JPH08139926A - Image forming device and its method - Google Patents

Abstract

PURPOSE: To improve the gradation stability and the preservation performance of gradation of a third generation copy in the dot processing where maximum density and intermediate tone picture elements are processed in mixture. CONSTITUTION: A dot processing section 106 converts a multi-value processing image into a dot image whose picture element is made up of plural unit picture elements and provides the output of a signal commanding the density growing direction of each of the unit picture elements. A PWM 109 generates a pulse width signal for each unit picture element of the dot image outputted from the dot processing section 106 based on the signal outputted from the dot processing section 106 and representing the density growing direction. A laser driver 110 stimulates a laser based on the pulse width signal. Since the density of each unit picture element is grown so as to be tangential to each adjacent black level picture element, the gravity center of the density represented in an area goes toward the center of the dot picture element thereby improving the image quality of the intermediate tone image, the gradation stability and the preservation performance of gradation of a third generation copy.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an image forming apparatus and method thereof, for example, an image forming apparatus and method for forming a halftone image.

[0002]

2. Description of the Related Art Office machines such as digital copying machines are becoming more multifunctional, and one of the functions is halftone dot processing. The halftone dot processing is a kind of modulation processing, and expresses gradation using a plurality of pixels.

FIG. 1 is a diagram showing three examples of halftone dot processing. The small quadrangle represents one pixel (hereinafter referred to as a "unit pixel"), and the number attached to the quadrangle becomes a black pixel according to the density level. Shows the order. Then, the plurality of unit pixels included in the area surrounded by the bold line in the figure function as one halftone pixel (hereinafter referred to as “halftone dot pixel”), and FIG.
Can express 9 gradations, (b) can express 14 gradations, and (c) can express 5 gradations. FIG. 2 is a diagram showing an example in which the halftone dot pixels of FIG. 1 (a) are combined.

Further, in the halftone dot processing, each unit pixel in the halftone dot pixels is density-modulated by a different density conversion curve so that a smoother density change can be obtained. Figure
3 and 4 are examples of the density conversion curve corresponding to each unit pixel in FIG. 1 (b). In the example shown in FIG. 3, the density of each unit pixel is the minimum density (for example, white) until the input density reaches the threshold value corresponding to each unit pixel, and becomes the maximum density (for example, black) when the input density exceeds the threshold value. Converted to binary. In the example shown in FIG. 4, there are two threshold values corresponding to each unit pixel,
White until the input density reaches the lower limit threshold, and black when the upper limit threshold is exceeded, but the unit pixel is set to halftone between the two thresholds to increase the number of gradations that can be expressed by halftone dots. . To make each unit pixel a halftone, a pulse width modulation (PWM) method that is generally used in digital copiers, that is, the density level of each pixel is converted to a pulse width to obtain a uniform gradation. The method of expressing as the area of a pixel is used.

FIG. 5 shows an example of a density conversion curve corresponding to each unit pixel of FIG. 1 (c), which has two threshold values as in FIG. As shown in FIG. 1 (c), if the density of each unit pixel of a pixel group formed by combining a plurality of unit pixels is changed using the same density conversion curve, the effect of noise components is reduced. can do.

FIG. 6 is a diagram showing an example of halftone dot pixels obtained by the above halftone dot processing. FIG. 6A is shown in FIG. 1B and FIG.
1A and 1B correspond to FIG. 1C, respectively, and show that some of the unit pixels inside the halftone dot pixel are in halftone.

[0007]

However, the above-mentioned conventional example has the following problems.

As described above, the halftone dot processing can increase the number of gradations that can be expressed by combining the maximum density (for example, black) pixel and the halftone pixel. It is possible to improve gradation stability and gradation preservation in grandchild copy, compared to the PWM method to be expressed. However, since some unit pixels are expressed in halftone,
To that extent, gradation stability and gradation storage stability in grandchild copy will deteriorate.

The present invention is to solve the above-mentioned problems, and its purpose is as follows. It is an object of the present invention to provide an image forming apparatus and method capable of improving gradation stability and gradation storage stability in grandchild copying in halftone dot processing in which maximum density pixels and halftone pixels are mixed. .

[0010]

[Means for Solving the Problems] and

The present invention has the following structure as one means for achieving the above object.

An image forming apparatus according to the present invention converts an input means for inputting a multi-valued image and a multi-valued image input by the input means into a halftone dot image having a plurality of unit pixels. Together with the conversion means for outputting an instruction signal for instructing the growth direction of the density of each unit pixel, and each unit of the halftone image output from the conversion means on the basis of the instruction signal output from the conversion means. And a forming unit that forms an image based on the halftone image by forming pixels.

