JP2872680B2 - Black character detection method - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a color image signal processing apparatus, and more particularly to a color image signal detecting a character portion from a color document and detecting a black character in the character portion with high accuracy. The present invention relates to a signal processing device.

2. Description of the Related Art In a color copying machine or a color facsimile, a color image read from a color original is subjected to various types of image processing for improving color image quality, and then the color image is transferred onto transfer paper with toner or ink. In such a color copying machine or color facsimile, it is desired to improve the reproduction quality of black characters as one method for improving color image quality.

[Conventional technology]

In a conventional color copying machine or the like, red (R) and green (G) obtained by scanning a color original with a scanner and performing photoelectric conversion.
And the three primary color signals of the additive color method of blue (B) are converted into three primary color signals of the subtractive color method of cyan (C), magenta (M) and yellow (Y), which are complementary colors thereof. At this time, since the C, M, and Y color materials (ink and toner) do not have ideal spectral reflection characteristics, faithful color reproduction cannot be performed even if the color materials are simply used as the complementary colors of R, G, and B as they are. After the color correction is performed on the three primary color signals of the subtractive color method, a black (BK) signal is generated based on these signals.

The three primary color signals (ink control data) Y, M, C and the black signal (ink control data) of the color-reduced color reduction method described above are subjected to a known image quality improvement process for halftone reproduction and image sharpening. After the application, the Y, M, and C single-color images are generated on transfer paper using the Y, M, and C color materials, and are superimposed to obtain a full-color image.

Here, the printing of black characters can be performed by superposing the same amount of color materials of the three primary colors of Y, M, and C, but in this case, the density of the high-density portion is mainly affected by the reflection of the color material surface. Shortage is noticeable. For this reason, underwater removal (UCR; Under Color Removal) has conventionally been used to compensate for this lack of density.
And inking.

The above UCR is, for example, subtracted from each of the three primary color ink control data y, m, c representing the character color by, for example, the minimum value and an equal amount (that is, the gray component is removed), and the black component of the same amount as the removed gray component. This is a color process for obtaining ink control data. Further, inking is performed using the black ink control data in addition to the ink control data of the remaining primary colors from which the gray component has been removed. The UCR can save the amount of color ink or the amount of color toner.

[Problems to be solved by the invention]

However, conventionally, black characters have not been detected. In the case of black characters, ink control data of three primary colors representing the character colors has been used.
Although printing was performed with y, m, and c indicating the same amount, these ink control data were not the same amount even in the case of black characters. Black characters were printed (reproduced) with some Y, M, and C inks or toners added to the image. For this reason, there has conventionally been a disadvantage that the reproduction quality of black characters is not good.

Accordingly, it is a general object of the present invention to provide a new and useful black character detection method which eliminates the above-mentioned disadvantages.

Another object of the present invention is to detect a black character by comparing a threshold value corresponding to a minimum level and a maximum value of a difference between the density signals among density signals of three primary colors read from a color original. And a method for detecting a black character.

[Means for solving the problem]

FIG. 1 is a block diagram showing the principle of the present invention. In the figure, 11
Is a maximum difference value detecting means. Of the three primary color signals of the additive color method or the subtractive color method read from the color original, two of different combinations are used.
The difference between the subtractive color signals is calculated, and the maximum value of the difference is obtained for each pixel.

Reference numeral 12 denotes a maximum level detecting means for detecting a minimum level for each pixel among the three primary color signals.

Reference numeral 13 denotes a determination unit that outputs a threshold value adaptively set in advance in accordance with the detected minimum level, and compares the threshold value with the maximum difference value to determine whether the pixel is a black pixel. Do.

[Action]

The level of the three primary color signals of the additive color method obtained by reading a color original with a scanner or the three primary color signals of the subtractive color method obtained by color correcting the three primary color signals of the additive color method represents the density of each pixel. I have.

The maximum difference value obtained by supplying the three primary color signals to the maximum difference value detection means 11 (this is represented by D)
The relationship between the minimum level and the minimum level obtained by supplying the minimum level detection means 12 (this is represented by S) is as shown in FIG. 2 for black characters. Confirmed by experiment.

That is, in the case of a black character, since the levels of the three primary color signals are the same in principle, the value of the maximum difference value D is small. However, due to the deterioration of the modulation transfer function (MTF) in the thin line part,
As shown in FIG. 2, the balance of the three primary color signals is lost, and the balance becomes maximum when the minimum level S is near the intermediate level, and has little effect when the minimum level S is small and large.

