The present invention relates generally to computer-implemented trapping processes and, more particularly, to the specification of device independent trap colors.
As the availability, affordability, and use of publishing systems increase, techniques for preparing and reproducing pages on a variety of output devices have been developed. Output devices may include image setters, printers, monitors, and digital printing presses. A page may include various types of objects such as text, line art, and images, and its appearance is generally described by page description language (PDL) instructions. PDL instructions are executed by an output device to generate physical output having the desired appearance.
A color page additionally includes information about the colors used in the page. Colors are described in accordance with a “color space,” which defines a data representation in terms of basic color components. The CMYK color space has four components (cyan (C), magenta (M), yellow (Y), and key (K) or black) and represents the color to be applied at a particular location on the page as a combination of four values: one representing the amount of cyan; one representing the amount of magenta; one representing the amount of yellow; and one representing the amount of black. In contrast, the CIELAB (Commission Internationale de l'Eclairage) color space represents color as a combination of three values: one representing luminosity (L); and two (A and B) which, in combination, represent chromaticity. Luminosity refers to the overall brightness of a color. Chromaticity refers to the quality of a color characterized by its dominant and complementary wavelength and purity taken together.
These color data representations are used to create color separations which, in turn, are used by output devices to generate a color output page. Generally, one separation is created for each color component in the device's color space. For example, the PDL representation of a page to be output by an image setter using the CMYK color space is used to create four color separations, one for each color component cyan, magenta, yellow, and black. Each separation indicates where on the page the corresponding ink should be applied. The same color data representation, if output on an image setter using an RGB (red-green-blue) color system, is used to create color separations for red, green, and blue inks.
Misalignment, also referred to as misregistration, of separations can cause unintentional gaps or overlaps in the generated output page where regions containing different colors abut. To minimize the effects of misalignment, a trapping technique can be applied to PDL representation to adjust the shapes of abutting color regions. Trapping is used to spread (expand) some separation color regions to prevent gaps, or choke (contract) separation color regions to prevent overlaps. The adjusted areas into which inks will be spread, or from which inks will be choked, are referred to as “trap regions.” Each trap region includes a trap color defining a color for the trap region.
Because trapping involves adjusting the shape of separation regions, and separations correspond to an output device's specific ink color, trapping has traditionally been a device dependent operation. Accordingly, the trapping process is typically forced into the later stages of print production where performance bottlenecks can become a problem. For instance, trapping is often performed in an output device's raster image processor which can have limited amounts of memory and, thus, can significantly slow output production. In addition, because each output device can implement its color components in a unique fashion, specification of trap colors at the time of output generation can result in a single page description looking distinctively different from output device to output device.