JPH02210966A - Picture forming device - Google Patents

Abstract

PURPOSE:To output a stable picture without deteriorating the picture quality of an original with a character picture and a photographic picture by applying linear gradation reproduction to a halftone picture area. CONSTITUTION:A picture area of a halftone and a picture area of a character and a thin line are discriminated, 1st picture forming means 11a, 12a shift a picture signal of a highlight part toward a dark level with respect to the character and thin line picture area to apply gradation reproduction with intensified edge. A switch circuit 18 is switched to the halftone picture area and 2nd picture forming means 11b, 12a apply linear gradation reproduction. Thus, an original mixed with a character picture and a photographic picture is reproduced stably.

Description

[Detailed Description of the Invention] [Industrial Applications] The present invention relates to an image forming apparatus, and particularly to an image forming apparatus that processes halftone dots, images in which photographs and characters are mixed, and forms images. .

[Prior Art] In recent years, full-color images have been increasing not only in the fields of printing and design but also in offices and the like, and color copying apparatuses that faithfully read and output color originals have become widespread. These color copying machines are required to simultaneously output full-color images with high gradation and output color text documents clearly and with high resolution.

Dithering methods, halftone dot modulation, and the like are known methods for outputting full-color images with high gradations, but when these methods are applied to characters and line drawings, the resolution drops significantly and the character quality deteriorates.

On the other hand, although binary processing is suitable for reproducing text and line drawings well, it is well known that when applied to halftone dots or photographic images, the gradation drops significantly and the image quality deteriorates. .

Therefore, many proposals have been made to achieve both the quality of characters and the quality of halftones.

For example, an image reading device first extracts a black area based on brightness information and saturation information of a document, then extracts the edge components of the document to identify black edges, and also refers to neighboring color information. The image forming device (printer) automatically distinguishes between black characters, color images, and black lines in halftone dots based on the degree of black edges and the degree of nearby color information.
It has been proposed to output an image signal including a signal specifying image forming conditions (Japanese Patent Application No. 63-2169489).
).

This printer uses first and second image forming circuits suitable for halftone image reproduction and line drawing reproduction to output respective images depending on the image forming conditions.For example, the printer has advantages such as high printing quality and high speed. Laser printers are widely used as output devices such as color copying machines and as ordinary printers, and use so-called pulse width modulation, which is a modulation that makes the emission time of a laser correspond to the magnitude of an image signal. When performing such pulse width modulation,
By changing the period of the reference pulse, a and b in FIG.
Select multiple tone reproduction characteristics as shown in . here,
The frequency of the reference pulse is larger in the reproduction characteristic a than in the reproduction characteristic a.

Although the reproduction characteristic a of the first image forming circuit shown in FIG. 2 has high resolution, it cannot guarantee the reproduction of highlight parts.
The reproduction characteristic of the second image forming means can guarantee gradation reproduction from highlight areas to solid areas, but the resolution is lower than that of reproduction characteristic a.

[Problems to be Solved by the Invention] However, in electrophotographic printers, output images vary due to various characteristic fluctuations, including temperature and humidity inside and outside the device, latent image characteristics, development characteristics, and transfer characteristics. Therefore, as described above, in a device that outputs an image by changing the gradation reproducibility of the printer depending on the image forming conditions from the image reading device, the first. The manner in which the reproduction characteristics change depending on the second image forming circuits differs from each other.

The manner in which the gradation reproduction characteristics a and b in FIG. 2 change is as shown in FIG. 3. In FIG. 3, when each condition of the printer is "1", for example when placed in a high temperature environment, the reproduction characteristics by the first image forming circuit are aI, and the reproduction characteristics by the second image forming circuit are bl. On the other hand, “2”
The reproducibility characteristics when placed in a low humidity environment, for example, are &2r bl, respectively.

If you pay attention to the highlighted part, in the second image forming circuit,
Although the copy density only increases or decreases with respect to the original density, in the first image forming circuit, the reproducible range varies greatly even if the original density is the same. That is, there is a problem in that character images (particularly thin, thin lines) that can be reproduced when the reproduction characteristic is a2 are not reproduced when the reproduction characteristic is a1. On the other hand, the variation in the reproduction characteristic of the second image forming circuit at this time is a density variation that does not pose a problem in image reproduction. This is because reproduction in the first image forming circuit, where the frequency of the reference pulse is higher, is susceptible to changes in development characteristics because each latent image forming a pixel is unstable.

