The present invention relates especially to an image processing method which is applied to a printing machine for performing pseudo-halftone processing and an image processing apparatus.
In a case where a black character is printed by a Bi-level monochrome laser beam printer, a quality of a character print has been improved by smoothing processing. Smoothing processing is to conduct printing by placing interpolating dots at an edge portion of a character in order that an outline of the character and an line image is made to appear smooth.
However, when character printing with a medium density is conducted, there has been a problem of increase in level of edge jaggedness since pseudo-halftone processing using a dither matrix method or a error diffusion method is adopted in the medium density printing.
For example, when a character with a 32 level gray scale is printed with a Bi-level printer, there is a need for constructing a dither pattern with at least 32 pixels in 4 times 8 matrix, as shown in FIG. 23. For example, a case where a gray character [B] with a 10% density developed into a bitmap as shown in FIG. 24 is printed is considered, where such a dither pattern is employed. FIG. 25 is an enlarged view of 32 pixels in a p portion of the character [B] of FIG. 24 and each of values in the table indicates a density (%) of a pixel. When the p portion is subjected to pseudo-halftone processing while using a dither pattern shown in FIG. 23, a density of a pixel exceeds three values of the dither pattern: [1, 2 and 3], wherein a density 10% is equivalent to [3.2] in the 32 representation levels. Hence, outputs after the pseudo-halftone processing are such that only the three pixels of the 32 pixels of FIG. 25 have a density 100% as shown in FIG. 26 and thereby, only the three pixels are printed with a density 100% and the other 29 pixels are printed with a density 0%, in other words the other 29 pixels are not printed.
Focus attention here on an edge portion of the character. While the p portion of the gray character [B] with a density 10% is located at the second column from left, pixels in the edge portion at the second column from left are not printed but pixels inwardly located at the fourth and fifth columns from left are printed in an image of FIG. 26 which have been subjected to the pseudo-halftone processing. Therefore, for example, if dither patterns shown in FIG. 23 are arranged as shown in FIG. 27 in the vicinity of the edge portion of the gray character [B] with a density 10%, the edge portion is printed in a pattern shaded with oblique lines as shown in FIG. 28. That is, a problem arises since the edge outline which is originally a perfectly straight line is out of order to show jaggedness, which makes it impossible to express an exact edge.
On the other hand, a measure is conceived in which smoothing processing is conducted through a procedure such that an outline of a character which has been subjected to pseudo-halftone processing is detected by pattern matching and then a proper correction is applied to the pseudo-halftone processed image. Specially, for example, many of matching patterns each in 5xc3x975 pixel matrix are stored in ROM and detection based on the matching patterns in the ROM is conducted to find matching patterns that coincide with an original image in 5xc3x975 pixel matrix through comparison, replacement of pixels in the problematic edge image is achieved by the selected matching patterns for data correction, whereby smoothing processing is completed.
Such a method is, however, problematic since the number of gray scale levels is large, a volume of data constituting dither patterns is increased as the number of pixels of a dither pattern is increased and in addition, there is a requirement that dither patterns including all kinds of arrangement pattern must be prepared to secure matching and as a result a volume of data necessary for all of the dither patterns is further increased.
Accordingly, it is an object of the present invention to provide a image processing apparatus which does not require a tremendous volume of data unlike pattern matching, which can be adapted to pseudo-halftone processing such as a dither method or an error diffusion method so that its versatility in application is improved, and which can improve a quality of edge printing for a character and a line image after pseudo-halftone processing is conducted.
The present invention is directed to an image processing apparatus comprises detection means for detecting an edge from pixel data having a given first number of gray scale levels; processing means for performing pseudo-halftone processing of the pixel data to output pseudo-halftone data having a second number of gray scale levels lower than the first number of gray scale levels; and synthesis means for synthesizing pixel data based on the pseudo-halftone data outputted by the processing means with the pseudo-halftone data at positions corresponding to the edge that is detected by the edge detection means to output the pseudo-halftone data having pixels of the pixel data which distribute over the positions corresponding to the edge of the pixel data.
With this configuration, the present invention detects an edge position of an original image in pseudo-halftone data which are dispersed without any relationship with the original edge positions and by performing synthesis of pixels according to a result of the detection, pixels of the pseudo-halftone data are distributed at positions corresponding to the edge position of the original image. With such a procedure implemented, pixels are distributed at positions corresponding to the edge position of the original image, which is a result of pseudo-halftone processing. There can be provided, therefore, an image processing apparatus by which high quality printing is realized with no jaggedness on the outline of a printed character.
Besides, in an image processing method of the present invention as well, high quality printing can be achieved according to the steps constituted of the same features as described above.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.