JP2840278B2 - Image processing device - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an image processing apparatus that compares an image signal with a pattern signal whose level continuously changes at a predetermined cycle and outputs a pulse width modulated signal. is there.

[Conventional technology]

When an image signal is binarized to form an image with a laser beam printer or the like, a pulse width is obtained by comparing the image signal with a periodic pattern signal such as a triangular wave in order to obtain halftone gradation. An image processing apparatus that performs modulation has been proposed.

FIG. 6 shows a specific example of this technique. FIG. 6A shows a reading unit which is an input side of digital copy original information, and FIG. 6B shows a printer unit which is an output side.

First, by turning on a start button (not shown), the illumination system 6 is turned on to illuminate the document 1. As a result, the information of the original 1 becomes optical information, reaches the imaging optical system 7 via the return mirrors 3, 4, and 5, and is imaged by the imaging optical system 7 onto the CCD 8 that converts the optical information into electrical information. , CCD 8 outputs an electric signal of a level corresponding to the incident light.

The electric signal as shown in FIG. 7A obtained by the CCD 8 is converted into a digital value through an A / D converter 31 as shown in FIG. 8, and then the image processing circuit 32 performs edge enhancement or smoothing. Image processing is performed, and γ correction is performed by the γ correction circuit 33, and the result is transmitted to the D / A converter 34. With D / A converter 34
The D / A converted analog signal is compared with a reference wave (triangular wave) as shown in FIG. 7B by a comparator 35, for example.

At this time, normally, as shown in FIG. 7B, one output pixel is output in association with one output pixel. As a result, a laser beam is generated from the laser generator 19 at a period as shown in FIG. 7 (C). The laser beam oscillated from the laser generator 19 is scanned by the scanner 9 in the direction of the rotation axis of the photosensitive drum 18, reaches the folding mirror 11 through the correction optical system 10, and is condensed on the photosensitive drum 18 from the folding mirror 11. Imaged. The photoconductor drum 18 is uniformly charged by the charger 12, and is formed on the photoconductor drum 18 by removing the electric charge with a laser beam. This latent image is developed by a developing device 14 according to a well-known electrophotographic process, and the developed image is transferred to a roller by the action of a transfer charger 15 and a transfer roller 17.
The paper is transferred onto the transfer paper conveyed at 16 and discharged.

Further, the photosensitive drum after the transfer is cleaned by the cleaner 13.

[Problems to be solved by the invention]

However, in the above conventional example, the reading cycle on the input side: 1 / f
Against _0, for example, no moire in the input side when the read document having a period of 2 / f ₀ (the sampling theorem) is, corresponding to the period 1 / f ₀ is read in the input side, the comparator When the period of the reference wave is set to 1 / f ₀ , moiré occurs in the output image, resulting in a very deteriorated image with respect to the original.

This is a pulse train where x is the reference wave period and τ is the pulse.
PWM (x) When is the Fourier series expansion _{PWM (x) = a 0 +} Σa n cos nK 0 x + Σb n sin nK 0 x ... (1) K ₀ = 2πf ₀ (pulse angular frequency) (3) When harmonic e _n and _{e n = a n cosnK 0 x} , is expressed as follows.

_{PWM (x) = Σe n ...} (4) Here, assuming that the input signal is a sine wave having a frequency f, the pulse width τ is τ = τ ₀ (1 + msinKx) (6) where K = 2πf (angular frequency of modulated wave) (7) From equation 5), Becomes Since the maximum value of the output pulse width is the pulse interval, τ ₀ = x / 2, m = 1 (9) Here, J _n (Z): nth-order Bethcell function K ₀ : angular frequency of pulse (= 2πf ₀ ) k: angular frequency of modulated wave (= 2πf) This modulates the spectral component of pulse train PWM (x). Assuming that the frequency of the wave is f ₀ and the frequency is f, it is expressed by the coefficient a _ij as follows.

ΣΣa _ij (± (if ₀ ± jf)) (13) In equation (13), the components having relatively large coefficients are f, ± (f ± 2f ₀ ), ± (2f ± 3f)… (14) f ₀ : reference wave frequency f: sine wave frequency If a low frequency appears, moiré is observed.

FIG. 10 shows this as a function of the frequency component and the spectrum. When the period on the output side is (1 / f ₀ ) and the period on the input side (1 / f) enters, ± (f ₀ −f) and ± (f ₀ −2f) are generated as aliasing components. . For this reason, the moire component that does not occur on the input side is ± (f ₀ −2f)
Appears by.

[Means for solving the problem]

The present invention has been made in view of the above points, and has an input unit for inputting an image signal represented by a digital value for each pixel, and generates a pattern signal of a predetermined cycle having an extreme value inside one cycle. Pattern signal generation means, and pulse width modulation signal generation means for comparing the value of the image signal with the value of the pattern signal to generate a pulse width modulation signal, wherein the cycle of the pattern signal is The present invention provides an image processing apparatus characterized in that the period is set to approximately 3/2 times as long as the input period of the digital value.