Further, the image forming method according to the present invention comprises an input step of inputting a multi-valued image, and converting the multi-valued image input in the input step into a halftone image in which the pixels are composed of a plurality of unit pixels. A conversion step of: and an instruction step of instructing a growth direction of the density of each of the unit pixels,
A forming step of forming an image based on the halftone dot image by forming each unit pixel of the halftone dot image obtained by the conversion in the converting step based on the instruction of the instructing step. And

[0013]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An image forming apparatus according to an embodiment of the present invention will be described in detail below with reference to the drawings. In the following,
An embodiment in which the present invention is applied to a digital copying machine will be described. However, the present invention is not limited to this, and any device or system that handles multi-valued pixel data therein, such as a printer, a facsimile device, and an image scanner, can be used. The present invention can be applied.

[0014]

First Embodiment FIG. 7 is a block diagram showing a configuration example of an image forming apparatus according to a first embodiment of the present invention.

In the figure, 101 is a line type CCD, for example.
Then, the original image is read by the reflected light from the original placed on the original table. At this time, a light source such as a halogen lamp is scanned in the longitudinal direction (sub-scanning direction) of the document table, and the CCD 101 sequentially reads an image line by line.

102 is a sample and hold circuit (S / H),
103 is an analog / digital converter (A / D), and CCD101
The analog image signal input from is sampled and held, A / D converted, and output, for example, an 8-bit digital image signal.

An input correction unit 104 performs shading correction on the image signal input from the A / D 103 to correct variations in gain and offset for each pixel due to uneven sensitivity of the light source, the optical system and the CCD.

Reference numeral 105 denotes an image processing unit, which performs edit processing and luminance-density conversion on the image signal input from the input correction unit 104. Although the editing process is generally performed as a digital copying machine, for example, zooming, negative / positive inversion,
This includes movement, trimming, masking, shading, shading, shadowing, contour extraction and gradation.

Reference numeral 106 denotes a halftone dot processing unit, which will be described in detail later, but applies halftone dot processing to the image signal input from the image processing unit 105.

An output correction unit 107 subjects the image signal input from the halftone dot processing unit 106 to non-linear processing such as gamma correction.

Reference numeral 108 denotes a digital / analog converter (D / A), 1
09 is a pulse width modulation circuit (PWM), 110 is a laser driver, the D / A 108 converts the digital image signal input from the output correction unit 107 into an analog image signal, and the PWM 109 is a D The laser driver 110 outputs a pulse width signal corresponding to the level of the image signal input from / A108.
Causes a laser element (not shown) to emit light according to the pulse width of the signal input from the PWM 109. The emitted laser light scans a photosensitive drum of an image forming unit (not shown) to form a latent image on the photosensitive drum, and an image is formed on recording paper by an electrophotographic process.

Next, details of the halftone dot processing section 106 will be described. In the following description, FIG. 1 (a) or (b)
2 is used, the halftone dot pixels are arranged in the arrangement shown in FIG. 2 (b), and the density conversion curve shown in FIG. 4 is used, but this embodiment is not limited to this. It is not something that will be done. It should be noted that which shape of the halftone dot pixel in FIG. 1A or 1B is to be used depends on information and instructions from an operation unit (not shown).

As shown in FIG. 2 (b), the halftone dot processing unit 106 utilizes the fact that the positional relationship of halftone dot pixels is periodic, from the value counted for each unit pixel and for each line, The position of the pixel of interest in the halftone dot pixel is determined. That is, the counter that counts pixels and the counter that counts lines can be reset at cycle 8 and the position within the halftone dot pixel can be obtained from the combination of the two count values. For example, if the count value of the unit pixel at the upper left corner of the 8 × 8 unit pixel block indicated by the bold line in FIG. 2 (b) is (line, pixel) = (0,0), the count value is (1,2) In the case of, it indicates the position of the unit pixel of pixel number 7. In the case where the shape of the halftone dot pixel is as shown in FIG. 1 (b), the position of the pixel of interest can be similarly obtained.

In this way, when the pixel number is obtained from the position of the pixel of interest within the halftone dot pixel, the density is converted by the density conversion curve corresponding to the pixel number. Specifically, after converting the 8-bit density data of the pixel of interest input to the halftone dot processing unit 106 into, for example, 6-bit density data by an arithmetic circuit according to the corresponding density conversion curve,
'00' is added to the lower 2 bits and output as 8-bit density data.

Further, when the halftone dot processing unit 106 uses the halftone dot pixels shown in FIG. 1 (a), the unit pixels of pixel numbers 0, 1, 6 and 7 concentrate the density near the center thereof. Instruct PWM109 to do so. That is, the black pixels are allowed to grow from the vicinity of the center of the unit pixel to both sides according to the density. Regarding the unit pixels of pixel numbers 2 and 3, the PWM 109 is instructed to concentrate the density in the vicinity of the left end of the unit pixel so that the black pixel grows from the vicinity of the left end of the unit pixel to the right according to the density, Conversely, for the unit pixels of pixel numbers 4 and 5, the PWM 109 is instructed to concentrate the density in the vicinity of the right end of the unit pixel so that the black pixel grows from the vicinity of the right end of the unit pixel to the left according to the density. To do. This is shown in FIG. 8 (a).

