Pascual Capilla

Image quality metric based on multidimensional contrast perception models

Abstract The procedure to compute the subjective difference between two input images should be equivalent to a straightforward difference between their perceived versions, hence reliable subjective difference metrics must be founded on a proper perception model. For image distortion evaluation purposes, perception can be considered as a set of signal transforms that successively map the original image in the spatial domain into a feature and a response space. The properties of the spatial pattern analyzers involved in these transforms determine the geometry of these different signal representation domains. In this work the general relations between the sensitivity of the human visual system and the perceptual geometry of the different representation spaces are presented. This general formalism is particularized through a novel physiological model of response summation of cortical cells that reproduce the psychophysical data of contrast incremental thresholds. In this way, a procedure to compute subjective distances between images in any representation domain is obtained. The reliability of the proposed scheme is tested in two different contexts. On the one hand, it reproduces the results of suprathreshold contrast matching experiences and subjective contrast scales (Georgeson and Shackleton, Vision Res. 34 (1994) 1061–1075; Swanson et al., Vision Res. 24 (1985) 63–75; Cannon, Vision Res. 19 (1979) 1045–1052; Biondini and Mattiello, Vision Res. 25 (1985) 1–9), and on the other hand, it provides a theoretical background that generalizes our previous perceptual difference model (Malo et al., Im. Vis. Comp. 15 (1997) 535–548) whose outputs are linearly related to experimental subjective assessment of distortion.

Corresponding-pair procedure: a new approach to simulation of dichromatic color perception

The dichromatic color appearance of a chromatic stimulus T can be described if a stimulus S is found that verifies that a normal observer experiences the same sensation viewing S as a dichromat viewing T. If dichromatic and normal versions of the same color vision model are available, S can be computed by applying the inverse of the normal model to the descriptors of T obtained with the dichromatic model. We give analytical form to this algorithm, which we call the corresponding-pair procedure. The analytical form highlights the requisites that a color vision model must verify for this procedure to be used. To show the capabilities of the method, we apply the algorithm to different color vision models that verify such requisites. This algorithm avoids the need to introduce empirical information alien to the color model used, as was the case with previous methods. The relative simplicity of the procedure and its generality makes the prediction of dichromatic color appearance an additional test of the validity of color vision models.

Vision defects in albinism

We have examined the possible presence of color vision anomalies in 9 individuals (17 eyes, 1 blind) with fundus findings suggesting ocular albinism using the Ishihara plates, the 28-hue Roth test, and the Davico anomaloscope. Results indicate that four of these individuals show no sign of the anomalies expected in an albino in either of the two eyes. Of the remaining cases, two are simple deuteranomals in both eyes, according to Pickfords classification criteria. The rest have protanomaly; however, in these the deviation toward red appears in both eyes in only one subject, whereas in the other two subjects it appears in only one eye, their binocular color vision being basically normal. Our study shows that a large proportion of these albinos have photophobia, pendular nystagmus, strabismus, noticeable refractive errors (astigmatism and high myopia), and poor visual acuity [usually less than 6/30 (20/100) with correction]. The measurement of contrast sensitivity function (CSF) indicates that the frequency of 12 cpd cannot be perceived, even in binocular vision.

Multichannel functional testing in normal subjects, glaucoma suspects, and glaucoma patients.

PURPOSE To evaluate visual function with a novel multichannel functional test named the ATD Multichannel Functional Test. METHODS This multicenter study had a prospective and cross-sectional design. A total of 186 eyes were included: 42 with glaucoma, 14 glaucoma suspects due to optic nerve characteristics, 25 ocular hypertensives, and 105 normal eyes. All patients performed standard visual fields (Humphrey 24-2) and ATD with eight stimuli configurations: four achromatic (A), two red-green (T), and two blue-yellow (D). To derive main outcome measures, mean sensitivity, mean defect (MD), and pattern standard deviation (PSD) were calculated and compared among groups and types of stimuli with the Kruskal-Wallis test. The percentage of cases outside normal limits (ONL) was calculated. RESULTS MD and PSD were significantly different in glaucoma eyes than in normal subjects for all types of stimuli except D-0.5 cycles per degree (cpd)/12Hz. PSD was also lower for normals than for all pathologic groups with A-4cpd/2Hz, A-4cpd/12Hz, D-0.5cpd/2Hz, and T-0.5cpd/2Hz. The highest percentage of ONL cases was obtained with the two low-spatial-frequency chromatic stimuli, with D-0.5cpd/2Hz and T-0.5cpd/2Hz using PSD, which classified as ONL 81.6% and 86.7% of glaucoma eyes, 51.8% and 44.5% of hypertensives, and 72.2% and 41.2% of optic disc suspects, respectively. CONCLUSIONS ATD assessed different aspects of visual function, and the most sensitive tests to detect glaucomatous damage were the low-temporal-frequency chromatic tests.

Red-green vs. blue-yellow spatio-temporal contrast sensitivity across the visual field

We measured contrast sensitivity (CS) to sinusoidal spatio-temporal patterns isolating the red-green and blue-yellow mechanisms, at 21 locations in the visual field (including the fovea). These measurements complete the available data for the red-green mechanism at fovea and for both mechanisms outside fovea with non-stationary patterns. Chromatic detection surfaces are low-pass at fovea and CS decreases with eccentricity at a rate that depends on the spatial and temporal frequencies. Our results confirm that, in general, sensitivities decrease with eccentricity at different rates for stationary red-green and blue-yellow patterns at each point of the spatio-temporal domain and that the chromatic detection surfaces are still low-pass at each spatial location. The results suggest some superiority of the temporal visual field for both mechanisms. Surprisingly the spatial region with maximum sensitivity for blue-yellow patterns for stimuli in the low-spatial–high temporal corner of the spatio-temporal domain is not the fovea, as happens always for red-green patterns, but shifted towards the upper-nasal quadrant of the retina.