On the other hand, when the present inventor measured the characteristics of the above-described maximum difference value D and minimum level S also for color characters other than black characters, the maximum difference value D versus minimum level S characteristics of black characters (shown in FIG. 2) The blue character indicating the characteristic indicated by the oblique line I in FIG. 3 showed the characteristic closest to the above (note that the characteristic of the yellow character is as indicated by the broken line II in FIG. 3).

Therefore, as can be seen by comparing FIGS. 2 and 3, if the threshold value to be compared with the maximum difference value D in the determination means 13 is constant regardless of the value of the minimum level S,
When the threshold is set to a small value, detection omissions are increased in black characters, and when the threshold is set to a large value, blue characters are often determined to be black characters.

Therefore, in the present invention, the threshold value generated by the determination means 13 (this is represented by TH) is adaptively set according to the minimum level as shown in FIG. 4 in view of the characteristics of FIG. (In other words, the threshold value TH is a function of the minimum level S). Then, the determination means 13 compares the threshold value TH with the maximum difference value D, and determines that the pixel is a black pixel when the maximum difference value D is smaller than the threshold value TH.

In the present invention, the determining means 13 has a counting means for counting the number of pixels determined as the black pixels in the local region and determining the black character when the counted value is equal to or more than a certain value. You may. In this case, black characters can be detected more accurately.

〔Example〕

Next, an embodiment of the present invention will be described. FIG. 5 is a block diagram of an embodiment of the present invention, and FIG. 6 is a block diagram of an embodiment of a main part in FIG. In FIG. 5, it is obtained by reading a color original with the scanner 15.
R, G, and B primary color signals (this is a digital signal obtained by sampling and quantizing an analog primary color signal)
Is supplied to the color correction unit 16 and a character color determination unit
Supplied to 17.

The color correction unit 16 converts the input three primary color signals B, G, and R into the three primary color signals Y, M, and C of the subtractive color method, which are complementary colors thereof, and performs color correction for performing color reproduction faithful to the original image. The three primary color signals Y, M, and C obtained as described above are input to a black signal generator 18 and a character color detector 19, respectively.

Black signal generator 18 the three primary color signals Y, and outputs to generate a black signal B _K based on the M and C, above three primary color signals Y,
Output M and C.

On the other hand, the character portion detection section 19 separately detects whether or not each of the three primary color signals Y, M, and C is a character portion, and when the Y signal is detected as a character portion, sets the output signal Ychar to "1", and similarly. When the M signal and the C signal are detected as character portions, the output signals Mchar and Cchar are set to "1". This character part detector 1
As a detection method according to 9, for example, an edge degree is obtained for each pixel, and when a pixel having a large edge degree is equal to or more than a set number in a local region including a plurality of pixels, the pixel is determined to be a character part There is.

Further, the three primary color signals Y,
The M and C and the black signal _BK are supplied to a gradation processing unit 20 and a resolution processing unit 21, respectively. Gradation processing unit 20 is a known circuit for dither processing smoothes the input signal, thereby decorated with the four signal Y gradation processing, M, and outputs the C and B _K.

Further, the resolution processing unit 21 in a known circuit for dither processing after edge enhancement with respect to the input signal, and outputs four signals Y having undergone resolution enhancement processing, M, C and B _K.

Husband From these gradation processing unit 20 and the resolution processor 21 people extracted signal Y, M, C and B _K are supplied to the selection circuit 22, where the signal from the character-detecting section 19 Ychar, Mchar and Cchar
And a character color determination unit 17 and a counter 23, which are essential parts of the present embodiment.
The black character detection signal B _K ch generated by the circuit consisting of
It is selectively output based on ar.

Here, the character color determination unit 17 is a circuit that determines whether the character part is a temporary black pixel and outputs a determination signal B _K char ′.
When it is determined that the pixel is a provisional black pixel, the logic of B _K char ′ is set to “1”, and
Otherwise, it is set to “0”. The counter 23 counts the number of the temporary black pixel determination signal B _K char ′ having the logic “1” among a plurality of pixels constituting the local area having a predetermined size. Determines that the pixel is a black character, outputs a black character detection signal B _K char of logic “1”,
If it is determined that the character is not a black character, a black character detection signal B _K char of logic “0” is output.

