JPH07104927B2 - Image processing device - Google Patents

Description

The present invention relates to an image processing apparatus for processing an input image signal according to a pulse signal having a predetermined pattern.

[Prior Art] Conventionally, for example, a dither method or a density pattern method using a threshold matrix is well known as a method of binarizing an image including a halftone image tone. However, especially when the halftone image is binarized by these methods, there is a drawback that moire fringes are generated due to the beat of the periodic structure of the halftone dot and the threshold matrix, and the image quality is remarkably deteriorated.

[Problems to be Solved by the Invention] The present invention has been made in view of the drawbacks of the above-described conventional example, and an object thereof is to obtain a high-quality reproduced image output from an input image signal partially having an edge portion. Another object of the present invention is to propose an image processing apparatus that faithfully reproduces an image of an original including at least one of a halftone image, a character image, a halftone image and the like.

[Means for Solving the Problems] The configuration of the present invention for achieving the above object is represented by an input means for inputting a digital image signal represented by a plurality of bits for each pixel, and by the digital image signal. Identification means for identifying characteristics of an image, smoothing means for smoothing the digital image signal, a pattern having a predetermined number of pixels of the digital image signal as one cycle, and a pattern having an extreme value inside the one week period A pulse width that has a pattern signal generating means for generating a signal, and generates a pulse width modulation signal processed according to the digital image signal smoothed by the smoothing means according to the output of the identifying means and the pattern signal. A modulation signal generation means, a synchronization signal generation means for generating a line synchronization signal synchronized with image recording for each line according to the pulse width modulation signal, and the synchronization signal A reference clock signal generating means for generating a reference clock signal according to the reference signal, and the pulse width modulation signal generating means according to the digital image signal synchronized with the reference clock signal and the pattern signal synchronized with the reference clock signal. It is characterized by generating a pulse width modulation signal.

[Operation] According to the image processing apparatus having the above-described configuration, the smoothing unit performs the smoothing processing according to the image tone of the image, so that even if images having various image tones are processed and reproduced, A high-quality reproduced image is selected. For example, in the reproduced image, the sharpness is increased and the resolution is improved.

Also, the pattern signal used for pulse width modulation is a signal "having a predetermined number of pixels as one cycle and having an extreme value at the center of that cycle", so the pulse width is not from the beginning of one cycle, Its “extreme” within “one cycle”
Start growing from the position. Therefore, the spatial frequency of the reproduced image becomes constant, and the pulse width modulation can be performed substantially continuously, so that it is possible to prevent the reproduced image for a character / line drawing from being cut off as in the conventional case.

Further, since one cycle of the pattern signal is set as the period of the predetermined number of pixels of the digital image signal, if the period for a plurality of pixels is set, for example, the pulse width modulation signal is generated in a concentrated manner around the extreme value position of the pattern signal. Therefore, it is possible to reproduce an image having excellent gradation.

Further, in the present invention, since the digital image signal synchronized with the reference clock signal according to the synchronizing signal generated for each line of image recording is subjected to the pulse width modulation processing with the pattern signal synchronized with the reference clock signal, the pulse width The generation timing of the modulation signal is the same in all lines, and an excellent reproduced image without image blur can be obtained.

Embodiments Embodiments of the present invention will be described in detail below with reference to the accompanying drawings.

<Principle> As an embodiment of the present invention, for example, an image processing apparatus shown in FIG. 1 has an edge detecting section (buffer memory 11 and buffer memory 11 for detecting whether or not an input digital image signal 1a corresponds to an edge section of an image. Identification circuit 13) and digital image signal 1a
Smoothing circuit 8b for smoothing and outputting a smoothed image signal 50
If the determination circuit 13 determines that the edge portion corresponds to the edge portion, the digital image signal (1a or 8a) is binarized, and if it is determined that the edge portion other than the edge portion corresponds to the smoothed image signal. And a binarization processing unit 51 for binarizing 50.

