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MULTITONE PROCESSOR FOR HIGHLY NONLINEAR SYSTEMS

IP.com Disclosure Number: IPCOM000028635D
Publication Date: 2004-May-25
Document File: 6 page(s) / 24K

Publishing Venue

The IP.com Prior Art Database

Abstract

This disclosure pertains to the field of digital printing, and more particularly to a method of multitoning digital image data for printing on a printer having a highly nonlinear tonescale response.

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MULTITONE PROCESSOR FOR HIGHLY NONLINEAR SYSTEMS

                        This disclosure pertains to the field of digital printing, and more particularly to a method of multitoning digital image data for printing on a printer having a highly nonlinear tonescale response.

                        This disclosure will be discussed in the context of an inkjet printer in which the size of the ink dots on the page is disproportionate to the spacing between the printing locations.  This situation is not uncommon in inkjet printers, as the strive to claim higher and higher resolution (in dots per inch, or DPI) has outpaced the technology to provide correspondingly smaller and smaller ink drops.  Thus, inkjet printers are used to print relatively large drops that overlap many output pixels.  This is illustrated diagramatically in FIG. 1, wherein ink dots 10 are printed on a grid of raster locations 20, and the relative size of the ink dots (measured by the dot diameter D) is disproportionately large compared to the raster spacing p, as each of the ink dots overlaps significantly into neighboring pixels.  This causes the tonescale response of the printer to be strongly nonlinear, as the page will become substantially covered with ink well before every available pixel in the image has received a dot at its location.  FIG. 2 shows an example of a nonlinear tonescale, wherein the maximum density is substantially obtained at an input tone level of 150 (out of 255 for an 8-bit input code value range).

                        Obtaining maximum density at such a low code value forces the output density range to be represented by fewer input levels.  In the example tonescale of FIG. 2, the entire density range must be printed by 150 tone levels, as opposed to 255 that are theoretically available.  This means that the density difference between any two input levels will become larger, leading to undesirable contouring artifacts in the image output.  To calibrate the tonescale of such a system to an aim response (assume that it is desired to have input code value linear with output density), then a calibration look-up table (LUT) as shown in FIG. 3 is created based on the measured tonescale response shown in FIG. 2.  The steep portion of the measured tonescale data forces the calibration LUT to have a very shallow slope initially.  If the output precision of the calibration LUT is the same as the input precision (both are 8-bits as shown in FIG. 3), then many input levels may be mapped to the same output level, resulting in a contouring a...