JP3046034B2 - Image forming device - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an image forming apparatus for forming an image of input image data.

2. Description of the Related Art Conventionally, in the case of forming an image by binarizing multivalued image data including a halftone image, as a binarization method, for example, a dither method, a density pattern method, or the like using a threshold matrix is often used. Are known. In a laser beam printer or the like, a so-called "pulse width modulation" binarization method in which multi-value image data is compared with a triangular wave or the like and the density of the image data is modulated so as to correspond to the pulse width is also proposed. Have been.

Further, when performing pulse width modulation in an electrophotographic image forming apparatus, in order to improve the gradation of a halftone image, the period of the pattern pulse signal for pulse width modulation is set to two times the period of the synchronization signal of the image signal. It was doubled.

[Problems to be Solved by the Invention] However, in the above-described conventional example, when a line drawing such as a character is subjected to pulse width modulation, there is a problem that an edge portion thereof becomes jagged or a resolution of image data is reduced. Was.

The present invention has been made to solve the above problems,
It is an object of the present invention to provide an image forming apparatus that can faithfully reproduce input image data and reproduce a higher quality image than before.

[Means for Solving the Problems] To achieve the above object, an image forming apparatus according to the present invention comprises: an input unit for inputting digital image data in which the density of each pixel is represented by a plurality of bits; Processing means for distributing density values between adjacent first and second pixels, generating means for generating a pulse width modulation signal in accordance with digital image data processed by said processing means, Image forming means for forming an image based on a width modulation signal, wherein the processing means determines that an added value of the density value of the first pixel and the density value of the second pixel is a predetermined maximum density value If the maximum density value exceeds the maximum density value, the difference value between the added value and the maximum density value is determined as the density value of the pixel having the higher density of the first and second pixels. Output as the value When the addition value is equal to or less than the maximum density value, the addition value is set as the density value of the higher density pixel of the first and second pixels, and the predetermined minimum density value is set as the density value of the other pixel. It is characterized by outputting.

Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the accompanying drawings.

<Description of Configuration (FIGS. 1 and 4)> FIG. 1 is a block diagram showing a schematic configuration of an image signal processing unit of a laser beam printer in the present embodiment.

In the figure, reference numeral 101 denotes a video data output unit, which is a CCD (not shown).
A / D conversion of image data from sensors and video cameras,
Digital multi-valued image data 114 of predetermined bits (8 bits in this embodiment) including density information is synchronized with a synchronous clock (RCLK) 115.
Output in synchronization with The digital image data 114 is temporarily stored in the buffer memory 102 and is read out as 8-bit video data 116 in synchronization with the horizontal synchronization signal (HSYNC) and the video clock signal (VCLK) from the timing signal generation circuit 112. Is done. Also, buffer memory 1
Through the 02, the image data from the video data output unit 101 is out of synchronization and speed-converted.

The video data 116 read out from the buffer memory 102 is corrected in a look-up table (LUT) 103 to a gradation suitable for the latent image and development characteristics of the laser beam printer, and then an addition operation circuit 129 and a density gradient detection circuit 130
Is input to First, the addition operation circuit 129 adds two pixel data in one cycle of the pattern pulse signal 123 described later, and when the added value is equal to or less than the maximum value of image data ([FFH] in this embodiment), the addition is performed. The value is output to port 131, and “0” is output to port 132. When the addition value is larger than the maximum value, the maximum value is output to the port 131, and the difference between the addition value and the maximum value is output to the port 132.

Next, in the density gradient detection circuit 130, the density gradient of two pixels is detected in synchronization with the above-described addition operation circuit 129, and when the data is the larger of the two pixels, the select circuit 104 selects the A side. A signal is output, and in the case of smaller data, a signal for selecting the B side is output to the select circuit 104. Data 119 selected by this select circuit 104 is D / A
The signal is input to a converter 105, converted into an analog video signal 120 by the D / A converter 105, and input to one input terminal of a comparator 106. To the other input terminal of the comparator 106, a pattern pulse 123 for binarizing the analog video signal 120 (PWM modulation) is input. Then, the modulation signal (P
WM signal) 124 passes through the gate circuit 107 to the laser driver 10
Entered in 8.

Here, the laser driver 108 receives the input modulated signal 1
The semiconductor laser 109 is driven at a constant current for a time corresponding to 25 pulse widths. Then, the laser beam emitted from the semiconductor laser 109 scans the photoconductor, and forms a copy image by an electrophotographic process.

FIG. 4 is a block diagram showing a schematic configuration of a recording unit of the laser beam printer. In the figure, reference numeral 108 denotes the above-described laser driver, and 109 denotes a semiconductor laser. 51 is a collimator lens, 52 is a polygon mirror, and 53 is an fθ lens. The laser light thus modulated and corrected for fθ scans the photoreceptor 50 by a raster scan to form an electrostatic latent image, and the image is transferred to a recording sheet by an electrophotographic method.
Reference numeral 110 denotes a beam detector (BD) provided near the start of scanning of one line of the laser beam and detecting the start of laser beam scanning.

Here, returning to FIG. 1, the BD generation circuit 111 generates the BD signal 121 based on the signal from the beam detector (BD) 110. This BD signal 121 and 4 of the input video data
A clock 127 having a frequency more than twice the frequency is input from the crystal oscillator 126 to the timing signal generation circuit 112, and the horizontal synchronizing signal (HSY
NC) and a video clock (VCLK) are generated and output. Further, the timing signal generating circuit 112 outputs a screen clock (SCLK) 122 to the pattern pulse generating circuit 113. In this embodiment, the screen clock (SCLK) 122 is a 50% duty clock synchronized with the video clock (VCLK) with a period twice that of the video clock (VCLK) 117.