Under the above configuration, for an image of an edge portion or a halftone image, since the original image signal is binarized as it is, the sharpness thereof is not lost, and the image signal after smoothing is converted into an image of a portion other than the edge portion.
Since the periodicity of halftone dots is removed by digitizing, moire or the like does not occur.

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

<Structure of Embodiment> FIG. 1 is a schematic view of an image processing apparatus according to this embodiment. In the figure, 1 is a video data output unit, which performs A / D conversion of image data from a CCD sensor or a video camera (not shown), and outputs a predetermined bit (6 bits in this example) of digital image data 1a having density information. Output. This digital image data 1a may be temporarily stored in a memory (not shown), or may be input from an external device by communication or the like. The image data 1a from the digital data output unit 1 is 6-bit image data as shown in the figure and is input to the buffer memory 11 in the next stage. The buffer memory 11 has four line memories 11 as shown in FIG.
The input image data 1a is selected by the selector 15a and one line is selected and written to the selected line memory. On the other hand, from the line memory where writing is not performed, the already written image signal is selected.
It is read by 15b and output as image signals 16a, 16b, 16c. In addition, writing to such line memory is 11a, 11b,
11c and 11d are sequentially performed, and finally the output image signals 16a to 16c output image data of continuous three lines. Further, by preparing four line memories, writing and reading can be performed simultaneously.

The outputs (16a, 16b, 16c) from the line memory are input to the smoothing circuit 8b in the subsequent stage, and on the other hand, a part of the original signal (image data 8a of the central pixel) in the line memory is also extracted and the selector 9 Entered in. The other input of the selector 9 is the output of the smoothing circuit 8b, and the selection signal of the selector 9 is the SELECT signal 28 obtained by the identification circuit 13. Identification circuit
The operation of 13 will be described later.

An output signal 10 from the selector 9 is converted into an analog quantity by a digital-analog converter (D / A converter) 2, and each picture element is sequentially connected to one terminal of a comparator circuit (comparator) 4. Entered in. On the other hand, the pattern signal generator 3 outputs the triangular wave pattern signal 12 at a cycle of once for every predetermined number of picture elements (pixels) of the digital data.
Is generated and is input to the other terminal of the comparator 4.

The master clock 52 from the oscillator (master clock generation circuit) 6 is synchronized with the horizontal synchronization signal 55 generated for each line from the horizontal synchronization signal (HSYNC) generation circuit 5 by the timing signal generation circuit 7, for example, 1/4. It is counted down to a cycle and becomes the pixel clock 11, which is used for the transfer clock of the image data and the latch timing of the D / A converter 2. The horizontal synchronizing signal may be generated internally or may be given from the outside. Further, since this embodiment is applied to a laser beam printer, the horizontal synchronizing signal corresponds to a well-known beam detect (BD) signal.

<Binarization Processing by PWM> In the comparator 4, the analog-converted image signal 53 and the level of the triangular wave pattern signal 12 are compared, and the PWM signal output 39 pulse-width modulated is output. Then, the PWM signal output 39 is input to, for example, a modulation circuit (for example, the laser driver 32 in FIG. 7) for modulating the laser beam. Then, the laser beam is turned on / off according to the pulse width, and a halftone image is formed on the recording medium (also the photosensitive drum 38).

FIG. 5 is a diagram for explaining signal waveforms of respective parts of the apparatus of FIG. Referring to FIG. 5, the master clock 52 is the output of the oscillator 6, and the BD signal is the horizontal synchronizing signal described above. Also, the pixel clock 11 is an oscillator 6
The master clock 52 is counted down by the timing signal generating circuit 7. That is, the pixel clock 11 is a signal which is synchronized with the horizontal synchronizing signal by the timing signal generating circuit 7 and counts down the master clock 52 to a quarter cycle. The screen clock 54 is a signal synchronized with the BD signal inside the timing signal generation circuit 7, and the pixel clock 11 obtained by the timing signal generation circuit 7 has a third cycle in the timing signal generation circuit 7. Thus, the three pixel clocks 11 have a cycle of length once. The screen clock 54 is a synchronizing signal for generating the triangular wave pattern signal 12, and is input to the pattern signal generator 3. Further, the digital data 10 is digital image data (code data), and the image signal output from the video data output unit 1 is an original image signal depending on its image tone (characteristic or property of the image). Any one of the smoothed image signals is selected. The analog video signal 53 indicates image data D / A converted by the D / A converter 2, and as can be seen from the figure, each pixel data of analog level is output in synchronization with the pixel clock 11. As shown in the figure, the higher the analog level, the higher the density.

