JP2555315B2 - Image processing device - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to an image processing apparatus, and more particularly, it performs gradation conversion of input image data with higher precision, and performs pulse width modulation processing on the converted image data. The present invention relates to an image processing device that does.

[Prior Art] Conventionally, various methods have been proposed for outputting an image including a halftone in a digital printer or the like. Examples of such a method include a dither method and a density pattern method. These methods can: Display images with halftones using a binary display.

: The hardware configuration of the device is easy.

: It is possible to obtain a certain level of image quality.

Therefore, it is widely used in many fields.
Specifically, as shown in FIGS. 2 (A) and 2 (B), the pixel (input pixel information) 8 of the input image and each component of the threshold matrix 5 are associated with each other to determine whether they are larger or smaller than the threshold. To determine whether it is white or black and output it to the display screen 6.

FIG. 2A illustrates the dither method, in which one pixel 8 of the input is made to correspond to one component of the threshold matrix 5. Further, FIG. 2B illustrates the density pattern method, in which one pixel 8 of the input is made to correspond to all the components of the threshold matrix 5. That is, in the density pattern method, a plurality of cells on the display screen 6 indicate one pixel of the input image.

The difference between the dither method and the density pattern method is that only one pixel of the input is made to correspond to one component of the threshold matrix or all the components, and there is no essential difference. Naturally, there is also an intermediate method, for example, one pixel of the input is used as a plurality of components (second component) of all components of the threshold matrix.
A method of corresponding to, for example, 2 × 2 4 components in FIG. Therefore, there is essentially no difference between the two, and henceforth, the dither method, the density pattern method, and the intermediate method will be referred to as the dither method.

[Problems to be Solved by the Invention] By the way, in such a dither method, since image output is output as a binary value, there are only two states of white or black, that is, "0" or "1", and of course, However, it is impossible to control the density of the image. However, there are various image output devices, for example, laser beam printers, ink jet printers, CRT devices, and the like. The problem is that in such a wide variety of output devices, there is an output characteristic peculiar to the device, and there is a difference in output density even with the same binary signal.

For example, a laser beam printer normally forms an electrostatic latent image by controlling irradiation or non-irradiation of laser light on a photosensitive drum, and develops the latent image to obtain a visible image. The latent image potential changes according to the on-time of the laser beam, and the relationship between the on-time of the laser beam and the potential is non-linear, and there is also non-linearity between the potential and the developing density. That is, since there are many variation factors even with one laser beam printer, there is a lot of variation between different types of output devices, and even if the same binary signal value is output, the reproduced reproduction Images may differ significantly from what you would expect. In particular, the image data binarizing means is only on the side of the control device (such as a host computer) outside the output device, and a plurality of output devices receive the same binary signal from the control device to reproduce the image. The time is even more so. The situation is the same whether the image signal received from the control device is not only a binary signal but also a multilevel signal such as a quaternary signal.

The present invention has been made in view of the above-mentioned difficulties of the conventional art, and an object of the present invention is to provide an apparatus that enables high-definition gradation processing for an image that has been halftone processed.

[Means for Solving the Problems] An image processing apparatus according to the present invention for achieving the above object is an image processing apparatus for supplying a pulse width modulation signal to an image forming means, and has an intermediate number of m bits. Image data generating means for generating tone-processed image data; converting means for converting the m-bit image data generated by the image data generating means into n-bit (n> m) image data; Adjusting means for adjusting the density of the n-bit image data output from the image forming means according to the output density characteristics of the image forming means; and converting the n-bit image data output from the adjusting means into analog image data. A pulse width modulation signal generating means for generating a pulse width modulation signal according to the converted analog image data, and a pulse width modulation signal generating means for outputting the pulse width modulation signal. It is characterized by having a supply means for supplying a width modulation signal to the image forming means.

[Operation] In the image processing apparatus of the present invention having the above-described configuration, the conversion means performs conversion for increasing the number of bits of the halftone-processed digital image data, and the adjustment means converts the converted digital image data into the image forming means. The density adjustment is performed according to the output density characteristics of, and the pulse width modulation signal generation means converts the adjusted digital image data into analog image data to perform pulse width modulation. On the other hand, it is possible to newly perform detailed gradation processing according to the output density characteristic of the image forming unit.

Further, in the pulse width modulation signal generating means of the present invention, since the digital image data is returned to the analog image data to perform the pulse width modulation, for example, even if the bit number of the digital image data is small, the pulse by the analog processing is processed. It is possible to achieve detailed gradation processing by width modulation. Further, since it is analog processing, a high-speed clock used for digital processing is not required, so that a high-definition image can be obtained at high speed.

