Perceived food quality under metameric vs. chromatic illumination changes

Are there regions in a color space where color categories are invariant across illuminant changes? If so, what characteristics make them more stable than other regions? To address these questions, we asked observers to give a color name to 424 colored surfaces, presented one at a time, under various chromatic illuminants. Results showed a high degree of categorical color constancy, especially under illuminants that occur in the natural environment. It was also shown that surfaces selected as a focal color (the best example of a color category) are more resistant to illuminant change than nonfocal color samples. We additionally ran an asymmetric color matching experiment to quantify the shift of color appearance induced by illuminant changes using surfaces that were all named gray, thereby disentangling the appearance-based color constancy from the categorical color constancy (which are often confounded). Results suggested that the appearance of color samples largely shifted owing to illuminant changes, even though all samples were named gray; showing that the constancy of a color category is substantially more robust than the constancy of color appearance.

Ratios of excitations in each cone-photoreceptor class produced by light reflected from pairs of surfaces in a scene are almost invariant under natural illuminant changes. The stability of these spatially defined ratios may explain the remarkable ability of human observers to efficiently discriminate illuminant changes from changes in surface reflectances. Spatial cone-excitation ratios are not, however, exactly invariant. This study is concerned with observers' sensitivity to these invariance violations. Simulations of Mondrian paintings with either 49 or two natural surfaces under Planckian illuminants were presented as images on a computer-controlled display in a two-interval experimental design: in one interval, the surfaces underwent an illuminant change; in the other interval, the surfaces underwent the same change but the images were then corrected so that, for each cone class, ratios of excitations were preserved exactly. Although the intervals with corrected images corresponded individually to highly improbable natural events, observers systematically misidentified them as containing the illuminant changes, the probability of error increasing as the violation of invariance in the other interval increased. For the range of illuminants and surfaces tested, sensitivity to violations of invariance was found to depend on cone class: it was greatest for long-wavelength-sensitive cones and least for short-wavelength-sensitive cones. Spatial cone-excitation ratios, or some closely related quantities, seem to be the cues preferred by observers for making inferences about surface illuminant changes.

Human observers can correctly attribute changes in the appearance of a scene either to changes in the incident light or to changes in the spectral-reflectance properties of the scene. This ability was assessed as a function of the time course of illuminant and spectral-reflectance changes. Observers were presented with computer simulations of Mondrian patterns of 49 randomly selected Munsell papers. On each trial a Mondrian pattern was presented for 1 s; the pattern then changed either instantaneously or gradually into another Mondrian pattern, also presented for 1 s, which was related to the first either by an illuminant change or by an illuminant change accompanied by additional changes in the spectral-reflectance functions of the individual papers. Illuminant and spectral-reflectance change were applied linearly in time (with respect to CIE coordinates) over intervals ranging from 0 to 7 s. Observers indicated whether there was a spectral-reflectance change. They were able to make reliable discriminations between illuminant and spectral-reflectance changes both when the changes were applied instantaneously and when they were applied gradually over time, but performance worsened progressively as the duration of the changes increased, that is, as their rate decreased. It is suggested that discrimination in this task depends on the extraction of a low-level transient signal which is generated in response to rapid changes in scene appearance and which is progressively attenuated as changes occur more and more gradually.

Color constancy refers to the ability of the human visual system to stabilize the color appearance of surfaces under an illuminant change. In this work we studied how the interrelations among nine colors are perceived under illuminant changes, particularly whether they remain stable across 10 different conditions (5 illuminants and 2 backgrounds). To do so we have used a paradigm that measures several colors under an immersive state of adaptation. From our measures we defined a perceptual structure descriptor that is up to 87% stable over all conditions, suggesting that color category features could be used to predict color constancy. This is in agreement with previous results on the stability of border categories [1,2] and with computational color constancy algorithms [3] for estimating the scene illuminant.

Most state-of-the-art approaches to object and face detection rely on intensity information and ignore color information, as it usually exhibits variations due to illumination changes and shadows, and due to the lower spatial resolution in color channels than in the intensity image. We propose a new color descriptor, derived from a variant of Local Binary Patterns, designed to achieve invariance to monotonic changes in chroma. The descriptor is produced by histograms of encoded color texture similarity measures of small radially-distributed patches. As it is based on similarities of local patches, we expect the descriptor to exhibit a high degree of invariance to local appearance and pose changes. We demonstrate empirically by simulation the invariance of the descriptor to photometric variations, i.e. illumination changes and image noise, geometric variations, i.e. face pose and camera viewpoint, and discriminative power in a face detection setting. Lastly, we show that the contribution of the presented descriptor to face detection performance is significant and superior to several other color descriptors, which are in use for object detection. This color descriptor can be applied in color-based object detection and recognition tasks.

One proposed mechanism for underpinning colour constancy is computation of the relative activity of cones within one class--cone ratios, or cone contrasts--between surfaces in a fixed scene undergoing a change in illuminant. Although there is evidence that cone ratios do determine colour appearance under many conditions, the site or sites of their computation is unknown. Here, we report that a cerebrally achromatopsic observer, MS, displayed evidence of colour constancy in asymmetric colour matching tasks and was able to discriminate changes in cone ratios for simple, but not complex scenes. We hypothesise that the site of local cone-ratio computation is therefore early in the visual system, probably retinal.

