Showing papers in "Journal of The Optical Society of America A-optics Image Science and Vision in 1990"
TL;DR: A definition of local band-limited contrast in images is proposed that assigns a contrast value to every point in the image as a function of the spatial frequency band and is helpful in understanding the effects of image-processing algorithms on the perceived contrast.
Abstract: The physical contrast of simple images such as sinusoidal gratings or a single patch of light on a uniform background is well defined and agrees with the perceived contrast, but this is not so for complex images. Most definitions assign a single contrast value to the whole image, but perceived contrast may vary greatly across the image. Human contrast sensitivity is a function of spatial frequency; therefore the spatial frequency content of an image should be considered in the definition of contrast. In this paper a definition of local band-limited contrast in images is proposed that assigns a contrast value to every point in the image as a function of the spatial frequency band. For each frequency band, the contrast is defined as the ratio of the bandpass-filtered image at the frequency to the low-pass image filtered to an octave below the same frequency (local luminance mean). This definition raises important implications regarding the perception of contrast in complex images and is helpful in understanding the effects of image-processing algorithms on the perceived contrast. A pyramidal image-contrast structure based on this definition is useful in simulating nonlinear, threshold characteristics of spatial vision in both normal observers and the visually impaired.
1,370 citations
TL;DR: A model of human preattentive texture perception that can predict the salience of texture boundaries in any arbitrary gray-scale image and Quantitative predictions of the degree of discriminability of different texture pairs match well with experimental measurements of discriminateability in human observers.
Abstract: We present a model of human preattentive texture perception. This model consists of three stages: (1) convolution of the image with a bank of even-symmetric linear filters followed by half-wave rectification to give a set of responses modeling outputs of V1 simple cells, (2) inhibition, localized in space, within and among the neural-response profiles that results in the suppression of weak responses when there are strong responses at the same or nearby locations, and (3) texture-boundary detection by using wide odd-symmetric mechanisms. Our model can predict the salience of texture boundaries in any arbitrary gray-scale image. A computer implementation of this model has been tested on many of the classic stimuli from psychophysical literature. Quantitative predictions of the degree of discriminability of different texture pairs match well with experimental measurements of discriminability in human observers.
1,037 citations
TL;DR: The purpose of this paper is to survey briefly the kinds of fractals that appear in scientific research, to discuss the application of Fractals to nonlinear dynamical systems, and to review more comprehensively the state of the art in numerical methods for estimating the fractal dimension of a strange attractor.
Abstract: Fractals arise from a variety of sources and have been observed in nature and on computer screens. One of the exceptional characteristics of fractals is that they can be described by a noninteger dimension. The geometry of fractals and the mathematics of fractal dimension have provided useful tools for a variety of scientific disciplines, among which is chaos. Chaotic dynamical systems exhibit trajectories in their phase space that converge to a strange attractor. The fractal dimension of this attractor counts the effective number of degrees of freedom in the dynamical system and thus quantifies its complexity. In recent years, numerical methods have been developed for estimating the dimension directly from the observed behavior of the physical system. The purpose of this paper is to survey briefly the kinds of fractals that appear in scientific research, to discuss the application of fractals to nonlinear dynamical systems, and finally to review more comprehensively the state of the art in numerical methods for estimating the fractal dimension of a strange attractor.
895 citations
TL;DR: In this paper, the principles of phase retrieval in crystallography are outlined and compared and contrasted with phase retrieval for general imaging, and the emphasis is on phase-retrieval algorithms and areas in which results in one discipline have, and may, contribute to the other.
Abstract: Phase problems occur in many scientific disciplines, particularly those involving remote sensing using a wave field. Although there has been much interest in phase retrieval in optics and in imaging in general over the past decade, phase retrieval has a much longer history in x-ray crystallography, and a variety of powerful and practical techniques have been developed. The nature of crystallography means that crystallographic phase problems are distinct from those in other imaging contexts, but there are a number of commonalities. Here the principles of phase retrieval in crystallography are outlined and are compared and contrasted with phase retrieval in general imaging. Uniqueness results are discussed, but the emphasis is on phase-retrieval algorithms and areas in which results in one discipline have, and may, contribute to the other.
