JPH04345268A - Device for forming picture - Google Patents

Abstract

PURPOSE:To make tone reproduction compatible with the high resolution of an edge part by making a screen unconspicuous in a pulse width modulating means. CONSTITUTION:An edge detecting circuit 9 outputs an edge detecting signal 10 in picture scanning and a delay circuit 11 delays for the portion of a processing clock in the circuit 9. A gamma converting circuit 12 switches conversion characteristic for the edge part and for another by the edge detecting signal. A timing signal generating circuit 15 generates VCLK 16 and SCLK 17 whose cycle is twice as many as that of VCLK 16. A pattern generating circuit (A) 18 generates the triangle wave pattern signal A 20 of the same cycle as VCLK 16 and the circuit (B) 19 generates the triangle wave pattern signal B 21 which has the same cycle as SCLK 17 and inverts a phase at every line. Comparators 23 and 24 respectively compares the analog picture signal 22 and the signals A 20 and B 21 and a selector 27 selects the PWM signal A 25 when the edge detecting signal is HIGH and selects the PWM signal B 26 when LOW so as to output it.

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 that binarizes a multivalued image by pulse width modulation to form an image.

0002

2. Description of the Related Art In recent years, image forming apparatuses such as laser beam printers are required to have both high resolution and excellent gradation.

An example of the above-mentioned conventional image forming apparatus will be described below with reference to the drawings.

FIG. 10 is a block diagram of a conventional pulse width modulation circuit. In FIG. 10, 102 is a D/A converter, which converts a raster scan digital image signal 101 into an analog image signal 103. A 1/2 frequency divider 104 divides the pixel clock 119 of the digital image signal 101 into 1/2 and outputs a screen clock 120. 105, 106, and 107 are pattern signal generation circuits A, B, and C, respectively, and the screen clock 12
0, pattern signals A108 and B109, respectively.
, C110. Pattern signal A108, B10
9. The period of C110 is twice that of the pixel clock, and the respective waveforms are different. Comparators 111, 112, and 113 compare the analog image signal 103 and pattern signals A108, B109, and C110, respectively.
Pulse width modulated PWM signals A121, B122, C
123 respectively. A density gradient detection circuit 116 detects the density gradient of the image signal 101 in the main scanning direction and outputs a density gradient detection signal 117. 114 is a selector, based on the concentration gradient detection signal 117, PWM
One of the signals A121, B122, and C123 is selected and the PWM signal 115 is output.

FIG. 11 is a timing diagram of the conventional pulse width modulation circuit shown in FIG. The pattern signal A108 is a ramp wave rising to the right. The pattern signal B109 is a triangular wave. The pattern signal C110 is a ramp waveform that slopes downward to the right. The density gradient detection circuit 116 2 in FIG. 10 detects the direction and magnitude of the density gradient of the image signal, and determines which PWM signal is selected by the selector 114. The bottom row of FIG. 11 shows the PWM selected by the selector 114.
Showing a signal.

The conventional pulse width modulation circuit as described above is
Even though the period of the pattern signal is twice that of the pixel clock, even when pulse width modulating an image signal that includes line drawings such as characters, the edges do not become jagged, and the resolution of the character part is prevented from decreasing. is possible. (For example, JP-A-2-47973).

[0007]

However, in the above configuration, the screen pattern of the formed image becomes a parallel screen of vertical lines, and the screen becomes visually obtrusive.

Furthermore, when there are diagonal lines or halftone dots in the image signal, although the density gradient is large in two dimensions, the density gradient in the main scanning direction is smaller than that in the case of vertical lines, so it is not optimal. It becomes impossible to select pattern signals.

Furthermore, if there is a horizontal line in the image signal, the horizontal line will be cut off by the period of the pattern signal.

Furthermore, since three types of pattern signals are used, three pattern signal generating means and three comparators are required.

In addition, when the frequency of the pixel clock increases, it becomes difficult to generate a short-period ramp wave because the pattern signal uses a ramp wave containing many high frequency components.

In view of the above problems, the present invention makes the screen visually inconspicuous, and even when line drawings such as diagonal lines, halftone dots, horizontal lines, characters, etc. are included in the image signal, the line drawing portions have high resolution. The present invention provides an image forming apparatus that can form an image and requires only two pattern signal generating means for generating a triangular wave.

