Input and Output Device Characterization in the Field of Color Facsimile

We propose a complete methodology for the control of the color responses of the input/output devices for color facsimile. An initial calibration of both scanner and printer is done offline. The color response of each device is then modelized by 3D non-linear transformations linking their specific RGB/YMC color space with the standard CIELAB color space. These transformations are then quickly calculated by using online a tri-linear interpolator with 3D look-up tables. Objectives and Backgrounds The first international standard (ITU) for color facsimile was approved in June 1994. It fixed JPEG as the encoding method for continuous-tone color images and chose the 1976 CIELAB color space [1] with CIE Standard Illuminant D50 [2] as standard color space for data transmission. The most important point of color facsimile products, which will soon appear on the market is the quality of color reproduction. In order to gain acceptance by potential users it is necessary that color consistency across the entire color facsimile process (scanning, coding, transmission, decoding and printing) can be guaranteed. Thus tools have to be developed which enable characterization of the input device (scanner) and of the output device (printer) and which transform the device dependent values provided by the input device into the device independent CIELAB values at the emission step and then transform these device independent values into the device dependent values necessary for the output device. During this processes the differences in the actual color gamuts have to be considered. Our aim is to define a flexible color calibration methodology which is general enough to work properly on both scanner and printer, whatever their technology, and which uses minimal hardware equipment. This color calibration problem is present in all color image processing systems, especially in the electronic prepress systems [3]. We propose an adapted methodological approach to solve this problem in the context of the color facsimile.