The effects of surface gloss and roughness on color constancy for real 3-D objects

Perception of colour depends on the spectral composition of light that reaches retina and depends as well on various mechanisms of visual system that processes information flow. The few important mechanisms can be distinguished in colour perception: colour adaptation, colour constancy and colour contrast. If the visual field has only one coloured object, then colour perception will be determined by spectral composition of the light and colour adaptation. Whereas mechanisms of colour constancy and colour contrast switches on when in the visual field there are at least two colour objects. Von Kries (1905) have attributed colour constancy phenomenon to theory of receptoral adaptation. But this is theory is applicable to local processes happening in relatively small size of visual field. Craven and Foster (1992) shows that receptor excitation ratios remains constant during change of illumination. But again remains unexplained colour change during adaptation. So, the hypothesis is raised that two processes attribute colour perception: local colour contrast calculation and global adaptation to the background. Therefore the experiments have been carried out to establish the colour contrast and background adaptation impact on colour perception.Four subjects with normal colour vision participated in the experiments. 40 Munsell samples (value 7 and chroma 4) illuminated with one reference illuminant (standard C) and 6 test illuminants (standard A, standard S, cardinal red, cardinal green, cardinal yellow and cardinal blue) were simulated on computer monitor. The dark-adapted subjects have been shown colour samples on neutral background illuminated with one of test illuminants. The task was to match the colour stimuli appearance under reference illuminant. The sequential asymmetric matching procedure was carried out. First subject adapts to neutral background under illuminan C then test stimulus and test background appears for limited time followed by readaptation to neutral background. Then subject is asked to adjust the colour mach under illuminant C. Two different experiment paradigms have been used. In the first one the stimulus had 2° on neutral background covering 20° of visual field. In the second paradigm the stimulus size was the same but the background was covering the all visual field. The adaptation and presentation timings were 1, 5, 30 or 60 seconds.Results under the first experiment paradigm show partial colour constancy for all subjects and for all test illuminants under various adaptation times. Despite quite a long adaptation time of 30 seconds subjects was unable to achieve full colour constancy. Colour constancy improves under second experiment paradigm. 60-second adaptation time is enough to achieve full colour constancy. The less adaptation time (1 or 30 second) gives partial colour constancy.The subject sees two colours (stimulus and background) during the experiment. The colour perception of stimulus and background changes during adaptation time. In the first 5 seconds of adaptation 40–60% of full adaptation level is reached (Fairchild and Reniff, 1995; Werner et al., 2000). Wesner and Shevell (1992, 1994) shown that signals from all visual field have influence on colour perception of object. The first part of our experiment shows that neutral background of 20° gives partial adaptation to background colour. Therefore, increase background illumination to full visual field gives full adaptation to background. So, subject perceives different coloured background under various illuminants as the same neutral (grey) background.We can state that two different systems have been distinguished in the process of colour perception. One system evaluates the colour difference between the stimulus and the background. Second system evaluates the colour of the background. Perceived colour difference during adaptation does not change much, but perceived colour of the background changes a lot and drifts towards neutral colour. The hypothesis of two level colour perception is supported by experimental data.

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.

Color constancy is the ability to assign a constant color to an object independent of changes in illumination. Color constancy is achieved by taking context information into account. Previous approaches that have used matching paradigms to quantify color constancy found degrees of constancy between 20% and 80%. Here, we studied color constancy in a color-naming task under different conditions of surround illumination and patch size. Observers categorized more than 400 patches for each illumination condition. This allows one to overcome inherent limitations in color naming and to study the changes in color categories under illumination changes. When small central test patches with a full context illumination were categorized, observers followed the illumination shift almost completely, showing a high degree of constancy (99%). Reducing the available context information or increasing the patch size decreased the degree of constancy to about 50%. Moderate degrees of constancy (66%) occurred even when the test patches were never viewed simultaneously but only in temporal alternation with the illumination. Boundaries between color categories were largely stable within and across observers under neutral illumination. Under changing illumination, there were small but systematic variations in the color category boundaries. Color category boundaries tended to rotate away from the illumination color. This variation was largest under full context conditions where highest degrees of color constancy were obtained.

Color constancy is of important for many computer vision applications, such as image classification, color object recognition, object tracking and so on. But unlike the human visual system, imaging device cannot be able to compute color constant descriptors which do not vary with the color of the illuminant, so solving color constancy problem is necessary. In the calculation of color constancy, illuminant estimation is the key. Because grey surfaces can perfectly reflect the color of the scene illumination, many methods have been proposed to identify grey surfaces to estimate the illuminant. But they either rely on the camera.s parameters, lacking universality, or work inaccurate in worse conditions. In order to solve these problems, in this paper, an iterative method is proposed. The quality of the proposed method is tested and compared to the previous color constancy methods on the Macbeth Chart and two data sets of synthetic and real images. Through MATLAB simulation, experimental pictures and quantitative data for performance evaluation were gotten. The simulated results show that the proposed algorithm is accurate and efficient in identification of the grey surfaces, even in worse condition. And it performs well in color constancy computation on both synthetic and real images.

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.

