Tim Bruylants

Objective and subjective evaluation of light field image compression algorithms

This paper reports results of subjective and objective quality assessments of responses to a grand challenge on light field image compression. The goal of the challenge was to collect and evaluate new compression algorithms for light field images. In total seven proposals were received, out of which five were accepted for further evaluations. For objective evaluations, conventional metrics were used, whereas the double stimulus continuous quality scale method was selected to perform subjective assessments. Results show competitive performance among submitted proposals. However, in low bitrates, one proposal outperforms the others.

JPEG 2000-based compression of fringe patterns for digital holographic microscopy

Abstract. With the advent of modern computing and imaging technologies, digital holography is becoming widespread in various scientific disciplines such as microscopy, interferometry, surface shape measurements, vibration analysis, data encoding, and certification. Therefore, designing an efficient data representation technology is of particular importance. Off-axis holograms have very different signal properties with respect to regular imagery, because they represent a recorded interference pattern with its energy biased toward the high-frequency bands. This causes traditional images’ coders, which assume an underlying 1/f2 power spectral density distribution, to perform suboptimally for this type of imagery. We propose a JPEG 2000-based codec framework that provides a generic architecture suitable for the compression of many types of off-axis holograms. This framework has a JPEG 2000 codec at its core, extended with (1) fully arbitrary wavelet decomposition styles and (2) directional wavelet transforms. Using this codec, we report significant improvements in coding performance for off-axis holography relative to the conventional JPEG 2000 standard, with Bjøntegaard delta-peak signal-to-noise ratio improvements ranging from 1.3 to 11.6 dB for lossy compression in the 0.125 to 2.00 bpp range and bit-rate reductions of up to 1.6 bpp for lossless compression.

Compression of medical volumetric datasets: physical and psychovisual performance comparison of the emerging JP3D standard and JPEG2000

The size of medical data has increased significantly over the last few years. This poses severe problems for the rapid transmission of medical data across the hospital network resulting into longer access times of the images. Also longterm storage of data becomes more and more a problem. In an attempt to overcome the increasing data size often lossless or lossy compression algorithms are being used. This paper compares the existing JPEG2000 compression algorithm and the new emerging JP3D standard for compression of volumetric datasets. The main benefit of JP3D is that this algorithm truly is a 3D compression algorithm that exploits correlation not only within but also in between slices of a dataset. We evaluate both lossless and lossy modes of these algorithms. As a first step we perform an objective evaluation. Using RMSE and PSNR metrics we determine which compression algorithm performs best and this for multiple compression ratios and for several clinically relevant medical datasets. It is well known that RMSE and PSNR often do not correlate well with subjectively perceived image quality. Therefore we also perform a psycho visual analysis by means of a numerical observer. With this observer model we analyze how compression artifacts actually are perceived by a human observer. Results show superior performance of the new JP3D algorithm compared to the existing JPEG2000 algorithm.

Wavelet coding of off-axis holographic images

Significant research efforts have been invested in attempting to reliably capture and visualize holograms since their inception in 1962. However, less attention has been given to the efficient digital representation of the recorded holograms, which differ considerably from digitally recorded photographs. This paper examines the properties of recorded off-axis holograms and attempts to find a suitable sparse representation for holographic data. Results show significantly improved Bjøntegaard delta PSNR of over 4.5 dB on average within a bit-rate range of 0.125 to 2 bpp when combining the direction-adaptive discrete wavelet transform with non-standard decomposition schemes for off-axis holographic recordings; up to 7.5% reduction of file size has been achieved in the lossless case.

The JPEG XT Suite of Standards: Status and Future Plans

The JPEG standard has known an enormous market adoption. Daily, billions of pictures are created, stored and exchanged in this format. The JPEG committee acknowledges this success and spends continued efforts in maintaining and expanding the standard specifications. JPEG XT is a standardization effort targeting the extension of the JPEG features by enabling support for high dynamic range imaging, lossless and near-lossless coding, and alpha channel coding, while also guaranteeing backward and forward compatibility with the JPEG legacy format. This paper gives an overview of the current status of the JPEG XT standards suite. It discusses the JPEG legacy specification, and details how higher dynamic range support is facilitated both for integer and floating-point color representations. The paper shows how JPEG XTs support for lossless and near-lossless coding of low and high dynamic range images is achieved in combination with backward compatibility to JPEG legacy. In addition, the extensible boxed-based JPEG XT file format on which all following and future extensions of JPEG will be based is introduced. This paper also details how the lossy and lossless representations of alpha channels are supported to allow coding transparency information and arbitrarily shaped images. Finally, we conclude by giving prospects on upcoming JPEG standardization initiative JPEG Privacy & Security, and a number of other possible extensions in JPEG XT.