743 citations
TL;DR: The data suggest strongly that the cells of the M-pathway form the physiological substrate for detection of luminance modulation and the Cells of the P- pathway the substrate for Detection of chromatic modulation, however, at high light levels, intrusion of a so-called luminance mechanism near 10 Hz in psychophysical detection ofchromatic modulation is probably due to responses in the M -pathway.
Abstract: We measured the sensitivity of macaque ganglion cells to luminance and chromatic sinusoidal modulation Phasic ganglion cells of the magnocellular pathway (M-pathway) were the more sensitive to luminance modulation, and tonic ganglion cells of the parvocellular pathway (P-pathway) were more sensitive to chromatic modulation With decreasing retinal illuminance, phasic ganglion cells' temporal sensitivity to luminance modulation changed in a manner that paralleled psychophysical data The same was true for tonic cells and chromatic modulation Taken together, the data suggest strongly that the cells of the M-pathway form the physiological substrate for detection of luminance modulation and the cells of the P-pathway the substrate for detection of chromatic modulation However, at high light levels, intrusion of a so-called luminance mechanism near 10 Hz in psychophysical detection of chromatic modulation is probably due to responses in the M-pathway, arising primarily from a nonlinearity of cone summation Both phasic and tonic ganglion cells responded to frequencies higher than can be psychophysically detected This suggests that central mechanisms, acting as low-pass filters, modify these cells' signals, though the corner frequency is lower for the P-pathway than for the M-pathway For both cell types, the response phase at different frequencies was consistent with the cells' description as linear filters with a fixed time delay
481 citations
TL;DR: In this paper, the authors studied the guided-mode resonance behavior of the evanescent and propagating fields associated with an unslanted, planar diffraction grating by means of the rigorous coupledwave theory.
Abstract: The guided-mode resonance behavior of the evanescent and propagating fields associated with an unslanted, planar diffraction grating is studied by means of the rigorous coupled-wave theory. For weakly modulated gratings, the condition on the guided-mode wave number of the corresponding unmodulated dielectric-layer waveguide may be used to predict the range of the incident angle or wavelength within which the resonances can be excited. Furthermore, the locations of the resonances are predicted approximately by the eigenvalue equation of the waveguide. As the modulation amplitude increases, the location and shape of the resonances are described in detail by the rigorous coupled-wave theory. The results presented demonstrate that the resonances can cause rapid variations in the intensity of the external propagating diffracted waves.
470 citations
TL;DR: A number of task-specific approaches to the assessment of image quality are treated, but only linear estimators or classifiers are permitted, and results are expressed as signal-to-noise ratios (SNR's).
Abstract: A number of task-specific approaches to the assessment of image quality are treated. Both estimation and classification tasks are considered, but only linear estimators or classifiers are permitted. Performance on these tasks is limited by both quantum noise and object variability, and the effects of postprocessing or image-reconstruction algorithms are explicitly included. The results are expressed as signal-to-noise ratios (SNR's). The interrelationships among these SNR's are considered, and an SNR for a classification task is expressed as the SNR for a related estimation task times four factors. These factors show the effects of signal size and contrast, conspicuity of the signal, bias in the estimation task, and noise correlation. Ways of choosing and calculating appropriate SNR's for system evaluation and optimization are also discussed.
408 citations
TL;DR: It is shown that observers can perceive their direction of self-motion during stationary fixations and pursuit eye movements and with displays that simulate the optical effects of eye movements, indicating that the visual system can perform the decomposition with both continuous and discontinuous fields on the basis of flow-field information alone but requires a three-dimensional environmental structure to do so.
Abstract: Translation of an observer through a static environment generates a pattern of optical flow that specifies the direction of self-motion, but the retinal flow pattern is confounded by pursuit eye movements. How does the visual system decompose the translational and rotational components of flow to determine heading? It is shown that observers can perceive their direction of self-motion during stationary fixations and pursuit eye movements and with displays that simulate the optical effects of eye movements. Results indicate that the visual system can perform the decomposition with both continuous and discontinuous fields on the basis of flow-field information alone but requires a three-dimensional environmental structure to do so. The findings are inconsistent with general computational models and theories based on the maximum of divergence, oculomotor signals, or multiple fixations but are consistent with the theory of reliance on differential motion produced by environmental variation in depth.
