Variant and invariant color perception in the near peripheral retina

Perceived shifts in hue that occur with increasing retinal eccentricity were measured by using an asymmetric color matching paradigm for a range of chromatic stimuli. Across nine observers a consistent pattern of hue shift was found; certain hues underwent large perceived shifts in appearance with increasing eccentricity, while for others little or no perceived shift was measured. In separate color naming experiments, red, blue, and yellow unique hues were found to be correlated with those hues that exhibited little or no perceptual shift with retinal eccentricity. Unique green, however, did not exhibit such a strong correlation. Hues that exhibited the largest perceptual shifts in the peripheral retina were found to correlate with intermediate hues that were equally likely to be identified by adjacent color naming mechanisms. However, once again the correlation was found to be weakest for the green mechanism. These data raise the possibility that perceptually unique hues are linked to color signals that represent the most reliable (minimally variant) chromatic information coming from the retina.     

Variations in normal color vision. II. Unique hues

We examined individual differences in the color appearance of nonspectral lights and asked how they might be related to individual differences in sensitivity to chromatic stimuli. Observers set unique hues for moderately saturated equiluminant stimuli by varying their hue angle within a plane defined by the LvsM and SvsLM cone-opponent axes that are thought to characterize early postreceptoral color coding. Unique red settings were close to the +L pole of the LvsM axis, while green, blue, and yellow settings clustered along directions intermediate to the LvsM and SvsLM axes and thus corresponded to particular ratios of LvsM to SvsLM activity. Interobserver differences in the unique hues were substantial. However, no relationship was found between hue settings and relative sensitivity to the LvsM and SvsLM axes. Moreover, interobserver variations in different unique hues were uncorrelated and were thus inconsistent with a common underlying factor such as relative sensitivity or changes in the spectral sensitivities of the cones. Thus for the moderately saturated lights we tested, the unique hues appear largely unconstrained by normal individual differences in the cone-opponent axes. In turn, this suggests that the perceived hue for these stimuli does not depend on fixed (common) physiological weightings of the cone-opponent axes or on fixed (common) color signals in the environment.     

Real-world stimuli show perceived hue shifts in the peripheral visual field

Certain hues undergo shifts in their appearance when they are viewed by the peripheral retina. This has often been shown on a 3-primary color CRT monitor. To investigate the possible role of metamerism, we replicated our peripheral color matching experiments using Munsell paper stimuli viewed under real and simulated daylight (using a 3-primary projection system). Using stimuli of constant value and chroma (7/4), observers adjusted the hue of a 3 deg target presented 18 deg nasally, until it matched a 1 deg target presented 1 deg nasally. The magnitude and pattern of measured hue shifts were similar to those measured using CRT stimuli. We conclude that the perceived hue shifts that have previously been reported in the peripheral retina are independent of the nature of the stimulus and of the illuminant.     

Seeing unique hues

Hue can be described by four separate sensations of red (R), green (G), yellow (Y), and blue (B). These are combined in the spectrally opponent RG and YB mechanisms, whose null points correspond to the unique sensations of Y, G, B. Participants used a form of magnitude estimation to describe color appearance of light flashes that were systematically varied in size, luminance, duration, purity, and retinal eccentricity. Wavelengths of the unique hues were derived from the hue and saturation scaling functions. Only unique Y remained invariant across all the viewing conditions. The shifts in unique hues with test conditions place strong restrictions on models of how the RG and YB mechanisms are assembled. Despite polymorphism of L and M cones and variation of their ratios across participants and across the retina, the frequency distribution of unique Y was very narrow, implying some reweighting of cone inputs to individuals' RG mechanisms.     

Direction in the color plane as a factor in chromatic flicker and chromatic motion

A percept of motion results when a chromatic grating, formed from a spatial alternation between two isoluminant hues, drifts across the visual field. With hue pairs chosen to be equally subjectively dissimilar, the motion is greater for alternation along some directions in color space (orange/blue) than others (green/purple), suggesting a specific interaction between the (L-M) and S(0) chromatic opponent channels. This phenomenon was explored systematically by choosing 24 pairs of hues across the color circle and using the method of paired comparisons to scale their movement-inducing contrast. The flicker-inducing contrast observed from rapid alternation between the pairs was measured in the same way. Both phenomena consistently drew upon both chromatic channels, though in different proportions, as if chromatic and temporal variation information are multiplexed along motion-processing pathways. Border-distinctness data were also collected to isolate the (L-M) channel.     

