A full color process image that can be printed with cyan, magenta, yellow, and black inks is produced as a duotone image using only two colors of ink. Color data defining the colors used in the full color image serve as input to a process that maps the colors into a duotone gamut defined by the two colors of ink that will be used to produce the duotone image. A user is able to select 0, 1, or 2 colors of ink for the duotone image, and the software program automatically chooses the remaining ink or inks so as to reproduce the image as accurately as possible; the software also produces the appropriate color separations automatically. In addition, the paper color can be specified by the user or optimally determined by the optimizing process of the software to optimize the duotone image reproducing a full color image that has little white. The paper color provides a free third color. A black separation can also optionally be included when printing the duotone image to expand the range of luminance available. When black ink is used, some amount of black is subtracted from the each color of the original image before the optimization and mapping process is applied, and the black separation is determined to add back the appropriate amount of black when printing the duotone image. The duotone images produced by the method are relatively high-quality, economical alternatives to full-color printing.
RELATED APPLICATION
This application is based on prior copending provisional application Ser. No. 60/022,974, filed Aug. 2, 1996, the benefit of the filing date of which is hereby claimed under 35 U.S.C. .sctn. 119(e).
An image-processing system (20) for mapping full-color images to highlight-color images includes an image-input terminal (30). The image-input terminal (30) reads an image represented in a full-color scheme defined by a three-dimensional color space. An image-processing unit (40) samples the image to obtain a sample color. The image-processing unit (40) maps the sample color from the full-color scheme to a highlight-color scheme. The highlight-color scheme is defined by a two-dimensional color space that includes a highlight-color component and a base-color component. The image-processing unit (40) includes at least a first two-dimensional look-up table (42), which is used in mapping the sample color from the full-color scheme to the highlight-color scheme. Finally, an image-output terminal renders the image in the highlight-color scheme.
A method of color printing is described in which only two transparent inks are used to reproduce a source image The original digitally encoded image in red, green and blue colors may be initially adjusted for contrast, brightness, color balance, and tonal value. This is then converted to cyan, magenta and yellow encoding. The two of these channels which best represent the dominant colors of original image are then selected. The critical colors of the original image to be reproduced with reasonable accuracy are determined and spot colors that best represent these are selected from an ink palette The spot colors are assigned to the selected channels to which they have the closest color relationship. These two channels are next superimposed to form a composite image and their greyscale values are adjusted to produce an image most closely resembling the desired final image. New positive separations are then made and inverted to negative images from which printing plates can be prepared. A third channel may be introduced approximating the color of a colored substrate before the final greyscale adjustment. This channel is not printed, however. An additional procedure useful with colored substrates is to prepare a masking channel which will deposit an opaque ink, usually white, to underlie the two transparent inks and further help control substrate color. Under this implementation, the third channel is printed. The method is capable of rendering very attractive color images on substrates such as the unbleached kraft paper used for corrugated shipping containers.
A system and method are provided for processing digital multi-color images for the reproduction thereof on a color printing and/or copying device provided with a plurality of process colors defining a gamut. The system includes a color processing module, which divides the gamut into multiple sub-gamuts, each defined by three chromatic process colors, and which selects for each pixel of the digital multi-color image the corresponding sub-gamut. For each pixel it is judged whether or not to add a non-white achromatic process color or blue to the process colors defining the corresponding sub-gamut.
A method of printing a highlight color image uses half-tone printing reference patterns for generating printing values for a print original image for a standard color and for generating printing values for a print original image for a highlight color. The information pertaining to the color values of the original image that are assigned to a standard color region or to a highlight color region is integrated into the half-tone printing reference patterns, whereby the printing of a highlight image is limited to a selection of the corresponding half-tone printing reference pattern and the acceptance of the printing value indicated in the half-tone printing reference pattern. Only one corresponding selection for determining the printing values for the standard color and for the highlight color is carried out for each image point.
The invention provides a method and apparatus for the reproduction of color images by multi-color dithering. Multi-color dithering is an extension of bi-level dithering for halftoning an input image using as primary colors an arbitrary number of inks. The inks may comprise standard inks or non-standard inks such as non-process color inks, opaque inks, metallic inks, variable color inks and fluorescent inks. Multi-color dithering provides a solution for creating artistic multi-color dithered images, whose screen elements are made of artistic color screen shapes such as micro-letters, symbols and ornaments. When printed at high resolution and at high registration accuracy, multi-color dithering using large dither arrays incorporating artistic dither shapes provides an effective solution for preventing counterfeiting. The use of non-standard inks offers additional protection. Multi-color dithering also offers a solution for printers requiring that all inks are printed side by side, without overlaps.