In order to achieve a compact robot vision system that can perform object identification in real environments exposed to large changes in illumination, it is essential to consider both the algorithm and the hardware architecture. In this study, we have developed a compact and low-power image recognition system that is robust to illumination changes, consisting of a CMOS image sensor, field-programmable gate array (FPGA), and graphics processing unit (GPU) for embedded devices. To minimize the effects of changes in the illumination, our system uses the center/surround (C/S) retinex model, which is a color constancy model of the visual nervous system. Since the C/S retinex model involves large-scale spatial filtering with high computational costs, we compared the processing speeds of different hardware implementations of this model. This comparison showed that the FPGA implementation of the C/S retinex model used in this study is more than 10 times faster than the GPU implementation. Using the output of this efficiently processed C/S retinex model, a relatively small convolutional neural network (CNN), which runs on a GPU for embedded devices, performs object classification. We also investigated the impact of the spatial parameters of the C/S retinex model on the classification accuracy of CNNs using a dataset of images acquired under various lighting conditions. This investigation revealed parameters that provide better classification accuracy, and these parameters were largely independent of the CNN architecture used. This system performed object classification under various illumination colors at 52.1 frames per second with a power consumption of approximately 10.9 W.

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.

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.

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.

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

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.

We investigated several sensory and cognitive determinants of colour constancy across 40 illumination hues. In the first experiment, we measured colour naming for the illumination and for the colour induced by the illumination on the colorimetric grey. Results confirmed that the induced colours are approximately complementary to the colour of the illumination. In the second experiment, we measured colour constancy using achromatic adjustments. Average colour constancy was perfect under the blue daylight illumination and decreased in colour directions away from the blue daylight illumination due to undershooting and a strong blue bias. Apart from this blue bias, colour constancy was not related to illumination discrimination and to chromatic detection measured previously with the same setup and stimuli. We also observed a strong negative relationship between the degree of colour constancy and the consensus of naming the illumination colour. Constancy coincided with a low naming consensus, in particular because bluish illumination colours were sometimes seen as achromatic. Blue bias and category consensus alone explained >68%, and all determinants together explained >94% of the variance of achromatic adjustments. These findings suggest that colour constancy is optimised for blue daylight.

Stereo retinex

This study assessed the relative effects of metameric (preserving illumination chromaticity) vs. chromatic illumination changes on the perceived quality of real fruits. 20 normal trichromats viewed four singly-presented fruits under three neutral metameric illuminations (differing in spectral power distribution with constant CCT of ∼6500K) and two broadband chromatic illuminations (∼2000K and ∼10000K) inside a white-walled lightroom illuminated by spectrally tuneable lamps. Participants’ ratings of nine fruit attributes were used to calculate Positive Attributes Scores (PAS) for each fruit-illumination pair. For two fruits (banana and pear), PAS varied significantly under metameric illumination changes, but not under chromatic illumination changes. PAS varied significantly under both metameric and chromatic illumination changes for orange, and under chromatic illumination changes only for red apple. The findings indicate that metameric illumination changes alter perceived food attributes via changes in fruit colour appearance, while changes in illumination chromaticity may influence perceived food quality independently of fruit colour appearance which is largely maintained via colour constancy.

The simplified ray-tracing technique reported in Part I of this paper is employed to compute the single-scattering properties of hexagonal columns with maximum dimensions ranging from 2 to 3500 mum with a size-bin resolution of 2 mum at wavelengths of 0.86 and 2.13 mum. For small ice crystals, the current treatment of surface roughness may not be adequate because the applicability of the principles of geometric optics breaks down for small roughness scale. However, for ice crystals smaller than 40 mum, the aspect ratios of these particles are close to one, and the effect of surface roughness is quite small. In this paper, the diffraction is accounted for in the same way as in the case of smooth particles. It is essentially unfeasible to incorporate the effect of surface roughness into the numerical computation of the diffraction contribution. The scattering properties of individual ice crystals are then averaged over 18 particle size distributions whose effective particle radii (re) range from 5 to 90 mum. The single-scattering properties of ice clouds are strongly sensitive to surface roughness condition. Lookup tables that are built for the correlation between the bidirectional reflectances at wavelengths of 0.86 and 2.13 mum with different roughness conditions are used to retrieve ice cloud optical thickness and effective particle size over oceans. Pronounced differences are noticed for the retrieved cirrus cloud optical thickness and effective particle sizes in conjunction with different surface roughness conditions. The values of the retrieved cirrus cloud optical thickness in the case of the rough surface are generally smaller than their counterparts associated with smooth surface conditions. The effect of surface roughness on the retrieved effective particle radii is not pronounced for slight and moderate roughness conditions. However, when the surfaces of ice crystals are substantially rough, the retrieved effective radii associated with roughened particles are larger and smaller than their smooth surface counterparts forlarge (re>50 mum) and small (re<35 mum) ice crystals, respectively, whereas the effect of surface roughness on the retrieved effective radii shows a nonmonotonic feature for moderate particle sizes (35 mum<re<50 mum). In general, the dominant effect of surface roughness on cloud property retrievals is to decrease the retrieved optical thickness and to increase the retrieved effective particle size in comparison with their counterparts in the case of smooth ice particles.