JPH01143486A - Color signal processing circuit - Google Patents

Abstract

PURPOSE:To eliminate picture quality deterioration by converting an input color information signal into a color information signal having a fewer bit number than the bit number of the former signal and keeping specific color information included in the input color information signal and using the color information signal after conversion so as to convert the input color information signal to a color information signal of other form. CONSTITUTION:R, G, B input signals of 8-bit each from a picture read section 113 are converted into C, M, Y signal of 8-bit each by logarithmic devices 101, 102, 103 of a table reference system. An 8-bit Y signal is given to a table system bit number converter 104 from which a 6-bit Yc signal is outputted. Similarly, a 6-bit signal is outputted from converters 105-109 with respect to the 8-bit input signal. A table reference system masking converter 110 receives an 8bit C signal and Mc,Yc signals of 8-bit each and outputs an 8-bit C' signal. Similarly, masking converters 111, 112 output M', Y, signals of 8-bit each. The C', M', Y' signals converted from the R, G, B signals by the converters as above are given to a picture output section 114.

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention relates to a color signal processing circuit in a color copying machine, a color original reading device, a color printer, etc.

(Prior art) In a human-powered device for color images, R (Red), G
In many cases, light transmitted through primary color filters such as (Green) and B (Blue) is detected by a light receiving device to obtain three independent color signals. Using these color signals, CR
When outputting color image information to an image output device such as a T-display or printer, it is necessary to convert color signals in order to match the color characteristics of the human-powered device and the output device.

For example, FIG. 2 shows an example of an algorithm for color signal conversion when used as a stand-alone type digital color copying machine.

The 8-bit R, G, and B signals from the image reading unit 210 are converted into C (Cyan) and M (Mag), which represent 8-bit density information, by table converters 201, 202, and 203, respectively.
enta).

Logarithmically transform the signal into a Y (Yellow) signal. Next, in order to match the characteristics between the input and output devices (image reading section 2101 and image output section 211), the C, M, and Y signals are inputted to each masking converter 204, 205, and 206, and thereby each 8-bit C ”, M', Y' flaw signal masking conversion.

The masking transform in the masking transformer often uses a first-order matrix transform, and the transform coefficients are usually
The values of off-diagonal terms are much smaller than those of diagonal terms. This is because the quantization interval of the human input signal required to achieve the accuracy required for the output signal is different between the two, and the number of bits of the human input signal for the off-diagonal terms can be smaller than that for the diagonal terms. It shows. An example of using this property to limit the number of input signal bits for masking conversion is JP-A-59-21.
Examples include those described in Japanese Patent No. 0770. FIG. 3 shows an example of this.

In FIG. 3, each masking converter 304, 305°3
06 limits the number of large bits of two of the three input R, G, and B signals. The other configurations are the same as in FIG. 2. The configuration shown in Figure 3 is effective in reducing the memory size in cases where the required memory size increases significantly as the number of bits increases, such as in conversion using a table reference method using ROM, RAM, etc. . Regarding the above example, if the number of input bits is not limited, each masking converter 204, 205, 206 in FIG.
Although a memory size of 4 bytes is required, by manually limiting the number of bits, each masking converter 3 in FIG.
04.305.306, the memory size can be reduced to 1716 in FIG.

FIG. 4 shows another example of limiting the number of input signal bits of the color signal conversion circuit as described above. FIG. 4 shows a block diagram of a color conversion circuit for converting a YIQ human input signal conforming to the NTSC system into an RGB signal and displaying the signal on a color display. 404 is a human power I/F, and 405 is a color display. The relationship between the NTSC-compliant RGB signal and YIQ signal can be expressed by the following matrix transformation.

YIQ external input signals are each 8-bit data, whereas the number of human input signal bits is limited to a total of 20 bits. Table reference type matrix converter 401, 402, 403
Assume that the I and Q signals input manually are limited to 6 bits each. Before limiting the number of input bits, the I and Q signals have a virtual value of 0 for 128 out of 256 gray levels from 0 to 255.
Assign the I and Q values of Oh) to 127 as negative, 129
255 to 255 are assigned to positive IQ values, but for I and Q signals limited to 6 bits, only the upper 6 bits are input to the matrix converter.

