701 research outputs found
Object size, spatial-frequency content and retinal contrast
No full textRecent interest in HDR scene capture and display has stimulated measurements of the usable range of contrast information for human vision. These experiments have led to a model that calculates the retinal contrast image. A fraction of the light from each scene pixel is scattered to all retinal pixels. The amount of scattered light decreases with distance from the other pixels. By summing the light falling on each retinal pixel from all the scene pixels we can calculate the retinal image contrast. As objects, such as text letters, get smaller, their retinal contrast gets lower, even though the scene contrast is constant. This paper studies the Landolt C data, a commonly used test targets for measuring visual acuity, using three frameworks. First, it compares the visual acuity measurements with the receptor mosaic dimension. Second, discusses the Campbell and Robson's experiments and the limits of the Contrast Sensitivity Function (CSF). Third, the paper reports the calculated retinal stimulus after intraocular scatter of both Landolt C and Campbell and Robson's stimuli. These three different frameworks are useful in understanding limits of human vision. Each approach gives only one piece of the puzzle. Retinal contrast, CSF, and retinal cone spacing all influence our understanding of human vision limits. We have analyzed Landolt C and CSF using retinal contrast. Glare effect on Landolt C shows that retinal images are significantly different from target images. Veiling glare of the sine-wave stimuli used by Campbell and Robson to measure CSF, results in a retinal contrast decrease. This, above 3-4 cpd, correlates well with the data reported by them
Tomographic inversion of focusing operators
No full textSeismic images of the structure of the earth are a prerequisite for finding new hydrocarbon reservoirs. The quality of a seismic images is highly dependent on the accuracy of the velocity model of the subsurface. Conventional imaging consists of an iterative process between obtaining the image using a velocity model, and updating this model by investigating the properties of the image. The Common Focal Point (CFP) method makes it possible to analyze and treat seismic data in a fundamentally different way as it uses a two-step approach: 1) two-way reflection data are transformed into one-way data by estimating focusing operators, and 2) these focusing operators are used to estimate the velocity model by tomographic inversion. This second step, the tomographic inversion of focusing operators, is the subject of this thesis. This research contains two important new aspects. First, the use of focusing operators in (3D) velocity model estimation. Second the data-driven approach of the method. Some additional concepts like the inclusion of a priori information, the joint inversion of P and S-wave operators, and the new concept of the focal point clouds, by which the adequacy of the velocity model can be analyzed, are also addressed. After evaluation of the method of tomographic inversion of focusing operators on both synthetic and real data it can be concluded that the method results in accurate velocity models and is capable of dealing with complex subsurface models.Applied Science
Influence of chroma variations on naturalness and image quality of stereoscopic images
No full textThe computational view on image quality of Janssen and Blommaert states that the quality of an image is determined by the degree to which the image is both useful (discriminability) and natural (identifiability). This theory is tested by creating two manipulations. Firstly, multiplication of the chroma values of each pixel with a constant in the CIELab color space, i.e., chroma manipulation, is expected to increase only the usefulness by increasing the distances between the individual color points, enhancing the contrast. Secondly, introducing stereoscopic depth by varying the screen disparity, i.e., depth manipulation, is expected to increase both the usefulness and the naturalness. Twenty participants assessed perceived image quality, perceived naturalness and perceived depth of the manipulated versions of two natural scenes. The results revealed a small, yet significant shift between image quality and naturalness as a function of the chroma manipulation. In line with previous research, preference in quality was shifted to higher chroma values in comparison to preference in naturalness. Introducing depth enhanced the naturalness scores, however, in contrast to our expectations, not the image quality scores. It is argued that image quality is not sufficient to evaluate the full experience of 3D. Image quality appears to be only one of the attributes underlying the naturalness of stereoscopic images.Man-Machine InteractionElectrical Engineering, Mathematics and Computer Scienc
The RaPID Perceptual Image Description Method (RaPID)
No full textThis paper contains a general description of the RaPID (rapid perceptual image description) method, and the results of a series of experiments on the attribute sharpness. The method aims at a rapid and perceptually meaningful description and quantification of the primary factors of image quality. The purpose of the project is primarily to develop the main dimensions of image quality, but also to determine how they combine into overall image quality, and to develop perceptual models for the main dimensions. The result of the project will be a technique that can be used to evoke, quantify, analyze and interpret subjective reactions to the characteristics of imaging systems. A preliminary experiment was conducted on sharpness related attributes of images. The results showed that the five TV sets under study had to be evaluated on 11 different terms and that they combine into two independent attribute dimensions for natural or test images. If both types of images are considered simultaneously, the 'sharpness' space consists of at least 3 dimensions. Physical measurements were used to calculate sharpness related measures (MTFs, step responses, line width) and these were correlated with the subjective results
Interactions of visual attention and quality perception