[0013]

Means for Solving the Problems In order to solve the above problems, the image forming apparatus of the present invention includes an edge detection means for detecting an edge of an image signal, and a first clock having the same period as the pixel clock of the image signal. a first pattern generating means for generating a pattern signal; a second pattern generating means for generating a second pattern signal having a period twice as long as the pixel clock of the image signal and having a phase inverted for each line; , first pulse width modulation means for pulse width modulating the image signal based on the pattern signal from the first pattern generation means;
a second pulse width modulation means for pulse width modulating the image signal based on a pattern signal from a second pattern generation means; Alternatively, it is characterized by comprising a selection means for selecting one of the modulation signals output from the second pulse width modulation means.

[0014] Furthermore, the first and second pattern signals are triangular waves. Furthermore, the edge detection means scans the image signal in a window of M×N pixels and calculates a two-dimensional second-order differential component, or calculates the difference between the maximum value and the minimum value of the image signal within the window. It is characterized by comprising a means to do so.

[0015]

According to the present invention, the screen angle can be set to 45 degrees with the above-described configuration, so that the screen can be visually inconspicuous in the formed image.

Furthermore, the edge portion of the image signal forms an image with the same high resolution as the resolution of the image signal, and the halftone portion forms an image with the same high resolution as the resolution of the image signal.
By lowering the resolution after pulse width modulation than the resolution of the image signal and improving the gradation properties, it is possible to prevent the edge portion from becoming jagged and to prevent a decrease in the resolution of the character portion.

Furthermore, by using a triangular wave for the pattern signal, the pattern signal can be easily generated even if the pixel clock frequency becomes high.

Furthermore, since edges in the image signal are detected two-dimensionally, edges with edges can be reliably detected.

[0019]

DESCRIPTION OF THE PREFERRED EMBODIMENTS An image forming apparatus according to an embodiment of the present invention will be described below with reference to the drawings.

FIG. 1 is a schematic block diagram of an image signal processing section of a laser beam printer, which is an image forming apparatus according to an embodiment of the present invention.

In FIG. 1, a line buffer memory 1 delays the input image signal 4 by one line and changes the clock rate and timing of writing and reading. The write clock of the line buffer 1 is RCLK5, which is the synchronization clock of the input image signal 4. The read clock of line buffer 1 is VCLK16. Line buffer memories 2 and 3 delay input signals 7 and 8 by one line. line buffer 2
and 3 write and read clocks are both V
It is CLK16. Reference numeral 9 denotes an edge detection circuit, which receives the outputs 6, 7, and 8 of the line buffer memory as input, scans the image signal in a 3×3 pixel window, and generates an edge detection signal 10 that becomes HIGH when the edge of the image signal is large. Output. Reference numeral 11 denotes a delay circuit, which transfers the image signal 7 read out from the line buffer memory 2 to an edge detection circuit 9.
There is a delay of the clock (VCLK) required for processing. 1
A gamma conversion circuit 2 converts the density characteristics of the input image signal and outputs the converted image signal 33. The gamma conversion circuit 12 has two conversion characteristics: an edge portion conversion characteristic and other conversion characteristics, and uses the character portion conversion characteristic when the edge detection signal is HIGH. 13 is a D/A converter;
The converted image signal 33 is converted into an analog image signal 22. 14 is a beam detection signal generation circuit, which generates a beam detection signal 32 when a laser beam is incident on the pin photodiode 30.

Reference numeral 15 denotes a timing signal generation circuit, which synchronizes the line with the beam detect signal 32 and outputs the VC signal.
Generate SCLK17 with twice the cycle of LK16 and VCLK16. 18 is a pattern generation circuit A, and VCLK
A triangular wave pattern signal A20 having the same period as 16 is generated. Reference numeral 19 denotes a pattern generation circuit B, which generates a pattern signal B21 which is a triangular wave having the same period as SCLK17 and whose phase is inverted for each line. Further, the pattern generation circuit B19 outputs a clock signal CSCLK34 corresponding to the phase of the pattern signal having the same period as SCLK. 23 is a comparator, and the analog image signal 22
The pattern signal A20 is compared with the analog image signal 22.
P becomes HIGH when is larger than pattern signal A20.
Outputs WM signal A25. A comparator 24 compares the analog image signal 22 and the pattern signal B21, and outputs a PWM signal B26 which becomes HIGH when the analog image signal 22 is larger than the pattern signal B21. 27 is a selector, and the edge detection signal 27 is HIGH.
When PWM signal A25 and edge detection signal 27 are LO
When the signal is W, the PWM signal B26 is selected and output as the PWM signal 28. A laser driver 29 drives the semiconductor laser 31 when the PWM signal 28 is HIGH.