341 citations
TL;DR: In this paper, a procedure to calculate a highly quantized, blazed phase structure is presented, which is based on iterative Fourier transform (IFT) algorithms with high diffraction efficiency and a large signal-to-noise ratio.
Abstract: A procedure to calculate a highly quantized, blazed phase structure is presented. Characteristics that are concentrated on are a high diffraction efficiency and a large signal-to-noise ratio. The calculation techniques are based on iterative Fourier-transform algorithms. Stagnation problems are discussed, and methods to overcome them are described.
340 citations
TL;DR: In this article, the Kaiser-Bessel window function is used to construct a rotationally symmetric basis function in n-dimensional space, which can be used for image reconstruction from line-integral data.
Abstract: Inverse problems that require the solution of integral equations are inherent in a number of indirect imaging applications, such as computerized tomography. Numerical solutions based on discretization of the mathematical model of the imaging process, or on discretization of analytic formulas for iterative inversion of the integral equations, require a discrete representation of an underlying continuous image. This paper describes discrete image representations, in n-dimensional space, that are constructed by the superposition of shifted copies of a rotationally symmetric basis function. The basis function is constructed using a generalization of the Kaiser-Bessel window function of digital signal processing. The generalization of the window function involves going from one dimension to a rotationally symmetric function in n dimensions and going from the zero-order modified Bessel function of the standard window to a function involving the modified Bessel function of order m. Three methods are given for the construction, in n-dimensional space, of basis functions having a specified (finite) number of continuous derivatives, and formulas are derived for the Fourier transform, the x-ray transform, the gradient, and the Laplacian of these basis functions. Properties of the new image representations using these basis functions are discussed, primarily in the context of two-dimensional and three-dimensional image reconstruction from line-integral data by iterative inversion of the x-ray transform. Potential applications to three-dimensional image display are also mentioned.
308 citations
TL;DR: A general description of phase measurement by digital heterodyne techniques is presented in which the heterodyning is explained as a filtering process in the frequency domain.
Abstract: A general description of phase measurement by digital heterodyne techniques is presented in which the heterodyning is explained as a filtering process in the frequency domain Examples of commonly used algorithms are given Special emphasis is given to the analysis of systematic errors Gaussian error propagation is used to derive equations for the random phase errors of common algorithms
TL;DR: Response times were measured for a visual search task in which the observer was required to find a target that differed from distracting stimuli only in color.
Abstract: Response times were measured for a visual search task in which the observer was required to find a target that differed from distracting stimuli only in color. In the first experiment the search time was measured as a function of display density for both small and large color differences. With small color differences response time increased with display density, indicating a serial search, but with large color differences response time was constant, indicating a parallel search. In the second experiment the color difference required for parallel search was measured in eight different directions from the distracter chromaticity. These color differences were much larger than threshold color differences and were not well represented by the ellipse used to describe the threshold contour around a point in color space.
TL;DR: In this article, the effect of various display parameters, such as resolution, addressability, contrast, luminance, display size, and viewing distance, on subjective image quality is taken into account.
Abstract: After a short survey of some other metrics of perceived image quality, the square-root integral (SQRI) is described. In this metric a fixed mathematical expression for the contrast sensitivity of the eye is used. With the SQRI method the effect of various display parameters, such as resolution, addressability, contrast, luminance, display size, and viewing distance, on subjective image quality can be taken into account. Experimental data of subjective image quality, measured by various authors, are compared with calculated SQRI values. From the comparison it appears that the calculated SQRI values show a good linear correlation with perceived subjective image quality not only at variation of resolution but also at simultaneous variation of other display parameters.
TL;DR: Estimating the absorption and transport-corrected scattering coefficients of dermis and subcutis from intensities measured from intact skin and predicting the magnitude of the optical-density variations measured by a photoplethysmograph show potential applications of the model.