Effect of color on contrast sensitivity with two different accommodative stimuli

We studied the influence of color and accommodation on the contrast sensitivity function (CSF). At the same time, we measured the effect of axial chromatic aberration (ACA) on the CSF. The CSFs of two observers were determined using red, green, blue, and white light, at 5- and 0.5-m viewing distances. At 5 m the CSFs were measured with natural vision and also with lenses which compensated the ACA. Results show that the effect of ACA on the CSF is to reduce the sensitivity to blue with respect to the red. The difference in sensitivity between these two colors is between 50 and 150% and varies with the frequency and the subject. When the ACA is compensated the influence of the color on the CSF is very small, in our experiment, and this result is discussed in relation to the compensating effects of chromatic adaptation of the color mechanisms. In near vision, the CSFs were measured with natural vision and also with the lenses required to give the same accommodative stimulus for all colors. We discuss the effect of accommodation on the CSF. For both far and near the results are compared with the CSF with white light.

Reliability in perimetric multichannel contrast sensitivity measurements.

In this study, the reliability of perimetric contrast sensitivity measurements favouring the achromatic, the red‐green and the blue‐yellow post‐receptorial mechanisms was analysed.The ATD multichannel perimeter was built thanks to the support of the Spanish Ministerio de Ciencia y Tecnologia Grants DPI2000-0116-P4-02 and PTR 1995-0909-OP, in collaboration with INDUSTRIAS DE OPTICA SA (San Cugat del Valles, Spain).

Effect of a yellow filter on brightness evaluated by asymmetric matching: measurements and predictions

We found that yellow filters reduce the mean brightness of a set of coloured samples significantly less with blue surrounds than with black, achromatic or yellow ones, in comparison with the naked eye and less than a yellow surround in comparison with a luminance-matched grey filter. The predictions of Hunts model and Guths ATD95 agree with these results. The yellow filter is actually capable of significantly increasing the brightness with the blue surround both in comparison with the naked eye (although only for our younger observer) and with the neutral filter. We show that in some samples the yellow filter causes global response increments in the chromatic mechanisms that compensate the response reduction in the achromatic mechanism, resulting in increased brightness. Although global chromatic response increments may arise from any chromatic mechanism, the blue–yellow mechanism determines in most cases the final result, even when the response of the red–green mechanism decreases.

Simulating images seen by patients with inhomogeneous sensitivity losses.

Purpose. We aim to simulate how colored images are perceived by subjects with local achromatic and chromatic contrast sensitivity losses in the visual field (VF). Methods. The spatiochromatic corresponding pair algorithm, introduced in a previous article (J Opt Soc Am (A) 2004;21:176–186), has been implemented with a linear model of the visual system. Spatial information is processed separately by the chromatic and achromatic mechanisms by means of a multiscale model, with sensors selective to frequency, orientation, and spatial position, whose mechanism-dependent relative weights change with the spatial location of the image. These weights have been obtained from perimetric data from a patient with Lebers Hereditary Optic Neuropathy and an age-matched sample of normal subjects, using achromatic, red-green, and blue-yellow gratings of different spatial frequencies. Distortion contours for each mechanism have been derived from the images simulating the perception of these subjects at different locations in the VF. Results. The images simulating the perception of normal subjects at different locations of the VF show a fast decrease in image quality with eccentricity. The same analysis carried out with the Lebers Hereditary Optic Neuropathy patient reveals worse overall image quality throughout the VF, plus a color vision defect resembling red-green dichromacy at fovea and trichromatic anomaly in the rest of the VF. Conclusions. In the present article, we show that implementing the algorithm with a spatial vision model that considers the changes in contrast sensitivity with spatial location of the stimulus may reveal the local effects that are suffered, in general, by pathological subjects, and that are ignored by simpler spatial vision models.

Images Perceived After Chromatic or Achromatic Contrast Sensitivity Losses

Purpose. We simulate how subjects with losses in chromatic and achromatic contrast sensitivity perceive colored images by using the spatiochromatic corresponding pair algorithm. Methods. This is a generalized version of the algorithm by Capilla et al. (J Opt Soc Am (A) 2004;21:176–186) for simulating color perception of color deviant subjects, which incorporates a simple spatial vision model, consisting of a linear filtering stage, with a band-pass achromatic filter and two low-pass chromatic ones, for the red-green and blue-yellow mechanisms. These filters, except for the global scaling, are the subjects contrast sensitivity functions measured along the cardinal directions of the color space. In its present form, the algorithm would serve to simulate alterations both in the spectral sensitivities and in the contrast sensitivities of the visual mechanisms. Results. After a preliminary theoretical study on the effect of frequency selective and overall reductions in the contrast sensitivity function of a single mechanism, we present cases of real subjects with glaucoma and diabetes, suffering alterations of different magnitude in the three mechanisms. Conclusions. The simulations allow us to learn about the different types of distortions that can be experienced by a subject with impaired contrast sensitivities (blur, haloes, color shifts, local or global contrast, brightness and colorfulness reductions, etc.) and highlight the difficulties arising when trying to predict the quality of the final image from the losses in the individual mechanisms.