JPH0343831B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to an image processing apparatus that processes a halftone image having multi-value luminance or multi-value brightness.

[Prior Art] Conventionally, display cells of a fixed size are arranged in a matrix, such as plasma panels and liquid crystal displays, and each cell can take only two values (on and off), and inkjet printers, etc. In addition, there are cases where the dot pattern formed by the ejected ink can take only two values, on and off.
There are devices that display images in discrete patterns of values, which are used in panel displays and facsimile image display devices, respectively.

In order to improve image quality in such a display device, it is necessary to be able to display halftones.

A conventional method for displaying halftones using a binary discrete dot pattern is a density modulation method, such as a systematic dither method. The systematic dither method is created by dividing the entire display screen consisting of a matrix-like dot arrangement into sub-matrices consisting of n x n dots, and combining different predetermined threshold levels into these sub-matrices. Prepare a dither pattern and set 2 for each dot.
This is the threshold for value digitization processing. References regarding the dither method include Iizuka and Arai, “Binary display of grayscale images using the dither method,” Nikkei Selectronics, May 1978.
Day, pages 50-65.

[Problems to be Solved by the Invention] However, conventionally, when attempting to reproduce halftones using a systematic dither method, for example, the outline of the image becomes blurred, making it impossible to reproduce the image well. Ta.

This is due to the fact that the halftones of an image do not have uniform properties, but have various properties depending on the area, such as the contour and other halftone parts. This is due to the inability to respond.

This is a problem that occurs not only in the dither method but also in other density modulation methods, such as the density pattern method.

SUMMARY OF THE INVENTION An object of the present invention is to solve the above-mentioned problems and provide an image processing apparatus that can satisfactorily process and reproduce halftones having various properties with a simple configuration and at high speed.

[Means for Solving the Problems] In order to achieve the above object, the present invention provides an image processing device for reproducing halftones of an image according to a given image signal by changing the area ratio of dots. a first generating circuit that outputs a signal for generating dots in a first period based on a result of comparing the image signal with a first threshold value of a predetermined period; A circuit, which is provided separately from the circuit and in parallel with the first generating circuit, generates a signal by comparing the image signal with a second threshold having a cycle longer than the first threshold. a second circuit that outputs a signal for generating dots at a second period larger than the first period;
a generating circuit, a detecting means for detecting an edge portion of an image indicated by the image signal from the image signal, and a signal output from the first generating circuit in the edge portion based on the detection by the detecting means; A portion other than the edge portion includes means for combining and outputting signals output from the second generating circuit.

[Operation] In the above configuration, the means for outputting the combination is based on the detection by the detecting means, and the edge portion outputs the signals output from the first generating circuit.
In areas other than the edge portion, the signals output from the second generating circuit are combined and output.

[Example] In the example described below, different halftone processing is applied to the vicinity of the outline of the image and the other halftone parts, and furthermore, in this example, the simultaneous color processing is applied especially to the outline. Both edge-highlighted display and halftone display are performed using a contrast phenomenon.

Here, the simultaneous color contrast phenomenon will be explained for reference. Simultaneous color contrast phenomenon refers to the phenomenon in which one color looks different depending on other colors juxtaposed, and this phenomenon is further classified into brightness contrast, hue contrast, and saturation contrast using chromatic colors. However, this will be discussed separately.

(1) Contrast of brightness in chromatic colors When a pure color is placed on a white background, the color with lower brightness will be seen more clearly.On the other hand, on a black background, yellow, which has the highest brightness, will be clearly visible, and gray, which has medium brightness, will be seen clearly. Above, it refers to a phenomenon in which any color can exhibit its own characteristics.

(2) Contrasting hues For example, on a screen where red and purple are in contact, the side where red is in contact with purple has an orange tinge, and the side where purple is in contact with red appears to be more bluish-purple. means. If a white line or a black line is drawn at the boundary between two colors, the phenomenon will disappear.

(3) Contrast of saturation For example, if you compare the case where a bright orange is placed on top of a pure orange, and the case where an orange with the same brightness is placed on a gray, the result is better when placed on a gray. On the other hand, when placing a dark red color on top of a pure red color, and when placing the same dark red color on top of a gray color, when placed on top of a gray color, This refers to the phenomenon in which people perceive color as having higher saturation.

