Relational Constancy Research Articles

Misidentifying illuminant changes in natural scenes due to failures in relational colour constancy.

The colours of surfaces in a scene may not appear constant with a change in the colour of the illumination. Yet even when colour constancy fails, human observers can usually discriminate changes in lighting from changes in surface reflecting properties. This operational ability has been attributed to the constancy of perceived colour relations between surfaces under illuminant changes, in turn based on approximately invariant spatial ratios of cone photoreceptor excitations. Natural deviations in these ratios may, however, lead to illuminant changes being misidentified. The aim of this work was to test whether such misidentifications occur with natural scenes and whether they are due to failures in relational colour constancy. Pairs of scene images from hyperspectral data were presented side-by-side on a computer-controlled display. On one side, the scene underwent illuminant changes and on the other side, it underwent the same changes but with images corrected for any residual deviations in spatial ratios. Observers systematically misidentified the corrected images as being due to illuminant changes. The frequency of errors increased with the size of the deviations, which were closely correlated with the estimated failures in relational colour constancy.

Color Constancy, Illumination, and Matching

Color constancy is a foundational and yet puzzling phenomenon. Standard appearance invariantism is threatened by the psychophysical matching argument, which is taken to favor variantism. This argument, however, is inconclusive. The data at best support a pluralist view: color constancy is sometimes variantist, sometimes invariantist. I add another potential explanation of these data, complex invariantism, which adopts an atypical six-dimensional model of color appearance. Finally, I prospect for a unifying conception of constancy among two neglected notions: discriminatory color constancy and relational color constancy. The former arguably marks a common core capacity that is present across widely differing viewing contexts.

Relational color constancy in the absence of ratio constancy: 3D scenes with spatially inhomogeneous illumination

Relational color constancy in the absence of ratio constancy: 3D scenes with spatially inhomogeneous illumination

The span of cone ratios and color naming

It has been suggested that ratios of photoreceptor catches for the light reflected by pairs of surfaces play an important role in color vision (Foster & Nascimento, 1994). We provide results from simulations showing that L, M and S ratios obtained from surface pairs consisting of a colored surface and an achromatic surface of the same luminosity reach their extreme values for a few specific colored surfaces, namely red, green, blue and yellow surfaces. There is a striking correspondence between the colors singled out in this way and the ones singled out in cross-cultural studies of color naming (Regier, Kay & Cook, 2005). These results will be compared with a related approach proposed in Philipona & O'Regan (in press), which further addressed the question of unique hues and hue cancellation. References - Nascimento Foster. (1994). Relational colour constancy from invariant cone-excitation ratios. Proc. R. Soc. Lond. B. 257, 115-121. - Kay Regier Cook. (2005). Focal colors are universal after all. Proc. Natn. Acad. Sci. USA. 102, 8386-8391. - O'Regan Philipona (in press). Color naming, unique hues and hue cancellation predicted from singularities in reflection properties. Vis. Neurosci.

Color constancy from invariant wavelength ratios: I. The empirical spectral mechanism

AbstractThe wavelengths of several constant hues over four illuminants (D95, D65, D50, A) are derived from several sets of published data. In the plane of wavelength and reciprocal illuminant color temperature (MK−1), the wavelengths of constant hues plot straight approximately parallel lines whose mean slope is about 87°. Parallel lines give invariant wavelength ratios, hence constant hues in this plane are near‐invariant wavelength ratios across illuminants. As recently demonstrated, the complementary wavelengths to a constant hue (across illuminants) represent the complementary constant hue; these complementary wavelengths also plot a near‐parallel line to the first constant hue. To confirm and further define the constant slope of these lines, it is shown that complementary wavelength pairs, per CIE data, can only plot parallel straight lines at the angle of 87° ± 1. In summary, near‐parallel sloping lines represent constant hues at near‐invariant wavelength ratios. This mechanism of color constancy is shown to relate to the well‐known theory of relational color constancy from invariant cone‐excitation ratios. In the visual process, the latter ratios are presumably the source of the former (invariant wavelength ratios). © 2008 Wiley Periodicals, Inc. Col Res Appl, 33, 238–249, 2008

Local and relational judgements of surface colour: constancy indices and discrimination performance

