Color Management for Color Facsimile

We propose a color management system for the color facsimile. It consists of protocols for the colorimetric calibration of the scanner and the printer by establishing the relationships between the device-dependent color coordinates and the device-independent CIELAB color space. The scanner calibration is based on 3rd order polynomial regression techniques. The printer calibration uses 3D triangulation techniques, which gives us high flexibility and which allows us to apply different color gamut mapping techniques in an efficient way. The online color transformations are calculated by a tetrahedral interpolator using 3D look-up tables provided by the calibration algorithms. Introduction The first international standard for color facsimile was approved in 1994. It fixed JPEG as the encoding method for continuous-tone color images and chose the CIELAB color space with CIE Standard Illuminant D50 as the default color space for data transmission. An important aspect of color facsimile products is the quality of the color reproduction. To obtain faithful color reproduction, colorimetric calibration of the I/O devices is needed, so that the device-dependent color coordinates of the scanner, the printer and the monitor can be linked to the device-independent CIELAB color space. Furthermore, efficient means for processing and converting images are needed. These are typically the tasks performed by a color management system. To calibrate I/O devices colorimetrically, three different approaches are typically used: • Theoretical/physical models. Classical examples of this approach are the Neugebauer printer model, and the monitor models based on relatively few parameters such as the specification of the chromaticity coordinates and gamma value of each channel, and the specification of the white point. The drawback with such models is that they are mostly technology dependent. • Analytical models. They are based on the minimizing of the mean square error of a set of measurement points by polynomial regression. These models are widely used, especially for input devices such as scanners. Important design parameters for this approach are the polynomial order as well as the choice of input/output color spaces requiring pre/post-processing. • Geometric models. Such models are based on lookup-tables and interpolations. They are used by numerous authors, and have the advantage to be general. However, they require mostly a larger amount of measured data, compared to the precedent models. We present a color management system for the color facsimile, consisting of procedures for the colorimetric calibration of the scanner, based on an analytical model, as well as for the calibration of the printer, based on a geometric model. The calibration of the monitor, based on a simple physical model of a CRT display is quite straightforward, and will not be presented in this document.