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

<Outline of Embodiment> The image output device 200 of FIG. 1 shows a configuration in which the present invention is applied to a laser beam printer. An image output system including the image output device of the embodiment shown in the figure, an image data sending device 100 for sending digital image data 101,
A plurality of image output devices 200, ..., Which are connected to the image data transmission device 100 and output digital image data 100 as an image
It consists of 201 and. The digital image data 101 is, for example, a binary signal. Each image output device has digital image data 10
Multi-value conversion unit 202 for converting 1 into multi-value density data 202a, density conversion unit 203 for density conversion of the multi-value density data 202a, and binarization for binarization of the multi-value density data 203a after density conversion Part 20
It has 4 mag. The image output device 200 and the like may be different output devices such as a laser beam printer and an ink jet printer.

According to the above configuration, even if the digital image data 101 is a binary signal in which the density cannot be expressed, once the multi-value density data
After conversion to 203a, the binarization unit 204 binarizes. Therefore, for example, even when the output device 200 or the like is a different output medium, it is possible to adjust the density inside the output device so that the final output density becomes a desired density.

As described above, the input digital image data 101
Is subjected to conversion processing such as multi-value conversion, density conversion, and binarization, and the binary signal is input to the laser driver 40, and laser light is imaged on the photosensitive drum through the imaging optical system. .
For processing such as multi-value conversion and density conversion, refer to FIG. 3 and FIG.
This will be described with reference to FIGS. 7, 7 and 9 and the processing such as binarization will be described with reference to FIGS. 3 and 4. or,
The optical system will be described with reference to FIG.

<Outline of Embodiment> FIG. 3 shows a block diagram of a process for converting dither data into a PWM output signal 4a. The multivalued processing of the input digital image data (dither data 10) is performed by the multivalued circuit 9, and the density conversion is performed by the LUT 13. Here, the dither data is an image signal that has been subjected to dither processing (halftone processing) in the broad sense described above, and is usually a binary signal.
Further, the binarization processing of the present embodiment uses the binarization by the PWM (pulse width modulation) method proposed by the present applicant in view of the simplicity of the circuit and the high definition of the output image.

<Multi-value conversion and density conversion> Explaining with reference to FIG. 3, dither data (binary data) 10 having a value of “0” or “1” is converted by the multi-value conversion circuit 9.
It is converted into 8-bit BCD display multilevel data 9a as follows.

Data “0” → Data “0” Data “1” → Data “FF” Such conversion is performed in ROM (Read Only Memory)
It can be easily realized by Thereafter, for density conversion, the 8-bit multi-valued data 9a is input to a look-up table (LUT) 13 and converted into 8-bit density correction data 13a as described below. In this case, the data is "0
Since there are only two states of 0 "" FF ", any two 8-bit data" A "," B ", that is, data" 00 "→ data" A "data" FF "→ data" B "are converted. In this way, since the binary dither data 10 is converted into multi-valued data, density conversion, that is, density adjustment becomes possible, and density conversion is performed by the LUT 13 according to desired density conversion characteristics. 6th as described later
Although it has the characteristics as shown in FIG. 7 or FIG. 7, which characteristic is selected depends on the select signal 15a from the select signal generating circuit 15.
Done by. Details of this density conversion will be described later.

<Binarization> The output signal 13a from the LUT 13 is converted into an analog amount by a digital-analog converter (D / A converter) 2, and each picture element is sequentially compared (comparator). 4 is input to one terminal. On the other hand, the pattern signal generation circuit 3 generates a pattern signal 3a having a triangular wave shape at a cycle of once for every predetermined number of picture elements (pixels) of the dither data 10, and inputs it to the other terminal of the comparator 4. To do. The shape of the pattern signal may be a triangular wave, a sine wave, a sawtooth wave, or the like. Here, in the case of the laser beam printer of the present embodiment, it is desirable that the pattern signal 3a has the same period as the pixel width, but for convenience of explanation, the period of the pattern signal 3a is as shown in the timing chart of FIG. In addition, it is performed as if it has a period three times as wide as one pixel width.

The pattern signal 3a input to the comparator 4 will be described. The oscillator (master clock generation circuit) 1 generates a master clock 1a in synchronization with the generation of the horizontal synchronization signal 14a for each line from the horizontal synchronization signal (HSYNC) generation circuit 14. The master clock 1a is counted down by the timing signal generation circuit 7 to, for example, a quarter cycle and becomes the pixel clock 7a, 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.
On the other hand, the timing signal generating circuit 7 further generates the screen clock 7b. This screen clock 7b also has a cycle of an integral multiple of the length of the master clock 1a, but in this example, it has a cycle three times that of the pixel clock 7a. The screen clock 7b is input to the pattern signal generation circuit 3. The pattern signal generating circuit 3 is composed of, for example, an integrating circuit including a capacitor and a resistor, and generates, for example, a triangular wave pattern signal 3a.