To provide an image forming apparatus which eliminates the above-mentioned conventional drawbacks and stably reproduces a document containing a mixture of character images and photographic images without degrading the image quality of either the halftone part or the character part.

Furthermore, there is provided an image forming apparatus including a pulse width modulation means for pulse width modulating an image signal to which the present invention is applied.

[Means for solving problem i11] In order to solve this problem, the image forming apparatus of the present invention has the following features:
An image forming apparatus that receives an image signal including a designation signal that designates different image forming conditions depending on the characteristics of the image, and forms an image corresponding to the image forming condition, the image forming apparatus forming a halftone image area and images of characters and thin lines. and a discriminating means for discriminating the area, and for the image area of the characters and thin lines,
a first image forming means for reproducing edge-emphasized gradation by shifting the image signal of the highlight portion to the dark side; and a second image forming means for performing linear gradation reproduction for the intermediate tone image area. and an image forming means.

Here, the first and second image forming means include pulse width modulating means for pulse width modulating the image signal, and the first and second image forming means include a reference frequency of the pulse width modulating means. are different.

Further, the reference frequency of the pulse width modulation means included in the first image forming means is higher than the reference frequency of the pulse width modulation means included in the second image forming means.

[Function] In this configuration, the discrimination means discriminates between a half-tone image area and an image area of characters and thin lines, and the first image forming means forms a highlight part for the image area of characters and thin lines. The image signal is shifted to the dark side to reproduce the gradation of edge emphasis, and for the intermediate tone image area, the second
This image forming means reproduces linear gradations. As a result, documents containing a mixture of character images and photographic images can be stably reproduced.

[Example] Hereinafter, an example of the present invention will be described in detail with reference to the drawings.

First, a method for identifying line drawings and halftone images in an image reading device will be described. In Figure 4, R (red),
The G (14) and B (blue) signals are color signals for one pixel read by the color document reading device. 51 is a luminance signal Y and a color signal 1.51 from the RGB signals. This is a circuit that calculates Q. The luminance signal Y is the source signal of the black character edge signal, and after being inverted by the inverter 53 and converted to the darkness signal y,
The black edge generation circuit 54 outputs edges extracted by Laplacian transformation.

Color signal 1. Q is a signal representing a color difference from an achromatic color, and is input to an achromatic signal calculation circuit 52, which outputs an achromatic signal W using a lookup table. The achromatic color signal W indicates that the larger the value, the more achromatic the color.The achromatic color signal W and the darkness signal y are input to the black level determination circuit 55, and the achromatic color signal W and the darkness signal y are input to the black level determination circuit 55, and the dark achromatic color, that is, the black level, is It is output as a black level signal T.

The black character edge generating circuit 54 generates a black character edge signal E1 + according to the black level signal T from 7 to the black edge signal.
"2 is output. The black character edge signal E1 is a signal that emphasizes the edges of black characters, and the black character edge signal E2 is a signal that removes the black shift of the edges of black characters.

The image area signal generation circuit 57 generates an achromatic color signal W and a luminance signal Y.
Based on this, the bright chromatic color and its vicinity are determined to be an image area, and an image area determination signal Z is output.

The black character correction circuit 58 obtains C (cyan), 1M (magenta), Ye (yellow), and K (black) signals from the R, G, and B signals, and produces a black character edge signal from which misjudgments have been removed by the image area determination signal 2. Of EI+E2, C, M, Y
A black character edge signal E is added to e, and a black character edge signal E1 is added to K as a correction signal, and the resultant signals are output to an output device such as a printer. Also, signal P is a printer line number signal, and C
It is sent to the PU and selects the frequency of the fundamental pulse of the pulse width modulation circuit in the image forming apparatus.

Next, FIG. 5 shows an example of the configuration of an image forming apparatus (laser printer) according to this embodiment which forms an image using an image signal output from the image reading apparatus.

In the figure, 22 is a CPU that controls each circuit, and 26 is a ROM in which operating programs are stored. 27 again
is a RAM used as a work area when the CPU 22 operates.

29 is an electrophotographic photoreceptor on a drum that rotates in the direction of the arrow. The photoreceptor 29 is first uniformly charged by a charger 30,
Next, the photoreceptor 29 is scanned and exposed in a direction substantially perpendicular to the direction of rotation of the photoreceptor 29 with a laser beam 31 that blinks in response to the modulation signal. As a result, the electrostatic latent image formed on the photoreceptor 29 is developed and visualized by the developing device 32.