In the pattern pulse generating circuit 113, a screen clock 122 generates a pattern pulse 123 having a predetermined shape.

In the present embodiment, the pattern pulse 123 is a triangular wave synchronized with the screen clock 122 and having the same cycle as the screen clock 122.

Also, as described above, the pattern clock 123 is a PWM
The analog image data 120 is input to the comparator 106 for binarization by (pulse width modulation).

<Description of Modulation Example (FIGS. 1 and 2)> FIG. 2 is a diagram for explaining signal waveforms at various parts of the image signal processing unit shown in FIG.

Referring to FIG. 2, a period clock 127 that is at least four times longer than the video clock 117 is input from the crystal oscillator 126 to the timing signal generation circuit 112, and the BD signal 121 and the clock are output.
HSYNC signal 128 synchronized with 127, video clock signal 117,
A screen clock signal 122 is output. As described above, this screen clock 122 is a synchronizing signal for generating a pattern pulse, and
3 is entered.

The analog video signal 120 is transmitted to the buffer memory 102.
FIG. 2 shows an example in which video data 116 read out from the D / A converter 105 is converted into an analog signal. As can be seen from FIG. 2, each pixel data of analog level is output in synchronization with the video clock 117. Is done.

As shown in the figure, the lower the analog level is, the darker the analog level is and the higher the density is.

The pattern pulse 123 output from the pattern pulse generating circuit 113 is generated in synchronization with the screen clock 122 as shown in FIG. Here, the modulation signal (PWM) 124 indicates a binary signal obtained by comparing the video signal 120 with the pattern pulse 123 by the comparator 106, and serves as a drive signal for the laser driver 108.

<Description of Image Modulation Example (FIG. 3)> FIG. 3 is a diagram showing an image modulation example in the present embodiment.

For comparison with the conventional example, the output of LUT118 is directly
An example in which the signal is input to the A converter 105 is also shown. The input image data 114 is the same.

As shown in the figure, in the present embodiment, according to the calculation result by the above-described addition calculation circuit 129 and density gradient detection circuit 130,
An analog video signal 302 is being output. That is, in the cases of and shown in FIG. 3, the added value is equal to or less than the maximum value, and “0” and the added value are output according to the density gradient. In addition, in the case of 〜, the addition value is larger than the maximum value, and the difference between the maximum value and the calculation result is similarly output.

Then, the modulated signals obtained by modulating the analog video signals 301 and 302 by the above-described pattern pulse signal 123 become like 303 and 304, respectively. This modulated signal 304
The ON signal is more concentrated in a portion where the image density is higher than in the modulation signal 303. For this reason, blurring of a line drawing such as a character can be prevented, and the output is particularly suitable for an electrophotographic image forming apparatus.

Next, another embodiment according to the present invention will be described.

In the above-described embodiment, the detection of the density gradient of the image data is detected based on the magnitude relationship of the two pixel data in one cycle of the pattern pulse signal 123. However, the detection may be performed on the peripheral image data adjacent to the two pixel data.

305 shown in FIG. 3 is an example of image modulation when detection is performed based on data of one pixel before and after two-pixel data. Also in this case, it can be seen that the ON signal is more concentrated in the portion where the image density is high.

[Effects of the Invention] As described above, according to the present invention, a pulse width modulation signal is more concentrated on a portion having a high image density, and the outline of a line drawing such as a character is not blurred, and a high-quality image is obtained. Can be reproduced.

[Brief description of the drawings]

FIG. 1 is a schematic block diagram showing the configuration of a laser beam printer in an embodiment, FIG. 2 is a timing chart for explaining an operation of pulse width modulation, FIG. 3 is a timing chart for explaining image modulation in this embodiment, FIG. 4 is a diagram for explaining the formation of a binary image in the recording unit when a laser beam printer is used as an example. In the figure, 101: video data output unit, 102: buffer memory, 103: LUT, 104: select circuit, 105: D / A
Transducer, 106 ... Comparator, 107 ... Gate, 108 ...
... Laser driver, 109 ... Semiconductor laser, 110 ... Beam detector, 111 ... BD generation circuit, 112 ... Timing signal generation circuit, 113 ... Pattern pulse generation circuit, 129 ...
.., An addition operation circuit, and a density gradient detection circuit.

Claims (1)

(57) [Claims] An input unit for inputting digital image data in which the density of each pixel is represented by a plurality of bits; and a process of distributing a density value between a first pixel and a second pixel adjacent to the digital image data. Processing means; generating means for generating a pulse width modulation signal in accordance with the digital image data processed by the processing means; and image forming means for forming an image based on the pulse width modulation signal; Means for, when a sum of the density value of the first pixel and the density value of the second pixel exceeds a predetermined maximum density value, changing the maximum density value between the first and second pixels; A density value of a pixel having a higher density is output, and a difference value between the added value and the maximum density value is output as a density value of the other pixel, and when the added value is equal to or less than the maximum density value, the addition is performed. Value to the first, An image forming apparatus, wherein a density value of a pixel having a higher density among the second pixels is output, and a predetermined minimum density value is output as a density value of the other pixel.