Abstract: A diffusion model describing the propagation of photon flux in the epidermal, dermal, and subcutaneous tissue layers of the skin is presented. Assuming that the skin is illuminated by a collimated, finite-aperture source, we develop expressions relating photon flux density within the skin and intensities re-emitted from the skin surface to the optical properties of the individual layers. Model simulations show that the rate at which re-emitted intensities diminish with radial distance away from the source can provide information about absorption and scattering in underlying tissues. Re-emitted intensities measured from homogeneous and two-layer tissue phantoms compare favorably with model predictions. We demonstrate potential applications of the model by estimating the absorption (Σa) and transport-corrected scattering
(Σs′) coefficients of dermis and subcutis from intensities measured from intact skin and by predicting the magnitude of the optical-density variations measured by a photoplethysmograph.
TL;DR: In this paper, a new technique of high-resolution imaging through atmospheric turbulence is described, where short-exposure images are recorded, but in addition the associated wave fronts are measured by a Hartmann-Shack wave-front sensor.
Abstract: A new technique of high-resolution imaging through atmospheric turbulence is described. As in speckle interferometry, short-exposure images are recorded, but in addition the associated wave fronts are measured by a Hartmann–Shack wave-front sensor. The wave front is used to calculate the point-spread function. The object is then estimated from the correlation of images and point-spread functions by a deconvolution process. An experimental setup is described, and the first laboratory results, which prove the capabilities of the method, are presented. A signal-to-noise-ratio calculation, permitting a first comparison with the speckle interferometry, is also presented.
TL;DR: In this article, an error analysis of a Mueller matrix polarimeter with dual rotating retarders is presented, where errors in orientational alignment of three of the four polarization elements are considered.
Abstract: An error analysis of a Mueller matrix polarimeter with dual rotating retarders is presented. Errors in orientational alignment of three of the four polarization elements are considered. Errors that are due to nonideal retardation elements are also included in the analysis. Compensation for imperfect retardation elements is possible with the equations derived, and the equations permit a calibration of the polarimeter for azimuthal alignment of polarization elements. An analytical treatment is given and is followed by numerical examples. The latter should prove useful in the laboratory in comparing precalibrated experimental results with theoretical predictions.
TL;DR: This paper investigates the question of which value of the exponent beta describes the power spectrum of the ensemble of images to which the visual system is optimally tuned, and results are consistent with, but do not provide direct evidence for, the theory that thevisual system is tuned to an ensemble of image with Markov statistics.
Abstract: In order to transmit information in images efficiently, the visual system should be tuned to the statistical structure of the ensemble of images that it sees. Several authors have suggested that the ensemble of natural images exhibits fractal behavior and, therefore, has a power spectrum that drops off proportionally to 1/f beta (2 less than beta less than 4). In this paper we investigate the question of which value of the exponent beta describes the power spectrum of the ensemble of images to which the visual system is optimally tuned. An experiment in which subjects were asked to discriminate randomly generated noise textures based on their spectral drop-off was used. Whereas the discrimination-threshold function of an ideal observer was flat for different spectral drop-offs, human observers showed a broad peak in sensitivity for 2.8 less than beta less than 3.6. The results are consistent with, but do not provide direct evidence for, the theory that the visual system is tuned to an ensemble of images with Markov statistics.
TL;DR: In this paper, the effect of intensity noise on the computed phase of phase-shifting interferometry is investigated using a simple Taylor expansion, one can easily show the sensitivity of the error in computed phase to frame-to-frame intensity noise correlation.
Abstract: The effect of intensity noise on the computed phase of phase-shifting interferometry is investigated. Using a simple Taylor expansion, one can easily show the sensitivity of the error in computed phase to frame-to-frame intensity noise correlation. The sensitivity of various algorithms to statistically independent intensity noise and intensity quantization noise is considered. Intensity-error cross correlations are found to increase the error in computed phase for the case of intensity quantization error.
TL;DR: In this article, the accuracy of orientation and spatial-frequency discrimination for sine-wave gratings that vary in either luminance or color was examined, and the color and luminance discrimination thresholds did not differ from the two types of color grating and reached values as low as 1 deg (orientation) or 4% (spatial frequency).