Similarly, when the halftone dot pixels shown in FIG. 1 (b) are used, the unit pixels of pixel numbers 0, 2, 4, 9, and 11 are located near the center and pixel numbers 1, 5, 6 For the unit pixels of 10 and 10, the PWM 109 is instructed to concentrate the density near the left end and for the pixel numbers 3, 7, 8 and 12 near the right end. FIG. 8 (b) shows this state.

The instruction sent from the halftone dot processing unit 106 to the PWM 109 is, for example, a 2-bit signal. For example, "00" is near the center, "01" is near the left end, and "11" is the right end. Indicates that the concentration is concentrated in the vicinity.

When the halftone dot processing is not required due to an instruction or information from the operation unit, the halftone dot processing unit 106 outputs the inputted density data as it is, and at the same time, it is a 2-bit value indicating the density growth method of the unit pixel. '0 as the signal of
Output 0 '.

FIG. 9 is a block diagram showing a detailed configuration example of the PWM 109, and FIG. 10 is a diagram for explaining the operation of the PWM 109. Below,
Details of the PWM 109 will be described with reference to these drawings.

In FIG. 9, 202 to 204 are comparators, respectively, which compare the analog image signal input from the D / A 108 with the triangular wave or sawtooth wave input to the other terminal, and show the comparison result. A signal with a corresponding pulse width is output. A selector 205 selects one of the pulse signals output from the three comparators according to the 2-bit signal SEL input from the halftone dot processing unit 106, and outputs the pulse signal to the laser driver 110.

When the signal SEL is '00', the selector 205 receives the triangular wave shown in FIG.
Since the output of (1) is selected ((b) in the figure), the pixel grows from near the center as shown in (c) in the figure. Also the signal
If SEL is '01', the output of the comparator 203 shown in Fig. 2 (d), which receives a sawtooth wave with a sharp rise (Fig.
Since (e)) is selected, the pixel grows from near the left end as shown in FIG. Also, the signal SEL is '11'.
If so, the output of the comparator 204 ((h) in the same figure) to which the sawtooth wave with a sharp fall shown in (g) in the figure is input is selected. It will grow from the vicinity of the right end.

Pixels actually formed on the recording paper do not become clearly separated black and white due to subtle variations in the circuit, laser, or electrophotographic process. FIG. 11 is a diagram showing an example of a pixel formed by an electrophotographic process. An example of two pixels with black on the right side and halftone on the left side is shown in the figure. become. Therefore, rather than arranging the halftone pixel growing from the vicinity of the center on the left side of the black pixel shown on the right side of FIG. 4A, it is necessary to contact the black pixel as in this embodiment shown in FIG. It is possible to form clearer pixels and improve the image quality of the halftone image by arranging the halftone pixels that grow from the vicinity of the right end of the halftone image.

As described above, according to this embodiment,
In the halftone dot processing in which the maximum density pixel and the halftone pixel are mixed, the center of gravity of the density represented by the area is made near the center of the halftone dot pixel by growing the density of the unit pixel so as to contact the adjacent black pixel. In addition, clearer pixels can be formed, and the image quality of halftone images, gradation stability, and gradation storage stability in grandchild copying can be improved.

[0034]

[Modification] The algorithm for setting the growth direction of a unit pixel of a halftone is not limited to the one described above, and another algorithm will be described below.

When the halftone dot processing unit 106 uses the halftone dot pixels shown in FIG. 1 (a), the unit pixels with pixel numbers 0, 1, 6 and 7 are not allowed to concentrate the density near the center. Same as Example 1, but for the unit pixels of pixel numbers 2 and 3, when the unit pixel of pixel numbers 1 and 0 is a black pixel, the concentration is concentrated near the left end, and when not, the concentration is concentrated near the center. Let Similarly, for the unit pixels of pixel numbers 4 and 5, when the unit pixels of pixel numbers 0 and 1 are black pixels, the densities are concentrated near the right end, and when not, the densities are concentrated near the center.

