METACOW: A public-domain, high-resolution, fully-digital, noise-free, metameric, extended-dynamic-range, spectral test target for imaging system analysis and simulation

Standard, easily accessible, test targets have long served the field of color imaging as a foundation for comparison of the performance of various imaging systems and algorithms and the open and meaningful exchange of research results. This paper details the creation and application of a new digital color test target useful for research and development of color imaging systems. The target has several advantages over previous types of targets that include spatial resolution, dynamic range, spectral resolution, metameric properties, lack of noise, and continuous tonal variations. All these features can be important for visual assessment, computational analysis, and colorimetric evaluation. This target, known as METACOW, is freely available to all performing research in color imaging. Introduction and Objectives Test targets of various sorts have been tremendously helpful in color imaging research and development. Perhaps the most widely used and recognized instantiation is the GretagMacbeth ColorChecker color rendition chart originally designed by McCamy et al.1 approximately 30 years ago. The ColorChecker was designed with an array of 24 color patches that could be easily evaluated visually and instrumentally. The design was such that the spectral reflectance characteristics of the chart, not just the colors, were created to simulate objects of special interest such as blue flowers (notoriously difficult to reproduce with photographic color film), skin tones, and a nonselective gray scale. The ColorChecker has been so widely used, and was so effectively designed, that its 24 color patches can often be considered as memory colors to scientists and engineers in color imaging. More recent evolution of digital photographic systems have prompted the evolution of the ColorChecker to a new second form embodied as the GretagMacbeth ColorChecker DC color reference chart.2 The ColorChecker DC serves similar purposes, but with an order of magnitude more patches (237 instead of 24) it was designed with some of the opensystems properties of digital color imaging in mind. It includes replicate gray scales around the chart to evaluate the spatial uniformity of color balance and tone reproduction. The large number of patches and extended gamut facilitate use of the ColorChecker DC in construction of ICC profiles for scanner or camera characterization. And while reproduction of the ColorChecker DC can be evaluated both visually and instrumentally, visual evaluation is far more difficult with such a large number of color patches. It is safe to assume that the 237 patches of the ColorChecker DC will never become widely recognized memory colors and that the usefulness of the target in psychophysical experiments is somewhat limited. Similar applications are served by test targets such as the well-known ANSI IT8 targets3,4 that consist of large numbers of well-characterized color patches that can be used for scanner characterization and related applications. Again, such targets are of limited practical use in psychophysical evaluation of imaging systems due the large number of patches although some versions have a small area of pictorial content more amenable to human judgement. For visual evaluation of imaging systems or imaging algorithms, standard (actual or de facto) pictorial images are often used by various investigators to allow comparison of research results and combination of data from various studies. Well-known examples include the SCID, standard color image data, and SHIPP, standard high precision picture data, images that are available in digital form in various color representations.5-7 Previous examples provide targets that well represent two categories of need. The first are well defined spectrally and instrumentally but often with too many patches for easy visual evaluation. The second are standard digital images that are well defined colorimetrically and appropriate for visual evaluation, but difficult to assess instrumentally. They are also potentially impacted by the original image capture technology. The objective of the test target described in this paper is two bridge the gap between those two categories and produce an image that can be used in research and development that is both well specified spectrally and contains continuously shaded areas that are useful for perceptual judgements. This image is not intended to replace both of the previous categories, but rather to supplement them with a new type of test image with significant practical utility. Additional objectives in the development of this target include eliminating noise in the target itself, including metameric reflectance pairs, and being of sufficient spatial and photometric resolution to be applicable to the evaluation of essentially any imaging technology. The addition of metameric reflectance pairs alone is a significant advance in practical utility over previous color charts.