Hereinafter, one embodiment of the present invention will be described in detail with reference to the drawings. FIG. 1 shows the configuration of the image processing apparatus of this embodiment, where 1 is an original image signal, 2 is a contour extraction processing circuit that extracts a contour image signal _S2 from the original image signal 1, and 3 is an original image signal. a contour removal circuit that obtains an output signal _S3 by removing the contour image signal _S2 from the signal 1;
4 is a color signal conversion processing circuit that forms an edge enhancement color signal _S4 for converting the edge portion from the edge image signal _S2 ; 5;
and 6 are supplied with the outputs of circuits 3 and 4, respectively, and perform processing for displaying a halftone image as a binary image. 7 is an output signal S of the halftone display processing circuits 5 and 6. This is an image conversion processing circuit that forms an image that has been subjected to edge enhancement processing and halftone display processing from _S5 and _S6 .

Here, the original image signal 1 is a color signal that reads color image information from the original image and converts it into an electrical signal.
It is assumed that the original image signal 1, which is a scanner signal, is composed of a signal separated into yellow, magenta, cyan, and black, and a luminance signal having only luminance distribution information. Note that, as can be seen from the fact that a figure can generally be recognized using only a luminance signal, it is understood that the luminance signal includes information on a contour image.

The contour extraction processing circuit 2 performs a process of extracting the contour image signal _S2 from the luminance signal described above. contour image signal
_S2 extracts a signal from a luminance signal where the amplitude value changes significantly over time, that is, a signal with a high frequency component. This can be done by, for example, taking a difference signal from a slightly delayed signal. The absolute value of the amplitude value of the output signal obtained by this processing represents the amount of change in the luminance signal, that is, the clarity of the contour, and the time represents the position of the contour.

In this embodiment, the outline of a color image is converted to a special color, for example, black, which visually emphasizes the outline, but if the conversion width is wide, part of the image will be cut off and the image quality will deteriorate. A decline occurs. Therefore, the width to be converted needs to be appropriately narrowed. Therefore, the contour extraction processing circuit 2 extracts only those signals whose absolute values of amplitude values are equal to or higher than a predetermined level. The amplitude value of this signal represents the density of the color signal to be converted, and the time is used as a signal representing the position of the color signal to be converted.

The contour removal circuit 3 removes signal components at locations to be converted into contour enhancement signals from each color signal of the original image, based on the output signal _S2 of the contour extraction processing circuit 2.
The color signal conversion processing circuit 4 forms yellow, magenta, cyan, and black color signals constituting an edge enhancement color signal S ₄ based on the output signal S ₂ of the edge extraction processing circuit 2 . The halftone display processing circuits 5 and 6 process the output signals S ₃ and S ₄ of the contour removal circuit 3 and the color signal conversion processing circuit 4, respectively, using a dither method to convert them into binary digital signals. Note that this processing differs from each other as will be described later. The image conversion processing circuit 7 combines the output signals S ₅ and S 6 of the halftone display processing circuits 5 and ₆ to form an image signal to be displayed.

FIG. 2 shows an example of the contour extraction processing circuit 2 shown in FIG. 1, in which a luminance signal S ₁ obtained by raster scanning the original image signal 1 is input to an input terminal 10. This luminance signal S ₁ is supplied directly to the differential amplifier 13 via a delay line 11 having a delay time of one scanning line. As a result, each signal on the adjacent scanning line is input to the differential amplifier 13, and the difference signal is taken out. This difference signal has information on the position and amount of change in the contour image, and has a positive amplitude value at a position where the brightness changes from a low place to a high place, and a negative amplitude value in the opposite case. The difference signal from the differential amplifier 13 is further supplied to a full-wave rectifier circuit 15 to convert the signal component with a negative amplitude value into an inverted signal with a positive amplitude value. This signal is a component of the contour image in a direction perpendicular to the scanning direction. Similarly, the luminance signal S ₁ is supplied directly to the differential amplifier 14 via the delay line 12 having a delay time approximately equal to the Nyquist interval of the luminance signal, and the differential amplifier 14 and the full-wave A rectifier circuit 16 obtains contour image components in the scanning direction. Full wave rectifier circuit 1
The outputs of 5 and 16 are supplied to an amplitude comparator 18 and an analog switch 17, and the amplitude comparator 18 compares the amplitudes of the aforementioned two contour image signals. from the analog switch 17. This signal is a two-dimensional contour image signal. This contour image signal is supplied to a limiter 19 and a differential amplifier 20, and only signal components whose amplitude value is equal to or higher than the slice level of the limiter 19 are taken out to an output terminal 21 as a contour image signal _S2 .