(Problem to be solved by the invention) Of the above two conventional examples of limiting the number of input bits, the former (
In Fig. 3), for example, for a C signal of 8-bit data, converter 304 selects 8 bits, converter 305 selects the upper 6 bits, converter 306 selects the upper 6 bits, and so on. It is supposed to be entered. When calculating the upper 6 bits manually, for example, the values 0° 1, 2.3 for 8-bit data all converge to the value 0, but if the probabilities of the values 0, 1, and 2.3 are the same, then the average value becomes 1.5, and the two values are different. This problem can be solved, for example, by using a method in which a human power value of 0 is virtually regarded as a human power value of 1.5 and the contents of the reference table are created. However, this further causes the following problems. That is, in the case of a blank original, C=M=Y=0 when the data is 8 bits, but when using the above method, in the reference table, C≠O, M≠O, Y≠0. Therefore, the output signal to the printer is also C'=
O, M'=O, Y'=0. In other words, even a blank document is output as a non-blank page. This phenomenon tends to cause grainy noise in printers that are not good at expressing highlight areas, which causes a chain of problems such as deterioration of image quality. Such problems cannot be solved by simply limiting the number of bits manually by the number of upper bits.

In addition, in the latter conventional example (Fig. 4), for example, among the 8-bit data I signal, the value 128.129 near the achromatic color,
130 and 131 are all 128 (32 for 6-bit values), and 124, 125, 126, and 127 are all 12.
It converges to a value of 4 (31 for a 6-bit value). Here, as shown in the former conventional example, it is possible to construct the contents of the reference table using the average value as the virtual human power value.

However, with this method, the I signal value at 8 bits, which should originally be achromatic, is 128 and its neighboring value 127.129.
etc. become chromatic values, and as a result, color areas to which human visual characteristics are sensitive, such as the achromatic parts of highlights, tend to become grainy noise related to color, which can lead to deterioration of image quality, etc. cause problems. Again, this problem cannot be solved as long as the number of human-powered bits is simply limited to the number of high-order bits.

SUMMARY OF THE INVENTION An object of the present invention is to solve the above-mentioned problems and provide a color signal processing circuit that does not cause deterioration in image quality.

(Means for Solving the Problems) The present invention provides a method in which the number of bits of an input color information signal is smaller than the number of bits, and specific input color information is color information that retains the specific color information. The apparatus includes a bit number conversion means for converting the input color information signal into a signal, and a means for converting the input color information signal into another form of color information signal using the converted color information signal obtained by the bit number conversion means.

[Effect]

According to the present invention, the number of bits of an input color information signal is smaller than the number of bits, and specific input color information is converted into a color information signal that retains the specific color information,
The input color information signal is converted into another form of color information signal using the converted color information signal.

[Embodiment 1] FIG. 1 shows a block diagram of a color signal conversion circuit according to the present invention applied to a color copying machine. 8 bits each of R and G from the image reading section 113.

The B input signal is a table reference type logarithm unit 101, 102.
103, each signal is converted into 8-bit C, M, and Y signals. The look-up table type bit number converter 104 inputs an 8-bit Y signal and outputs a 6-bit YcC signal. Similarly, converters 105, 106°107.10
8,109 also outputs a 6-bit signal in response to an 8-bit human input signal. In the table lookup type masking converter 110, an 8-bit C signal and 6-bit M
c, Y The C signal is used as a human input signal, and an 8-bit C' signal is output. Similarly, masking converters 111, 112
M' of 8 bits each.

Outputs Y' signal. With the above converter, RlG, B
The C', M', and Y' signals converted from the signals are sent to the image output section 114 and output as an image.

Next, the contents of the bit number converter and the masking converter that receives its output signal will be described.

Table 1 shows some of the correspondences of the human output signals of the transducer 104.

