The present invention relates to an image processing method which processes a multi-level input gradation image data on the basis of a multi-level dither method so as to convert into gradation image data having the smaller number of gradations, and an image processing apparatus therefore.
Conventionally, in an image forming apparatus which employs a line head such as a line LED (light emitting diode), a line thermal head and a line ink jet head, such as a printer and the like, a two-value image is formed by printing dots having the same size on a recording sheet according to a resolving power of the head, that is, according to an interval in a raster direction of a plurality of LED light emitting elements serving as recording elements arranged in a line in the case of the line LED head, according to an interval in a raster direction of a plurality of heat generating resistances serving as recording elements arranged in a line in the case of the line thermal head and according to an interval in a raster direction of a plurality of ink discharge ports serving as recording elements arranged in a line in the case of the line ink jet head. Further, the head is scanned plural times with changing a position in the raster direction so as to print the dots in a resolution more than the intervals of the respective recording elements, whereby the two-value image is formed.
In these image forming apparatuses, images of characters and lines are simply reproduced as a two-value image corresponding to the resolution of the head, and a photo image is reproduced according to a halftone process such as an ordered dither method or an error diffusing method. In the halftone process in this case, it is very hard to achieve both of a maintenance of high resolution and a reproduction of high gradation, and particularly in the ordered dither process, the resolution and the gradation have opposite characteristics.
To the contrary, in the image forming apparatus provided with the line head mentioned above, in recent years, there is known a structure which can express one pixel at some steps of gradations by modulating a printing area within one pixel with using the multi-level image data generated by processing the input gradation image data according to the multi-level dither method. FIG. 14 shows a recording head 71 constituted by arranging a plurality of recording elements in a line, and a state of dots recorded by this recording head 71. In this drawing, there is shown for simplification an example of the case of recording one pixel by three values including white. In this case, by arranging, for example, four or three recording heads 71 mentioned above in parallel, it is possible to record a color image constituted by a combination of four colors comprising C (cyan), M (magenta), Y (yellow) and K (black), or a combination of three colors comprising C, M and Y.
In the image forming apparatus which can record the multi-level image data, in order to reproduce the predetermined number of gradations peculiar to a printer engine actually performing an image recording operation after executing various image processes such as a color conversion process, an under color removal (UCR) process or a xcex3 correction, the structure is made such as to execute a multi-level halftone process such as a multi-level dither process employing a screen angle or a multi-level error diffusing process at each of colors so as to obtain the multi-level image data having some bits per one pixel. Then, it is intended to improve an image reproduction by concentrating more amount of information onto one pixel.
Further, the ordered dither process is generally light in processing, has a high speed and can reduce a cost, however, the error diffusing process is more excellent with respect to an image quality. However, in the case of the multi-level halftone process, a less difference in the image quality is generated in comparison with the case of two values. In particular, the more the number of gradation expressed by one pixel in a high resolution is, the smaller the difference of the image quality is.
In the multi-level dither process which is one of a multi-level halftone processes, there are two methods which are obtained by widely classifying according to a method of distributing a threshold value arrangement overriding each of planes. One is a method shown in FIG. 16A, that is, a method of filling up the threshold value in order of size from a smallest one at a unit of plane, and the other is a method shown in FIG. 16B, that is, a method of filling up the threshold value in order of size from a smallest one with respect to one optional pixel to be a subject to be processed.
For example, a multi-level dither process for converting image data of input 8 bits into an image having four values per one pixel (2 bits) according to the method shown in FIG. 16A by using a basic dither matrix of 2xc3x972 shown in FIG. 15 is a dither process used for a printer such as an ink jet printer, which is basically hard to be influenced by an appearance of a dot of an adjacent pixel and can stably reproduce an image formation at each pixel. A resolution thereof is substantially the same as a resolution performance of the printer engine section. That is, it is an ideal method in the case that the resolution is very high, the dot density is high and the image is reproduced in a density modulation. In this case, since the image is easily reproduced on a whole of a screen at an pixel of the same size or the similar size, it is easily affected by a printing accuracy.
Further, a multi-level dither process for converting image data of input 8 bits into an image having four values per one pixel (2 bits) according to the method shown in FIG. 16B by using a basic dither matrix of 2xc3x972 shown in FIG. 15 is a dither process frequently used for a printer such as a laser printer or a thermal printer, which is easily influenced by an appearance of a dot of an adjacent pixel and can be hard and unstable to form an image at each single pixel. A resolution thereof is low and a dot density is rough, and when setting a basic threshold value arrangement of the dither to a dot concentration type, an image called a dotted screen is formed. Since the resolution is low, a fine printing accuracy unevenness at a pixel unit is adsorbed.
To the contrary, with respect to a relationship between a recording head and a printing accuracy such as a printing position and a printing size, for example, in the case of the ink jet printer, a volume and a direction of an ink discharged from an ink discharge port corresponding to a recording element are in general frequently dispersed at each of ink discharge ports. It is possible to restrict the dispersion to a level equal to or less than a fixed value which generates no problem, however, a manufacturing cost becomes very expensive.
Further, in the case of forming an image at a high resolution equal to or more than an interval of the respective recording elements by scanning the head at plural times with changing a position in a raster direction, there is a possibility that a starting position for each of scanning is shifted, and a very high mechanical control is required for completely correcting this, so that it is a problem on cost.
