JPH0722327B2 - Color image processor - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention inputs a general image including halftone density, and outputs halftone to an output device such as a thermal transfer printer or a liquid crystal display, which is difficult to perform halftone display. The present invention relates to a color image processing device that processes an image including a.

[Problems of Prior Art] Binary density display is most stable in output devices such as thermal transfer printers and liquid crystal displays, and when displaying images including halftone density in these devices, conventionally, density display by dither method is used. Was used.

However, when the halftone is reproduced by using the dither method, the display resolution is generally lowered, and further, irregular noise is noticeable.

On the other hand, a block-based image area separation method has been proposed and improved (IE 81-57, IEICE Image Engineering Research Group), but this method requires complicated block-based determination processing. When there is a change point of image density between blocks, there is a drawback that noise is likely to occur.

[Object of the Invention] It is an object of the present invention to provide a halftone image display device which eliminates the drawbacks found in the conventional method and which is capable of expressing an image with high resolution and halftone.

[Outline of the Invention] For human eyes, even if a general image such as a photograph is decomposed into a line component and a halftone component and displayed in a display like an illustration, it does not feel so unnatural. Therefore, the color image signal input is subjected to color conversion in accordance with the ink density characteristics of an output device such as a printer, then the local variation rate is obtained for each color signal, and the size of this value determines the contours of characters and objects. Detects the line component, performs high-frequency correction of the color signal based on the local fluctuation rate, and outputs the high-resolution image signal without lowering the display resolution when the local fluctuation rate is larger than a predetermined value.
When the local fluctuation rate is smaller than a predetermined value, a signal expressing an intermediate gradation component is output.

That is, a portion having a large local variation rate can be binarized by a predetermined fixed threshold value, and a portion having a small local variation rate can express the halftone density of the image by dithering.

[Effects of the Invention] When a color image is displayed with good reproducibility up to the halftone, a blurred image with low resolution is generally obtained. However, in the present invention, a line component having a large local variation rate is detected and The line component is expressed without blurring by performing simple binarization of the signal. Therefore, even in an image in which characters and photographic images are mixed, halftone densities can be expressed without impairing the resolution of the outline portion of the characters, and a sharp image can be obtained.

Further, the local variation rate of the color signal obtained by separating the input color image signal and the local variation rate of the color signal converted according to the density characteristics of the ink used in the output device such as a printer do not always match. By switching the output signal processing depending on the local variation rate of the color signal converted according to the ink density characteristics of the printer, it is possible to obtain a sharp image in the output image from the printer.

Further, since the determination of switching between the simple binarization and the systematic dither method is performed depending on the magnitude of the local variation rate of the image density, the determination process becomes easy. The local variation rate can be easily obtained by the differential processing.

Next, since there is no concept of a block as seen in the block-by-block image area separation method, there is an effect that there is no problem such as generation of noise between blocks.

Embodiment of the Invention (Basic Principle) The basic principle of the present invention will be described below with reference to the drawings.

In FIG. 1, an image of an original 1 is illuminated by a linear light source 2 such as a fluorescent lamp, and an image is formed on a one-dimensional sensor 4 such as a CCD sensor by a self-locking lens array 3. The image signal output by the one-dimensional sensor is converted into a digital signal by the A / D converter 5.

Next, the density characteristics of the display printer 22 are adjusted through density conversion ROMs 6, 7, and 8 for matching the density characteristics of the ink. At this time, depending on the characteristics of the original document 1, that is, a dark original document, a thin original document, or an original document having no contrast, the output of the density conversion ROMs 6, 7 and 8 is selected by the switch 9 to select an appropriate one. Specifically, as shown in FIG. 2, (a) is for a normal document, (b) is for a thin document, and (c) is.
Is a characteristic of the density conversion ROM for dark originals.

Next, based on this signal, the high frequency component emphasizing circuit 10 obtains a local variation rate. Specifically, two-dimensional differentiation is performed by a convolution calculation circuit as described later. In the first half of this circuit, of the signals input as one-dimensional information,
Line memory unit 11 for preparing some 2D data
And a product-sum operation unit 12 that performs two-dimensional differentiation from this partial two-dimensional data.

Here, in order to facilitate the explanation, consider a one-dimensional signal. It is assumed that a signal as shown in FIG. 3A is input as an input image. Then, an electric signal obtained by the optical transfer characteristic of the self-locking lens array 3 or the like becomes a signal as shown in FIG. 3 (b). When this signal is input to the differentiating circuit 10, it becomes a differentiated signal as shown in FIG. This signal is input to the simple binarization or dither method switching code conversion unit 13 (made up of ROM). In the switching code conversion unit 13, for the input signal having a level higher than the broken line 31 or a level lower than the broken line 32 among the signals shown in FIG. Generate a first code signal that selects When there is an input signal having a level between the broken line 31 and the broken line 32, a second code signal for selecting a signal by the dither method described later is generated.

