1. Field of the Invention
The present invention relates to trapping which prevents exposure of a background color caused by misregistration.
2. Description of the Related Art
In printing a color image, a given color is presented by superposing a plurality of colors, for example, the four colors cyan C, magenta M, yellow Y, and black K, which are known as process colors. The colors have a potential misregistration problem. In the boundary of the different process colors, even when the width of the boundary is ideally zero, exposure (to be referred to as a blank portion, hereinafter) of the white color of the background paper sheet or of unnatural artifacts occurs due to misregistration. This misregistration almost always arises from mechanical problems in a print system.
As a technique called trapping for correcting misregistration is known. In a general approach, a gap between colors or a boundary position of misregistration is detected, and the region of one color is extended to overlap the region of the other color so that adverse visual effects after printing are minimized.
One flaw of trapping is that the color changes in the overlap region (generally corresponding to the edge portion of an image) of different colors. Therefore, misregistration tolerance and color change are in a trade-off relationship.
Japanese Patent Laid-Open No. 2000-165693 discloses a technique of minimizing the adverse visual effects of overlap regions. This technique determines whether to perform trapping by converting a color into a color based on human perception or determines the color to undergo region extension and implements trapping with fewer adverse visual effects.
On the other hand, an edge operation such as an edge enhancement process is sometimes performed on the edge portion of an image. That is, a printing apparatus also has a spatial filter processing function. An edge enhancement process is sometimes performed not only to a picture image and a scan image but also to objects such as text, lines, and figures.
Japanese Patent Laid-Open No. 8-237472 discloses a technique which applies filter processing to trapping. This technique applies a softening filter and mask process to a portion to be trapped (a portion in which a blank portion is likely to appear due to misregistration) in order to prevent the blank portion. However, a blank portion caused by misregistration is more clearly noticeable in an edge-enhanced image rather than in an image which does not undergo edge enhancement.
FIGS. 1A to 1C are views showing an example of a text image (density of 80%) with a sharp edge in a light background (density of 10%).
When the image shown in FIG. 1A undergoes edge enhancement, the density of the text image in the edge portion increases and that of the background decreases as shown in FIGS. 1B and 1C. As a result, the edge portion of the image is well-defined.
FIGS. 2A to 2E are views showing an example of an image formed with a plurality of colors. For example, a text image is formed with the color K in a light background and the periphery of the text image is colored with the color M.
When the image shown in FIG. 2A undergoes edge enhancement, the density of the edge portions of colors K and M increase and that of the background of the edge portions decreases as shown in FIGS. 2B and 2C. When misregistration occurs, a clear blank portion appears as shown in FIG. 2E, as compared with a case shown in FIG. 2D wherein no edge enhancement is performed.
FIGS. 3A to 3D are views showing a case wherein edge enhancement is performed for the edge of an image which underwent trapping.
When trapping is performed for the image shown in FIG. 3A, as shown in FIGS. 3B and 3C, the colors overlap with each other in the trap width and an edge which does not exist in the original image appears. When edge enhancement is performed for this image, as shown in FIG. 3D, no blank portion appears but an unnatural stripe-patterned color change, which does not exist in the original image, occurs in the edge portion. Note that the same problem arises even when the processing order is reversed, that is, even when the edge-enhanced image undergoes trapping.
Such a potential problem exists because whether to perform edge enhancement is not considered in the post processing or preprocessing of trapping.
Since both of spatial filter processing and trapping are generally performed for each window, in order to obtain a satisfactory processing result, it is necessary to buffer an image for a plurality of lines. Accordingly, a processing system which performs both spatial filter processing and trapping must have independent line memories for respective processings, and this leads to an increase in memory cost.