Colour constancy is generally assumed to arise from a combination of perceptual constancy mechanisms operating to partially discount illumination changes and relational mechanisms involved in judging the colour relationships between object surfaces. Here we examined the characteristics of these mechanisms using a 'yes/no' task. Subjects judged whether a target colour patch embedded in an array of coloured patches (a) stayed the same across a simulated temporal illuminant change (local colour judgement), or (b) changed in a manner consistent with the illuminant change (relational colour judgement). The colour of the target patch remained constant in one-third of the trials, changed in accord with the illuminant shift in another third, and shifted partially with the illuminant change in the remaining third. We found that perceptual constancy was relatively weak and relational constancy strong, as assessed using a modified colour constancy index. Randomising the spatial positions of coloured patches across the illuminant change did not affect subjects' constancy indices. Application of signal detection analysis revealed some otherwise hidden effects. In the case of relational judgements, subjects adopted more conservative criteria (fewer true and false positives) with randomisation, maintaining a constant level of discrimination performance (d'). For local judgements, randomisation led to small increases in performance but no changes in criteria. We conclude that signal detection theory provides a useful tool to supplement conventional approaches to understanding colour constancy.

Minimum-variance cone-excitation ratios and the limits of relational color constancy

Relational color constancy refers to the constancy of the perceived relations between the colors of surfaces of a scene under changes in the spectral composition of the illuminant. Spatial ratios of cone excitations provide a natural physical basis for this constancy, as, on average, they are almost invariant under illuminant changes for large collections of natural surfaces and illuminants. The aim of the present work was to determine, computationally, for specific surfaces and illuminants, the constancy limits obtained by the application of a minimum-variance principle to cone-excitation ratios and to investigate its validity in predicting observers' surface-color judgments. Cone excitations and their changes due to variations in the color of the illuminant were estimated for colored surfaces in simulated two-dimensional scenes of colored papers and real three-dimensional scenes of solid colored objects. For various test surfaces, scenes, and illuminants, the estimated levels of relational color constancy mediated by cone-excitation ratios varied significantly with the test surface and only with certain desaturated surfaces corresponded to ideal matches. Observers' experimental matches were compared with predictions expressed in CIE 1976 (u',v') space and were found to be generally consistent with minimum-variance predictions.

Color constancy from physical principles

A well known property of human vision, known as color constancy, is the ability to correct for color deviations caused by a difference in illumination. A common approach to investigate color constant behavior is by psychophysical experiments, regarding the human visual system as a black box responding to a well defined change in an laboratory setup.A fundamental problem in psychophysical experiments is that significant conclusions are hard to draw due to the complex experimental environment necessary to examine color constancy. An alternative approach to reveal the mechanisms involved in color constancy is by modeling the physical process of spectral image formation. In this paper, we aim at a physical basis for color constancy rather than a psychophysical one.By considering spatial and spectral derivatives of the Lambertian image formation model, object reflectance properties are derived independent of the spectral energy distribution of the illuminant. Gaussian spectral and spatial probes are used to estimate the proposed differential invariant. Knowledge about the spectral power distribution of the illuminant is not required for the proposed invariant.The physical approach to color constancy offered in the paper confirms relational color constancy as a first step in color constant vision systems. Hence, low-level mechanisms such as color constant edge detection may play an important role in front-end vision. The research presented raises the question of whether the illuminant is estimated at all in pre-attentive vision.

Cone contributions to colour constancy.

Colour constancy refers to the stable perception of object colour under changing illumination conditions. This problem has been reformulated as relational colour constancy, or the ability of the observer to discriminate between material changes and changes in illumination. It has been suggested that local cone excitation ratios play a prominent role in achieving such constancy. Here we show that perceptual colour constancy measured by achromatic adjustments is to a large part complete after 25 ms. This speaks against a prominent role for receptor adaptation, which takes significantly longer. We also found no difference in colour constancy between colour changes that were compatible with a change of illuminant, and between colour changes where local cone ratios were uncorrelated between the two illuminants. Our results show that constant cone ratios are not necessary for colour constancy.

How temporal cues can aid colour constancy

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.

How temporal cues can aid colour constancy

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.