The formed visible toner image is transferred onto a transfer material 34 by a transfer charger 33. The visible toner image transferred to the transfer material 34 is fixed by a fixing device (not shown). After transfer, photoreceptor 2
The toner remaining on the toner 9 is removed and cleaned by a cleaning device 35. Thereafter, the charge remaining on the photoreceptor 29 is eliminated by the discharge light from the lamp 36, and the above steps are repeated again.

Laser beam 31 is emitted from semiconductor laser 37 .

The semiconductor laser 37 is driven by a drive circuit 41 to which a pulse width modulation signal E outputted from the comparator 4 is applied, and emits a laser beam 31 that is converted into a blinking state in response to this modulation signal. It is. Here, in the comparator 4, the digital video signal latched in the latch section 1 is converted into a voltage by the A/D converter 2, and the level signal whose range is adjusted by the dynamic range adjustment circuit 25 and the clock from the clock generator 23 are input. Based on this, a triangular wave adjusted by the bias adjustment circuit 24 and generated from the triangular wave generating circuit 8 is input and compared.

The laser beam 31 emitted from the semiconductor laser 37 is
The image is scanned by a scanner 38 such as a rotating polygon mirror or a galvano mirror. 39 is a lens for forming a point-like image of the laser beam 31 on the photoreceptor 29, and 40 is a mirror for folding the optical path.

FIG. 1 shows in detail an example of a circuit used when forming an image by performing the above-mentioned pulse width modulation in the image forming apparatus of this embodiment. Note that the output from the switch circuit 18 and subsequent parts are the same as in FIG. 5.

Digital video Vln from the interface is latched by latch 1 with video clock VCLK and synchronized. This image signal DvIn is converted into an analog video signal AV by a D/A converter 2.

The output of the D/A:7 inverter 2 is converted to a voltage level by a resistor 3, and then input to one of the input terminals of two comparators 4a and 4b.

In addition, in this example, two systems of triangular wave generation circuits, each of which has an integral circuit as its basic configuration, are prepared, each synchronizing with VCLK and having two different cycles of PHCIJ and TXCLK.
Integrate the output of the flip-flop 5.13 into the frequency divided J/. Here, the frequency of TXCL is the frequency α where resolution is important. On the other hand, the frequency of PHCL is set to be frequency β (however, α>β). These divided 50% duty clocks are sent to the buffer 6a.

6b, variable resistors 7a, 7b and capacitor 8a,
A triangular wave is generated by the triangular wave generating circuit 11a and 1lb (integrator circuit) composed of the triangular wave generator 8b.

Then, capacitors 9a, 9b and variable resistors 10a, 10
A bias adjustment circuit 12a consisting of b.

12a adjusts the bias amount, and the aforementioned comparator 4a
, 4b, and is compared with the analog video signal AV, and two pulse width modulation signals E, and E
It becomes b.

These two signals E1 and Eb are input to the switch circuit 18, which sets/resets the D flip-flop 22a based on the data and address from the CPU 22, and sets/resets the D flip-flop 22a in accordance with the output tdHJjJ signal cps in the case of a character part. In the case of a photo (halftone image) portion, the selector circuit 18 switches to Eb.

An example of a method for always stably obtaining images formed by pulse width modulation signals E, . . . Eb will be described below.

The method of matching the level of the analog video signal and the level of the triangular wave is to detect the amount of light emitted from the light emitting element modulated by the triangular wave, and calibrate the characteristics of the analog video signal (input image signal) or the triangular wave according to the detection level. , the level of the analog video signal and the level of the triangular wave are matched (Japanese Patent Application No. 63-095841).

By the way, regarding a text document, the relationship between the digital video signal Vln after image processing is performed by the image reading device in this embodiment (the line number signal P of the printer selects the one with the highest frequency) and the line width of the document is as follows. , 1st in Figure 6
as shown in the quadrant. This is M in the image reading device.
When this digital video signal representing TF is received by an image forming apparatus and an image is formed, the relationship with the copy density is as shown in C in the second quadrant of FIG. 6. This is the same as the limiting characteristic a (enlarged view of the highlighted part) due to the first image forming circuit (how to adjust the frequency of the reference pulse) in FIG. 3.