Abstract: We have examined the accuracy of orientation and spatial-frequency discrimination for sine-wave gratings that vary in either luminance or color. The equiluminant chromatic gratings were modulated along either a tritanopic confusion axis (so that they were detectable on the basis of activity in only the short-wavelength-sensitive cones) or an axis of constant short-wavelength-sensitive cone excitation (so that they could be detected on the basis of opposing activity in only the long- and medium-wavelength-sensitive cones). Grating contrasts ranged from the detection threshold to the highest levels that we could produce; the contrasts of the luminance and color patterns were equated for equal multiples of their respective detection thresholds. Discrimination thresholds for all patterns showed a similar dependence on stimulus contrast, rising sharply at low contrasts and becoming nearly asymptotic at moderate contrasts. However, even at threshold contrasts, observers could still reliably discriminate sufficiently large differences in the orientation or spatial frequency of all patterns, and they could also reliably identify the type of variation (luminance or which color) defining the grating. For most conditions the discrimination thresholds did not differ from the two types of color grating and reached values as low as 1 deg (orientation) or 4% (spatial frequency). Thus observers were able to make accurate spatial judgments on the basis of either type of chromatic information. However, these thresholds were slightly but consistently higher than the thresholds for comparable luminance gratings. This difference in the color and luminance discrimination thresholds may reflect somewhat coarser orientation and spatial-frequency selectivity in the mechanisms encoding the chromatic patterns.
TL;DR: In this article, the problem of scattering of optical or electromagnetic waves from a family of irregular rough surfaces characterized by band-limited fractal functions was considered and a unified and realistic method for examining rough surfaces without the use of random or periodic functions was proposed.
Abstract: We consider the problem of scattering of optical or electromagnetic waves from a family of irregular rough surfaces characterized by band-limited fractal functions. This method provides a unified and realistic method for examining rough surfaces without the use of random or periodic functions. We relate the angular distribution and the amount of energy in the specularly scattered field to the fractal characteristics of the surfaces by finding their analytical expressions under the Kirchhoff limit and calculating the scattering patterns.
TL;DR: In this paper, the generalized Lorenz-Mie theory was applied to numerical computations in the framework of the generalized Mie theory, and a new double set of coefficients gn,TMm and gn,TEm (n = 1, …, ∞; m = − n, + n, n + n) was designed to permit fast and accurate computations on microcomputers.
Abstract: Numerical computations in the framework of the generalized Lorenz–Mie theory require the evaluation of a new double set of coefficients gn,TMm and gn,TEm (n = 1, …, ∞; m = − n, … +n). A localized interpretation of these coefficients is designed to permit fast and accurate computations, even on microcomputers. When the scatter center is located on the axis of the beam, a previously published localized approximation for a simpler set of coefficients gn is recovered as a special case. The subscript n in coefficients gn and gnm is associated with ray localization and discretization of space in directions perpendicular to the beam axis, while superscript m in coefficients gnm is associated with azimuthal wave modes.
TL;DR: The proposed vector-subspace model includes a simple relation between the inner products and conventional color coordinates, which provides a way to estimate the spectrum of an object that has known chromaticity coordinates.
Abstract: In multispectral imaging it is advantageous to compress spectral information with a minimum loss of information in a way that permits accurate recovery of the spectrum. By use of the simple vector-subspace model that we propose, spectral information can be stored and recovered by the use of a few inner products, which are easy to measure optically. Two large data sets, the first consisting of 1257 Munsell colors and the other of 218 naturally occurring spectral reflectances, were analyzed to form two bases for the model. The Munsell basis can be used to represent the natural colors, and the basis derived from the natural data can be used to represent the Munsell data. The proposed vector-subspace model includes a simple relation between the inner products and conventional color coordinates. It also provides a way to estimate the spectrum of an object that has known chromaticity coordinates.
TL;DR: In this article, a rigorous integral theory of scattering by metallic or dielectric nonperiodic rough surfaces leading to a single integral equation is proposed, and numerical results of enhanced backscattering from perfectly conducting, metallic, and dielectoric random rough surfaces are given.