FIG. 3 shows an example of the color signal conversion processing circuit 4 shown in FIG. 1, in which an input terminal 100 is supplied with the contour image signal S ₂ from the contour extraction processing circuit 2.
This signal S ₂ is connected to amplifier 101 and limiter 102.
is converted into a pulse waveform signal by Furthermore, by supplying this pulse signal to the differentiating circuit 103, a trigger signal indicating the rising and falling positions of the pulse signal is formed. This trigger signal represents the position of the edge of each contour of the original image. Further, this trigger signal is supplied to a pulse generator 106 via a half-wave rectifier 104 and to a pulse generator 107 via a half-wave rectifier 105, and is supplied from each pulse generator 106 and 107 to the start and end points of each contour. A pulse signal representing the position of is extracted. The sample and hold circuit 1 receives a pulse signal from the pulse generator 106 representing the position of the starting edge of the contour portion.
14, 116, 118 and 120, and sample and hold circuits 115, 11 supply a pulse signal representing the end of the contour section from the pulse generator 107.
7, 119 and 121. Furthermore, the yellow, magenta, cyan, and black color signals Y, M, C, and B of the original image are input to sample and hold circuits 114, 115, 116, and 1 through input terminals 110, 111, 112, and 113, respectively.
17, 118 and 119, and 120 and 121, respectively. As a result, the sample and hold circuits 114, 116, 118 and 120 hold each color signal at the start end of the contour, and the sample and hold circuits 115, 117, 119 and 121 hold each color signal at the end.

The output signals of the pulse generator 107 and the sample and hold circuits 114 to 121 are sent to the color calculation circuit 12.
Supply to 2. This color calculation circuit 122 can be composed of a microcomputer and a storage device, and calculates yellow, magenta, and cyan so that color signals for exchanging the contour can be obtained based on the characteristics of each color signal at the start and end of each contour. and controls the amplification factors of amplifiers 123 to 126 for each color signal of black. Note that, based on the measurement results, the color calculation circuit 122 preliminarily applies a color signal having an edge enhancement effect to two color signals sandwiching the outline due to the above-mentioned contrast phenomenon of brightness, saturation, and hue. At the same time, the above-mentioned medium-brightness gray signal having an edge enhancement effect is stored without depending on the color signal, and control is performed so that both can be used properly depending on the purpose. In this manner, color signals for converting the contour portion are output from the output terminals 130 to 133.

FIG. 4 shows an embodiment of the contour removal circuit 3 and halftone display processing circuit 5 shown in FIG. In FIG. 4, the contour removal circuit 3 is connected to an analog switch 3.
5, 36, 37, and 38, and the input terminals 30 to 34 receive the yellow, magenta, cyan, and black color signals Y, magenta, cyan, and black of the original image, respectively.
M, C and B and a contour image signal _S2 are supplied.
Contour image signal S ₂ input from input signal terminal 34
has a positive amplitude value at the position that emphasizes the contour, and the amplitude is zero at other positions, and the analog switches 35, 36, 37 and 38 operate according to this signal, and the input terminals 30, 31, Analog switches 35, 36, 37 and 38 via 32 and 33
Processing is performed to set the signal components of the contour parts of the yellow, magenta, cyan, and black color signals Y, M, C, and B of the original image inputted to each of the original images to zero amplitude values.

The halftone display processing circuit 5 includes a pulse generator 39 and dither processing circuits 40 to 43.
The output _S2 of the contour extraction processing circuit 2 inputted from the input signal terminal 34 is supplied to the pulse generator 39 to generate a pulse synchronized with the starting edge of the contour portion, and the dither processing circuits 40, 41, 42 and 43 Forms a pulse signal for sampling processing. Dither processing circuit 4
0, 41, 42, and 43, each color signal of yellow, magenta, cyan, and black is converted into a dithered binary signal and output terminal 44,
45, 46 and 47, respectively.

Here, the operating principle of the systematic dither method will be explained with reference to FIG. In FIG. 5, numeral 51 indicates a pixel pattern when the pixel level of the input image can take on 16 different values from 0 to 15. 52 is a mask pattern that provides a threshold for binarization processing, and values from 0 to 15 are arranged at predetermined positions. 53 is the basic mask pattern 4
This is a ×4 dither matrix. In the organized dither method, the basic dither matrix is 2n×2n
It is given by a matrix (n is an integer). 54 is a display device.

In the dither method, the threshold for each pixel varies depending on the pixel's location, is independent of the pixel level, and is determined only by the pixel's coordinates. For example, the threshold 56 for pixel 55 in the 4th row and 1st column of the input image is 15;
Since the pixel level of is 4, the pattern 57 of the display device 54 is off (white).

