JPH01200964A - Apparatus for forming color image - Google Patents

Abstract

PURPOSE:To eliminate the turbidity or gradation of the color of a black character, by discriminating whether the pixel concerned is an almost black one on the basis of the achromatic signal detected from a color separation signal and the brightness signal extracted thereform and performing printing only by black when said pixel is the almost black one. CONSTITUTION:A reader 102 separates a color manuscript image into the three primary colors R, G, B to output a digital image (brightness) signal. Next, said signal is converted to a density signal by a logarithmic converter circuit 103 to be passed through a min. value extraction circuit 104 as it is and the min. value signal therein is detected to be outputted and, on the basis of this min. value signal, a black signal K2 is formed from an inking/UCR circuit 105. Brightness signals R-B are converted to an NTSC standards by a matrix converting circuit 108 and become an achromatic signal W by a look-up table 109 and said signal W is multiplied by a brightness signal (y) by a multiplier 116 to be converted to an achromatic brightness signal V. When a judge circuit 112 judges a black character region, a printer 114 prints only black on the basis of the signal S of said circuit 112 and performs printing by the superposition of the four primary colors at a time other than that.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Application in Industry] The present invention relates to a color image forming apparatus, and in particular, to a color image forming apparatus for dividing black areas of an original image into 11! The present invention relates to a color image forming apparatus for processing.

[Prior art] Japanese Patent Application Laid-Open No. 59-205876 discloses color-separated R, G, B
After converting the signal into four primary color signals of Y, M, and C, only the K signal is used, and the image area is a high resolution area such as a character image, or does not require high resolution such as a photographic image. Based on the determination result, if it is a high resolution area, Y, M.

The present invention discloses an apparatus that outputs all four colors of C and C in a binary format, and in the case of a photographic area, outputs all four colors of Y, M, and C as multivalued.

In addition to this, in the past, image area identification was performed independently for Y, M, and C signals, and based on the identification results, it was determined whether to binarize each signal or output it as a multi-value signal. There is.

[Problems to be Solved by the Invention] If the image area is identified independently for the black, Y, M, and C signals, the identification results may differ for each color, and color unevenness, blurring, etc. are likely to occur in the output image. Become.

Furthermore, if image area identification is performed using only signals, the following problems will occur, for example. That is, the pixel of interest is now represented by 8-bit digital signals of Y, M, and C. For example, Y
=255. M=255°C=150, and = m
i n (Y, M.

Suppose that the color is blackish red with C)=150.

If the pixel of interest is determined to be a pixel in a character area based on the signal status of its neighboring pixels, Y, M,
Since all of the signals are binarized at C, if we now set the binarization threshold to the middle level of 128, the binarization output of the pixel of interest is Y=M=C==1 (255 levels) Then,
It will be replaced with completely dark black.

The present invention eliminates the above-mentioned drawbacks of the prior art, and its purpose is to provide an image forming apparatus that can effectively detect only true black in a human-powered image and reproduce high-quality black. There is a particular thing.

[Means for Solving the Problems] In order to achieve the above object, the color image forming apparatus of the present invention converts color separation signals of red, green, and blue into four primary color signals of yellow, magenta, cyan, and black. A color image forming apparatus that forms a color image by overlay printing, the apparatus comprising: an achromaticity detection means for detecting the degree of achromatic color based on the color separation signal; a luminance signal extracting means for extracting a luminance signal of the pixel; a determining means for determining whether the pixel is a substantially black pixel based on the achromatic color signal and the luminance signal; and when the determining means determines that the pixel is a substantially black pixel; The outline thereof is to include a printing means that prints only in the black, and in other cases prints in a superposition of the four primary colors.

[Function] In such a configuration, the achromaticity detection means may detect, for example, R,
The degree of achromatic color is detected based on the G and B three color separation signals. The luminance signal extraction means extracts a luminance signal corresponding to the brightness of the image from the color separation signals. The determining means determines whether or not the pixel is a substantially black pixel, based on the achromatic color signal and the luminance signal, for example, by calculating the product of the achromatic color signal and the luminance signal and comparing this with a predetermined threshold value. . The printing means prints only the black when the discriminating means determines that the pixel is substantially black, and otherwise prints by superimposing the four primary colors.

[Description of Embodiments] Hereinafter, embodiments of the present invention will be described in detail with reference to the accompanying drawings.

