1.Field of the Invention
The present invention relates to an image processing apparatus for recording or displaying an image, such as a laser beam printer or an ink jet printer, which records or displays halftone images with a high resolution and a high gradation.
2.Description of the Prior Art
A scanning system using a rotary polyhedral mirror or an oscillation mirror can provide a great scanning angle and small color dispersion. Such a scanning system is therefore widely used in facsimile systems using lasers, various types of display devices, printers and the like. The system is particularly used as a high-speed scanning device with a rotary polyhedral mirror.
In order to record or display a halftone image with such a scanning system, one picture element on the recording or display surface consists of an n.times.n matrix of subelements or an n.times.n dot matrix where n is a positive integer. A halftone image is reproduced by printing the respective subelements in black or leaving them blank, that is, by suitably distributing black and white subelements. For example, as shown in FIG. 1, one picture element 1 consists of a 2.times.2 matrix consisting of four subelements 2. As shown in FIGS. 1(0) to 1(4), five gray levels of 0 to 4 are obtained by sequentially increasing the number of subelements which are printed in black. In general, n.times.n+1 gray levels may be reproduced by constituting a picture element with an n.times.n matrix.
The halftone image processing apparatus adopting the method as described above has a number of advantages, described below:
(1) Since recording of the respective subelements may be controlled with digital signals of white or black (binary signals), the system configuration may be simplified.
(2) If a photosensitive body is used, the .gamma.-characteristics of the photosensitive body can be non-linear so that the photosensitive body may be freely selected.
FIG. 2 schematically shows the configuration of an image processing system of the image processing apparatus of the type described above. In this system, an input signal Xi (designated by reference numeral 3) representing the lightness or gray level information at a sampling point i (not shown) of an image is compared with a threshold level Ci (designated by reference numeral 5) by a comparator 4. The comparator 4 produces an output signal Yi (designated by reference numeral 6) in accordance with the comparison result. The threshold level Ci is set by a comparison matrix (threshold matrix) having a j.times.k matrix. The input signal Xi shown in FIG. 3(A) is compared with the threshold levels of the subelements of the comparison matrix (threshold matrix). When the input signal Xi exceeds the threshold level, the corresponding subelement is determined to have a black level or logic level "1". By comparing the input signals Xi with the threshold levels, j.times.k output signals Yi are produced. A density pattern consisting of a combination of j.times.k subelements is obtained by dividing each picture element into j.times.k subelements and systematically printing them in black or leaving them blank.
The output signals Yi obtained in this manner are temporarily stored in a memory 7 shown in FIG. 2 and recorded or displayed by an output device. Alternatively, the output signals Yi are directly supplied to the output device for recording or display. For example, as sequentially shown in FIGS. 3(A) to 3(C), assume that the input signal Xi is given as: EQU Xi=5.3.times.(1/16)
and the comparison matrix Ci comprises a 4.times.4 matrix of: ##EQU1## Assume that the maximum value Ximax of the input signal Xi is 1.0. As a result, the obtainable output signal Yi is given by: ##EQU2## The comparison results for the respective picture elements of the image are stored in the memory 7.
FIG. 4 shows the mode of operation of the comparator 4 for performing comparison operation as described above. FIG. 5 shows an example of a comparison matrix, and FIG. 6 shows an example of a density pattern obtained.
As may be seen from the respective figures, the input signal Xi is of 6-bit configuration and represents 65 gray levels of 0 to 64. The comparison matrix Ci for comparing and determining the 65 gray levels comprises a 8.times.8 matrix. Sixty-four threshold levels 1 to 64 for providing 65 gray levels are set in the respective subelements as shown in FIG. 5. In the comparison matrix as shown in the figure, the 8.times.8 matrix consists of four 4.times.4 submatrices. If each subelement in which the density level of the picture element exceeds the threshold level is to be printed in black, the black and white pattern of the output signals Yi becomes as shown in FIG. 6 if Xi=13. The resolution of the output image determined by comparison of each of four submatrices with the comparison matrix is thus represented in units of 4.times.4 matrices. On the other hand, gradation of the output image is represented in units of 8.times.8 matrices each having 4 submatrices. Thus, the resolution and gradation of the output image conveniently correspond to human perception characteristics. According to human perceptual characteristics, high gradation is required for a low spatial frequency region such as the skin of a person. However, in a high spatial frequency region such as an outline of a person, a high resolution is required but not so high a gradation is required. For this reason, better results may be obtained by using a 4.times.4 matrix as units for determining resolution and an 8.times.8 matrix which allows comparison with a greater number of threshold levels as units for determining gradation.
The relationship between the spatial frequency and the threshold levels of a matrix is described in detail in, for example, application Ser. No. 349,168.
However, in such a method for determining a density level with a subelement matrix, reproducibility is impaired at any image portion where the number of black dots is small. When the density pattern represented by the subelement matrix of a picture element sequentially changes as shown in FIGS. 7(A) to 7(D), the pitch of dots becomes smaller as the number of black dots increases. Thus, the resolution of an image consisting of a combination of such density patterns increases with an increase in the density level of the picture element. More specifically, in the state shown in FIG. 7(A), the pitch of black dots is 8a where a is the size of each subelement. In the state shown in FIG. 7(B), the oblique pitch of the black dots is 4.sqroot.2a. In the state shown in FIG. 7(C), pitches of 4a and 8a are involved vertically and horizontally. In the state shown in FIG. 7(D), the pitch of black dots is 4a vertically and horizontally.
If the pitch of black dots changes with an increase in the number of black dots (which, in turn, increases with an increase in the density) in the manner as described above, the density level of the recorded or displayed image does not result in a faithful reproduction of an image in correspondence with the number of black dots. The density level may become denser or lighter than the desired level, or the gradation of the output image may be significantly lowered. Accordingly, with such a change in resolution, the image quality of the output image is significantly degraded and an image of good quality may not be obtained.