Abstract: Using a generalization of the integral theory of metallic and dielectric gratings developed in our laboratory 15 years ago, we propose a rigorous integral theory of scattering by metallic or dielectric nonperiodic rough surfaces leading to a single integral equation. The numerical implementation has been carried out despite strong difficulties for TM polarization and metallic surfaces because of propagation of surface plasmon waves outside the illuminated region of the rough surface. Numerical results show the influence of the statistical parameters of the asperities on the absorption phenomena for metallic surfaces. Then the influence of asperities on the total transmission around Brewster incidence is studied. Finally numerical results of enhanced backscattering from perfectly conducting, metallic, and dielectric random rough surfaces are given.
TL;DR: In this paper, the Stokes parameters of the average scattered light from two-dimensional rough surfaces show the presence of an unpolarized component, which lends support to the multiply scattering ray model.
Abstract: An experimental study of backscatter enhancement from rough surfaces is presented. The Stokes parameters of the average scattered light from two-dimensional rough surfaces show the presence of an unpolarized component, which lends support to the multiply scattering ray model. Experimental data from one-dimensional rough surfaces are compared with numerical calculation.
TL;DR: A prediction of the model, namely, that discrimination asymmetry will be reduced when textural elements have identical orientation, was tested psychophysically and confirmed and found remarkably good correlation with human performance.
Abstract: Texture-discimination tasks reveal a pronounced performance asymmetry depending on which texture represents the foreground region (small area) and which represents the ground (large area). This asymmetry implies that some global processes are involved in the segmentation process. We examined this problem within the context of the texture-segmentation algorithm, assuming two filtering stages. The first stage uses spatial frequency and orientation-selective (Gabor) filters, whereas the second stage is formed by low-resolution edge-detection filters. The presence and location of texture borders are indicated by significant responses in the second stage. Spurious texture borders may occur owing to textural local variabilities (such as orientation randomization), which are enhanced by the first stage. We suggest that these spurious borders act as background noise and thus limit performance in texture-discrimination tasks. The noise level depends on which texture occupies the ground in the display. We tested this model on numerous pairs of textures and found remarkably good correlation with human performance. A prediction of the model, namely, that discrimination asymmetry will be reduced when textural elements have identical orientation, was tested psychophysically and confirmed.
TL;DR: The findings suggest that color contrast and Luminance contrast are coded in similar ways in the visual system but that the neural signals used in letter recognition are carried by different pathways for color and luminance.
Abstract: Text can be depicted by luminance contrast (i.e., differences in luminance between characters and background) or by color contrast (i.e., differences in chromaticity). We used a psychophysical method to measure the reading speeds of eight normal and ten low-vision subjects for text displayed on a color monitor. Reading speed was measured as a function of luminance contrast, color contrast (derived from mixtures of red and green), and combinations of the two. When color contrast is high, normal subjects can read as rapidly as with high luminance contrast (>300 words/min). Curves of reading speed versus contrast have the same shape for the two forms of contrast and are superimposed when contrast is measured in multiples of a threshold value. When both color and luminance contrast are present, there is no sign of additive interaction, and performance is determined by the form of contrast yielding the highest reading rate. Our findings suggest that color contrast and luminance contrast are coded in similar ways in the visual system but that the neural signals used in letter recognition are carried by different pathways for color and luminance. We found no advantages of color contrast for low-vision reading. For text composed of 6° characters, all low-vision subjects read better with luminance contrast than with color contrast.
TL;DR: In this paper, the intensity spectra and variances for waves emanating from a point source and propagating through extended three-dimensional random media by simulation were calculated and the results agree reasonably closely with the results of laser propagation experiments over kilometer-length paths in the atmosphere.
Abstract: We have calculated intensity spectra and variances for waves emanating from a point source and propagating through extended three-dimensional random media by simulation. Spectra of the medium fluctuations considered were power-law, power-law with inner scale, and Gaussian spectra. The simulations covered the regimes of weak fluctuations and strong focusing, including the peak of the intensity variance and beyond. The intensity variances are substantially larger than both the corresponding results for plane-wave incidence and the theoretical calculations for point sources by other authors. Our simulation results agree reasonably closely with the results of laserpropagation experiments over kilometer-length paths in the atmosphere.
