Showing papers on "Color constancy published in 1993"

Color constancy. I. Basic theory of two-stage linear recovery of spectral descriptions for lights and surfaces

Abstract: Changing a scene’s illuminant causes the chromatic properties of reflected lights to change. This change in the lights from surfaces provides spectral information about surface reflectances and illuminants. We examine conditions under which these properties may be recovered by using bilinear models. Necessary conditions that follow from comparing the number of equations and the number of unknowns in the recovery procedure are not sufficient for unique recovery. Necessary and sufficient conditions follow from demanding a one-to-one relationship between quantum catch data and sets of lit surfaces. We present an algorithm for determining whether spectral descriptions of lights and surfaces can be recovered uniquely from reflected lights.

Diagonal transforms suffice for color constancy

Abstract: The main result is to show that under the conditions imposed by the Maloney-Wandell color constancy algorithm, color constancy can be expressed in terms of a simple independent adjustment of the sensor responses, so long as the sensor space is first transformed to a new basis. The overall goal is to present a theoretical analysis connecting many established theories of color constancy. For the case where surface reflectances are two-dimensional and illuminants are three-dimensional, it is proved that perfect color constancy can always be solved for by an independent adjustment of sensor responses, which means that the color constancy transform can be expressed as a diagonal matrix. In addition to purely theoretical arguments, results from simulations of diagonal-matrix-based color constancy, in which the spectra of real illuminants and reflectances along with the human cone sensitivity functions were used, are presented. The simulations demonstrate that when the cone sensor space is transformed to its new basis in the appropriate manner, a diagonal matrix supports close to optimal color constancy. >

Color constancy. II. Results for two-stage linear recovery of spectral descriptions for lights and surfaces

Abstract: Our analysis of color constancy in a companion paper [J. Opt. Soc. Am A 10, 2148 (1993)] provided an algorithm that lets one test how well linear color constancy schemes work. Here we present the results of applying the algorithm to a large parametric class of color constancy problems involving bilinear models that relate photoreceptoral spectral sensitivities, surface reflectance functions, and illuminant spectral power distributions. These results, supported by simulation and further analysis, provide a detailed classification of two-stage linear methods for recovering the spectral properties of reflectances and illuminants from reflected lights.

How much does illuminant color affect unattributed colors

Abstract: Does the light coming from a surface (as opposed to the surface color) appear the same after adaptation to a new illumination as it did before the illumination changed? Many answers have been proposed over the past century, but all have been unsatisfactory. The main measurement problem is to provide a comparison stimulus that is unaffected by the adaptation being tested. My observers used a mental standard. Under 4000-, 6500-, and 10,000-K adaptations (the extremes and the average of daylight) they produced on a CRT unique hues with a constant saturation that was memorized during training. The main evaluation problem is how to determine the theoretical chromaticity shifts that represent illumination invariance for comparison with the data. Like most previous investigators, I used light sources rather than actual surfaces and illuminants. Using a new technique, I determined theoretical surfaces that would have unique hues under the test illuminants. Using Cohen's basis vectors, I derived theoretical reflectances that under 6500 K would produce the chromaticities that the observer chose as unique hues. The chromaticities of those same reflectances under 4000 and 10,000 K are theoretical points representing illuminants-invariant appearance of the light coming from the surfaces. Even for this small range of illuminants the adaptive shifts were too small for invariance, i.e., the appearance of the light was different even after full adaptation. This result sharpens the question of the basis for humans' concept of color as a stable property of surfaces.

Simulated Studies of Color, Arousal, and Comfort

Abstract: Color psychology is applied all the time in our everyday environments, in the form of paint and graphics both outdoors and indoors, and also in the form of artificially lit spaces. Are the effects of color on the human organism so extreme that our physical and mental hygiene depends on them? Could lack of color in our internal and external spaces not only cause ugliness and boredom but also be responsible for physiological discomfort or stress reactions? The aim of the present chapter is to review the research on color in three areas that have caused considerable confusion during the last 50 years. Most of this research has been carried out in simulated situations, some of which seem to have been barren of any contact with reality.

Color constancy: enhancing von Kries adaption via sensor transformations

Abstract: Von Kries adaptation has long been considered a reasonable vehicle for color constancy. Since the color constancy performance attainable via the von Kries rule strongly depends on the spectral response characteristics of the human cones, we consider the possibility of enhancing von Kries performance by constructing new `sensors' as linear combinations of the fixed cone sensitivity functions. We show that if surface reflectances are well-modeled by 3 basis functions and illuminants by 2 basis functions then there exists a set of new sensors for which von Kries adaptation can yield perfect color constancy. These new sensors can (like the cones) be described as long-, medium-, and short-wave sensitive; however, both the new long- and medium-wave sensors have sharpened sensitivities -- their support is more concentrated. The new short-wave sensor remains relatively unchanged. A similar sharpening of cone sensitivities has previously been observed in test and field spectral sensitivities measured for the human eye. We present simulation results demonstrating improved von Kries performance using the new sensors even when the restrictions on the illumination and reflectance are relaxed.

