Color copiers and printers account for a growing share of the reprographics market now that there are moderately priced and easy to use computerized tools for creating color source documents. Xerographic highlight color printers and copiers have enjoyed substantial commercial success with this increased demand for color printing and copying because they fill a need for higher speed, lower priced, color capable, copying equipment.
Four-color copiers and printers provide much broader color gamuts than two-color machines, but the two colors that are used for conventional highlight color printing can be printed in a single pass by a single station xerographic print engine through the use of Xerox' patented tri-level xerography. Furthermore, recent advances in xerography, such as described in commonly assigned Kovacs et al. U.S. Pat. No. 5,347,303 on "Full Color Xerographic Printing System with Dual Wavelength, Single Optical System ROS and Dual Layer Photoreceptor" (which is hereby incorporated by reference), have created quad-level xerography (sometimes referred to as "xerocolography") that enables the printing of three colors (for example, black plus two highlight colors) in a single pass by a single xerographic station. Accordingly, the throughput (i.e., copies or prints per minute) of these 2 or 3 highlight color machines are comparable to those of monochromatic print engines, including high speed print engines (i.e., 90+ pages per minute), and significantly higher than traditional four color print engines.
Additionally, 2 or 3 color printing or copying can be achieved with a full color four-pass print engine at improved throughput speed by reducing the number of passes respectively. For example, when compared to full color printing on a four pass printer, 2 color printing can be done with twice the throughput of full color. Likewise 3 color printing can be done with 33% more throughput than four-pass full color printing.
A 2 or 3 color print engine is referred to herein as a highlight color printer. As pointed out above, two color printing can be achieved with either tri-level xerography in a single pass or with a four-pass/full-color printer in two (instead of four) passes. Three color printing can be achieved with quad-level xerography in a single pass or with a four-pass/full-color printer in three (instead of four) passes. Tri-level and quad-level technologies offer 2 or 3 color printing, respectively, in a single pass at speeds equivalent to black-only printing (90+ pages per minute). Four-pass/full-color printers can produce 2 or 3 color prints with twice or one-and-a-third times more throughput, respectively, than full color printing. These concepts have been previously applied to highlight color printers to achieve relatively high speed, functionally compatible printing of full color, computer generated images using n-to-m color gamut transformations (where m&lt;n) which are selected based on the image characterizing information that is contained by those computer generated images.
To carry out these color gamut transformations, several full-color to 2 or 3 color mappings have been proposed and others may be developed. For example, a commonly assigned U.S. patent application of Harrington, which was filed Aug. 29, 1990 under Ser. No. 07/574,145 on "Color Printing Having a Highlight Color Image Mapped from a Full Color Image" now U.S. Pat. No. 5,237,517 (D/88330), not only describes what are sometimes referred to as a "pictorial" mapping and "presentation" mapping algorithms for mapping full-color images to two-color (typically black plus highlight color) images, but more generally teaches techniques for creating full color-to-highlight color mappings.
Harrington's "pictorial" mapping maintains the integrity of hues that match the highlight color. Thus, for example, if the highlight color in the 2-color space is blue, blues in the original full-color image are mapped into shades of highlight blue; all other colors of the full-color space are mapped into appropriate shades of gray. This means that the two color output image has essentially the same appearance as the full-color source image would have when viewed through a filter having the same color as the highlight color. Thus, the pictorial mapping effectively preserves certain of the natural colors in the source image.
On the other hand, Harrington's "presentation" mapping maps the different intensity levels of a full-color image into a two-color highlight color space. This mapping preserves the distinctiveness of the differently colored regions in the source image, so it is well suited for use with business graphics that are typically composed of text, line-art and synthetic graphical objects.
As will be appreciated, extensions of both the presentation mapping and the pictorial mapping could be employed for mapping full-color images to three-color (typically black plus two highlight colors) space. Regardless, however, of the specific color mappings that are employed, source image information is lost while doing these mappings, so an intelligent selection of the color mapping is the key to producing functionally compatible highlight color renderings of full color images.
Scanned-in full color image files contain far less image characterizing information than computer generated image files. These scanned-in files digitally define images at the pixel level in anyone of a number of different full-color spaces. Examples include RGB (red, green, blue) space, CMYK (cyan, magenta, yellow, black) space, CIE-LAB space and so on. Moreover, color space transformation algorithms exists that can transform a digitally stored color document from one color space to another. Scanned-in image files do not, however, include the image type characterizers that are needed to intelligently select functionally compatible full color-to-highlight color mappings. Thus, the throughput advantages offered by these full color-to-highlight color mappings have not previously been available for copiers (i.e., machines that replicate hardcopy source images), even though the throughput advantages of utilizing these color mappings, as well as the functional compatibility of such mappings, have been demonstrated by the successful highlight color printing of computer generated full color images. Clearly, therefore, there is a need to close this gap.
A concurrently filed, commonly assigned Beach et al U.S. patent application on "Digital Highlight Color Copier" provides an approach for building this needed functionality into digital highlight color copiers for applications in which copies are made from full color versions of the source images. However, still more is needed to provide satisfactory highlight color copying of full color source images that may have experienced significant color degradation, such as by being subjected to a black and white copying process.