No full textSeveral attempts to integrate visual saliency information in quality metrics are described in literature, albeit with contradictory results. The way saliency is integrated in quality metrics should reflect the mechanisms underlying the interaction between image quality assessment and visual attention. This interaction is actually two-fold: (1) image distortions can attract attention away from the Natural Scene Saliency (NSS), and (2) the quality assessment task in itself can affect the way people look at an image. A subjective study was performed to analyze the deviation in attention from NSS as a consequence of being asked to assess the quality of distorted images, and, in particular, whether, and if so how, this deviation depended on the distortion kind and/or amount. Saliency maps were derived from eye-tracking data obtained during scoring distorted images, and they were compared to the corresponding NSS, derived from eye-tracking data obtained during freely looking at high quality images. The study revealed some structural differences between the NSS maps and the ones obtained during quality assessment of the distorted images. These differences were related to the quality level of the images; the lower the quality, the higher the deviation from the NSS was. The main change was identified as a shrinking of the region of interest, being most evident at low quality. No evident role for the kind of distortion in the change in saliency was found. Especially at low quality, the quality assessment task seemed to prevail on the natural attention, forcing it to devi te in order to better evaluate the impact of artifactsElectrical Engineering, Mathematics and Computer Scienc
Veiling glare : the dynamic range limit of HDR images
No full textHigh Dynamic Range (HDR) images are superior to conventional images. However, veiling glare is a physical limit to HDR image acquisition and display. We performed camera calibration experiments using a single test target with 40 luminance patches covering a luminance range of 18,619:1. Veiling glare is a scene-dependent physical limit of the camera and the lens. Multiple exposures cannot accurately reconstruct scene luminances beyond the veiling glare limit. Human observer experiments, using the same targets, showed that image-dependent intraocular scatter changes identical display luminances into different retinal luminances. Vision's contrast mechanism further distorts any correlation of scene luminance and appearance. There must be reasons, other than accurate luminance, that explains the improvement in HDR images. The multiple exposure technique significantly improves digital quantization. The improved quantization allows displays to present better spatial information to humans. When human vision looks at high-dynamic range displays, it processes them using spatial comparisons
Post-filtering for Color Appearance in Synthetic Images Tone Reproduction
No full textIn the Photorealistic Image Synthesis process the spectral content of the synthetic scene is carefully reproduced, and the final output contains the exact spectral intensity light field of the perceived scene. This is the first important step toward the goal of producing a synthetic image that is indistinguishable from the actual one, but the real scene and its synthetic reproduction should be studied under the same conditions, in order to make a correct comparison and evaluate the degree of photorealism. To simplify this goal, a synthetic observer could be employed to compensate differences in the viewing conditions, since a real observer can not enter into a synthetic world. Various solutions have been proposed to this end. Most of them are based more on perceptive measures of the Human Visual System (HVS) under controlled conditions rather than on the HVS behavior under real conditions, e.g. observing a common image and not a controlled black and white striped pattern. Another problem in synthetic image generation is the visualization phase, or tone reproduction, whose purpose is to display the final result of the simulation model on a monitor screen or on a printed paper. The tone reproduction problem consists of finding the best solution to compress the extended dynamic range of the computed light field into the limited range of the displayable colors. We would like to propose a working hypothesis to solve the appearance and the tone reproduction problems in the synthetic image generation, integrating the Retinex model into the photorealistic image synthesis context, including in this way a model of the human visual system in the synthesis process
Separating the effects of glare from simultaneous contrast
No full textAppearance in High Dynamic Range images is controlled by intraocular glare and physiological spatial contrast. Increasing the number of high luminance pixels in a display increases glare and reduces the dynamic range of luminances on the retina. Simultaneous contrast makes areas with higher glare related luminances look darker. Previous experiments measured the range needed for the appearance black in surrounds with variable percentage of white pixels in the background. In these test targets it was 2.0 log units with 100% white pixels, 2.3 log units with 50% white pixels, 2.9 log units with 8% white pixels, and 5.5 log units with 0% white pixels. We want to calculate the intensity of veiling glare in these test scenes and relate retinal luminances to the magnitude estimates of appearance reported by observers. This paper uses a glare spread function to calculate the retinal luminances after intraocular scatter. By modeling the actual luminances on the retina we can compare them with appearance
Measuring perceptual contrast in a multilevel framework
No full textThis is the copy of journal's version originally published in Proc. SPIE 7240. Reprinted with permission of SPIE: http://spie.org/x10.xml?WT.svl=tn7In this paper, we propose and discuss some approaches for measuring perceptual contrast in digital images. We start from previous algorithms by implementing different local measures of contrast and a parameterized way to recombine local contrast maps and color channels. We propose the idea of recombining the local contrast maps and the channels using particular measures taken from the image itself as weighting parameters. Exhaustive tests and results are presented and discussed, in particular we compare the performance of each algorithm in relation to perceived contrast by observers. Current results show an improvement in correlation between contrast measures and observers perceived contrast when the variance of the three color channels separately is used as weighting parameter for local contrast map
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