Color constancy and color perception: the linear-models framework

Abstract: A butterfly carton made from a single blank with an auto-erecting lock bottom, and a recloseable top portion including an attractive butterfly wing shaped design. The butterfly carton is of a knock-down variety which can be shipped in a flattened configuration and is further provided with an auto-erecting lock bottom to facilitate erection of the carton. The carton is provided with movable triangular shaped locking portions, pivotally connected to the top of the carton, which are interengaged with V-shaped slots provided in the side wall of the carton, to securely lock the carton in the closed condition. The top portion of the carton is also provided with butterfly wing shaped tabs which are interengaged to provide secondary locking of the carton. In addition, the butterfly wing shaped tabs are maintained at an angle relative to the top of the carton to create an aesthetically pleasing design.

Chromatic adaptation in hard copy/soft copy comparisons

Abstract: The human visual system has evolved with a sophisticated set of mechanisms to produce stable perceptions of object colors across changes in illumination. This phenomenon is typically referred to as chromatic adaptation or color constancy. When viewing scenes or hard-copy reproductions, it is generally assumed that one adapts almost completely to the color and luminance of the prevailing light source. This is likely not the case when soft-copy image displays are viewed. Differences in the degree of chromatic adaptation to hard-copy and soft- copy displays point to two types of chromatic-adaptation mechanisms: sensory and cognitive. Sensory mechanisms are those that act automatically in response to the stimulus, such as retinal gain control. Cognitive mechanisms are those that rely on observers' knowledge of scene content. A series of experiments that measured the spatial, temporal, and chromatic properties of chromatic-adaptation mechanisms are reviewed and a mathematical model for predicting these chromatic adaptation effects is briefly described along with some practical recommendations, based on psychophysical experiments, on how to approach these problems in typical cross-media color reproduction situations.

Surface color and color constancy

Abstract: Color constancy is often treated as the tendency of surfaces to stay the same perceived color under changing illumination or context (removing/adding/replacing surrounding objects). But these types of color constancies are not basic ones and there is another kind of color constancy that is fundamental for the explanation of all color constancy phenomena. We experience it when looking at a curved uniformly colored surface or when changing the shape of the surface. A new concept of surface color is developed and the variety of all perceived colors is suggested to be described as a nine-dimensional set of 3 X 3 matrices corresponding to different surface colors. Examples of color matrices calculated for some colored surfaces being viewed by the standard viewer are presented and arguments supporting the concept are discussed. It is shown that the set of color matrices represents all perceived colors quite adequately.

Conformal transplantation of lightness to varying resolution sensors

Abstract: Many standard computer vision algorithms for shape from shading, optical flow, color constancy or surface reconstruction depend upon the solution of Poisson equations. Using retinex lightness computation as an example, it is shown how these algorithms can be dramatically sped up using a varying resolution sensor, so long as the sensor architecture is based on a conformal mapping. The varying resolution lightness algorithm is developed by the technique of using a conformal transplant. In the final analysis, very little of the algorithm needs to be changed, but a speed up in running time of roughly 50 times is obtained, even though the total amount of data is reduced by only a factor of 28. The speed increases at the expense of peripheral resolution. >

Can color-space transformation improve color computations other than von Kries?

Abstract: Three-dimensional objects in an image, which appear with shading and cast shadows, can be difficult to recognize as single entities, and there can also be problems recognizing the colors of the objects independent of the spectrum of illumination. The removal of shading and cast shadows has often been done in remote sensing by the band-ratio algorithm. A ratio of red to green bands cancels variations of incident light intensity between different points on the same matte object. Finlayson et al. showed that, for physically reasonable illuminant and reflectance spectra, von Kries adaptation gives exact color constancy if a particular linear transformation on the color-matching functions is performed prior to adaptation. The present paper extends this approach to band ratios, and also to the related color-constancy model of Judd (which subtracts the white-reflectance chromaticity instead of dividing by the white-reflectance tristimulus values as von Kries adaptation does). In both cases, invariance requires the illuminant basis functions to be metameric (up to a scale factor) -- with respect to the reference white in the case of Judd adaptation, and with respect to all reflectances in the case of band ratios. The von Kries theory thus seems unique among the simple processing methods.

Distinctness of perceived three-dimensional form induced by modulated illumination : effects of certain display and modulation conditions

Abstract: The present study concerns the distinctness of the 3-D shape-induced on flat displays by spacemodulated illumination (“shape from shading”) The displays that we used varied in structure, hue contrast, lightness contrast, and in the number of reflectance edges present The modulations of the illumination were a square-wave grating, a gradual modulation (a blurred grating), and an “O’Brien modulation,” which combines an edge and a gradient The displays were compared, using the paired comparison method, as to the distinctness of the perceived 3-D form The results showed that the structure and chromatic color of the display were important factors facilitating the distinctness of the induced 3-D shape under all the modulation conditions The results are discussed in relation to the “vector model” for color constancy, proposed in earlier papers

An energy-minimization approach to color constancy

Abstract: A computational approach to color constancy is formulated as an energy minimization problem. The energy function for the problem consists of error terms (data fitting) of the contrast and the local DC values between input image and recovered surface reflectance, and a gray-world constraint term. A neural network model which works to minimize the energy is constructed and the performance of the model is evaluated using the images under the illuminants with color and spatial luminance variations. It turned out that the illuminant variations were removed and color constancy was achieved in the output image. It is shown that the color contrast effect can also be simulated by the proposed model.

