Advances in imaging and display engineering have given rise to new and improved image and video applications that aim to maximize visual quality under given resource constraints (e.g., power, bandwidth). Because the human visual system is an imperfect sensor, the images/videos can be represented in a mathematically lossy fashion but with enough fidelity that the losses are visually imperceptible—commonly termed “visually lossless.” Although a great deal of research has focused on gaining a better understanding of the limits of human vision when viewing natural images/video, a universally or even largely accepted definition of visually lossless remains elusive. Differences in testing methodologies, research objectives, and target applications have led to multiple ad-hoc definitions that are often difficult to compare to or otherwise employ in other settings. We present a compendium of technical experiments relating to both vision science and visual quality testing that together explore the research and business perspectives of visually lossless image quality, as well as review recent scientific advances. Together, the studies presented in this paper suggest that a single definition of visually lossless quality might not be appropriate; rather, a better goal would be to establish varying levels of visually lossless quality that can be quantified in terms of the testing paradigm.
We take a look at crowdsourcing for subjective image quality evaluation using real image stimuli with nonsimulated distortions. Our aim is to scale the task of subjectively rating images while ensuring maximal data validity and accuracy. While previous work has begun to explore crowdsourcing for quality assessment, it has either used images that are not representative of popular consumer scenarios or used crowdsourcing to collect data without comparison to experiments in a controlled environment. Here, we address the challenges imposed by the highly variable online environment, using stimuli that are subtle and more complex than has traditionally been used in quality assessment experiments. In a series of experiments, we vary different design parameters and demonstrate how they impact the subjective responses obtained. Of the parameters examined are stimulus display mode, study length, stimulus habituation, and content homogeneity/heterogeneity. Our method was tested on a database that was rated in a laboratory test previously. Once our design parameters were chosen, we rated a database of consumer photographs and are making this data available to the research community.
Purpose: The study evaluated the accuracy of depth perception afforded by static and dynamic stereoscopic three-dimensional (S3D) images with proportional (scaled to disparity) and constant size cues. Methods: Sixty adult participants, 18 to 40 years (mean, 24.8 years), with good binocular vision participated in the study. For static S3D trials, participants were asked to indicate the depth of stationary S3D images rendered with 36, 48 and 60 pixels of crossed disparity, and with either proportional or a constant size. For dynamic S3D trials, participants were asked to indicate the time when S3D images, moving at 27, 32 and 40 pixels/sec, matched the depth of a reference image which was presented with 36, 48 and 60 pixels of crossed image disparity. Results: Results show that viewers perceived S3D images as being closer than would be predicted by the magnitude of image disparity, and correspondingly they overestimated the depth in moving S3D images. The resultant depth perception and estimate of motion speed were more accurate for conditions with proportional and larger image size, slower motion-in-depth and larger image disparity. Conclusion: These findings possibly explain why effects such as looming are over stimulating in S3D viewing. To increase the accuracy of depth perception, S3D content should match image size to its disparity level, utilize larger depth separation (without inducing excessive discomfort) and render slower motion in depth.
KEYWORDS: Eye, Target detection, 3D image processing, Visualization, Diamond, 3D image enhancement, Stereoscopic displays, Signal detection, 3D acquisition, Eyeglasses
Purpose: Stereoscopic three-dimensional (S3D) viewing enhances depth perception of two-dimensional (2D) images.
The present study measured viewer’s ability to discern depth differences and depth change afforded by image disparities presented on an S3D display. Methods: Sixty adults (age 24.8 +/- 3.4 years, 28% male) with binocular acuity better than 20/25 and stereoacuity better than 60 arcsec viewed test images presented on a 55” 3D TV (1920 x 1080 pixels) at 2.4m distance. In each trial, three of the four circles in the test image were with the same crossed baseline disparity of 12, 24, 36, 48, and 60 pixels, and the other (target) circle with added crossed disparity (delta disparity) of 2, 4, and 6 pixels. A subsequent change in delta disparity of the target circle (+/-2 pixels) was presented. Participant's response time and accuracy for identifying the target circle and its direction of depth change, as well as their vergence eye position, was recorded. Results: Larger baseline disparity resulted in lower accuracy and longer response times in identifying the target circle. The change of delta disparity was more accurately discerned when the delta disparity was larger and the change resulted in increased crossed disparity. Direction of vergence change and the final vergence error reflected an averaging of screen and image depths. Conclusion: S3D-induced depth difference and motion in depth is more easily discerned with smaller crossed disparity for background objects (< 43.3 arcmin or 48 pixels) and larger separation between image disparities (>3.6 arcmin or 4 pixels).
