV-HEVC to improve the compression efficiency of LF data. To further increasethe compression efficiency of views captured using an MCS, an LF reconstructionscheme based on shearlet transform is introduced in LF compression. A sparse set of views is coded using MV-HEVC and later used to predict the remaining views by applying shearlet transform. The prediction error is also coded to further increase the compression efficiency. Publicly available LF datasets are used to benchmark the proposed compression schemes. The anchor scheme specified in the JPEG Plenocommon test conditions is used to evaluate the performance of the proposed scheme. Objective evaluations show that the proposed scheme outperforms state-of-the-art schemes in the compression of LF data captured using a plenoptic camera and an MCS. Moreover, the introduction of shearlet transform in LF compression further improves the compression efficiency at low bitrates, at which the human vision sys-tem is sensitive to the perceived quality.The work presented in this thesis has been published in four peer-reviewed con-ference proceedings and two scientific journals. The proposed compression solu-tions outlined in this thesis significantly improve the rate-distortion efficiency forLF content, which reduces the transmission and storage resources. The MV-HEVC-based LF coding scheme is made publicly available, which can help researchers totest novel compression tools and it can serve as an anchor scheme for future researchstudies. The shearlet-transform-based LF compression scheme presents a compre-hensive framework for testing LF reconstruction methods in the context of LF com-pression.
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| 6. |
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| 7. |
- Ahmad, Waqas, et al.
(författare)
-
Interpreting Plenoptic Images as Multi-View Sequences for Improved Compression
- 2017
-
Ingår i: ICIP 2017. - : IEEE. - 9781509021758 ; , s. 4557-4561
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Konferensbidrag (refereegranskat)abstract
- Over the last decade, advancements in optical devices have made it possible for new novel image acquisition technologies to appear. Angular information for each spatial point is acquired in addition to the spatial information of the scene that enables 3D scene reconstruction and various post-processing effects. Current generation of plenoptic cameras spatially multiplex the angular information, which implies an increase in image resolution to retain the level of spatial information gathered by conventional cameras. In this work, the resulting plenoptic image is interpreted as a multi-view sequence that is efficiently compressed using the multi-view extension of high efficiency video coding (MV-HEVC). A novel two dimensional weighted prediction and rate allocation scheme is proposed to adopt the HEVC compression structure to the plenoptic image properties. The proposed coding approach is a response to ICIP 2017 Grand Challenge: Light field Image Coding. The proposed scheme outperforms all ICME contestants, and improves on the JPEG-anchor of ICME with an average PSNR gain of 7.5 dB and the HEVC-anchor of ICIP 2017 Grand Challenge with an average PSNR gain of 2.4 dB.
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| 8. |
- Ahmad, Waqas, et al.
(författare)
-
Matching Light Field Datasets From Plenoptic Cameras 1.0 And 2.0
- 2018
-
Ingår i: Proceedings of the 2018 3DTV Conference. - 9781538661253
-
Konferensbidrag (refereegranskat)abstract
- The capturing of angular and spatial information of the scene using single camera is made possible by new emerging technology referred to as plenoptic camera. Both angular and spatial information, enable various post-processing applications, e.g. refocusing, synthetic aperture, super-resolution, and 3D scene reconstruction. In the past, multiple traditional cameras were used to capture the angular and spatial information of the scene. However, recently with the advancement in optical technology, plenoptic cameras have been introduced to capture the scene information. In a plenoptic camera, a lenslet array is placed between the main lens and the image sensor that allows multiplexing of the spatial and angular information onto a single image, also referred to as plenoptic image. The placement of the lenslet array relative to the main lens and the image sensor, results in two different optical design sof a plenoptic camera, also referred to as plenoptic 1.0 and plenoptic 2.0. In this work, we present a novel dataset captured with plenoptic 1.0 (Lytro Illum) and plenoptic 2.0(Raytrix R29) cameras for the same scenes under the same conditions. The dataset provides the benchmark contents for various research and development activities for plenoptic images.
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| 9. |
- Ahmad, Waqas, et al.
