Showing papers by "Brian A. Wandell published in 1993"

Appearance of colored patterns: pattern-color separability.

Abstract: We have measured how color appearance of square-wave bars varies with stimulus strength and spatial frequency. Observers adjusted the color of a uniform patch to match the color appearance of the bars in square-wave patterns. We used low-to-moderate square-wave patterns, from 1 to 8 cycles per degree (c/deg). The matches are not photoreceptor matches but rather are established at more central neural sites. The signals at the putative central sites obey several simple regularities. The cone contrast of the uniform patch is proportional to square-wave stimulus strength (color homogeneity) and additive with respect to the superposition of equal-frequency square waves containing different colors (color superposition). We use the asymmetric matches to derive, from first principles, three pattern–color-separable appearance pathways. The matches are explained by two spectrally opponent, spatially low-pass mechanisms and one spectrally positive, spatially bandpass mechanism. The spectral mechanisms that we derive are similar to luminance and opponent mechanisms that are derived with entirely different experimental methods.

Color Constancy: From Physics to Appearance

Abstract: 1. P.C. Glick, Fifty years of family demography: A record of social change, Journal of Marriage and the Family, 50, 861-873 (1988); J.A. Seltzer and S.M. Bianchi, Children's contact with absent par ents, Journal of Marriage and the Family, 50, 663 677(1988). 2. P. Bronstein and C.P. Cowan, Eds., Father hood Today: Men's Changing Role in the Family (Wiley, New York, 1988); M.E. Lamb, Ed., The Role of the Father in Child Development, rev. ed. (Wiley, New York, 1981); M.E. Lamb, Ed., The Father's Role: Cross-Cultural Perspectives (Wiley, New York, 1987). 3. P.j. Caplan, Don't Blame Mother: Mending the Mother-Daughter Relationship (Harper & Row, New York, 1989); P.J. Caplan and I. Hall McCorquodale, Mother-blaming in major clinical journals, American Journal of Orthopsychiatry, 55, 345-353 (1985). 4. V. Phares and B.E. Compas, The role of fa thers in child and adolescent psychopathology: Make room for daddy, Psychological Bulletin, 111, 387-412(1992). 5. V. Phares, Where's Poppa?: The relative lack of attention to the role of fathers in child and ado lescent psychopathology, American Psychologist, 47,656-664(1992). 6. K. Dodge, Developmental psychopathology in children of depressed mothers, Developmental Psychology, 26, 3-6 (1990); G. Downey and J.C. Coyne, Children of depressed parents: An integra tive review, Psychological Bulletin, 108, 50-76 (1990).

Water into wine: converting scanner RGB to tristimulus XYZ

Abstract: A simple method of converting scanner (RGB) responses to estimates of object tristimulus (XYZ) coordinates is to apply a linear transformation to the RGB values. The transformation parameters are selected subject to minimization of some relevant error measure. While the linear method is easy, it can be quite imprecise. Linear methods are only guaranteed to work when the scanner sensor responsivities are within a linear transformation of the human color- matching functions. In studying the linear transformation methods, we have observed that the error distribution between the true and estimated XYZ values is often quite regular: plotted in tristimulus coordinates, the error cloud is a highly eccentric ellipse, often nearly a line. We will show that this observation is expected when the collection of surface reflectance functions is well-described by a low-dimensional linear model, as is often the case in practice. We will discuss the implications of our observation for scanner design and for color correction algorithms that encourage operator intervention.

Color appearance: the effects of illumination and spatial pattern

Abstract: The color we perceive at each point in an image depends on information spread across the three spatial arrays of cone photoreceptors. I describe experiments aimed at clarifying how information is integrated across the spatial arrays to yield a color experience. We have found that changes of color appearance due to changes of the ambient illumination and the pattern's spatial frequency can be described by using a simple set of optical and neural transformations. Each transformation can be thought of as having two parts. First, the transformation converts the color representation into a new coordinate frame that is independent of the image contents. Second, the transformation scales the neural responses in the new coordinate frame by a gain factor that depends on the image contents.

Rethinking the White Point.

Abstract: 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.