Senescent changes in parafoveal color appearance: saturation as a function of stimulus area.

The chromatic content (saturation) of monochromatic stimuli (480, 505, 577, and 650 nm) was scaled as a function of field size at three different retinal locations by 58 observers ranging from 18 to 83 yr of age. The different retinal locations (6 deg nasal, 2.5 deg inferior and 6 deg temporal eccentricity) were chosen according to anatomical studies demonstrating different degrees of senescent losses of cones or ganglion cells. Nine field sizes were tested, ranging from 0.0096 to 0.96 deg in diameter. The subjects used a percentage scale to judge the saturation of the flashed stimulus presentations (2 s on, 5 s off). The data analysis demonstrated that older observers require larger field sizes than younger observers to perceive hue as well as larger field sizes to reach the same level of scaled saturation. The spatial dependency of color appearance for younger and older observers was not correlated with senescent losses in retinal cells reported for the different retinal locations. The data were modeled by using an impulse-response function (i.e., Naka-Rushton equation) so that perceptive fields could be compared to electrophysiological measures of receptive fields or dendritic fields of retinal and cortical cells.     

Bezold-Brücke effect in normal trichromats and protanopes

Luminance-dependent change in color appearance--the Bezold-Brücke effect--was investigated in protanopes and related to that in normal trichromats. Spectral lights were presented at six luminance levels covering mesopic, low, and high photopic vision-across three log steps from 0.76 to 760 Td. To judge color appearance, a variant of the color-naming method was used with four primary basic color terms and a "White" response. This modification enabled us to examine apparent saturation changes along with the Bezold-Brücke hue shift. Color-naming frequency functions were acquired across ten presentations of each stimulus. Since protanopes name colors idiosyncratically, changes in color appearance cannot be quantified directly from the color-naming functions. To circumvent the difficulty, these functions were transformed into color similarity measures for analysis with multidimensional scaling purported to reconstruct individual color spaces. In these, luminance-dependent shifts in color appearance were represented by means of geometric displacements. We found that for normal trichromats, shifts measured in this way agreed with those derived in our study directly, and with the hue shifts reported in earlier studies. For protanopes, contrary to some models of dichromatic vision, changes in color appearance are significant and indicate superimposed shifts in hue and saturation. The results obtained for normal trichromats, especially for protanopes, imply that nonlinearity in the yellow-blue opponent system is insufficient to explain the Bezold-Brücke effect, given the nature of the saturation shift and the demonstrated divergence between unique hues and invariant hues.     

Uniform color spaces and natural image statistics

Many aspects of visual coding have been successfully predicted by starting from the statistics of natural scenes and then asking how the stimulus could be efficiently represented. We started from the representation of color characterized by uniform color spaces, and then asked what type of color environment they implied. These spaces are designed to represent equal perceptual differences in color discrimination or appearance by equal distances in the space. The relative sensitivity to different axes within the space might therefore reflect the gamut of colors in natural scenes. To examine this, we projected perceptually uniform distributions within the Munsell, CIE L(*)u(*)v(*) or CIE L(*)a(*)b(*) spaces into cone-opponent space. All were elongated along a bluish-yellowish axis reflecting covarying signals along the L-M and S-(L+M) cardinal axes, a pattern typical (though not identical) to many natural environments. In turn, color distributions from environments were more uniform when projected into the CIE L(*)a(*)b(*) perceptual space than when represented in a normalized cone-opponent space. These analyses suggest the bluish-yellowish bias in environmental colors might be an important factor shaping chromatic sensitivity, and also suggest that perceptually uniform color metrics could be derived from natural scene statistics and potentially tailored to specific environments.     

Color appearance in the entire visual field: color zone map based on the unique hue component

To provide the fundamental data for a color zone map, the color appearances of nearly unique hue stimuli presented over the entire visual field were qualitatively and quantitatively evaluated by hue and saturation judgments, blackness evaluation, and categorical color naming. The hue of red and green stimuli shifts toward a unique yellow, while that of yellow and blue does not change with an increase in eccentricity. The saturation of all the stimuli falls with an increase in the eccentricity in all directions. On the basis of the unique hue component, color zone maps for red, dark yellow, yellow, green, and blue stimuli are drawn. All the color zone maps extend over a wider region in the temporal and lower directions than in the nasal and upper directions of the visual field. The results are compared with the color zones of previous studies. The relationship between the color zones and the color categorization, as well as the underlying mechanisms of reduced saturation and hue shift, is discussed.     