On the other hand, the triangular wave 12 which is the output of the pattern signal generator 3 is generated in synchronization with the screen clock 54 as shown by the solid line of "input to comparator" in FIG. The broken line in the figure is image data analogized by D / A2, and is compared with the triangular wave 12 from the pattern signal generator 3 by the comparator 4, and the image data is pulse width modulated as shown in the PWM output signal 39. It becomes the binarized data.

As described above, in the binarization processing in this embodiment, after the digital image data is once converted into the analog image data, it is possible to perform almost continuous pulse width modulation by comparing the digital image data with the triangular wave pulse 12 having a predetermined period, and thus the high gradation is obtained. The image output of is obtained.

Further, according to the present embodiment, the screen clock synchronized with the horizontal synchronizing signal 55 is generated by using the master clock 52 having a frequency (for example, 12 times the frequency of the triangular wave) higher than the frequency of the synchronizing signal for generating the pattern signal (for example, the triangular wave). Since 54 are formed, the “fluctuation” of the pattern signal (triangular wave) 12 generated from the pattern signal generation circuit 3 (for example, in the first line and the second line).
The deviation of the pattern signal on the line) is 1/12 of the pattern signal period in this embodiment. Therefore, since the grayscale information is pulse-width modulated almost steplessly by using the pattern signal with little fluctuation, a high-quality reproduced image can be obtained.

The pulse-width-modulated video output generated as described above obtains 64-level analog output if the D / A converter 2 has an input of 6 bits, so that 64-level pulse-width modulated output is obtained.

<Image Discrimination> Next, details of the discrimination circuit 13 will be described. Figure 3 (a),
(B) shows a filter used in the discrimination circuit, and a temporary differential filter is used as shown in the figure. As is well known, the temporary differential filter is directional, and in order to detect the first-order differential amount in the two-dimensional direction, it is necessary to combine the two filters shown in FIGS. The physical property of the identification circuit 13 is to detect the edge portion of the image.

FIG. 4 (a) is a partial block diagram for performing such a filter operation by hardware. Such a block is a selector.
Output signals 16a, 16b, 16c from 15b are constructed by taps 17a-17c, 18a-18c, 19a-19c corresponding to each element of a 3 × 3 matrix by using 1-pixel clock delay circuits 20a-20f. Hold. The circuits shown in FIGS. 4 (b) to 4 (d) show a configuration for performing an operation from the output from such a tap and obtaining the output of the primary differential filter as the SELECT signal 28. The circuit of FIG. 4 (b) executes the matrix operation corresponding to FIG. 3 (a), and the circuit of FIG. 4 (c) executes the matrix operation corresponding to FIG. 3 (b). In the figure, 21a to 21d, 22a and 22b are adders, which are added in consideration of the sign. Reference numerals 23a and 23b denote outputs of respective matrix operations, and absolute value circuit (ABS) 2 shown in FIG. 4 (d).
Input to 4a and 24b, and added by adder 25. The resulting data is compared with the reference data 27 as a digital signal in the comparator 26 to obtain an output of 1 when X> D and 0 when X ≧ D. However, X is the data of the calculation result, and D is the reference data. When the select signal 28 is "1", the sum of the differential values is large, so it is considered that the image is an edge portion image.