When the volume and the direction of the ink are dispersed at each of the ink discharge ports, a density becomes higher at a portion having a large dot or a portion at which adjacent dots are closely arranged, in comparison with the other portion, whereby a density unevenness called as a black streak is generated, and further, a density becomes lower at a portion having a small dot and a portion at which adjacent dots are arranged apart from each other, in comparison with the other portions, whereby a density unevenness called as a white streak is generated. Accordingly, there is a problem that an image quality is deteriorated.
In order to prevent this, conventionally, the structure is made such that like a checkers-shaped thinned-out printing and the like, the same line in a direction perpendicular to the raster direction corresponding to a arranging direction of the respective recording elements of the ink jet head is not printed by the ink discharged from the same ink discharge port, but is printed by alternately using the inks discharged from a plurality of ink discharge ports, thereby preventing the density unevenness from generating. However, this method has a problem that a printing speed is low since a printing control is complicated.
Further, in the case of the image forming apparatus which employs the multi-level image data and can express one pixel at some stages of gradations by modulating the printing area within one pixel, there is a problem that a streak-shaped density unevenness is visually conspicuous particularly in the case of reproducing an image having a flat gradation on a whole surface at a dot of a medium size at which the adjacent dots are nearly brought into contact with each other. Particularly, in a human visual characteristic, there is a problem that a little positional shift is erroneously recognized as a streak-shaped density unevenness since a visual sensibility is very high in a horizontal direction and a vertical direction.
Further, there are various images printed by the printer, for example, there are a character/line image, a graph/CG, a photo image and the like. Optimum output systems for the respective images are different from each other, the character/line image emphasizing a resolution preferably employs a printing at higher resolution, the photo image emphasizing gradation preferably employs a printing which can obtain a more stable gradation reproducibility, and the graph/CG preferably employs a printing which has both of the high resolution and the stable gradation reproducibility.
In order to optimally print in this manner, there is conventionally known a method of switching contents of the halftone process according to the sort of the image. For example, Jpn. Pat. Appln. KOKAI Publication No. 9-233323 describes a structure which judges whether an image field of an input image is a character area or a pattern area and switches processes on the basis of the result between a two-value process according to a fixed threshold value having a priority to the resolution and a two-value process according to a dither process having a priority to the gradation.
Further, a structure described in Jpn. Pat. Appln. KOKAI Publication No. 10-191054 is made such as to judge after separating the input image into an area of an edge portion, an area adjacent to the edge and the other flat area, employ a dot dispersion type multi-level dither matrix for the area of the edge portion, employ a dot concentration type multi-level dither matrix for the flat area, and employ a multi-level dither matrix obtained by mixing the dot dispersion type and the dot concentration type for the area adjacent to the edge, thereby performing the multi-level dither process.
However, the structure described in Jpn. Pat. Appln. KOKAI Publication No. 9-233323 is made such as to switch the method of forming two values on the basis of the image field discrimination, and accordingly there is a problem that it can not process the multi-level output. Further, since the structure in Jpn. Pat. Appln. KOKAI Publication No. 10-191054 basically switch the sort of the basic dither matrix at the switching portion of the area, there is a problem that it is impossible to sufficiently delete a transient discontinuity of the image. Further, there is a problem that the process is made complex.
An object of the present invention is to provide an image processing method which can perform a multi-level dither process with keeping a sufficient resolution in the case that an input gradation image data is an image such as a character/line image setting a resolution as import and with keeping a sufficient gradation reproducibility in the case that it is an image such as a photo image emphasizing a gradation, and an image processing apparatus.
Further, according to the method and the apparatus, it is possible to sufficiently secure a continuity of an image on a boundary of the sorts of images.
In accordance with the present invention, there is provided an image processing method comprising the steps of: storing a plurality of multi-level dither threshold value arrangements employed for a multi-level dither process; discriminating a given first gradation image data expressing one pixel by a first bit number into an image field emphasizing a resolution and an image field emphasizing a gradation; selecting an optimum multi-level dither threshold value arrangement from a plurality of multi-level dither threshold value arrangements stored in the storing step on the basis of a discrimination result by the discriminating step; and converting the gradation image data into a second gradation image data expressing one pixel by a second bit number less than the first bit number according to a multi-level dither process on the basis of the optimum multi-level dither threshold value arrangement selected by the selecting step.
The present invention is structured, as mentioned above, such that a multiplicity of multi-level dither threshold values for the multi-level dither process are previously prepared, and the multi-level dither process is performed on the basis of the selected arrangement, for example, the multi-level dither threshold value arrangement emphasizing a resolution is selected for the character and line areas which require the resolution and the multi-level dither threshold value arrangement emphasizing a gradation is selected for the photo image field which requires the gradation. Accordingly, it is possible to perform the optimum multi-level dither process according to the field characteristics.
Further, since the same basic dither matrix is used even when the sort of the image field is replaced, a great strain which was generated in the conventional apparatus is not generated on the image even when the field discrimination is erroneously performed.
Further, the operation and effect of the present invention are also generated in an image processing apparatus employing the method according to the present invention for the same reason.
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.