The relationship between peripherally matched invariant hues and unique hues: a cone-contrast approach

A characteristic shift in hue and saturation occurs when colored targets are viewed peripherally compared with centrally. Four hues, one in each of the red, blue, green, and yellow regions of color space, remain unchanged when presented in the peripheral field. Apart from green, these peripherally invariant hues correspond almost exactly in color space with the unique hues. We explore this puzzling observation using asymmetric color-matching and color-naming experiments and computing cone contrasts for peripheral and central stimuli. We find that the difference between cone contrasts for the peripheral and central stimuli reaches a maximum at the chromatic axis corresponding to peripherally invariant green. We speculate that the effect is linked to a weakened signal from M-cones and probably associated with a reduced number of M-cones in peripheral retina.     

Role of perceptual organization in chromatic induction

Color matches between two small patches were made in a display containing ten larger regions of different chromaticities. The spatial organization of the ten regions was varied while keeping constant the immediate surround of each patch as well as the space-average chromaticity of the entire stimulus. Different spatial arrangements were designed to alter the perceptual organization inferred by the observer without changing the ensemble of chromaticities actually in view. For example, one arrangement of the ten regions was consistent with five surfaces under two distinct illuminations, with one edge within the display (an "apparent illumination edge") dividing the stimulus into two areas, one under illuminant A and the other under illuminant C. Another spatial arrangement had the ten regions configured to induce an observer to infer ten surfaces under a single illumination. When the ten regions were arranged with an apparent illumination edge, the patch within the area of illuminant C was perceived as bluer than when the same patch and immediate surround were presented without an apparent illumination edge. The results are accounted for by positing that observers group together regions sharing the same inferred illumination, with a consequent effect on color perception: A fixed patch-within-surround shifts in hue and saturation toward the perceived illumination. We suggest that the change in color perception in a complex scene that results from a difference in real illumination may be caused by the inferred illumination at the perceptual level, not directly by the physical change in the light absorbed by photoreceptors.     

Aftereffect of contrast adaptation to a chromatic notched-noise stimulus

One of the most challenging topics in the study of human color vision is the investigation of the number of hue-selective channels that are necessary for the representation of color appearance at the post-opponent level and the bandwidth of their sensitivity. The present study aims to elucidate this issue by using a chromatic version of the notch-filtered noise (herein, notched-noise) stimulus for contrast adaptation. After adaptation to this stimulus, some color-sensitive mechanisms that selectively respond to missing hues in the notched-noise stimulus may remain sensitive, while the other mechanisms may be desensitized. The shifts in the color appearance of a gray test field after the adaptation to such a notched noise were measured using the method of adjustment. The results showed systematic shifts in the hue and saturation. They showed neither point nor line symmetric profiles with respect to the achromatic point in an isoluminant plane. The fittings of the results, obtained by using a tiny numerical model for assessing the hue-selective mechanisms, suggested that there are at least two narrowly tuned and at least three broadly tuned mechanisms. The narrowly tuned mechanisms are the most sensitive along the blue and yellow directions. The present study confirmed the variation of multiple channels at the post-opponent level and suggested that this variation may be responsible for the processing of color appearance.     

CIE94 CIEDE2000色差公式均匀性分析

利用孟塞尔(Munsell)颜色系统,在视觉上为等色差系统以及色调、彩度和明度可以单独改变的特性,选择明度V=5的色块,分别从色调、彩度两方面对CIE94、CIEDE2000色差公式的均匀性进行了研究.数据计算结果表明,它们计算出的色差值是比较一致的;但它们的均匀性是不同的.需要针对不同的彩度、色调选择使用不同的色差公式.     

Variations in normal color vision. IV. Binary hues and hue scaling

We used hue cancellation and focal naming to compare individual differences in stimuli selected for unique hues (e.g., pure blue or green) and binary hues (e.g., blue-green). Standard models assume that binary hues depend on the component responses of red-green and blue-yellow processes. However, variance was comparable for unique and binary hues, and settings across categories showed little correlation. Thus, the choices for the binary mixtures are poorly predicted by the unique hue settings. Hue scaling was used to compare individual differences both within and between categories. Ratings for distant stimuli were again independent, while neighboring stimuli covaried and revealed clusters near the poles of the LvsM and SvsLM cardinal axes. While individual differences were large, mean focal choices for red, blue-green, yellow-green, and (to a lesser extent) purple fall near the cardinal axes, such that the cardinal axes roughly delineate the boundaries for blue vs. green and yellow vs. green categories. This suggests a weak tie between the cone-opponent axes and the structure of color appearance.     