KEYWORDS: Video, Data modeling, Quality measurement, Standards development, Statistical analysis, Statistical modeling, Performance modeling, Analog electronics, Video processing, Semantic video
Subjective assessment methods have been used reliably for many years to evaluate video quality. They continue to provide the most reliable assessments compared to objective methods. Some issues that arise with subjective assessment include the cost of conducting the evaluations and the fact that these methods cannot easily be used to monitor video quality in real time. Furthermore, traditional, analog objective methods, while still necessary, are not sufficient to measure the quality of digitally compressed video systems. Thus, there is a need to develop new objective methods utilizing the characteristics of the human visual system. While several new objective methods have been developed, there is to date no internationally standardized method. The Video Quality Experts Group (VQEG) was formed in October 1997 to address video quality issues. The group is composed of experts from various backgrounds and affiliations, including participants from several internationally recognized organizations working in the field of video quality assessment. The majority of participants are active in the International Telecommunications Union (ITU) and VQEG combines the expertise and resources found in several ITU Study Groups to work towards a common goal. The first task undertaken by VQEG was to provide a validation of objective video quality measurement methods leading to Recommendations in both the Telecommunications (ITU-T) and Radiocommunication (ITU-R) sectors of the ITU. To this end, VQEG designed and executed a test program to compare subjective video quality evaluations to the predictions of a number of proposed objective measurement methods for video quality in the bit rate range of 768 kb/s to 50 Mb/s. The results of this test show that there is no objective measurement system that is currently able to replace subjective testing. Depending on the metric used for evaluation, the performance of eight or nine models was found to be statistically equivalent, leading to the conclusion that no single model outperforms the others in all cases. The greatest achievement of this first validation effort is the unique data set assembled to help future development of objective models.
Ann Rohaly, Philip Corriveau, John Libert, Arthur Webster, Vittorio Baroncini, John Beerends, Jean-Louis Blin, Laura Contin, Takahiro Hamada, David Harrison, Andries Hekstra, Jeffrey Lubin, Yukihiro Nishida, Ricardo Nishihara, John Pearson, Antonio Pessoa, Neil Pickford, Alexander Schertz, Massimo Visca, Andrew Watson, Stefan Winkler
The Video Quality Experts Group (VQEG) was formed in October 1997 to address video quality issues. The group is composed of experts from various backgrounds and affiliations, including participants from several internationally recognized organizations working int he field of video quality assessment. The first task undertaken by VQEG was to provide a validation of objective video quality measurement methods leading to recommendations in both the telecommunications and radiocommunication sectors of the International Telecommunications Union. To this end, VQEG designed and executed a test program to compare subjective video quality evaluations to the predictions of a number of proposed objective measurement methods for video quality in the bit rate range of 768 kb/s to 50 Mb/s. The results of this test show that there is no objective measurement system that is currently able to replace subjective testing. Depending on the metric used for evaluation, the performance of eight or nine models was found to be statistically equivalent, leading to the conclusion that no single model outperforms the others in all cases. The greatest achievement of this first validation effort is the unique data set assembled to help future development of objective models.
In comparison to conventional displays, 3D stereoscopic displays convey additional information about the 3D structure of a scene by providing information that can be used to extract depth. In the present study we evaluated the psychovisual impact of stereoscopic images on viewers. Thirty-three non-expert viewers rated sensation of depth, perceived sharpness, subjective image quality, and relative preference for stereoscopic over non-stereoscopic images. Rating methods were based on procedures described in ITU- Rec. 500. Viewers also rated sequences in which the left- and right-eye images were processed independently, using a generic MPEG-2 codec, at bit-rates of 6, 3, and 1 Mbits/s. The main finding was that viewers preferred the stereoscopic version over the non-stereoscopic version of the sequences, provided that the sequence did not contain noticeable stereo artifacts, such as exaggerated disparity. Perceived depth was rated greater for stereoscopic than for non-stereoscopic sequences, and perceived sharpness of stereoscopic sequences was rated the same or lower compared to non-stereoscopic sequences. Subjective image quality was influenced primarily by apparent sharpness of the video sequences, and less so by perceived depth.
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