FIG. 2 is a schematic diagram of the recording section of the laser beam printer. In FIG. 2, 31 is a semiconductor laser, 115 is a collimator lens, 116 is a polygon mirror, 117 is an fθ lens, 118 is a photosensitive drum, and 30 is a pin photodiode. Laser light from the pulse width modulated semiconductor laser 31 passes through the collimator lens 115.
collimated by. Next, the laser beam is reflected by a rotating polygon mirror 116, corrected by fθ by an fθ lens 117, and scanned on a photoreceptor drum 118. The photosensitive drum 118 rotates in the direction of the arrow in the figure. An electrostatic latent image is formed on the photoreceptor drum 118. Based on the electrostatic latent image on the photoreceptor drum 118, an image is formed on recording paper by a well-known electrophotographic method. The pin photodiode 30 is provided near the scanning start position of one line of the laser beam, and detects line scanning of the laser beam.

FIG. 3 is a timing diagram of the image signal processing section shown in FIG. 1. VCLK16 and SCLK17 are signals synchronized with the beam detect signal 32. The analog signal 22 has a black level at a high potential. The pattern signal A20 is a triangular wave synchronized with VCLK16. The pattern signal B21 is a triangular wave synchronized with SCLK17, and the phase is actually inverted for each line. The edge detection signal 28 becomes HIGH when an edge is detected in the image signal.

As described above, the edge portion of the image signal can form an image with the same resolution as the image signal, and the halftone region other than the edge portion can be reproduced with half the resolution of the image signal. You can improve your sexuality. Although the edge portion has high resolution, gradation reproducibility is poor. However, in terms of human visual characteristics, this does not pose a problem because the gradation recognition characteristics of areas with high spatial frequencies, such as edge areas of an image, deteriorate.

FIG. 4 is a block diagram of the edge detection circuit 9 of FIG. 1. Reference numeral 40 denotes an edge component extraction circuit which inputs image signals 6, 7, and 8 of three adjacent lines and outputs an edge component signal 41 indicating the size of an edge in the image signal. A comparator 42 compares the edge component signal 41 with a predetermined threshold 48 and outputs a signal 43 that becomes HIGH when the edge component signal 41 is larger. 44 is D
It is a flip-flop, uses VCLK16 as a clock input, and outputs a signal 49 obtained by delaying the signal 43 by one pixel. 45 is an OR gate, which outputs a signal 50 which is the logical sum of the signal 43 and the signal 49. 47 is a D flip-flop which uses SCLK17 as a clock input, latches the signal 50 at the rising edge of SCLK, and outputs the edge detection signal 10.
Output.

As described above, the edge detection circuit outputs the edge detection signal 10 which becomes HIGH when an edge is included within one cycle of the pattern signal B21, every cycle of the pattern signal B21. Therefore, in the middle of one period of the pattern signal B, the PWM signals A25 and B by the selector 27
26 switching does not occur. This eliminates disturbances in the PWM signal 28 due to pattern signal switching, and improves the quality of the formed image.

FIG. 5 is an explanatory diagram of the operation of the first embodiment of the edge component extraction circuit 40 of FIG. 4. 6, 7, and 8 are image signals of adjacent lines. 60 converts the image signal into 3×3
Scan with a window of pixels, (value of image signal at the center of the window) - (sum of image signals at the four corners of the window/4)
This is a second-order differential circuit that performs the following calculation. Here, the pixel at the center of the window is the pixel of interest. Reference numeral 62 denotes an absolute value circuit that calculates the absolute value of the second-order differential signal 61 output from the second-order differential circuit 60. The absolute value circuit 62 outputs the edge component signal 41 which is the absolute value of the second-order differential signal 61.