FIG. 1 is a block diagram of a digital color copying apparatus according to an embodiment. In the figure, 101 is a human image, for example, a color original image in which text images, photographic images, etc. are mixed. 102 is a reader which separates and reads the color original image 101 into the three primary colors R, G, and B, converts the read signal from A/D to digital image signals (luminance signals) R, G;
, B (8 bits each). A logarithmic conversion circuit 103 logarithmically converts the luminance signals R, G, and B and outputs the resulting density signals C+, t, and Yt. 104 is a minimum value extraction circuit, which receives input concentration signals C+, Mr,
YI is passed through as is, and the minimum value signal m1n (Yl.

Mr, CI) is detected and output. 105 is inking/
It is a UCR circuit, and the input minimum value signal min (YI
, Mr , Cr ), and the minimum value signal min (YI , Mr
, C1), the density signals C2, M2, Yl are subjected to under color removal (UCR) from the input density signals C,,M,,Y,
Generate and output. 106 is a masking circuit;
Concentration signal c2. according to the characteristics of printer ink (toner, etc.). A known masking matrix calculation is performed to match M, , Y, with ink, etc.

A gradation correction circuit 107 corrects the density signals Cs, Ms, and Ys after masking by 2 in accordance with the gradation of the printer. In this way, 4 is obtained for the output signals C4, M4, Y4, for normal printer printing. Reference numeral 113 denotes a gate circuit, which receives the above-mentioned sending signals C4, M4 . Y4. Input 4 to the black pixel, and input only 5 to the black signal (in case of determination as a black pixel) or 4 according to the black pixel determination signal S described later.
The primary color signals Cs, Ms, Ys, and Ks (in the case of determination that the pixel is other than black) are selected and output. 114 is a laser beam printer, for example, which outputs four primary color signals Cs, Ms, Y
s, to form a color image according to step 5. 115 is an output image.

On the other hand, 108 is a matrix conversion circuit that converts the human-powered luminance signals R3G and B into y, t, and q signals according to the NTSC standard, which are general color television signals. This matrix conversion formula is, for example, as shown in formula (1). , where y is a luminance signal (8 bits) corresponding to the brightness of the image, and i and q are color difference signals (8 bits each). Reference numeral 109 denotes a look-up table, which manually transforms the color difference signals i and q into a table and outputs an achromatic chromaticity signal W representing the degree of achromatic color. The achromatic chromaticity signal W of this embodiment is set to be maximum when the human power is achromatic (i=q=o). That is, the achromatic chromaticity signal W of the embodiment is related to the color difference signals i and q according to equation (2), for example.

w = 255 X exp[-(i/1o24)”
-(Q/1024)2] In the above equation (2), the term (exp) on the right side has the maximum value of 1 when the human power is achromatic (i=q=o), so the achromatic chromaticity signal W is maximum value of 25
5 (8 bits). A multiplier 116 multiplies the luminance signal y and the contents of the achromatic chromaticity signal W to convert it into an achromatic luminance signal ■. Specifically, v=(255-y)XW/255 (3).

In the above equation (3), the brightness signal y on the right side is introduced in the form (255-y) so that it takes a large value in the low brightness portion. Therefore, the term (255-y) is minimum when the input image is bright (white portion) and maximum when the input image is dark (black portion). On the other hand, the term (W/255) is 1 when the color is achromatic, and takes a value of 1 or less when the color is chromatic.

Therefore, by taking the product of both, the achromatic luminance signal (2) has a maximum value of 255 when the input is achromatic and black, and has a small value when the input is achromatic and white or gray. Incidentally, it is not necessarily necessary to take the product of both as in the achromatic luminance signal (2) above. For example, set an appropriate threshold and first set (255-y)
It is clear that it is also possible to determine a dark condition from (W) and a substantially achromatic color condition from (W), and then take the logical product of these.

A line buffer 110 stores four lines of achromatic luminance signals V required for Hadamard transformation, which will be described later.

Black pixels may be determined using the achromatic luminance signal ■ for each pixel individually, but in general, images such as black characters are distributed over a fairly wide area compared to the size of one pixel (for example, 1 apel). Rather than judging each pixel alone, we should consider the nature of the image surrounding the pixel (characters,
It is much more stable to determine and control black pixels if the determination is made by taking into account whether the pixel is a line drawing image area or a photographic image area, etc. Therefore, in this embodiment, a line buffer for 4 lines is prepared in order to perform 4×4 pixel determination. A Hadamard transform circuit 111 includes, for example, a 4×4 pixel Hadamard transform matrix.

FIG. 2 is a diagram illustrating Hadamard transform processing in the embodiment. In the figure, the value of the achromatic luminance signal ■ of each pixel is
J, and the pixel of interest is pixel a22 at an appropriate intermediate position. Then, Hadamard transform yk in this block area
l is expressed by equation (4).