基于数学形态学的HSI空间彩色边缘检测方法

针对在RGB空间中很难有效区分颜色相似性的问题,选择更加符合颜色视觉特性的HSI颜色空间进行图像处理,提出一种基于HSI空间的多结构多尺度自适应彩色图像边缘检测方法.首先对H、S、I三个分量采用不同结构和不同尺度的结构元素进行形态学边缘检测,然后对三个边缘分量利用信息熵加权得到融合后的彩色边缘信息.实验结果表明,该方法可以充分利用彩色图像的色度,饱和度和亮度信息,有效地抑制噪声,自适应地提取完整的边缘信息.     

Characteristics of grouping colors for figure segregation on a multicolored background

A figure is segregated from its background when the colored elements belonging to the figure are grouped together. We investigated the range of color distribution conditions in which a figure could be segregated from its background using the color distribution differences. The stimulus was a multicolored texture composed of randomly shaped pieces. It was divided into two regions: a test region and a background region. The pieces in these two regions had different color distributions in the OSA Uniform Color Space. In our experiments, the subject segregated the figure of the test region using two different procedures. Since the Euclidean distance in the OSA Uniform Color Space corresponds to perceived color difference, if segregation thresholds are determined by only color difference, the thresholds should be independent of position and direction in the color space. In the results, however, the thresholds did depend on position and direction in the OSA Uniform Color Space. This suggests that color difference is not the only factor in figure segregation by color. Moreover, the threshold dependence on position and direction is influenced by the distances in the cone-opponent space whose axes are normalized by discrimination thresholds, suggesting that figure segregation threshold is determined by similar factors in the cone-opponent space for color discrimination. The analysis of the results by categorical color naming suggests that categorical color perception may affect figure segregation only slightly.     

矿物颜料颗粒度对色彩的影响

选取不同颗粒度的岩肌、白翠末等矿物颜料,采用简化的Mie散射理论,研究矿物颜料颗粒度对色彩的影响。研究结果表明,随着颜料颗粒度的减小,白翠末和岩肌的光谱反射率明显增加,颜色的明度值也均随之增加,而其颜色饱和度随之降低,且对于同种颜料而言,随着颗粒度的改变,颜色的色相有微小变化。     

基于颜色属性的光谱重建训练样本正交优化

选择具有代表性的颜色作为光谱重建的训练样本可以有效减少样本冗余,提高光谱重建精度。采用正交试验方法,基于色相、明度和饱和度在Munsell颜色集中选择具有代表性的颜色样本,并分析颜色三属性对光谱重建精度的影响。结果表明,采用主成分分析(PCA)法重建得到的反射率与原反射率的平均均方差(RMS)最大可达0.120 4,而采用违逆(PSE)法和R矩阵(R-matrix)法重建得到的平均RMS相对较小。三属性的优先级别R极差分析中,明度明显大于色相和饱和度。颜色三属性对P C A法的影响大于对PSE法和R-matrix法。明度对光谱重建精度的影响较大,而色相和饱和度对光谱重建精度的影响相对较小。     

基于色调单维度的相关色温计算

在考察现有相关色温的定义及在CIE1960UCS、CIE1976UCS和CIE1976LAB 3种不同均匀色品图中计算值的不一致性基础上,提出了一种基于色调维度来规范计算相关色温的方法。该方法借助孟塞尔表色系统,建立色温与色调的对应关系,忽略饱和度对色温的影响,重构了3个色品图的等色温线。通过与传统相关色温计算方法比较,该方法具有物理意义更明确、在不同色品图中的计算结果一致性更好的优点。     

Theory of spatiochromatic image encoding and feature extraction

We consider how to interpret, filter, and cross-correlate complex-value color (hue and saturation) images by using a single discrete Fourier transform: the spatiochromatic discrete Fourier transform. The model defines new types of spatiochromatic oriented sinusoidal gratings, termed rainbow gratings, which encode the variation of color over space. We demonstrate how color-opponent detectors observed within the vertebrate visual system can be easily defined by linear filters within this representation. This model also allows us to filter and detect both spatial and chromatic patterns in images by using a single cross-correlation procedure. In doing so, we explore a new form of the Cauchy-Schwartz inequality applied to complex-valued scalar products. Results demonstrate the power of this form of spatiochromatic matched filtering in detecting signals embedded in such a significant amount of noise that they are not visible to the unaided human eye.