Image processor and color image processor

Details Image processor and color image processor Image processor and color image processor

2000-06-14

2004-03-30

Rogers, Scott (Department: 2626)

Facsimile and static presentation processing

Static presentation processing

Attribute control

C358S003160, C358S003190, C358S003120, C358S003260, C358S533000, C358S535000

Reexamination Certificate

active

06714320

ABSTRACT:

CROSS-REFERENCE TO RELATED APPLICATIONS
This application is based upon and claims the benefit of priority from the prior Japanese Patent Applications No. 11-166878, filed Jun. 14, 1999; and No. 2000-123969, filed Apr. 25, 2000, the entire contents of which are incorporated herein by reference.
BACKGROUND OF THE INVENTION
The present invention relates to an image processor and a color image processor for dithering multi-tone-level input image data to convert the data into data of fewer tone levels, used in a printer, a copier, a facsimile machine, an MFP (Multi-Function Peripheral) and the like.
Conventionally, a binary image output printer employing a line head, such as a line LED (light emission diode) head, a line thermal head and a line ink jet head, forms a binary image by printing dots equal to the resolution of the head. Namely, if a line LED head is employed, dots having a size coincident with the distance between a plurality of recording elements (LED) linearly arranged in main scan direction are printed on a recording paper sheet to thereby print a binary image. If a thermal head is employed, dots having a size coincident with the distance between a plurality of recording elements (heating resistor) linearly arranged in main scan direction are printed on a recording paper sheet to thereby print a binary image. If an ink jet head is employed, dots having a size equal to the distance between recording elements (ink jet nozzles) linearly arranged in the main scanning direction are printed on a recording paper sheet to thereby print a binary image. It is also well known to shift the head slightly in the direction of the main scanning to form an image on the same paper sheet repeatedly and to thereby realize a higher resolution than that corresponding to the distance between the recording elements.
In the image forming apparatus provided with the recording head of this type, a character/line image is reproduced as a binary image simply corresponding to the resolution of the head. A graphic/photograph image is reproduced as a binary image by a halftone processing such as an ordered dither method or an error diffusion method. In the halftone processing, it is difficult to both maintain a high resolution and reproduce a high tone level. In case of the ordered dither processing using the same threshold matrix repeatedly, in particular, resolution and tone property are contradicting property. The halftone processing is also used for color characters, shading colors and the like.
Further, as the image forming apparatuses provided with a recording head as stated above, there is proposed one for modulating the printing area of one pixel (or adjusting dot size) based on multi-level image data converted by the multi-level dither processing, thereby allowing expressing one pixel with several tone levels. An example of a recording head constituted by a plurality of recording elements used for such an apparatus as well as the state of dots is shown in FIG.
33
. In
FIG. 33
, reference symbol
1
denotes a recording head,
2
denotes an ink discharge port and
3
denotes an output dot (printed dot).
For brevity,
FIG. 33
illustrates an example of the output of dots of the image forming apparatus capable for expressing one pixel with three levels including white (output
0
). In addition, by arranging four or three lines of these recording elements in parallel, it is possible to record a color image of C (cyan), M (magenta), Y (yellow) and K (black) or a CMY-color image.
The image forming apparatus capable of printing such multi-level image data conducts various image processing including a color conversion processing, a UCR (under-color removal) processing and a gamma correction processing, to input RGB image data. Thereafter, the apparatus conducts a multi-level halftone processing such as a multi-level dither processing employing screen angles for the respective colors or a multi-level error diffusion processing so as to reproduce the number of tones intrinsic to a printer engine, to thereby obtain multi-level image data. The apparatus then outputs pixels having tone properties so as to enhance image reproducibility.
Generally, the ordered dither processing is relatively simple, has a high degree of freedom for configuration and has high processing speed, and the cost of the apparatus can be held down. However, it is said that the error diffusion processing is superior in image quality to the ordered dither processing. The ordered dither processing truncates quantization error in a comparison processing between input tone levels and thresholds, whereas the error diffusion processing diffuses quantization errors to peripheral pixels. Thus, they greatly differ in algorithm. As a result of the difference, compared with the ordered dither processing, the error diffusion processing can advantageously provide an output pattern having high frequency characteristics least conspicuous in light of human visibility, has a high edge holding effect and excellent image quality.
On the other hand, in a case of the halftone processing of a multi-level image output printer, it is known that the ordered dither processing and the error diffusion processing do not differ in image quality compared with the output of a binary image. This is because the truncated quantization error becomes far smaller than that of binary image data as the number of levels of the multi-level dither processing increases. In case of a high resolution printer, in particular, if the number of tones which one pixel can express is higher, the difference in image quality between the ordered dither processing and the error diffusion processing becomes less.
In addition, a method, such as a dither processing method employing fixed mask dither improved from stochastic dither or cluster dither, of realizing output characteristics comparable to that of the error diffusion processing at the same high speed as that of the ordered dither processing, is recently developed.
An ordinary binary output dither processing obtains binary output pixels by comparing input pixels with dither matrix thresholds at corresponding positions while basically, only taking into consideration a threshold array in a dither matrix on one plane. This state is shown in FIG.
34
.
FIG. 34
is a typical view showing a binary dither processing employing a well-known 4×4 Bayer dither matrix. To simplify description, input pixels of 4-bit tone level are compared with corresponding thresholds in a dither matrix. If the input tone level is equal to or higher than the corresponding threshold in the dither matrix, 1 (black) is output and if it is lower than the corresponding threshold, 0 (white) is output, thus obtaining a binary output image in combination of 1 and 0.
As shown in
FIG. 34
, the dither matrix has a configuration in which a unit dither threshold matrix of, for example, 4×4 (to be simply referred to as “unit matrix” hereinafter) is repeatedly used regularly and performs the above-described processing to all input pixels. Further, a normal output apparatus, such as a printer, often outputs a pixel similar to a circle rather than a square pixel due to the process limitations of the apparatus. The output state in this case is shown in FIG.
35
. When all pixels are printed, the shapes of the printed pixels are designed to be ones completely covering ideal square pixels, i.e., circles with a diameter equal to or larger than {square root over (2)} times as large as a resolution pitch like dot “1” shown in FIG.
35
.
On the other hand, in the multi-level dither processing, it is necessary to consider not only a plane threshold array in the above-stated basic dither matrix but also depth (pixel level) direction. For example, in case of conducting a multi-level, e.g., N-level dither processing, (N−1) threshold planes are required. Dither thresholds on each of the threshold planes are compared with input tone levels, to thereby obtain an N-level output image. The state of this multi-level dither processing is shown in FIG.
36
and the state of

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