Here, the contents of the ROM constituting the code conversion unit 13 are determined as shown in FIG. In accordance with this content, when the switching multiplexer 14 described later is 0, it is simply binarized and 1
When, the signal by the dither method is selected.

On the other hand, a signal on the same line as the output signal from the product-sum calculation unit 12 is obtained from the line memory unit 11. This signal processes the output of the line sensor 4 as shown in FIG. 3 (b) by simple binarization or the dither method.

However, the signal in which the high-frequency component is reduced generated in the optical system such as the self-locking lens array 3 is the output signal of the product-sum calculation unit 12,
That is, a clearer image can be obtained by correcting the signal by adding only the high frequency component. However, if all are added, overcorrection results and the image quality deteriorates.
(0 <K <1) is applied to the signal by the multiplier 15 to reduce it, and the result is input to the adder 16. If the multiplier 15 is simplified and the correction coefficient K is limited to 1 to a power of 2, the multiplier 15 is omitted and only the upper bits are summed.
The purpose can be achieved by entering in 17.

The signal in which the high frequency component is detected is delayed from the signal of the line memory unit 11 due to the influence of the product-sum calculation unit 12 and the like. Therefore, the signal of the line memory unit 11 is delayed by the delay circuit 17 and combined with the correction signal, and then input to the adder 16. Since the adder 16 adds the correction signals, a signal having a higher high frequency characteristic than that shown in FIG. 3B is output.

This signal is then input to the two comparators 18, 19. The signal input to the comparator 18 is compared with a predetermined value 20 and converted into a binarized signal by simple binarization. The purpose can be achieved by omitting the comparator 18 and using the upper 1 bit of the output signal of the adder 16 as the binarized output signal.

On the other hand, the signal input to the comparator 19 is sequentially compared with the data of the reference matrix memory 21, which is called a dither pattern, and is output from the comparator 19 as a dithered binarized signal expressing halftone density.

These two types of signals are input to the multiplexer 14, and one of the signals is selected based on the magnitude of the local variation rate of the image signal. That is, the input signal is switched by the output signal from the switching code conversion unit 13 described above, and the signal binarized or the signal processed by the dither method is selected.

The output signal of the multiplexer 14 is displayed on the image output device 22 such as a thermal transfer printer. For example, when an image as shown in FIG. 3 (a) is input, this image input device 22
The output image of is the image as shown in FIG. However, in this display, the binary densities are averaged and expressed as the density state when viewed.

In order to understand the effect of this embodiment, the image is output by an image processing apparatus that performs only simple binarization processing without performing such determination, or an image processing apparatus that only performs halftone density expression processing by the dither method. Here is an example. Fifth
FIG. 6A shows a line sensor output signal. This is simple 2
When it is displayed by the digitization process, the output result is as shown in FIG. With this, it is not possible to represent a signal that is equal to or less than a predetermined threshold value. Further, FIG. 5C is displayed when only the halftone density expression processing by the dither method is displayed. This makes it possible to reproduce halftone densities, but the resolution of the edges of characters, line drawings, etc. is impaired.

On the other hand, according to the present invention, as shown in FIG. 3 (d), an output signal with a high resolution is generated by binarizing the high frequency region with a large local variation rate, and a low local variation rate is generated. Since the area part is displayed by dither, halftone density can be expressed.

Next, the high frequency emphasis circuit 10 will be described. In a line sensor 4 such as a CCD, pixels of individual detection elements generally have variations in sensitivity. Therefore, if excessive high-frequency enhancement of an image signal is performed in the line direction, the variation causes deterioration of the image signal. However, when the high-frequency emphasis of the image signal is performed in the document feeding direction or the moving direction of the sensor 4, the variation in the pixel sensitivity is canceled out, so that the output signal does not vary much. Therefore, relatively large high-frequency emphasis is possible in this direction.

Therefore, it is practically preferable to change the degree of high-frequency emphasis in the line direction and the direction perpendicular to the line direction. The matrix in FIG. 6 shows a group of parameters for high-frequency emphasis, and it is possible to control the degree of high-frequency emphasis in directions orthogonal to each other. The parameters a1 to c5 in this figure are sequentially producted on the two-dimensional image data, and the sum of the results is called a convolution operation. High-frequency emphasis can be performed by selecting the parameters.

In order to execute this product, for example, considering the matrix shown in FIG. 6, data for 5 lines are required at the same time. The line memory unit 11 prepares this data.
Is. This configuration is shown in FIG. 7, and the line memory 70 has 6
Lines are prepared, and while one line is writing, the other five phosphorus are reading.