Relational color constancy in achromatic and isoluminant images.

Relational color constancy, which refers to the constancy of perceived relations between surface colors under changes in illuminant, may be based on the computation of spatial ratios of cone excitations. As this activity need occur only within rather than between cone pathways, relational color constancy might be assumed to be based on relative luminance processing. This hypothesis was tested in a psychophysical experiment in which observers viewed simulated images of Mondrian patterns undergoing colorimetric changes that could be attributed either to an illuminant change or to a nonilluminant change; the images were isoluminant, achromatic, or unmodified. Observers reliably discriminated the two types of changes in all three conditions, implying that relational color constancy is not based on luminance cues alone. A computer simulation showed that in these isoluminant and achromatic images spatial ratios of cone excitations and of combinations of cone excitations were almost invariant under illuminant changes and that discrimination performance could be predicted from deviations in these ratios.

Background articulation and relational colour constancy

A visual search experiment was performed to examine whether the detection of constant chromatic relationships occurs in parallel. We used 6 stimulus conditions, corresponding to different combinations of three variables, relevant to description of displays made of different coloured regions: (i) the number of items; (ii) the number of background colour ratios; (iii) the total number of colour ratios. An illumination change was simulated in all trials. In half the trials, the simultaneous reflectance change of one item was also simulated. When the number of colour ratios is constant, detection becomes worse as the number of items increases, as predicted by serial processing of reflectance changes. However, background articulation favors relational colour constancy, by providing information about illumination changes. © 2000 John Wiley & Sons, Inc. Col Res Appl, 26, S201–S204, 2001

Relational colour constancy under rapid changes of illuminant on natural scenes

Relational colour constancy under rapid changes of illuminant on natural scenes

Detecting natural changes of cone-excitation ratios in simple and complex coloured images.

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.

Discrimination of Reflectance Changes under Simultaneous Illumination Changes

Relational colour constancy refers to the perception of constant relations among surface colours under illumination changes. We tested whether the ability of human observers to discriminate illumination from reflectance changes in a scene involves high-level attentional mechanisms. Observers were shown pairs of computer-generated Mondrian patterns with the number of patches varying between 2 and 49. On each trial a Mondrian pattern was presented for 1 s and replaced by a second pattern. In half of the trials in each experimental condition, the chromaticity of one patch was changed. Observers were required to discriminate whether a reflectance change occurred. Three conditions were run. In condition A, the illumination of the second Mondrian underwent a chromaticity change. In condition B, the chromaticity of the second Mondrian illumination remained constant, whereas its overall level was either increased or decreased. In condition C (control), the illumination was exactly the same in both Mondrians. Functions relating reaction time to number of patches in the display were compared. Data suggest that different mechanisms are involved in discriminating reflectance changes associated with different types of illuminant changes.

Four issues concerning colour constancy and relational colour constancy

Four issues concerning colour constancy and relational colour constancy are briefly considered: (1) the equivalence of colour constancy and relational colour constancy; (2) the dependence of relational colour constancy on ratios of cone excitations due to light from different reflecting surfaces, and the association with von Kries' coefficient rule; (3) the contribution of chromatic edges to colour constancy and relational colour constancy; and (4) the effects of instruction and observer training. It is suggested that cognitive factors affect colour constancy more than relational colour constancy, which may be an inherently more robust phenomenon.

Dependence of relational colour constancy on the extraction of a transient signal.

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.

Relational colour constancy from invariant cone-excitation ratios

Quantitative measurements of perceptual colour constancy show that human observers have a limited and variable ability to match coloured surfaces in scenes illuminated by different light sources. Observers can, however, make fast and reliable discriminations between changes in illuminant and changes in the reflecting properties of scenes, a discriminative ability that might be based on a visual coding of spatial colour relations. This coding could be provided by the ratios of cone-photoreceptor excitations produced by light from different surfaces: for a large class of pigmented surfaces and for surfaces with random spectral reflectances, these ratios are statistically almost invariant under changes in illumination by light from the sun and sky or from a planckian radiator. Cone-excitation ratios offer a possible, although not necessarily unique, basis for perceptual colour constancy in so far as it concerns colour relations.