TL;DR: In this paper, the authors measured the visibility of a 1 degree, 200-msec flash on a large yellow field as a function of the intensity of a coincident pedestal flash (a flash that was the same in both temporal intervals of a two-alternative forced-choice trial).
Abstract: The visibility of a 1 degree, 200-msec flash on a large yellow field was measured as a function of the intensity of a coincident pedestal flash (a flash that was the same in both temporal intervals of a two-alternative forced-choice trial). The various flashes were incremental (+Lum) or decremental (-Lum) yellow luminance flashes or green (+Chr) or red (-Chr) isoluminant chromatic flashes. With uncrossed conditions (Lum tests on Lum pedestals or Chr tests on Chr pedestals), we obtained the conventional dipper function, that is, the function of threshold test intensity was highly asymmetric about zero pedestal intensity, and strong pedestals induced strong masking. Crossed conditions produced neither effect: for example, with Chr tests on Lum pedestals, there was no dipper function: the function of threshold test intensity was symmetric about zero pedestal intensity, and strong pedestals produced no masking. Instead, the suprathreshold luminance pedestals facilitated chromatic detection by as much as 2-3X and also linearized the chromatic psychometric function, further enhancing sensitivity to weak chromatic stimuli. (Chromatic sensitivity on the suprathreshold luminance pedestal was approximately 25X higher than luminance sensitivity on the uniform field.) A pedestal consisting of a thin luminance ring that surrounded the chromatic test produced facilitation equal to that of the uniform-luminance pedestal: the pedestal may thus act to demarcate the test spatially and promote chromatic comparison with the surround. Removing the uniform yellow surround eliminated this crossed facilitation but did not eliminate the uncrossed facilitation (the dipper function), suggesting that different mechanisms mediate the crossed and uncrossed facilitations.
TL;DR: In this paper, the spectral unique hues (blue, green, and yellow) were determined for 50 observers ranging in age from 13 to 74 years, each unique hue was measured at three luminance levels (0.5-log-unit steps).
Abstract: Spectral unique hues (blue, green, and yellow) were determined for 50 observers ranging in age from 13 to 74 years. Each unique hue was measured at three luminance levels (0.5-log-unit steps). There were no significant changes in the spectral locations of red–green equilibrium hues (unique blue and yellow) as a function of luminance level or age. In contrast, significant shifts in unique green loci occurred as a function of both age and luminance. Unique green loci shifted toward shorter wavelengths with age. These results are consistent with the hypothesis that with advancing age there is a parallel decline in the input of all three cone types to the red–green chromatic channel and either a selective decline in short-wave-sensitive cone input to the yellow–blue chromatic channel or a change in the way in which cone signals are combined within the yellow–blue channel.
TL;DR: In this paper, a generic optical flow can be approximated by using a constant term and a suitable combination of four elementary deformations of the time-varying image brightness, namely, a uniform expansion, a pure rotation, and two orthogonal components of shear.
Abstract: We show that optical flow, i.e., the apparent motion of the time-varying brightness over the image plane of an imaging device, can be estimated by means of simple differential techniques. Linear algebraic equations for the two components of optical flow at each image location are derived. The coefficients of these equations are combinations of spatial and temporal derivatives of the image brightness. The equations are suggested by an analogy with the theory of deformable bodies and are exactly true for particular classes of motion or elementary deformations. Locally, a generic optical flow can be approximated by using a constant term and a suitable combination of four elementary deformations of the time-varying image brightness, namely, a uniform expansion, a pure rotation, and two orthogonal components of shear. When two of the four equations that correspond to these deformations are satisfied, optical flow can more conveniently be computed by assuming that the spatial gradient of the image brightness is stationary. In this case, it is also possible to evaluate the difference between optical flow and motion field—that is, the two-dimensional vector field that is associated with the true displacement of points on the image plane. Experiments on sequences of real images are reported in which the obtained optical flows are used successfully for the estimate of three-dimensional motion parameters, the detection of flow discontinuities, and the segmentation of the image in different moving objects.