In FIG. 6, similarly to FIG. 3, the reproduction characteristic when the state is "1" is CI, and the reproduction characteristic when the state is "2" is C1. Here, when using Cl, thin lines with a line width of 11 or more can be reproduced, but thin lines with a line width of 1 or less cannot be reproduced, and when using C2, thin lines with a line width of 22 or more can be reproduced. However, my father, <intersecting sentence 1. That is, thin lines with line widths between 2 and 1 may or may not be reproduced due to variations. Therefore, the thin lines that can be stably reproduced are those of Sentence 1 or higher.

Therefore, in order to always maintain stable tone reproduction characteristics,
The offset value V of the digital video signal vln when the reproduction characteristic is C is shifted to the data ■l when reading the original with the line width or. Therefore, when adjusting the level of the analog video signal and the level of the triangular wave, the bias adjustment circuit 12b sets the offset value to the value for the digital video signal VI. The value of this digital video signal V can be determined in advance from the emission characteristics of the semiconductor laser, the charging characteristics of the photoreceptor, the development characteristics, etc.

FIG. 7 shows the gradation reproduction characteristics of the first image forming circuit set in this manner. Here, the reproduction characteristic C is the same as the reproduction characteristic a in FIG. 2, and the reproduction characteristic C is the gradation reproduction characteristic after being set by the above method. Since the output density changes almost linearly with respect to the input data, a stable image can be output even if the state of the apparatus changes.

Another example of stabilizing the characteristics of the formed image using the frequency of the reference pulse in the first image forming circuit0 A lookup table stored in the ROM for the digital video signal sent from the image reading device (
LUT) to output data in unstable areas in a stable area.

This will be explained using FIG. In Figure 8, the first
The quadrants are the same as in FIG. 3 and show the gradation reproduction characteristics based on the reproduction characteristics a. The fourth quadrant is the contents of the LUT described above, and B is the LUT when this embodiment is not used. The reproduction characteristic a is made linear through this table. In this embodiment, B'' is eliminated from the digital video signal vI and below, and only the stable region of the reproduction characteristic 8 in the first quadrant is used.

When image formation is performed through this LUT, as shown in Figure 0, where the second quadrant is a'', thin lines below the data vI are not reproduced, but character images larger than that are stably reproduced.

Although this embodiment has been described with respect to a color copying apparatus, it is not limited to this, and may be applied to a monochrome copying apparatus or a black-and-white copying apparatus, which is an apparatus that reproduces an image containing a mixture of text images and photographic images using different image forming means. This embodiment can also be used.

[Effects of the Invention] According to the present invention, it is possible to provide an image forming apparatus that can stably reproduce a document containing a mixture of character images and photographic images without degrading the image quality of either the halftone portion or the character portion.

[Brief explanation of the drawing]

Figure 1 is a block diagram showing the characteristic parts of the image forming apparatus of this embodiment, Figure 2 is a diagram showing differences in tone reproduction characteristics, Figure 3 is a diagram showing variations in tone reproduction characteristics, and Figure 4 is a diagram showing differences in tone reproduction characteristics. 5 is a block diagram of the image forming apparatus of this embodiment. FIG. 6 is a diagram showing the image conversion characteristics of this embodiment. FIG. 7 is a diagram showing the image conversion characteristics of this embodiment before and after implementation. FIG. 8 is a diagram showing image conversion characteristics of another embodiment. In the figure, 1...Latch, 2...D/A converter, 3...
・Resistor, 4a, 4b...Comparator, 5.13・
...JK flip-flop, lla, fib...triangular wave generation circuit, 12a, 12b...bias adjustment circuit,
16.17...NAND circuit, 18...Switch circuit, 22a...D flip-flop, 23...Inverting circuit. nl, 1-Sakaki Huanguchi b-! 1 town

Claims (3)

[Claims] (1) An image forming apparatus that receives an image signal including a designation signal that specifies different image forming conditions depending on the characteristics of the image, and forms an image corresponding to the image forming condition, which includes a halftone image area, characters, and a discrimination means for discriminating between an image area of a thin line and a first image forming unit for reproducing the gradation of edge emphasis by shifting an image signal of a highlight part to a dark side with respect to the image area of characters and thin lines; An image forming apparatus comprising: a means for forming an image; and a second image forming means for linearly reproducing gradations in the image area of the intermediate tone. (2) The first and second image forming means include pulse width modulating means for pulse width modulating the image signal, and the first and second image forming means include a reference frequency of the pulse width modulating means. The image forming apparatus according to claim 1, wherein the image forming apparatus has different numbers. (3) The reference frequency of the pulse width modulation means included in the first image forming means is higher than the reference frequency of the pulse width modulation means included in the second image forming means. image forming device.