FIG. 6 shows the dither processing circuit 40 shown in FIG.
43, in which 60 is a color signal input terminal, and 61 is a pulse generator 39 (FIG. 4).
62 is an A/D converter that converts the color signal from the input terminal 60 into a digital signal at the timing of the above-mentioned pulse signal;
63 is an arithmetic circuit that counts the number of the above-mentioned pulse signals and calculates the position coordinates (i, j) of each pixel; 64
6 is a memory for storing a threshold corresponding to the position coordinates (i, j) of each pixel; 65 is a comparator for comparing the A/D conversion output with the threshold; and 66 is an output terminal of the comparator 65. In FIG. 6, when the size of the dither matrix is 4.times.4, the input image signal has 16 pixel levels from 0 to 15, that is, 4 bits of information per pixel. In this case, a 4-bit A/D converter 62 can be used. Furthermore, the arithmetic circuit 63
calculates the two-dimensional position coordinates (i, j) by counting the number of pulses from the output of the pulse generator 39, specifies the address of the memory 64, and stores the memory 64.
The threshold of the position coordinates (i, j) stored in is read out. Memory 64 is programmable.
Can be configured with read-only memory (PROM).
Incidentally, since the mask pattern can be expressed by repeating a 4.times.4 basic dither matrix, the memory 64 requires a capacity of 16.times.4 bits corresponding to the basic dither matrix. Comparator 65
is a 4-bit color signal xij of position coordinates (i, j)
and the aforementioned 4-bit threshold cij are input, and a binary signal of 1 when xijcij: 0 when xij<cij is outputted to the output terminal ₆₆ as an output signal S5.

The halftone display processing circuit 6 can be constructed in the same manner as the above-described halftone display processing circuit 5, but the signal for converting the contour part does not require halftone display compared to the normal image signal, and the width of the contour is Since the size of the basic dither matrix is smaller than that of a normal image, the size of the basic dither matrix can be equal to or smaller than that of the processing circuit 5 for halftone display.

A major feature of this embodiment is that the dither matrix is changed in accordance with the characteristics of the image in this way.

The image conversion processing circuit 7 is the halftone display processing circuit 5
The AND circuit is supplied with the yellow, magenta, cyan, and black color signals S ₅ and S ₆ outputted from 6 and 6, respectively, and extracts the combined color signals from these AND circuits and provides this color signal for display. .

In the embodiment of the present invention described above, the dither method is used as the process for displaying halftones. However, for the process for displaying the halftones, in addition to the dither method, for example, an image is divided into minute units, and the A dot method that expresses shading similar to continuous tone by changing the area ratio occupied by minute elements such as points and lines, a pulse width modulation method that uses a variable dot pattern by modifying this dot method, and a constant dot method. A pulse frequency modulation method using a dot pattern, a delta-sigma modulation method, etc. can also be used.

In the above-mentioned embodiment, halftone display processing is also performed on the color signal for converting the signal of the outline in order to emphasize the outline. Of course, sufficient effects can be obtained.

Furthermore, in the embodiments of the present invention described above, an example was taken where an image is displayed in a binary discrete pattern. By ejecting ink so that it overlaps three times,
Images can also be displayed in a discrete pattern of multivalued brightness. Even in this case, the image quality is inferior to continuous tone images, so in order to improve the image quality it is necessary to perform halftone display processing. It can be further improved.

Furthermore, in the above embodiments, in addition to changing the matrix in the outline and other parts to make the halftone processing different, the outline enhancement processing that utilizes the simultaneous contrast phenomenon of colors has been explained, but the simultaneous contrast phenomenon also occurs for black and white images, so
If the present invention is applied to a monochrome image as well, by performing different halftone display processing for the outline image signal and the original image signal, the same effect as in the case of color display can be obtained.

In addition, in the embodiments of the present invention described above, image processing for general images has been described, but in an image display device for pattern recognition, features such as image shape or outline information are extracted and displayed as an image. In this case, the contour image signal is processed for halftone display, and
For other image signals, normal quantization processing may be performed without performing halftone display processing.

As is clear from the above, according to the embodiment, without sacrificing the characteristics (sharpness) of the contour,
It can be used in conjunction with edge enhancement to display halftones, and furthermore, it performs processing for halftone display that is suitable for both the image area and the contour image area where the edges are emphasized, making the edges even more effective. be able to.

As explained above, in this embodiment, the first generation circuit that outputs a signal for generating dots in the first period based on the result of comparing the image signal and the first threshold value is shown in FIG. The halftone display processing circuit 6 shown in FIG.

