Hewlett-packard Company

Alexander Perumal, Jr

Paul H Dillinger

Qian Lin

This system selectively enables a printer to deposit primary or black color dots at pixel locations on a sheet to produce a color image--in such a way as to increase the overall number of dots used in the image, thereby minimizing contrast between dots on the sheet and adjacent areas and so providing a better simulation of a continuous-tone image. The result is print quality often near that of error diffusion, usually better than any dither technique, but with the greater speed of a dither technique. The system includes a memory for storing a first color vector for each pixel in the image, respectively, and a processor coupled to the memory for determining a second color vector from each of the first vectors, respectively. The processor accomplishes this determination by replacing a fraction of the black color component of the first vector with primary colors. Preferably the processor also removes from the first color vector a quantity of white, in conjunction with the black replacement; in this case the processor sets the fraction of the black component to a value between zero and one inclusive, depending on the quantity of white that is available, in the first color vector, for removal. Preferably the fraction is the smaller of (1) the quantity of black present and (2) a certain fraction of the quantity of white available for removal. Analogously a fraction of the secondary color component of a color vector is replaced with primary colors.

Color digital halftoning using black and secondary color replacement

Methods and apparatus generate, and use in printing, a dither matrix that incorporates a vivid-response function, that follows a generally S-shaped response curve. Printing with this pregenerated dither cell is just as fast as with a conventional cell, but printed color has a slightly exaggerated vividness, more pleasing to most users than more-accurately reproduced color. Color printed with this curve alone, however, is found unsatisfactorily dark; the invention also encompasses including in the overall response another function to lighten the colors--particularly at the low end of the brightness range. We define both functions as so-called 'gamma functions', but with different arguments and exponents, and construct the S-shaped curve as two separate sections blended at a generally central point. Preferably the argument of the first (low-brightness) section is an expression that is proportional to an input color signal; the function raises this argument to a power greater than unity (ideally 1.6). The second (high-brightness) section of the first function is essentially a mirror image of the first, flipped over first upward and then to the 'right' (i.e., in both steps toward higher brightness); and is defined by an equation closely analogous to the first but measuring the signal from the high end of the input range and subtracting the curve value from the maximum output. The second gamma function has as its argument the output value of the first, and an exponent smaller than one--ideally 0.6.

Halftone dither cell with integrated preferred color matching

Provided is a system and method for selectively enabling a printer to deposit on a sheet cyan, magenta, yellow and black (C,M,Y and K) primary or black color dots at each of the plurality of pixel locations to produce a color image. The system comprises a memory for storing C, M, Y and K values for each pixel in the color image and a processor for controlling selective deposition of the C, M, Y and K colors. The system includes a memory for holding a color vector for each pixel, respectively in the color image; a memory for storing parameters of a dither cell; and a processor coupled to the memory for comparing the dither cell with each color vector, to select a color component to print at the corresponding pixel location based on the comparison. The system provides a halftoning technique that produces print quality near error diffusion quality with the speed of a dither by dithering on a color vector. The replacement of black and secondary colors by the system produces better print quality than other dither techniques.

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Poway, CA, United States of America

Alexander Perumal, Jr