(författare)
-
Shearlet Transform-Based Light Field Compression under Low Bitrates
- 2020
-
Ingår i: IEEE Transactions on Image Processing. - : IEEE. - 1057-7149 .- 1941-0042. ; 29, s. 4269-4280
-
Tidskriftsartikel (refereegranskat)abstract
- Light field (LF) acquisition devices capture spatial and angular information of a scene. In contrast with traditional cameras, the additional angular information enables novel post-processing applications, such as 3D scene reconstruction, the ability to refocus at different depth planes, and synthetic aperture. In this paper, we present a novel compression scheme for LF data captured using multiple traditional cameras. The input LF views were divided into two groups: key views and decimated views. The key views were compressed using the multi-view extension of high-efficiency video coding (MV-HEVC) scheme, and decimated views were predicted using the shearlet-transform-based prediction (STBP) scheme. Additionally, the residual information of predicted views was also encoded and sent along with the coded stream of key views. The proposed scheme was evaluated over a benchmark multi-camera based LF datasets, demonstrating that incorporating the residual information into the compression scheme increased the overall peak signal to noise ratio (PSNR) by 2 dB. The proposed compression scheme performed significantly better at low bit rates compared to anchor schemes, which have a better level of compression efficiency in high bit-rate scenarios. The sensitivity of the human vision system towards compression artifacts, specifically at low bit rates, favors the proposed compression scheme over anchor schemes. The proposed compression scheme performed significantly better at low bit rates compared to anchor schemes, which have a better level of compression efficiency in high bit-rate scenarios. The sensitivity of the human vision system towards compression artifacts, specifically at low bit rates, favors the proposed compression scheme over anchor schemes. The proposed compression scheme performed significantly better at low bit rates compared to anchor schemes, which have a better level of compression efficiency in high bit-rate scenarios. The sensitivity of the human vision system towards compression artifacts, specifically at low bit rates, favors the proposed compression scheme over anchor schemes.
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| 10. |
- Ahmad, Waqas, et al.
(författare)
-
Shearlet Transform Based Prediction Scheme for Light Field Compression
- 2018
-
Konferensbidrag (refereegranskat)abstract
- Light field acquisition technologies capture angular and spatial information ofthe scene. The spatial and angular information enables various post processingapplications, e.g. 3D scene reconstruction, refocusing, synthetic aperture etc at theexpense of an increased data size. In this paper, we present a novel prediction tool forcompression of light field data acquired with multiple camera system. The captured lightfield (LF) can be described using two plane parametrization as, L(u, v, s, t), where (u, v)represents each view image plane coordinates and (s, t) represents the coordinates of thecapturing plane. In the proposed scheme, the captured LF is uniformly decimated by afactor d in both directions (in s and t coordinates), resulting in a sparse set of views alsoreferred to as key views. The key views are converted into a pseudo video sequence andcompressed using high efficiency video coding (HEVC). The shearlet transform basedreconstruction approach, presented in [1], is used at the decoder side to predict thedecimated views with the help of the key views.Four LF images (Truck, Bunny from Stanford dataset, Set2 and Set9 from High DensityCamera Array dataset) are used in the experiments. Input LF views are converted into apseudo video sequence and compressed with HEVC to serve as anchor. Rate distortionanalysis shows the average PSNR gain of 0.98 dB over the anchor scheme. Moreover, inlow bit-rates, the compression efficiency of the proposed scheme is higher compared tothe anchor and on the other hand the performance of the anchor is better in high bit-rates.Different compression response of the proposed and anchor scheme is a consequence oftheir utilization of input information. In the high bit-rate scenario, high quality residualinformation enables the anchor to achieve efficient compression. On the contrary, theshearlet transform relies on key views to predict the decimated views withoutincorporating residual information. Hence, it has inherit reconstruction error. In the lowbit-rate scenario, the bit budget of the proposed compression scheme allows the encoderto achieve high quality for the key views. The HEVC anchor scheme distributes the samebit budget among all the input LF views that results in degradation of the overall visualquality. The sensitivity of human vision system toward compression artifacts in low-bitratecases favours the proposed compression scheme over the anchor scheme.
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