Stereo retinex

When objects are viewed in different illuminants, their color does not change or changes little in spite of significant changes in the wavelength composition of the light reflected from them. In previous studies, we have addressed the physiology underlying this color constancy by recording from cells in areas V1, V2, and V4 of the anesthetized monkey. Truly color-coded cells, ones that respond to a patch of a given color irrespective of the wavelength composition of the light reflected from it, were only found in area V4. In the present study, we have used a different approach to test the responses of V4 cells in both anesthetized and awake behaving monkeys. Stimuli of different colors, embedded within a Mondrian-type multicolored background, were used to identify the chromatic selectivity of neurons. The illumination of the background was then varied, and the tuning of V4 neurons was tested again for each background illumination. With anesthetized monkeys, the psychophysical effect of changing background illumination was inferred from our own experience, whereas in the awake behaving animal, it was directly reported by the monkey. We found that the majority of V4 neurons shifted their color-tuning profile with each change in the background illumination: each time the color of the background on the computer screen was changed so as to simulate a change in illumination, cells shifted their color-tuning function in the direction of the chromaticity component that had been increased. A similar shift was also observed in colored match-to-sample psychometric functions of both human and monkey. The shift in monkey psychometric functions was quantitatively equivalent to the shift in the responses of the corresponding population of cells. We conclude that neurons in area V4 exhibit the property of color constancy and that their response properties are thus able to reflect color perception.

In this talk we will discuss calibration transforms that map the XYZ values generated by the same surface under different illuminants. We use the phrase calibration transforms to distinguish between analyses based on the physical properties of surfaces and illuminants, and to distinguish them from appearance transforms based on measurements of color appearance. Calibration transforms describe how the XYZ coordinates measured for a surface change with illumination. Appearance transforms describe how the XYZ coordinates of a particular appearance change with illumination. The change in XYZ values due to calibration and appearance transforms do not generally coincide.Calibration and appearance transforms serve different and useful functions in color management systems. Calibration transforms can be used to correct the device-independent color descriptors of surfaces that have been calibrated for one illuminant but will be rendered under a different illuminant. These calculations play an important role in device calibration. Appearance transforms share a similar computational structure, but they have a different goal and are not XYZ matches.One step in performing automated calibration transforms is to estimate the illuminant spectral power distribution (SPD). Performing the transform is much simpler if one can estimate the illuminant SPD from a three-sensor device, rather than using a spectroradiometer. We first analyze an illuminant estimation method based on using linear models of the illuminant SPD. This solution is taken from the color constancy literature and assumes very little or no information about the objects in the image.In many practical applications the need for accurate calibration transforms outweighs the advantages of algorithms based on little or no information about the surfaces and illuminants in a scene. In this paper we describe how a measurement of a single calibration with known surface target, such as the Macbeth Color-Checker, improves our ability to estimate the illuminant SPD. This method may be useful in practical applications where a single calibration measurement is permitted.

Color-based object recognition

Invariance of surface color representations across illuminant changes in the human cortex

The appearance of the “green” areas of Mars has been discussed from the standpoint of visual physiology. The conditions for perception of colors on Mars are favorable insofar as the observation is foveal and the luminance range is within that of daytime. Because of the smallness of some details normal color vision may reach an area-intensity limit beyond which it becomes very unreliable. In order to elucidate whether the variable blueness, bluegreenness or greenness of the dark areas are real or not the following factors have been discussed: (1) the transmittance of the atmospheres of Mars and Earth in its effect on the appearance of coloration on Mars; (2) the possibility of interpreting the coloration of the dark areas as a contrast phenomenon. Here we have to consider (a) the sun as a chromatic illuminant; (b) the yellow-red bright areas as potential contrast inducing areas.— The production of contrasts was studied experimentally by using colored papers simulating the bright and dark areas. The experiments permit us to deduce that a contrast induction is possible on the surface of Mars, the contrast depending on hue, brightness and saturation of contrast inducing and contrasting area. Blurredness of contours enhances contrasts. An attempt has been made to explain the difference in the induced hue on small (10′ to 20′) and large (1° and up) contrasting neutral gray areas. A method is shown for reducing contrast development which may be helpful in deciding about the real nature of the “green” areas of Mars.