The present embodiment is characterized in that black characters are detected using the character color determination unit 17 and the counter 23. Next, the configuration and operation of the character color determination unit 17 will be described in further detail. FIG. 6 shows a block diagram of an embodiment of the character color judgment section 17. In the figure, the same components as those of FIG. 1 are denoted by the same reference numerals, and description thereof will be omitted. In FIG.
Of the primary color signals R, G, and B, two different primary color signals of different combinations are supplied to arithmetic units 26, 27, and 28, respectively, where they are subtracted and the absolute value is obtained.

The comparator 29 selects and outputs the larger value of the output signal | R−G | of the calculator 26 and the output signal | G−B |
30 where it is compared with the signal | BR−R | The comparator 30 selects and outputs the higher-level signal of these input signals. Therefore, the arithmetic unit 26
28, comparators 29 and 30, the comparator 30 outputs the maximum level signal of | R−G |, | G−B | and | B−R | Will be output.

On the other hand, among the input three primary color signals R, G and B, R and G
Is supplied to the comparator 31, where the primary color signal having the lower level is selectively output and supplied to the comparator 32. Comparator 32
Compares the magnitude of the output signal of the comparator 31 with the magnitude of the input primary color signal B, and selectively outputs the lower-level signal. Therefore, the comparators 31 and 32 constitute the above-described minimum level detecting means 12, and the signal S indicating the minimum level among the input three primary color signals R, G and B is extracted.

This signal S is input to a read only memory (ROM) 33 as an address signal. Since the threshold value TH shown in FIG. 4 is previously stored in the ROM 33 at an address corresponding to the level of the signal S, the data corresponding to the input address signal S is supplied from the ROM 33 to the threshold value TH (S ) And input to the comparator 34.

The comparator 34 calculates the maximum difference value D and the threshold value T from the ROM 33.
H (S) and the maximum difference value D is equal to the threshold value TH (S)
More logical "1" is determined as a black pixel when the small becomes, when made other large outputs a determination signal B _K char 'of logic "0" is determined not to be a black pixel. Therefore, the ROM 33 and the comparator 34 constitute the above-described black pixel determination means 13.

Such black pixel determination signal obtained by the B _K char 'is supplied to the black character determination signal B _K char and has been selected circuit 22 as described above is counted by the counter 23 shown in FIG. 5.

The selection circuit 22 shown in FIG. 5 detects the detection signals Ychar, Mchar, Ccha
When r is logic “1”, the corresponding primary color signals Y, M, and C extracted from the resolution processing section 21 are selectively output, and Ychar, Mchar, Cch
When ar is logic “0”, the corresponding primary color signals Y, M, C extracted from the gradation processing section 20 are selectively output.

However, this selection operation is based on the detection signals Ychar, Mchar, Cchar
And when all of the black character determination signals B _K char are not logic “1” at the same time. When all of these four signals are simultaneously logic “1”, the primary color signals Y, M and C are not output. outputs, for example, only a black signal B _K from resolution processing unit 21. That is, according to the present embodiment, when a black character is detected, only the black signal _BK is output from the selection circuit 20.

As described above, when the selection circuit 22 does not detect a black character, the selection circuit 22 selectively outputs one of the three primary color signals of the gradation processing unit 20 and the resolution processing unit 21 depending on whether or not it is a character portion. As a result, pixels with a character portion are subjected to high resolution processing, and pixels without a character portion are subjected to gradation processing, so that contradictory color reproduction and gradation can be achieved without lowering the resolution of the character portion. Have maintained.

Primary color signals Y taken out from the selection circuit 22, M, C or black signal B _K is output to the printer to control the color material of the printer as an ink control data. Although this printer is not directly related to the gist of the present invention, for example, there is a laser printer having a configuration as shown in FIGS. 7 and 8. During FIGS. 7 and 8, the same components are denoted by the same reference numerals, also in response to the input signal Y, M, are given C, and subscript B _K.

The ink control data of the above-described Y, M, C and B _K, the laser light source 37Y shown in FIG. 8, 37M, is input to 37C and 37B _K,
Modulate the laser beam. The modulated laser beam emitted from the laser light source 37B _K is incident on the rotating polygon mirror 38B _K shown in FIGS. 7 and 8, respectively, where it is deflected and formed into an imaging lens 39B _K (only shown in FIG. 7). ) Through the photosensitive drum
It is irradiated onto 40B _K.

Since the rotary polygon mirror 38B _K is rotated in a certain direction by the motor 41B _K, where deflected laser beam is linearly scanned repeatedly along the upper surface of the photosensitive drum 40B _K in the drum rotation axis direction (the scanning Is called main scan). Further, by the photosensitive drum 40B _K is rotated in the arrow direction, said laser beam is the main is scanned in a direction orthogonal to the direction of scanning (the scanning is called the sub-scanning).

