Edward J. Giorgianni

Digital Color Management Encoding Solutions, Second Edition

This PDF file contains the editorial “Digital Color Management Encoding Solutions, Second Edition” for JEI Vol. 18 Issue 04

G: Transformations for Color Interchange

Transformations for color interchange can be developed for any pair of systems having fully defined color encoding specifications. For system-specific interchange, various restricted interchange methods and data metrics can be used. For general color interchange, the communication of color information must be unrestricted, i.e., the interchange method itself must not impose limitations on luminance dynamic range, color gamut, or color interpretation. In this appendix, several example transformations based on both restricted and unrestricted color interchange will be developed. Many of the examples will link Kodak PhotoYCC color interchange space to other color encoding specifications (Figure G.1). This particular interchange space was selected for inclusion here because it is representative of scene-based encoding and because its properties support both general and system-specific color interchange. Other important transformations using the color encoding specification developed for the example color-management systems described in Chapters 20 and 21 also will be described. Figure G.2 illustrates the colorimetric relationships of original scenes, the various color encoding specifications, and reproduced images. The characteristics of the individual color encoding specifications are as follows: PhotoYCC space: Kodak PhotoYCC color interchange space was used for color encoding on the Kodak Photo CD system. A number of transforms to and from the space are given here for two principal reasons. First, there currently are billions of PhotoYCC files in image archives throughout the world, and there is considerable interest in transforming these files to other formats. Second, PhotoYCC space is a scene-based color encoding, and the example transforms illustrate the general principles involved in transforming between scene-based and rendered-image-based color encodings. PhotoYCC space is based on color appearance defined in terms of colorimetry and a specified set of encoding reference viewing conditions. The conditions are consistent with those of outdoor scenes. Encoded colorimetric values are expressed according to the properties of a specified reference imagecapture device. This hypothetical device produces color values representative of the colorimetry of original scenes. The device is defined in terms of a set of reference primaries and an optoelectronic transfer characteristic based on Recommendations ITU-R BT.709 and ITU-R BT.601. However, unlike the electronic cameras described by those recommendations, the reference image-capture device has an extended luminance dynamic range, and its color gamut is not restricted to that defined by positive intensities of its reference RGB primaries.

Physics and psychophysics of color reproduction

The successful design of a color-imaging system requires knowledge of the factors used to produce and control color. This knowledge can be derived, in part, from measurements of the physical properties of the imaging system. Color itself, however, is a perceptual response and cannot be directly measured. Though the visual process begins with physics, as radiant energy reaching the eyes, it is in the mind of the observer that the stimuli produced from this radiant energy are interpreted and organized to form meaningful perceptions, including the perception of color. A comprehensive understanding of color reproduction, therefore, requires not only a knowledge of the physical properties of color-imaging systems but also an understanding of the physics, psychophysics, and psychology of the human observer. The human visual process is quite complex; in many ways the physical properties of color-imaging systems are easier to understand.