Introducing Wpt (Waypoint): A color equivalency representation for defining a material adjustment transform

Abstract

The basis of this research is the manipulation of sensor excitation values to account for differences in observer or illuminant when spectral data are unknown. This touches on several related topics: color constancy, chromatic adaptation, and white balancing. The central premise is that these concepts make use of some form of intermediate color equivalency representation or waypoint system that makes comparison and color transformations possible. Differences between these concepts are related to the kind of color equivalency representation system used and how transformations are made into and out of it. A new sensor excitation normalization method was derived that has been optimized to predict changes in material color as Wpt (pronounced Waypoint) coordinates, which can be used to form a material adjustment transform. The prediction of such changes is also known as least dissimilar color matching. This is contrasted with a chromatic adaptation transform, which is optimized to predict corresponding color changes. As such a distinction is made between adaptation (which is based on corresponding color) and adjustment (which is based on other criteria). V

Waypoint shift manifolds for object color evaluation and comparison

“…The Wpt color space is described by the coordinates (W, p, t) where W represents perceptive lightness, p and t represent perceptive chromaticness, that is, a combination of perceptive chroma and hue at constant perceptive lightness . The determination of Wpt coordinates for an arbitrary indexed observer, object, and illuminant (or light source) can be expressed as follows: where ( i , j , k ) are identifiers for the i th observer, j th object, and k th illuminant; w i , j , k represents a Wpt vector for the i th observer, j th object, and k th illuminant; A i , k represents a Wpt normalization matrix specific to the i th observer and k th illuminant; C i represents a matrix defining the sensor sensitivities or color matching functions for the i th observer; O j represents a spectral reflectance matrix for the j th object; l k represents vector for the SPD of the k th illuminant.…”

“…However, material color shifts can also result due to changes in observers as well as observing conditions. Derhak and Berns introduced the concepts of intermediate color equivalency representation space (ICERS) and material adjustment transform (MAT) to analyze sensor excitation changes between different observing conditions . An ICERS is any color coordinate system in which achromatic colors, chromatic colors, and white point are singularly defined.…”

“…Sensor adjustments are made by applying ICERS as a bridge with chromatic adaptation transforms (CAT) and MAT being examples. A MAT can estimate the sensor excitations of a material object under one set of observing conditions (destination) based on the sensor excitations of the same material object under different observing conditions (source) . Derhak and Berns proposed Wpt (Waypoint) normalization as a method that establishes an ICERS for defining a MAT.…”

“…Second, a plane of sensor excitation coordinates for reflectances of Munsell colors having constant Munsell value and chroma is then fitted and sheared to be orthogonal to the W axis, and finally rotation and scaling are performed around the W axis so that differences in distances from the W axis of the same set of Munsell reflectances are minimized as well as their angular alignment with axes p and t are roughly similar to those used by CIELAB a* and b* axes. The details of how to calculate the normalization matrix can be referred to Derhak and Berns' article and example Excel spreadsheets and Matlab code can be found at this website…”

Review and evaluation of color spaces for image/video compression

“…Color equivalency based spaces transform the tristimulus values to obtain color spaces whose coordinates represent actual “material color.” In other words, coordinates of such a color space are approximately identical even if there are changes in the color of the object due to change in observer and/or lighting conditions . Color equivalency based spaces are generally derived from XYZ tristimulus values by a Material Adjustment Transform (MAT) with further amendments (optional). MATs are used to predict how the sensor excitations (cone responses in the human eye) due to a material object (or material color) change when the observing conditions change .…”