Similarly, in the case of using the halftone dot pixels shown in FIG. 1 (b), regarding the unit pixels of pixel numbers 0, 2, 4, 9, and 11, it is the first to concentrate the density near the center. Same as the example, the unit pixel of the pixel number 1 is near the left end when the unit pixel of the pixel number 0 is a black pixel, otherwise the concentration is concentrated near the center, pixel number 5,6, Concentrations of 10 and 10 unit pixels are concentrated near the left end when the unit pixels of pixel numbers 4, 2, and 1 are black pixels, and near the center otherwise. Similarly, for the unit pixel of pixel number 3, the density is concentrated near the right end when the unit pixel of pixel number 0 is a black pixel, and near the center otherwise, for unit pixels of pixel numbers 7, 8 and 12. Respectively concentrates the density near the right end when the unit pixels of pixel numbers 2, 4, and 3 are black pixels, and concentrates it near the center otherwise.

By doing so, the pixel density growth method is determined according to the densities of the adjacent unit pixels. Therefore, in the edge portion of a line drawing or character image, outside the edge,
That is, it is possible to prevent the problem that the density is deviated to the side other than the line drawing or the character.

In the above description, the density growth direction is determined according to the position of the unit pixel within the halftone dot pixel.
It is also possible to determine the growth direction of the density according to the density of the adjacent unit pixels. For example, when one of the adjacent unit pixels is close to the black saturation level and the other is close to the white saturation level, the density of the pixel of interest is grown from the edge of the adjacent pixel side close to the black saturation level. To do. Even in this case, the center of gravity of the density can be arranged near the halftone dot pixel.

The present invention may be applied to a system composed of a plurality of devices or an apparatus composed of a single device.

Further, it goes without saying that the present invention can be applied to the case where it is achieved by supplying a program to a system or an apparatus.

[0041]

As described above, according to the present invention,
It is possible to provide an image forming apparatus and a method thereof, which are improved in gradation stability and gradation preservation in grandchild copy in halftone dot processing in which maximum density pixels and halftone pixels are mixed.

[Brief description of drawings]

FIG. 1 is a diagram showing three examples of halftone dot processing,

FIG. 2 is a diagram showing an example in which halftone dot pixels of FIG. 1 (a) are combined,

FIG. 3 is an example of a density conversion curve corresponding to each unit pixel in FIG. 1 (b),

4 is an example of a density conversion curve corresponding to each unit pixel in FIG. 1 (b),

5 is an example of a density conversion curve corresponding to each unit pixel in FIG. 1 (c),

FIG. 6 is a diagram showing an example of halftone dot pixels obtained by halftone dot processing;

FIG. 7 is a block diagram showing a configuration example of an image forming apparatus according to an embodiment of the invention.

FIG. 8 is a diagram showing an example of a growth state of the density of a unit pixel,

9 is a block diagram showing a detailed configuration example of the PWM shown in FIG. 7,

10 is a diagram for explaining the operation of the PWM shown in FIG. 9,

FIG. 11 is a diagram for explaining the effect of the present embodiment.

[Explanation of symbols]

 101 CCD 102 Sample-and-hold circuit (S / H) 103 Analog / digital converter (A / D) 104 Input correction section 105 Image processing section 106 Halftone dot processing section 107 Output correction section 108 Digital / analog converter (D / A) ) 109 Pulse width modulation circuit (PWM) 110 Laser driver

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁶ Identification code Office reference number FI technical display location H04N 1/40 B 103 A

Claims (5)

[Claims] 1. Input means for inputting a multi-valued image, and a multi-valued image input by the input means is converted into a halftone image in which the pixels are composed of a plurality of unit pixels, and each unit pixel Based on the conversion means for outputting an instruction signal for instructing the growth direction of the concentration, based on the instruction signal output from the conversion means,
An image forming apparatus comprising: a forming unit that forms an image based on the halftone image by forming each unit pixel of the halftone image output from the converting unit. 2. The conversion means outputs the instruction signal according to the position of a pixel of interest in the pixels of the halftone dot image, so that the density of the pixel near the center of each pixel of the halftone dot image. The image forming apparatus according to claim 1, wherein a center of gravity is arranged. 3. The conversion unit outputs the instruction signal according to the position of the pixel of interest in the pixels of the halftone image and the densities of the adjacent unit pixels, so that there are many unit pixels with high density. 2. The image forming apparatus according to claim 1, wherein the density centroid of pixels of the halftone dot image is arranged. 4. The conversion means outputs the instruction signal in accordance with the densities of the unit pixels adjacent to the pixel of interest, so that the density centroid of the pixels of the halftone dot image is increased toward the side having many high density unit pixels. 2. The image forming apparatus according to claim 1, wherein the image forming apparatus is arranged. 5. An input step of inputting a multi-valued image, a conversion step of converting the multi-valued image input in the input step into a halftone image in which the pixels are composed of a plurality of unit pixels, and each of the unit pixels Based on the halftone dot image, by forming each unit pixel of the halftone dot image obtained by conversion in the conversion step based on the instruction step of instructing the growth direction of the density of An image forming method comprising: a forming step of forming an image.