FIG. 6 is an explanatory diagram of the operation of the second embodiment of the edge component extraction circuit 40 of FIG. 4. 6, 7, and 8 are image signals of adjacent lines. 70 converts the image signal into 3×3
It scans a pixel window, performs the calculation (maximum value of marked pixels within the window) - (minimum value of marked pixels within the window), and outputs an edge component signal 41. This is a concentration difference calculation circuit. Here, the pixel at the center of the window is the pixel of interest.

As described above, edges are detected by scanning the image signal in a two-dimensional window, so even if the image signal contains line drawings such as diagonal lines, halftone dots, horizontal lines, and characters, edges are reliably detected. can be detected.

FIG. 7 is a block diagram of the gamma conversion circuit 12 of FIG. 1. 80 is a memory used as a lookup table. 81 is an input image signal to the gamma conversion circuit, address input of the memory 80, (ADR0 to A
DR7). Edge detection signal 10 is stored in memory 8
It is input to ADR8 of 0. Data output of memory 80 (
Image signal 33 converted from DATA0 to DATA7)
is output. Conversion characteristic data is stored in the memory 80 in advance.

By configuring the gamma conversion circuit as described above, the gamma characteristics at the time of image formation when the pulse width is modulated based on the pattern signal A20 and when the pulse width is modulated based on the pattern signal A20 can be changed. The difference between
This can be eliminated by switching between two types of gamma conversion characteristics. Alternatively, it is possible to arbitrarily change the gamma characteristics at the edge portion and other areas during image formation.

FIG. 8 is a block diagram of the pattern signal generation circuit B19 in FIG. 1. In FIG. 8, 90 is a D flip-flop, and since the beam detect signal 32 for each line is input as a clock, the signal 91 toggles for each line. 92 is an exclusive OR gate, which receives SCLK17 and signal 91 as inputs. The output signal CSCLK34 of the exclusive OR gate 92 is S
This is a signal obtained by inverting the phase of CLK17 for each line. 9
4 is a triangular wave generation circuit, which has the same phase as CSCLK32,
A triangular wave having the same period is generated and output as a pattern signal B21.

FIG. 9 shows the pattern generation circuit B shown in FIG.
19 is a diagram of an image formed when a halftone image of 50% of the maximum density is printed when pulse width modulation is performed using a triangular wave from No. 19. FIG. As shown in Figure 9, the screen angle is set to 4
Since the angle can be set at 5 degrees, the screen can be visually less noticeable in the formed image.

Note that in the embodiment of the edge component extraction circuit, 3
Although a x3 pixel window is used, a window of any size may be used.

Furthermore, in the second embodiment of the edge component extraction circuit, five pixels in a 3×3 pixel window were used;
All pixels may be used.

[0037]

As described above, according to the present invention, since the screen angle can be set to 45 degrees, the screen can be made less visually noticeable in the formed halftone image.

Furthermore, an image can be formed at the edge portion of the image signal with the same resolution as the image signal, and the halftone region other than the edge portion is made half the resolution of the image signal, improving the gradation reproducibility of the halftone. can.

Furthermore, since the edge detection signal does not change during one period of the pattern signal with the longer period, there is no disturbance in the PWM signal due to switching of the pattern signal, and the quality of the formed image is improved.

In addition, since edges are detected by scanning the image signal in a two-dimensional window, edges can be reliably detected even if the image signal contains line drawings such as diagonal lines, halftone dots, horizontal lines, and characters. can.

In addition, the difference in gamma characteristics during image formation when pulse width modulated based on pattern signal A20 and when pulse width modulated based on pattern signal A20 is explained by two types of gamma conversion characteristics. It can be eliminated by switching. Alternatively, it is possible to arbitrarily change the gamma characteristics at the edge portion and other areas during image formation.

[Brief explanation of drawings]

FIG. 1 is a schematic block diagram of an image signal processing section of a laser beam printer, which is one of the image forming apparatuses in an embodiment of the present invention.

FIG. 2 is a schematic configuration diagram of a recording section of a laser beam printer.

FIG. 3 is a timing diagram of the image signal processing section shown in FIG. 1;

FIG. 4 is a block diagram of the edge detection circuit 9 of FIG. 1.