<Smoothing Process> Next, the process of the smoothing circuit 8b will be described. Smoothing circuit
In 8b, the outputs of all the taps shown in FIG. 4 (a) are added and averaged. That is, the average output x that x = (1/9) Σxi is obtained. Here, xi is nine tap outputs. The circuit diagram is shown in FIG. Tap with adder 40
18a to 18c are added, and the output of adder 40 and tap are added by adder 41.
29a (output of adder 21a in FIG. 4 (b)) and tap 29b
(The output of the adder 21b in FIG. 4 (b))
The division ROM 42 obtains the average value output 50.

<Characteristics of Spatial Filter> The frequency characteristics of the above two spatial filters (first-order differential filter and smoothing filter) are as follows.

The first-order differential filter is configured so that the frequency at its peak position is lower than the spatial frequency of the halftone dots, is set so as not to spread the halftone dots, and detects the edge portion such as characters.
This filter is used for image tone recognition.

: The smoothing filter passes low frequencies (low-pass filter). Used for averaging image signals.

The characteristics of the above two filters are shown in FIG. As can be seen from the frequency characteristics in FIG. 8, it is considered that the magnitude relationship between the total number of signals that have passed through the bandpass filter (first derivative filter) and a predetermined number represents the image tone.

FIG. 9 shows the spatial frequency spectra of various originals. In general, halftone dot originals periodically have sharp peaks for halftone dot pitches.

<Image processing according to image tone recognition> To summarize the above, the image processing in this embodiment is: The peak frequency of the first-order differential filter (for image tone recognition) is set so as not to spread the halftone dots. The output of the filter determines the image tone of the image, in other words, the edge portion such as a character is determined.

A region where the output of the first derivative filter is small (the select signal 28 is "0", that is, a halftone or halftone dot region) is passed through the smoothing filter.

The original signal 8a is output to the area where the output of the primary fine powder filter is small (the select signal 28 is "1", that is, the character image area).

It goes without saying that the image processing method described above is compatible with the image processing apparatus shown in FIG. The following results are obtained by the above processing.

The halftone image is smoothed. That is, the dot periodicity is removed.

For character line drawing, without smoothing the edge part,
In other words, it is reproduced without losing the sharpness.

Smoothing and emphasis are applied to photographic images depending on the presence or absence of edges. Therefore, the halftone part of the photograph does not lose its naturalness.

The result of the above-described image tone determination and filter processing is the output 10 from the selector 9 in FIG. When the smoothed signal is pulse width modulated as described above, moire fringes no longer occur. The reason is that the image signal of the original is not halftone dots due to smoothing,
This is because the beat with the cycle of the screen pulse 54 does not occur even when the line screen output is performed.

FIG. 7 shows an example in which the signal processing result of the present invention is applied to a laser beam printer. The PWM output signal 39 described above modulates the laser driver 32, and causes the semiconductor laser 33 to perform pulse width modulation and emit light. The light beam emitted from the semiconductor laser 33 is collimated by the collimator lens 34, is deflected by the rotating polygon mirror 36, and is scanned on the photosensitive drum 38 by the fθ lens 37. The image of light formed on the photosensitive drum forms an electrostatic latent image, and the image is output by a normal copying machine process.

<Effects of Embodiment> As described above, in the present embodiment, high-quality output can be achieved regardless of the quality of the original.

Further, the operation of the discrimination and smoothing filter is sufficiently reflected because the pulse width modulation output of 64 levels is performed. Therefore, the hardware can be realized with a relatively small matrix size.

Although the above embodiment has been described by taking the image processing apparatus having the binarization processing by PWM modulation as an example, it goes without saying that the embodiment can be applied to the binarization processing by the conventional dither method or the like.

Furthermore, although the embodiment is shown with a matrix of 3 × 3, this is a matrix size larger than that, for example, 5 × 5,7 ×.
7 can be sufficiently implemented, and the output pattern from the pattern signal generating circuit 3 is not limited to the triangular wave, and the same applies to a sine wave, a sawtooth wave, or the like.