In the comparator 4, the analog-converted image signal 2a and the level of the pattern signal 3a are compared, and a PWM signal output 4a which is pulse width modulated is output. And this P
The WM signal output 4a is, for example, a modulation circuit for modulating a laser beam (for example, the laser driver shown in FIG. 1 or 8).
40) is input. The laser beam is turned on / off according to the pulse width and the recording medium (photosensitive drum 35 in FIG. 8).
A halftone image is formed on top.

In this embodiment, a frequency (for example, a triangular wave) higher than the frequency of a synchronization signal for generating a pattern signal (for example, a triangular wave) is generated.
Since the screen clock 7b synchronized with the horizontal synchronizing signal 14a is formed by using the master clock 1a of 12 times frequency, the "fluctuation" of the pattern signal (triangular wave) 12 generated from the pattern signal generating circuit 3 (for example, 1 In this embodiment, the difference between the pattern signals of the second line and the second line is 1/12 of the pattern signal period. Therefore, a high-quality reproduced image can be obtained by using a pattern signal with little fluctuation.

FIG. 4 is a diagram for explaining a signal waveform of each part of the apparatus shown in FIG. Referring to FIG. 4, the master clock 1a is the output of the oscillator 1 and the BD signal 14a is the horizontal synchronizing signal described above. Further, the pixel clock 7a is the master clock 1a of the oscillator 1 and the timing signal generating circuit 7
It is what was counted down in. Screen clock
7b is obtained by the timing signal generating circuit 7. The screen clock 7b has a period of a width of three pixel clocks 7a. The screen clock 7b is a synchronizing signal for generating the pattern signal 3a and is input to the pattern signal generating circuit 3.

For the dither data 10, for example, "10011" is input. Then, the multi-valued data 9a becomes (FF) by the multi-value conversion.
_H (00) _H (00) _H (FF) _H (FF) _H. Here, () _H represents hexadecimal notation. Then, according to the density conversion example described above, the digital density correction data 13a becomes "BAABB" by the LUT 13.

Data “00” → Data “A” Data “FF” → Data “B” Analog data 2a indicates image data D / A converted by the D / A converter 2. In the case of this example, the high density value is "B" and the low density value is "A". As shown in the figure, the higher the analog level, the higher the density.

On the other hand, the pattern signal 12 which is the output of the pattern signal generating circuit 3 has a triangular wave shape as shown by the solid line of "input to comparator" in FIG. 4, and is compared with the analog data 2a. The result of the comparison is the pulse width modulated binarization and becomes the PWM output signal 4a.

As described above, when the data is input to the image output apparatus, the dither data 10 that is only binary is converted into multi-valued data by being subjected to conversion processing, and can be converted into an arbitrary pulse width after density correction. Adjustment is possible on the output control side.

<Output> The PWM output signal 4a modulates the laser driver 40 to pulse-width-modulate the semiconductor laser 30 to emit light. The light beam emitted from the semiconductor laser 31 is collimated by the collimator lens 32, is deflected by the rotary polygon mirror 33, and is scanned on the photosensitive drum 35 by the fθ lens 34. 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.

As described above, in the signal processing in the present embodiment, after the dither data is once converted into analog image data, the pulse width modulation is variable by comparing with the triangular wave pattern signal 12 of a predetermined cycle, and the density controllable image can be obtained. The output is obtained.

<Image Output System> FIG. 5 is an example of a configuration in which the present invention is applied to an image output system. One controller (host computer, etc.)
It is a method to output the same image to a plurality of printers (22A to 22D) by an output signal from 21. Printer (22
A to 22D) are individually equipped with the above-mentioned image output devices.
The same dither data 10 is output from the controller 21 to each printer. The problem here is that the output density of each printer is different. That is, if the condition setting of each printer is different and the maximum density value Dmax is different for each printer, the density of the output image will be different even if the same data is given.

In general, in the case of dither data, such density control cannot be performed by the data itself. For example, in a printer or the like, density level control, such as potential setting on the photosensitive drum 35, has been performed. However, in the present embodiment shown in FIG. 5, the output densities of the respective printers can be made uniform only by setting and adjusting the LUT 13 shown in FIG.
The density adjustment characteristics are selected by the image output devices (22
It is done by the select dial 20 provided in A to 22D). This select dial 20 is used to select one of the density correction characteristics shown in FIG. 6 or 7.