The rotation of the photosensitive drum 40B _K, its surface is uniformly charged by the charger 42B _K shown in FIG. 7, the laser beam is performed in the main scanning and sub-scanning mentioned above writing black image is performed, the black image Is formed. The electrostatic latent image is visualized and developed by the developing device 43B _K.

On the other hand, the tray 44, the photosensitive drum 43B _K, 4 in a state where the transfer sheet distribution feeder 45 and the roller 46 respectively through and is clamped to the roller 46
It is fed onto the conveyor belt 47 in synchronization with the rotation of 3Y, 43M, 43C. The transfer paper on the transfer belt 47, after being electrostatically transferred by the action of a visible image is transferred discharger 48B _K on the surface of the photosensitive drum 40B _K on the surface thereof, is conveyed to the next photosensitive drum 40Y direction .

Hereinafter, in the same manner as above, a yellow image is formed by the photosensitive drum 40Y and the transfer discharger 48Y, and the photosensitive drum 40M and the transfer discharger 48M.
Magenta image, the photosensitive drum 40C and the transfer discharger 48C
Then, the cyan image is successively superimposed on the transfer paper surface and electrostatically transferred, and then fixed by the fixing unit 49, and discharged onto the tray 51 via the discharge roller 50.

By applying the present embodiment to a laser printer having such a structure, a character portion determined to be a black character is electrostatically transferred on a transfer paper using only black toner, and electrostatic transfer of the other three primary color images is not performed. Therefore, the reproduction quality of black characters can be improved as compared with the related art.

Note that the present invention is not limited to the above embodiment. For example, the detection of the maximum difference value and the detection of the minimum level
Of course, it may be performed based on the three primary color signals of the subtractive color method.

〔The invention's effect〕

As described above, according to the present invention, the threshold value adaptively set according to the minimum level of the three primary color signals is compared with the maximum difference value of the three primary color signals, and the maximum difference value is smaller than the threshold value. Is determined as a black pixel, the black pixel can be detected with high accuracy. Further, when the number of pixels determined as the black pixel is equal to or more than a predetermined value in the local area, the pixel is determined as a black character. This makes it possible to more accurately detect a black character, and when a black character or a black pixel is detected, printing can be performed on transfer paper using only black ink or black toner. It has features such as the ability to improve the reproduction quality of black characters.

[Brief description of the drawings]

FIG. 1 is a block diagram of the principle of the present invention, FIG. 2 is a maximum difference value versus minimum level characteristic diagram of black characters, FIG. 3 is a maximum difference value minimum level characteristic diagram of blue characters and yellow characters, and FIG. , FIG. 5 is a block diagram of an embodiment of the present invention, FIG. 6 is a block diagram of an embodiment of a main part of the present invention, and FIGS. 7 and 8 illustrate the present invention, respectively. FIG. 2 is a diagram illustrating a structure of a laser printer as an example of an applicable printer. In the figure, 11 is a maximum difference value detecting means, 12 is a minimum level detecting means, 13 is a judging means, 15 is a scanner, 17 is a character color judging section, 23 is a counter, 26 to 28 are arithmetic units, 29 to 32, 34 Indicates a comparator, and 33 indicates a read only memory (ROM).

──────────────────────────────────────────────────続 き Continued on the front page (58) Field surveyed (Int.Cl. ⁶ , DB name) H04N 1/40-1/409 H04N 1/46 H04N 1/60

Claims (2)

(57) [Claims] 1. A method for comparing a maximum value of a difference between color image signals of three primary colors to a threshold adaptively set in advance in accordance with a minimum value of the color image signal to determine whether a pixel of interest is a black pixel. Black pixel determining means for determining whether the pixel of interest is a pixel constituting a character portion, and a character determining means for determining whether the pixel of interest is a pixel constituting a character portion. A color image signal processing device comprising: a black character determination unit that determines whether a pixel is a pixel forming a black character. 2. An image input means for obtaining a color image signal by scanning an input image, a maximum difference detection means for detecting a maximum difference value of the color image signal, and detecting a minimum signal level of the color image signal. A signal level detection unit, a threshold setting unit that sets a threshold value for determining a black pixel in the input image based on a detection result of the signal level detection unit, and a threshold value and the maximum difference value set in the threshold value setting unit. A color image signal processing device comprising: a determination unit that determines a black pixel by comparison.