5 is an explanatory diagram of the operation of the first embodiment of the edge component extraction circuit 40 of FIG. 4; FIG.

6 is an explanatory diagram of the operation of a second embodiment of the edge component extraction circuit 40 of FIG. 4. FIG.

7 is a block diagram of the gamma conversion circuit 12 of FIG. 1. FIG.

[Fig. 8] Fig. 8 shows the pattern signal generation circuit B1 in Fig. 1.
9 is a block diagram of FIG.

FIG. 9 shows the pattern generation circuit B19 shown in FIG.
FIG. 4 is a diagram of an image formed when a halftone image of 50% of the maximum density is printed when pulse width modulation is performed using a triangular wave from .

FIG. 10 is a block diagram of a conventional pulse width modulation circuit.

FIG. 11 is a timing diagram of the conventional pulse width modulation circuit of FIG. 10;

[Explanation of symbols]

1 Line buffer memory 9 Edge detection circuit 12 Gamma conversion circuit 18 Pattern generation circuit A 19 Pattern generation circuit B 22 Analog image signal 25 PWM signal A 26 PWM signal B 27 Selector 29 PWM signal 32 Beam detect signal 34 CSCLK 40 Edge component extraction circuit 42 Comparator 44 D flip-flop 45 Or Gate 60 Second order differential circuit 62 Absolute value circuit 70 Concentration difference calculation circuit 80 memory 90 D flip-flop 92 Exclusive or Gate 94 Triangular wave generation circuit

Claims (8)

[Claims] 1. An image forming apparatus that forms an image by pulse width modulating a multivalued image signal, which generates a pattern signal having a cycle twice as long as a pixel clock of the image signal and inverting the phase for each line. An image forming apparatus comprising: a pattern generating means; and a pulse width modulating means for pulse width modulating the image signal based on a pattern signal from the pattern generating means. 2. The image forming apparatus according to claim 1, wherein the pattern signal is a triangular wave. 3. An image forming apparatus that forms an image by pulse width modulating a multivalued image signal, comprising: edge detection means for detecting an edge of the image signal; and a first pattern having the same period as a pixel clock of the image signal. a first pattern generating means for generating a signal; a second pattern generating means for generating a second pattern signal having a period twice as long as the pixel clock of the image signal and inverting the phase for each line; 1st
a first pulse width modulation means for pulse width modulating the image signal based on a pattern signal from a pattern generation means; and a first pulse width modulation means for pulse width modulating the image signal based on a pattern signal from a second pattern generation means. a selection means for selecting one of the modulation signals output from the first or second pulse width modulation means based on the magnitude of the edge component signal output from the edge detection means; An image forming apparatus comprising: 4. An image forming apparatus that forms an image by pulse width modulating a multivalued image signal, and an edge detection means for detecting an edge of the image signal, and a different edge detection signal from the edge detection means. density conversion means for performing density conversion of characteristics; first pattern generation means for generating a first pattern signal having the same period as the pixel clock of the image signal; and first pattern generation means having a period twice the pixel clock of the image signal. , a second pattern generating means for generating a second pattern signal whose phase is inverted for each line, and a pattern signal from the first pattern generating means,
A first pulse width modulator for pulse width modulating the image signal from the density converting means.
a second pulse width modulation means for pulse width modulating the image signal from the second density conversion means based on a pattern signal from the second pattern generation means; and a second pulse width modulation means for pulse width modulating the image signal from the second density conversion means; An image forming apparatus comprising: selection means for selecting one of the modulation signals outputted from the first and second pulse width modulation means based on the magnitude of the edge component signal outputted by the means. 5. The image forming apparatus according to claim 3, wherein the first and second pattern signals are triangular waves. 6. The image forming apparatus according to claim 3, wherein the edge detection means outputs an edge detection signal every cycle of the second pattern signal. 7. The edge detection means converts the image signal into M×N
Claim 3 or 4, further comprising means for scanning a pixel window and calculating a two-dimensional second-order differential component.
The image forming apparatus described above. 8. The edge detection means converts the image signal into M×N
5. The image forming apparatus according to claim 3, further comprising means for scanning a pixel window and calculating a difference between a maximum value and a minimum value of a plurality of predetermined pixels within the window.