Microscopic off-axis holographic image compression with JPEG 2000

With the advent of modern computing and imaging technologies, the use of digital holography became practical in many applications such as microscopy, interferometry, non-destructive testing, data encoding, and certification. In this respect the need for an efficient representation technology becomes imminent. However, microscopic holographic off-axis recordings have characteristics that differ significantly from that of regular natural imagery, because they represent a recorded interference pattern that mainly manifests itself in the high-frequency bands. Since regular image compression schemes are typically based on a Laplace frequency distribution, they are unable to optimally represent such holographic data. However, unlike most image codecs, the JPEG 2000 standard can be modified to efficiently cope with images containing such alternative frequency distributions by applying the arbitrary wavelet decomposition of Part 2. As such, employing packet decompositions already significantly improves the compression performance for off-axis holographic images over that of regular image compression schemes. Moreover, extending JPEG 2000 with directional wavelet transforms shows even higher compression efficiency improvements. Such an extension to the standard would only require signaling the applied directions, and would not impact any other existing functionality. In this paper, we show that wavelet packet decomposition combined with directional wavelet transforms provides efficient lossy-to-lossless compression of microscopic off-axis holographic imagery.

An optimized 3D context model for JPEG2000 Part 10

The JPEG2000 standard is currently widely adopted in medical and volumetric data compression. In this respect, a 3D extension (JPEG2000 Part 10 - JP3D) is currently being standardized. However, no suitable 3D context model is yet available within the standard, such that the context-based arithmetic entropy coder of JP3D still uses the 2D context model of JPEG2000 Part 1. In this paper, we propose a context design algorithm that, based on a training set, generates an optimized 3D context model, while avoiding an exhaustive search and at the same time keeping the space and time complexities well within the limits of today hardware. The algorithm comes as a solution for the situations in which the number of allowable initial contexts is very large. In this sense, the three-dimensional 3x3x3 context neighborhood investigated in this paper is a good example of an instantiation that would have otherwise been computationally unfeasible. Furthermore, we have designed a new 3D context model for JP3D. We show that the JP3D codec equipped with this model consistently outperforms its 2D context model counterpart, for an extended test dataset. In this respect, we report a gain in lossless compression performance of up to 10%. Moreover, for a large range of bitrates, we always obtain gains in PSNR, sometimes even over 3dB.

Subjective quality assessment of numerically reconstructed compressed holograms

Recently several papers reported efficient techniques to compress digital holograms. Typically, the rate-distortion performance of these solutions was evaluated by means of objective metrics such as Peak Signal-to-Noise Ratio (PSNR) or the Structural Similarity Index Measure (SSIM) by either evaluating the quality of the decoded hologram or the reconstructed compressed hologram. Seen the specific nature of holograms, it is relevant to question to what extend these metrics provide information on the effective visual quality of the reconstructed hologram. Given that today no holographic display technology is available that would allow for a proper subjective evaluation experiment, we propose in this paper a methodology that is based on assessing the quality of a reconstructed compressed hologram on a regular 2D display. In parallel, we also evaluate several coding engines, namely JPEG configured with the default perceptual quantization tables and with uniform quantization tables, JPEG 2000, JPEG 2000 extended with arbitrary packet decompositions and direction-adaptive filters and H.265/HEVC configured in intra-frame mode. The experimental results indicate that the perceived visual quality and the objective measures are well correlated. Moreover, also the superiority of the HEVC and the extended JPEG 2000 coding engines was confirmed, particularly at lower bitrates.

On the use of directional transforms for still image coding

Significant research has been performed on the use of directional transforms for the compression of still imagery, in particular on the application of block-based and segmentation driven directional adaptive discrete wavelet transforms. However, all of the proposed methodologies suffer from the extra side-information that needs to be encoded. This encoding overhead and added complexity is unfortunately not negligible. This paper describes various considerations and trade-offs that were made during the search towards a practical solution for using directional adaptive transforms in still image coding. We propose two codec instantiations respectively based upon quadtree-coding (QT-L) and JPEG 2000s EBCOT engine and discuss various experimental results.

Perceptual quality assessment of JPEG, JPEG 2000, and JPEG XR

In this paper, we examine the rate-distortion performance in terms of perceptual quality of JPEG XR (ISO/IEC 29199-2 | ITU-T Rec. T.832)1 and earlier standardized image compression algorithms such as JPEG (ISO/IEC 10918-1 | ITU-T Rec. T.81)2 and JPEG 2000 (ISO/IEC 15444-1 | ITU-T Rec. T.800)3. Unfortunately, objective visual quality metrics (like MSE, PSNR, VQM, SSIM, etc.) do not always correlate well with the actual perceived image quality. In some specific cases, it is even possible that certain visible coding artifacts remain undetectable by these objective visual quality tests. As such, we conducted a series of subjective visual quality assessment tests to measure the visual performance of JPEG XR, JPEG 2000 and JPEG. This paper describes the design of the subjective visual quality assessment experiments, addressing the encountered difficulties and potential pitfalls. Our results indicate that for high bit-rates (i.e. more than 1 bpp) all three codecs more or less have an equal overall performance. However, as expected, at lower bit-rates JPEG performs significantly weaker for every tested image than JPEG 2000 and JPEG XR. On the other hand, both JPEG 2000 and JPEG XR appear to be very competitive at these low bit-rate ranges. Only for specific image content types (e.g. smooth gradient surfaces), JPEG XR appears to have some difficulties. Nevertheless, discarding the fact that JPEG 2000 offers more functionality features than JPEG XR, the latter performed very good for most images and almost in par with JPEG 2000. As a conclusion, the results of the subjective visual quality assessment tests show that JPEG XR successfully passed our verification experiments for low dynamic range imagery.