That is, one write line is selected by the switching multiplexer 71, and the data is written in the line memory. Next, the line multiplexer 70, which is not writing by the decoding multiplexer 72, is switched so that the order of the data is not disturbed in the paper feeding direction, and output out1 to output
Output out5. Next, every time one data is input to the input I0, 5 outputs are read out to the respective outputs out1 to out5 to prepare 5 × 5 data. This data is input to the product-sum calculation unit 12.

FIG. 8 shows the structure of the product-sum calculation unit 12. As shown in FIG. 6, the calculation matrix has the same parameters in the first and fifth rows and the second and fourth rows. Therefore, the outputs out1 to out5 of the line memory 70 are set to SI1 to S of FIG.
Input to I5, first calculate the sum of SI1 and SI5 with adder 80,
The adder 81 calculates the sum of SI2 and SI4. Next, these results and SI3 are input to the product-sum operation circuits 82, 83, 84, respectively. This product-sum operation circuit has parameters 85, 86,
Take 87 and perform product and sum. That is, it is executed five times each time one data is input to I0.

Using this result, all the sums are obtained using the adder 88 and the adder 89, respectively, and the convolution operation is executed.
Then, the high-frequency emphasized signal is output to the output SO.
In this way, high-frequency emphasis is executed at high speed.

(Example) Next, the case of a color image processing apparatus will be described. In FIG. 9, an analog electric signal (for example, white, separated into a plurality of color signals from the color original input device 90)
The three colors (yellow and cyan) are converted into digital signals by the A / D converter 91. Meanwhile, color printer
It is supplied to a circuit 93 for correcting the density characteristics of the ink used in 92 and the input signal characteristics of the color original input device 90. Next, a matrix circuit 94 for color conversion is passed through a signal obtained by reading the original so as to match the spectral characteristics of the ink. This circuit
Reference numeral 94 is composed of, for example, a product-sum operation circuit as shown in FIG. 10, and performs the following operations.

M = a11w + a12y '+ a13c' Y = a21w + a22y '+ a23c' C = a31w + a32y '+ a33c' where w, y ', and c'correspond to the color signals separated from the input device 90, respectively to white, yellow, and cyan. Correspond. M, Y, and C are signals converted by the color conversion matrix circuit 94 into signals that match the ink density characteristics of the color printer 92.
Corresponds to yellow and cyan. a11 to a33 are matrix coefficients for conversion.

The operation of this circuit will be described with reference to FIG. Gate circuit
101 produces a w signal at terminal 102. Next, the product-sum calculation circuit
The 103 extracts the coefficient of a11 from the parameter memory 104 for this signal, calculates the product, and stores the product in the internal accumulator. Then, the gate circuit 101 generates the y'signal at the terminal 102, and the same operation as described above is carried out.
Multiply by the coefficient a12 and add by the internal accumulator. Further, c'is selected by the gate circuit 101, the coefficient a13 is multiplied in the same manner as the above-mentioned operation, and the result is added by the internal accumulator. If this result is output, M can be obtained. Y
And C are calculated in the same manner. In this way, it is possible to convert into a signal that matches the ink density characteristics of the color printer 92.

For each of the converted signals M, Y, and C, a circuit similar to the circuit described in the above-mentioned basic principle is used, and first, in the high frequency component emphasizing circuit 10, a color signal matched to the ink density characteristic of the printer is generated. Calculate the local fluctuation rate and adder 16
The high-frequency correction of the color signal is performed by the comparator 18, the simple binarization processing is performed by the comparator 18, the signal processing capable of expressing the halftone density by the dither method is performed by the comparator 19, and the local variation previously obtained by the multiplexer 14 is performed. Color printers that selectively output image signals with different processing depending on the size of the
Enter in 92.

A signal processing circuit adapted to the ink density characteristics of the color printer 92 is a circuit 95 surrounded by a broken line in FIG. 9, which is a combination of the circuits used in the explanation of the basic principle described above.
It has the same configuration as 21. This circuit 95 is M, Y, C respectively
It is provided independently for. According to this circuit, the parameters can be independently selected for each color that has been color-separated, so that delicate color adjustment is possible.

For example, by arranging the contents of the reference memory for dithering so that the position of the starting point is different for each color,
It is possible to reduce the color turbidity. It is also possible to make the degree of high-frequency emphasis different depending on the type of color, which makes it possible to clearly display a specific color.

By changing the parameters for dithering, it is possible to change the density gradient for each color, and set the parameters according to the density characteristics of the ink used in the printer to reproduce more natural colors. Can be realized.

If the contrast of the original is poor or there is noise on the white background, use a circuit that works like the density conversion ROMs 6, 7, and 8 shown in FIG. 1 used in the explanation of the basic principle above. 96 should be used.