Here, Ck+(i,j) is the order of Hadamard transform (k
, 1) are the coefficients for obtaining the components of 1), and some examples of 1) are as follows.

The Hadamard transform ykl in equation (4) above represents the spatial structure of the black image in the block of interest, and if the order (k,
1) If the value of the small Hadamard transform yk is large, the block of interest is an image with flat luminance, and the order (k, 1
) is large If the value of Hadamard transform yk1 is large, it can be seen that the image contains edge components.

Returning to FIG. 1, reference numeral 112 denotes a determination circuit, which determines whether the block of interest belongs to a character or line drawing area, for example, by the following determination method.

FIG. 3 is a diagram showing the arrangement of the Hadamard transform '/kl of the embodiment. In the figure, yr+ represents the intra-block average value, and as the order k increases, edge components in the horizontal direction are detected. Furthermore, as the order 1 increases, edge components in the vertical direction are detected. Furthermore, as the orders k and 1 increase, edge components (halftone dots, etc.) in the horizontal and vertical directions are detected.

Therefore, in the Kinomoto example, the Hadamard transformation ``jk
Find the next value from r.

Y v ” 'j 12+ y13+ 'l ImYo
"y2r+'/s++: J4+ Ys "'/a4+ys<+'J43 Therefore, from the above, when Yv is large, a vertical edge exists in the block of interest, and when Y is large, a horizontal edge exists, and YS If it is large, a complicated pattern such as halftone dots will exist.

Therefore, a predetermined threshold value "rl, . . . , T3 is set, and when the following condition is satisfied, it is determined that the block of interest is a black character or black line diagram area.

(Y V > T I or Y n > 72) and Y s < T 5 That is, there is a vertical edge or a horizontal edge and there is no complicated pattern such as a halftone dot. For area detection of black flat parts, Y v > T I or Y ) I
> This can be done by detecting T2 from the start to the end of detection, or by directly checking 'j11 using a predetermined threshold. As described above, when an achromatic black character area etc. is determined,
The determination result signal S is input manually to the gate circuit 113, and normally the concentration signal C4°M4. Y4. 4 as is and the density signals Cs, M5, Y5 . On the other hand, when the determination of the black area is negative, the density signal C,=O,M is output.

=O, YS =O and the constituent gate circuits of each color are turned ON10FF so that the density signal becomes 5=4. In this way, the above control shifts the pixel of interest one pixel at a time,
And, it is performed based on the Hadamard transform for the target block of surrounding pixels at each time point.

In addition, although Hadamard transform was adopted in the above-mentioned example, it is not limited to this. For example, edge determination may be performed by detecting the maximum and minimum values of the signal (2) in another orthogonal transform or the block of interest, and determining the magnitude of the difference.

Furthermore, the method of obtaining the achromatic chromaticity signal W and the method of obtaining the luminance signal Y are not limited to those of the above embodiment.

[Effects of the Invention] As described above, according to the present invention, black character areas can be detected with high accuracy, thereby preventing muddiness of colored characters due to misjudgment and color unevenness due to judgment errors for each color. Since the image itself can be output in a single black color, it is possible to remove the color of black characters and remove blur.

[Brief explanation of the drawing]

FIG. 1 is a block diagram of a digital color copying apparatus according to an embodiment, FIG. 2 is a diagram explaining Hadamard transform processing according to an embodiment, and FIG. 3 is a diagram showing an arrangement of Hadamard transform '/kr according to an embodiment. . In the figure, 101...human image, 102...reader, 1
03... Logarithmic conversion circuit, 104... Minimum value extraction circuit, 105... Inking/UCR circuit, 106... Masking circuit, 107... Gradation correction circuit, 108...
Matrix conversion circuit, 109... Lookup table, 110... Line buffer, 111... Hadamard conversion circuit, 112... Judgment circuit, 114... Printer, 115... Output image. Figure 3

Claims (1)

[Scope of Claims] In a color image forming apparatus that converts color separation signals of red, green, and blue into four primary color signals of yellow, magenta, cyan, and black and forms a color image by overlay printing, the color separation signals include: achromaticity detection means for detecting the degree of achromatic color based on the color separation signal; brightness signal extraction means for extracting a luminance signal corresponding to the brightness of the image from the color separation signal; and a luminance signal extraction means for extracting a luminance signal corresponding to the brightness of the image from the color separation signal; a determining means for determining whether the pixel is a substantially black pixel; and printing for printing only the black when the determining means determines that the pixel is a substantially black pixel, and otherwise printing by superimposing the four primary colors. A color image forming apparatus comprising: means.