〔Example〕

 FIG. 1 is a diagram showing an example of processing an image signal according to the present invention.

FIG. 6A is a diagram in which information of a document read by a digital copy reading unit shown in FIG. 6A is converted into an electric signal, and shows a pixel output of a CCD.

(B) shows a case where the period of the reference wave in the comparator 35 in FIG. 8 is set to (1 / 2f ₀ ) with respect to the period (1 / f ₀ ) on the input side.

 (C) shows a pulse width modulated signal.

FIG. 2 (C) shows a case where the post-processing is performed after interpolating the pixel column of FIG. 2 (A), that is, the adjacent pixel x and pixel x + 1
Is linearly interpolated to the pixel x ',
FIG. (D) shows the data of FIG. 2 (C) subjected to pulse width modulation.

FIG. 3 is a configuration example of an image processing unit according to the present invention,
It replaces the D / A converter 34 and the comparator 35 in FIG.

The digital video signal is latched by a video CLK at a latch 41 and synchronized. This Video CLK is the master CLK
A clock obtained by dividing the frequency by 2 with a K flip-flop 45.
This video signal is converted into an analog video signal by the D / A converter 42. The output of the D / A converter 42 is converted to a voltage level by a resistor 43 and then input to one input terminal of a comparator 44.

On the other hand, the master CLK undergoes a predetermined frequency division by the frequency conversion circuit 46, and is further divided into two by the JK flip-flop 48 to become a clock signal having a duty ratio of 50%.

The frequency conversion circuit 46 divides the frequency of the master CLK at a fixed rate, and multiplies the video CLK by a fixed number (for example, 1 /
3,2,1.0,1.5 ...). This clock passes through a buffer 49 and a variable resistor 50 and a capacitor.
A triangular wave as shown in FIGS. 1 (b), 2 (A) and 2 (C) is obtained by the integration circuit constituted by 51. Then adjust the bias with the capacitor 52 and the variable resistor 53, and
Through the 54 and buffer amplifier 55, the comparator
Input to the other input terminal of 44, D / A converter 4
2 is compared with the analog video signal from FIG. 2 and pulse-modulated as shown in FIGS. 1 (c), 2 (B) and 2 (D).

This makes it possible to remove moire (shaded portions) generated by the pulse width modulation method as shown in FIG. 4 (A).

Further, the generation region of f ₀ −2f, which is the aliasing noise of the adjacent pixel, is expanded as shown in FIG. 4 (B).

In general, the sensitivity characteristics of the human eye are shown in FIG.
It is known that there is a peak on the low frequency side as shown in FIG.

For this reason, moire that has conventionally occurred in the low-frequency region is pushed to the high-frequency region, and higher-quality image can be obtained. Further, the data between the adjacent pixels of the pulse width modulation is linearly interpolated and then compared with the reference wave as shown in FIG. Can be faithfully reproduced.

Further, when the period of the reference wave is set to be 3/2 times the period of the reading period (1 / f ₀ ) on the input side, moire can be removed most efficiently.

〔The invention's effect〕

As described above, according to the image processing apparatus of the present invention, the period of the pattern signal used for the pulse width modulation process is approximately 3 / the period of the input digital image signal.
Since it is set to twice, moire removal can be performed most efficiently.

[Brief description of the drawings]

1 and 2 are diagrams showing an example of image signal processing according to the present invention, FIG. 3 is a diagram showing an example of the configuration of an image processing unit, and FIG. 4 is a diagram showing the conventional PWM system and the PWM system of the present embodiment. FIG. 5 is a diagram showing a comparison, FIG. 5 is a diagram showing characteristics of human eyes, FIG. 6 is a configuration diagram of a digital copy, FIG. 7 is an explanatory diagram of a PW system, FIG. 9 is a diagram showing the relationship between the frequency component and the spectrum, FIG. 9 is a diagram showing the relationship between the frequency component and the spectrum, 42 is a D / A converter, 44 is a comparator, 50 is a variable resistor, 51 Is a condenser.

──────────────────────────────────────────────────続 き Continued on front page (58) Field surveyed (Int.Cl. ⁶ , DB name) B41J 2/52

Claims (1)

(57) [Claims] An input unit for inputting an image signal represented by a digital value for each pixel; a pattern signal generating unit for generating a pattern signal of a predetermined cycle having an extreme value inside one cycle; Pulse width modulation signal generation means for generating a pulse width modulation signal by comparing the value of the pattern signal with the value of the pattern signal. An image processing apparatus characterized in that the setting is approximately 3/2 times higher.