It is also possible to reduce the number of representation bits by making the circuit 93 for correcting the ink density characteristic of the color printer and the input signal characteristic of the color input device a circuit having a logarithmic conversion property. Specifically, a conversion ROM having the characteristics shown in FIG. 11 is prepared, and the logarithmic conversion is performed by addressing the ROM data. However, for example, from 6 bits to 4
In the case of converting into bits, it is possible to obtain better characteristics by using a dead region as shown by the following equation, than by simply performing logarithmic conversion. That is, when x> a, y = 15.lnx / (ln64-lna) -15.lna / (ln64-lna) ...
(2) When x <a or x = a y = 0 However, x is an input signal and y is an output signal. Further, a is the upper limit value of the dead zone, and about 2 to 3 is appropriate.

The reason for reducing the number of expression bits here is as follows. First, it is necessary to match the input color signal with the gradation of the color printer 92 capable of output display. Furthermore, the calculation with the color conversion matrix 94 found in the above-described embodiments may not provide good color reproduction depending on the ink characteristics.

In such a case, in particular, all the color combinations are calculated in advance to create a table, and the color is calculated by drawing this table. By doing so, even extremely complicated calculations can be processed in real time. It is sufficient that the table created here corresponds to the combination of colors that can be expressed by the color printer.
For these reasons, it is possible to reduce the number of expression bits. Further, when performing color conversion, it is only necessary to subtract the corresponding data from the table, and therefore the circuit can be further simplified.

The color conversion matrix circuit 94 can also be constructed by drawing a color conversion table. The color conversion table is composed of RAM, and the contents of this table can be input from ROM to enable free conversion. For example, when the contrast of the original is low, or the background of the original is too dark, the contents of this table can be exchanged to provide an intelligent printer device or copy device.

Even if the local fluctuation rate is sufficiently large, inversion does not occur from white to black or from black to white in the above-described embodiment unless the threshold value for simple binarization is exceeded. However, to the human eye, the above-mentioned color inversion may be recognized as a sharp image. In such a case, the decision circuit for binarization by the simple binarization / dithering of the previous embodiment may be modified to use the circuit shown in FIG.

In this circuit, when the local variation rate is sufficiently large, the value of the local variation rate is used to determine black, and when the local variation rate is sufficiently small, the local variation rate is used to determine white. Further, it is determined that the simple binarization is selected when the local variation rate is large, and the dither method is selected for the smaller portion. By doing so, if there is a large local fluctuation, the inversion of the density always occurs and a sharp image can be obtained.

[Brief description of drawings]

FIG. 1 shows an embodiment of the present invention, and FIG. 2 shows density conversion.
FIG. 3 is a diagram for explaining the contents of the ROM, FIG. 3 is a diagram for explaining the signal processing of the present invention, FIG. 4 is a diagram for showing an example of the contents of the switching code conversion ROM, and FIG. 5 is a conventional signal processing. FIG. 6, FIG. 6 shows a matrix of convolution operation, FIG. 7 shows a line memory section, FIG. 8 shows a product-sum operation section, and FIG. 9 shows another embodiment of the present invention. , FIG. 10 is a diagram showing a circuit for performing a color conversion matrix operation, FIG. 11 is a diagram for explaining bit reduction conversion, and FIG. 12 is a diagram showing a modification of the simple binary / dither switching circuit. . 10 …… High frequency component emphasis circuit (convolution circuit) 13 …… Judgment ROM 14 …… Binary / dither switching multiplexer 15 …… Multiplier 16 …… Summer 17 …… Delay circuit 18, 19 …… Comparator 20 …… Predetermined value storage circuit 21 …… Reference matrix memory 22 …… Printer

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Kiyoshi Yamada 1 Komukai Toshiba Town, Komu-ku, Kawasaki City, Kanagawa Prefecture, Shibaura Electric Co., Ltd. (72) Inventor Shuzo Miura Komukai, Kawasaki City, Kanagawa Prefecture Toshiba Town 1 Tokyo Shibaura Electric Co., Ltd. (56) Reference JP-A-57-185446 (JP, A) JP-A-57-54985 (JP, A) JP-A-57-78275 (JP, A) JP-A-56-68872 (JP, A)

Claims (2)

[Claims] 1. A color image signal input section, a color conversion section for converting a plurality of color signals from the input section according to recording characteristics of an output device, and a local for each color signal converted by the color conversion section. A high-frequency component correction unit that obtains a variation rate and corrects the high-frequency component of the converted color signal based on the local variation rate, and the high-frequency-corrected color when the local variation rate is larger than a predetermined value. An output unit which converts the signal into a binarized signal and outputs the converted signal, and when the local variation rate is smaller than a predetermined value, converts the high-frequency-corrected color signal into a signal representing a halftone density and outputs the signal. A color image processing apparatus comprising: 2. The color image processing apparatus according to claim 1, wherein the recording characteristic of the output device is determined according to the density characteristic of ink used in the output device.