We have examined visual functions, including colour vision, in a patient with bilateral cortical lesions involving mainly the fusiform and lingual gyri, areas known to be involved in the central processing of chromatic stimuli. The patient has near normal (6/9) acuity, and his responses to tests of binocular function and spatial vision are normal, as are his discrimination of changes in target speed and surface lightness. He does, however, exhibit minor losses in the upper visual field, mild prosopagnosia and topographical agnosia, all conditions commonly associated with cerebral achromatopsia. Colour matches and spectral response data establish that his cone photoreceptors have normal spectral characteristics and his spectral sensitivity measured against a white background reveals normal postreceptoral chromatic function. The patient's colour discrimination for differences in wavelength, hue or saturation is, however, impaired and his colour naming is significantly disturbed, particularly for blues and greens. We have determined the areas of the chromaticity chart that correspond to his naming categories for surface colours, and show that changes in illuminant cause him to alter the names of surface colours in a manner consistent with the changes in their chromaticities. Other subjects with normal or congenital red-green deficient colour vision make many fewer name changes under changes in illuminant. We conclude that the patient's colour constancy is impaired as a consequence of abnormal central processing of colour vision.

Event Abstract Back to Event The effect of surface material on colour constancy in real world scenes Annette Werner1* and Lara Zebrowski1, 2 1 Tübingen University, Ophthalmology, Germany 2 Ophthalmology, Germany The visual appearance of real world objects depends on their reflectance properties, illumination and geometrical factors. We asked how human colour constancy performance deals with the depth-mesostructure of surfaces, which may provide additional cues or, alternatively, worsen colour constancy. In our experiments we presented samples (circular patches, 90 mm) of different materials (paper, fur, tile, cloth, leather), each mounted on a black sample holder (front 30 x 15 cm, 60 deg slope) and presented in the middle of a black viewing box (1.0 x 1.0 x 0.8 m). All samples appeared nearly achromatic under daylight. The samples and the box were illuminated by a computer controlled, calibrated LCD-projector (Panasonic PT AE 1000E) which was mounted above the observers' head. The observers (2 male, 2 female) viewed the samples frontally (viewing distance 90 cm). The chromaticity of the standard illuminant was D65, the equiluminant test-illuminants were chosen from one of the cardinal axes. After viewing the samples under the standard illuminant for 15 s, the respective test-illuminant was presented for 5 s. Colour constancy was quantified by a hue-cancellation method, whereby the observer adjusts the appearance of the sample so that it appears achromatic (“gray”) under the respective test illuminant. Colour constancy for the different materials was found not to differ significantly, but there was a trend for compensating illumination changes best for fur and worst for tile surfaces, across all illuminants and observers, with paper ranking in the middle. The possible role of depth-mesostructure as a cue for colour constancy is discussed. Acknowledgements Supported by BMBF grant FKZ 01GQ1002. Keywords: colour constancy, material &amp, surfaces, mesostructure, Perception Conference: Bernstein Conference 2012, Munich, Germany, 12 Sep - 14 Sep, 2012. Presentation Type: Poster Topic: Sensory processing and perception Citation: Werner A and Zebrowski L (2012). The effect of surface material on colour constancy in real world scenes. Front. Comput. Neurosci. Conference Abstract: Bernstein Conference 2012. doi: 10.3389/conf.fncom.2012.55.00051 Copyright: The abstracts in this collection have not been subject to any Frontiers peer review or checks, and are not endorsed by Frontiers. They are made available through the Frontiers publishing platform as a service to conference organizers and presenters. The copyright in the individual abstracts is owned by the author of each abstract or his/her employer unless otherwise stated. Each abstract, as well as the collection of abstracts, are published under a Creative Commons CC-BY 4.0 (attribution) licence (https://creativecommons.org/licenses/by/4.0/) and may thus be reproduced, translated, adapted and be the subject of derivative works provided the authors and Frontiers are attributed. For Frontiers’ terms and conditions please see https://www.frontiersin.org/legal/terms-and-conditions. Received: 25 May 2012; Published Online: 12 Sep 2012. * Correspondence: Dr. Annette Werner, Tübingen University, Ophthalmology, Tübingen, 72070, Germany, annette.werner@uni-tuebingen.de Login Required This action requires you to be registered with Frontiers and logged in. To register or login click here. Abstract Info Abstract The Authors in Frontiers Annette Werner Lara Zebrowski Google Annette Werner Lara Zebrowski Google Scholar Annette Werner Lara Zebrowski PubMed Annette Werner Lara Zebrowski Related Article in Frontiers Google Scholar PubMed Abstract Close Back to top Javascript is disabled. Please enable Javascript in your browser settings in order to see all the content on this page.

Colour constancy assessed by asymmetric simultaneous colour matching usually reveals limited levels of performance in the unadapted eye. Yet observers can readily discriminate illuminant changes on a scene from changes in the spectral reflectances of the surfaces making up the scene. This ability is probably based on judgements of relational colour constancy, in turn based on the physical stability of spatial ratios of cone excitations under illuminant changes. Evidence is presented suggesting that the ability to detect violations in relational colour constancy depends on temporal transient cues. Because colour constancy and relational colour constancy are closely connected, it should be possible to improve estimates of colour constancy by introducing similar transient cues into the matching task. To test this hypothesis, an experiment was performed in which observers made surface-colour matches between patterns presented in the same position in an alternating sequence with period 2 s or, as a control, presented simultaneously, side-by-side. The degree of constancy was significantly higher for sequential presentation, reaching 87% for matches averaged over 20 observers. Temporal cues may offer a useful source of information for making colour-constancy judgements.