[Effects of the Invention] As described above, according to the present invention, high-quality reproduced images can be obtained even when images with various image tones are reproduced, and for example, halftone images, character images, halftone images, etc. It is possible to faithfully reproduce an original including one or more of the above.

Specifically, since the pattern signal used for pulse width modulation is a signal "having a predetermined number of pixels as one cycle and having an extreme value at the center of that cycle", its pulse width is from the beginning of one cycle. Instead of that "extreme" that is "inside one cycle"
Start growing from the position. Therefore, the spatial frequency of the reproduced image becomes constant, and the pulse width modulation can be performed substantially continuously, so that it is possible to prevent the reproduced image for a character / line drawing from being cut off as in the conventional case.

Further, since one cycle of the pattern signal is set as the period of the predetermined number of pixels of the digital image signal, if the period for a plurality of pixels is set, for example, the pulse width modulation signal is generated in a concentrated manner around the extreme value position of the pattern signal. Therefore, it is possible to reproduce an image having excellent gradation.

Further, in the present invention, since the digital image signal synchronized with the reference clock signal according to the synchronizing signal generated for each line of image recording is subjected to the pulse width modulation processing with the pattern signal synchronized with the reference clock signal, the pulse width The generation timing of the modulation signal is the same in all lines, and an excellent reproduced image without image blur can be obtained.

[Brief description of drawings]

FIG. 1 is a diagram showing the basic configuration of the embodiment, FIG. 2 is a diagram showing the configuration of a buffer memory, FIGS. 3 (a) and 3 (b) are matrix configuration diagrams of a one-dimensional differential filter, and FIG. 4 (a). ) To (d) are block circuit diagrams constituting an identification circuit, FIG. 5 is a timing chart explaining the operation of pulse width modulation, FIG. 6 is a block circuit diagram of a smoothing circuit, and FIG. 7 is a laser beam printer. FIG. 8 is a diagram illustrating output of a binary image in the case of an example. FIG. 8 is a frequency characteristic diagram of each spatial filter, and FIG. 9 is a section frequency characteristic diagram of various images. In the figure, 1 ... Video data output section, 2 ... D / A converter, 3 ...
Pattern signal generator, 4, 26 ... comparator, 5 ... horizontal sync signal generation circuit, 6 ... oscillator, 7 ... timing signal generation circuit, 8b ... smoothing circuit, 9, 15a, 15b ...
Selector, 11 ... Buffer memory, 11a-11d ... Line memory, 13 ... Identification circuit, 20a-20f ... Single pixel delay circuit, 21a-21d, 22a, 22b, 25, 40, 41 ... Adder , 24a, 24b
...... Absolute value circuit, 42 ...... Division ROM, 32 ...... Laser driver, 33 …… Semiconductor laser, 34 …… Collimator lens, 36
...... Rotating polygon mirror, 37 …… fθ lens, 38 …… Sensitized drum, 50 …… Smoothed image data, 51 …… Binarization processing unit, 52
...... Master clock, 53 ...... Analog image data, 54 ...
… Screen clock, 55 …… BD signal.

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Claims (1)

[Claims] 1. Input means for inputting a digital image signal represented by a plurality of bits for each pixel, identification means for identifying characteristics of an image represented by the digital image signal, and smoothing for the digital image signal. And a pattern signal generating means for generating a pattern signal having an extreme value inside the one cycle, with a predetermined number of pixels of the digital image signal as one cycle, and an output of the identifying means. Pulse width modulation signal generating means for generating a pulse width modulation signal processed according to the digital image signal smoothed by the smoothing means and the pattern signal, and an image for each line according to the pulse width modulation signal Sync signal generating means for generating a line sync signal in synchronization with recording; and reference clock generating means for generating a reference clock signal according to the sync signal. The image processing, wherein the pulse width modulation signal generation means generates a pulse width modulation signal according to the digital image signal synchronized with the reference clock signal and the pattern signal synchronized with the reference clock signal. apparatus.