Furthermore, in this embodiment, the first generating circuit is
A circuit that is provided separately and in parallel with the first generation circuit, and that generates a first cycle by comparing the image signal with a second threshold having a cycle larger than the first threshold. A second generation circuit that outputs a signal for generating dots at a second period larger than the second period is the halftone display processing circuit 5 shown in FIG. In particular, the processing circuit has a dither matrix having a different size from that of the first generation circuit.

Further, in this embodiment, the contour extraction processing circuit 2 is used as a detection means for detecting an edge portion of the image signal indicated by the image signal from the image signal.

Further, in this embodiment, based on the detection by the detection means, in the edge portion, the signal output from the first generating circuit is combined with the signal output from the second generating circuit in the edge portion other than the edge portion. The image conversion processing circuit 7 is used as a means for this purpose.

[Effects of the Invention] As explained above, according to the present invention, the first generation circuit and the second generation circuit, which process the edge portion and the image signals other than the edge portion by comparing them with threshold values, are connected in parallel. Since the two circuits are set up and operated in parallel, and the outputs of the two circuits are combined and output, the edge efficiency is lower than the method of simply selecting the optimal matrix according to the characteristics of the image. Adaptive processing suitable for the characteristics of each image signal can be performed at high speed on halftone and halftone image signals.

[Brief explanation of drawings]

FIG. 1 is a block diagram showing an outline of the configuration of an image processing apparatus according to an embodiment of the present invention, FIG. 2 is a block diagram showing an example of the contour extraction processing circuit shown in FIG. 1, and FIG.
FIG. 4 is a block diagram showing an example of the color signal conversion processing circuit shown in FIG. 1, FIG. 4 is a block diagram showing an example of the contour removal circuit and halftone display processing circuit shown in FIG. 1, and FIG. A diagram for explaining the operating principle of the organized dither method, FIG. 6 is a block diagram showing a specific example of the dither processing circuit shown in FIG. 1... Original image signal, 2... Contour extraction processing circuit, 3... Outline removal circuit, 4... Color signal conversion processing circuit, 5, 6... Halftone display processing circuit, 7...
Image conversion processing circuit, 10... Input terminal, 11, 1
2...Delay line, 13, 14...Differential amplifier, 1
5, 16... Full wave rectifier circuit, 17... Analog switch, 18... Amplitude comparator, 19... Limiter,
20... Differential amplifier, 21... Output terminal, 100
... Input terminal, 101 ... Amplifier, 102 ... Limiter, 103 ... Differentiation circuit, 104, 105 ...
Half-wave rectifier circuit, 106, 107... Pulse generator, 110, 111, 112, 113... Input terminal, 114, 115, 116, 117, 118,
119, 120, 121... Sample hold circuit, 122... Color calculation circuit, 123, 124, 1
25, 126...Amplifier, 130, 131, 13
2,133...Output terminal, 30,31,32,3
3, 34...Input terminal, 35, 36, 37, 3
8, 39...Pulse generator, 40, 41, 42,
43...Dither processing circuit, 44, 45, 46, 4
7... Output terminal, 51... Image pattern, 52
... Mask pattern, 53 ... Dither matrix that is the basis of the mask pattern, 54 ... Display device, 55 ... Pixels of input image, 56 ... Threshold for pixels of input image, 57 ... Pattern of display device, 60, 61...input terminal, 62...
...A/D converter, 63 ... Arithmetic circuit, 64 ... Memory, 65 ... Comparator, 66 ... Output terminal, S ₁ ...
...brightness signal, S ₂ ... signal of contour image, S ₃ ... output signal from which the contour image signal has been removed, S ₄ ... color signal for contour enhancement, S ₅ ... output signal of halftone display processing circuit 5, S ₆ ... Output signal of halftone display processing circuit 6, Y
...Yellow signal, M...Magenta signal, C...
Cyan signal, B...black signal.

Claims (1)

[Scope of Claims] 1. An image processing device for reproducing halftones of an image according to a given image signal by changing the area ratio of dots, which comprises: a first generating circuit that outputs a signal for generating dots in a first period based on the result of comparing the dots with a threshold value; A circuit installed in parallel with
outputting a signal for generating dots at a second period greater than the first period by comparing the image signal with a second threshold value having a period larger than the first threshold value; a second generation circuit; a detection means for detecting an edge portion of an image indicated by the image signal from the image signal; and a signal output from the first generation circuit in the edge portion based on the detection by the detection means. ,
An image processing apparatus comprising means for combining and outputting signals output from the second generating circuit in areas other than the edge portion.