Each printer has a circuit as shown in FIG. For example, the LUT 13 including a ROM has a plurality of tables (a to e) as shown in FIG. In FIG. 9, table a has addresses (00) _H to (FF) _H , and table b has addresses (10).
0) _H to (1FF) _H , table c has address (200)
_{From H} to (2FF) _H , table d has address (300) _H to
(3FF) Up to _H ... FIG. 6 is a density conversion characteristic diagram obtained by the conversion table of the LUT 13.
The LUT 13 has a conversion characteristic such that the horizontal axis is multi-valued data 9a and the vertical axis is digital data 13a as shown in FIG. Now, returning to FIG. 3, when the operator of the output device (22A to 22D) selects the select dial 20 to instruct an appropriate density conversion characteristic,
The select signal generation circuit 15 generates the select signal 15a according to the dial position. In the example of FIG. 6, five types of tables are provided, so if the select signal 15a is 3 bits, it is sufficient. The LUT 13 uses this select signal as the upper address (MSD address in FIG. 9) and multivalued data 9a.
Is input as a lower address, and the digital data 13a is output according to the table configuration of FIG. In this way, the output density can be made constant by selecting an arbitrary curve in each printer using the select dial 20.

<Other Embodiments> The above embodiments have been described with respect to the case of inputting binary data, but multivalued data (for example, ternary, quaternary, ...) Can be similarly implemented by the circuit of FIG. At this time, the multilevel circuit 9
For example, it is assumed that (“0” “1/2” “1”) is input in case of 3 values. Input “0” → output “00” input “1/2” → output “7F” input “1” → Convert to output "FF" and then perform the same as for binary. Further, the LUT 13 changes linearly by changing it as shown in FIG. Only the intermediate data (corresponding to 1/2) of the multi-valued data 9a can be converted. This may be done by making a LUT as shown in FIG.

In the embodiment, the triangular wave is described as a 3-pixel cycle, but a 1-pixel cycle is the most desirable.

As described above, according to the image processing apparatus of the present invention,
Since the density of n-bit image data is adjusted according to the output density characteristics of the image forming means, it is possible to form an image according to the output characteristics of the image forming means, and various output density characteristics can be obtained. An image having a constant output density can be reproduced by adding the image forming means.

Since the density-adjusted n-bit image data is converted into analog image data to generate the pulse width modulation signal, the density of the output image of the image forming unit is reduced even if the n-bit is small. It is possible to achieve constant and high-definition gradation processing.

Therefore, it is possible to provide a device that enables high-definition gradation processing for an image that has undergone halftone processing.

[Brief description of drawings]

FIG. 1 is a basic configuration diagram of an embodiment according to the present invention, FIGS. 2A and 2B are diagrams for explaining the principle of a conventional dither method and a density pattern method, and FIG. 3 is a circuit of the embodiment. Configuration diagram, FIG. 4 is a timing chart for explaining the operation of the embodiment, FIG. 5 is a configuration diagram of an embodiment of the image output system, FIGS. 6 and 7 are characteristic diagrams of density conversion in the embodiment, FIG. FIG. 8 is a configuration diagram of an output portion of the laser beam printer of the embodiment, and FIG. 9 is a memory map diagram of the LUT. In the figure, 1 ... Oscillator, 2 ... D / A, 3 ... Pattern pulse generation circuit, 4 ... Comparator, 7 ... Timing signal generation circuit, 7a ... Pixel clock, 7b ... Screen clock, 9 ...... Multi-valued circuit, 9a …… Multi-valued data,
10 ... Dither data, 13 ... LUT, 13a ... Density correction data, 15 ... Select signal generation circuit, 20 ... Select dial, 21 ... Controller, 22A-22D ... Image output device, 202a, 203a ... This is multi-valued density data. ,

Claims (1)

(57) [Claims] 1. An image processing device for supplying a pulse width modulation signal to an image forming means, comprising image data generating means for generating image data subjected to m-bit halftone processing, and said image data generating means. A conversion unit that converts the m-bit image data into n-bit (n> m) image data, and the n-bit image data output from the conversion unit is darkened according to an output density characteristic of the image forming unit. Adjusting means for adjusting, and pulse width modulation signal generating means for converting the n-bit image data output from the adjusting means into analog image data and generating a pulse width modulation signal according to the converted analog image data. And an image supply means for supplying the pulse width modulation signal output from the pulse width modulation signal generating means to the image forming means. Apparatus.