Quantifying Human Color Constancy.

Abstract: The visual system generates object color on the basis of spectral information mediated by reflected light. Although the color of this light, and hence, surface color, varies with the color and intensity of the ambient illumination, the visual system is capable of maintaining a fair degree of color fidelity, a phenomenon known as color constancy. Color constancy was already discussed by Helmholtz,1 but there still is surprisingly little known about the various underlying mechanisms. The current state of the art is rather confusing, since theoretical and experimental studies tend to follow different routes. Initiated by the work of Buchsbaum,2 a new class of computational models has emerged,3,4 but so far, no attempts have been made to confront these models with experimental data. As if in retribution, no attempts have been made in the recent experimental studies5,6 to quantify the data within the context of these new models. As a matter of fact, modeling of color constancy data is hardly ever done. A notable exception is the study by McCann et al.7 in which a sizeable data set was not only collected, but also theoretically accounted for, the latter within the context of the well-known retinex model.8,9 In the following we present some of the results of research that started as an “RGB analogue” of the McCann et al. study,10 but that eventually turned into a much more extensive PhD project.11 Here only two of the variables tested will be discussed: the chromaticity and the spectral composition of the illuminant. The data we obtained not only allowed the derivation of a simple (but accurate) quantitative model, but also provided the first experimental test of the basic principles underlying the aforementioned computational approaches to color constancy.

Color image enhancement using color constancy based on modified IHS coordinate system

Abstract: Color image enhancement to restore natural color by excluding the effect of the ambient illumination is important in recent image processing. In this paper, a new color image enhancement method using color constancy based on pseudo-linearly modified IHS coordinate system is proposed. Since the color constancy processing preserves only hue while reducing the dynamic range of lightness and saturation, the technique of dynamic range increase is used to compensate them. The proposed method, which analyzes the relationship between the RGB and modified IHS coordinate system, transforms and increases lightness and saturation simultaneously to avoid the complexity in the related transformation.

Color recovery from biased illumination: color constancy

Abstract: An algorithm for achieving color constancy is presented. The algorithm consists of four models: the finite-dimensional model, the homomorphic model, the statistical model, and the recovery model. From them, it is possible to estimate the surface reflectance, even when the spectral distribution of the ambient light is unknown. Thus color constancy can be achieved under biased illumination. To prove the correctness of the authors' algorithm, some experiments are conducted under different illuminative conditions. >

A framework for segmentation using physical models of image formation

Abstract: : Most approaches to computer image segmentation group sets of pixels according to visible features of an image such as edges, color, brightness, and curvature. Such approaches exploit specialized object properties to obtain satisfactory groupings, which can force those techniques to be domain specific. Furthermore, they do not provide a physical explanation for the image, nor do they group regions that have a single physical structure yet differing visible features. This paper presents a new approach to segmentation using explicit hypotheses about the physics that creates images. We propose an initial segmentation that identifies image regions exhibiting constant color, but possibly varying intensity. For each region, hypotheses are proposed that specifically model the illumination, reflectance, and shape of the 3-D patch which caused that region. An image region may have many hypotheses simultaneously, and each hypothesis represents a distinct, plausible explanation for the color and intensity variation of that patch. Hypotheses for adjacent patches can be compared for similarity and merged when appropriate, resulting in more global hypotheses for grouping elementary regions. This approach to segmentation has the potential to provide a list of possible explanations for a given image; to group together regions with coherent physical properties; and to provide a framework for applying specific operators such as shape-from-shading, color constancy, and roughness evaluation as part of the overall process of low- level vision. However, many profound unsolved problems are raised in determining the most plausible explanations for a given image region. In this paper, we present the approach, working through an example by hand, and discuss the implications of this approach for physics-based vision.

Object Identification and Search: Animate Vision Alternatives to Image Interpretation

Abstract: We are accustomed to thinking of the task of vision as being the construction of a detailed representation of the physical world. However, a paradigm that we term animate vision argues that vision is more readily understood in the context of the tasks that the system is engaged in, and that these tasks may not require elaborate categorical representations of the 3-D world. As an example, we show how the general problem of image interpretation can be replaced in many cases by a combination of two simpler problems, identification and search. Both tasks use multidimensional color histograms to represent the model and images. Color histograms are shown to permit efficent matching and a sufficiently rich representation to distinguish among a large number of objects.