Table 1 In this conversion, only the human power value 0 corresponds to the output value 0, and the human power value 1, 2, 3, etc. corresponds to the output value 1.
4.5, and for output value 2, input value 6,
By making 7, 8, 9, and 10 correspond, only the manual value 0 is treated as a value corresponding to the blank area of the document and is not affected by the limit on the number of bits, and in order to keep the output bit number to 6 bits. The remaining values are appropriately assigned to other output values. A similar conversion is performed by converter 105.
106, 107, 108, and 109.

In the masking converter, the average value of the values assigned to each output value of the bit number converter (for example, M of the value 1)
If M signals with values of 1, 2, 3, and 4°5 are assigned to the C signal, the average value will be 3. ) is regarded as a virtual input value and a conversion table is constructed.

By configuring each converter as described above, the read data (Y=M=C=O) of a white original, for example, is converted to Yc, Mc with a value of 0 by the bit number converters 104 and 105, for example.
signal and is converted into a value O by masking converter 110.
(The same goes for M' and Y''). Therefore, it is possible to prevent the deterioration of the output image corresponding to the white background part of the document that occurs when simply thinning out the lower bits. Moreover, in this case, the circuit added as a bit number converter is a human-powered 8-bit output 6-bit memory 6.
You only need one piece.

(Embodiment 2) FIG. 5 shows an NTSC-compliant Y
, I, and Q signals to R, G, and B signals for display on a color display.

Each 8-bit Y, I obtained through the input I/F 510
, Q table reference type matrix converters 507, 508, and 509 are assumed to manually input an 8-bit Y signal and 6-bit I' and Q' signals respectively. Table reference type bit number converter 501, 502, 5
03, 504, 505, 506, each 8 bit I
, Q signals into 6-bit I' and Q' signals.

Next, the contents of the bit number converter and the matrix converter to which the output signal is input will be described.

Table 2 shows some of the correspondences between the human output signals of the transducers 501-506.

Table 2 In this conversion, the output value is 32, while the human power value is 127.
128 and 129, and in the matrix converter, the output value 32 of the bit number converter is virtually converted to I of value 0.
, and construct a conversion table by regarding them as Q signals.

With such a configuration, achromatic color and I near achromatic color,
Even if the number of bits is limited by the bit number converter for the Q input signal value, R, G, and B signals of achromatic color values can be obtained by the matrix converter, and therefore, achromatic colors can be displayed on the display 511. Can be done. In other words, it is possible to display a natural color image by preventing the unnatural chromaticization of colors near achromatic colors caused by simple thinning of the lower bits, and in addition, the circuit added in this case requires only 8 bits of manual input and 6 bits of output. 6 memories (bit number converters 501 to 5
06) is enough.

(Embodiment 3) FIG. 6 shows another embodiment of the present invention, in which R, G, and B input signals from a color document reading device are converted to CIE-compliant X, Y, and Z signals.
A block diagram of a signal conversion circuit for converting into a signal is shown.

In FIG. 6, 610 is an image reading unit (color original reading device), 601 to 606 are table reference type bit number converters, 607 to 609 are table reference type masking converters, and 611 is an output I/F. It is.

This embodiment is similar to [Embodiment 1], and in the bit number converter, the 8-bit manual signal value R=G=B=255 corresponding to the blank section of the original from the image reading section 610 is used. In this case, the configuration is such that even if the number of bits is limited to 6 bits, the corresponding values are kept independent. The masking converter converts human input signals (eg, 6-bit Bx, Gx signals and 8-bit R signals in converter 607) to 8-bit X, Y, Z signals. The converted X, Y, Z signals are output 1/F61
Output to the outside via 1.

[Embodiment 4] FIG. 7 shows a case where CIE-compliant l,, *, B*, l)* signals are converted into C, M, and Y signals and output to a printer as still another embodiment of the present invention. 1 shows a block diagram of the signal conversion circuit of FIG.

In FIG. 7, 710 is a human power I/F, 701 to 706
are table reference type bit number converters, 707 to 709
711 is a table reference type masking converter, and 711 is a printer.

This embodiment is similar to (Embodiment 2), and in the bit number converter, the 8-bit human input signal values corresponding to achromatic colors and colors near achromatic colors from the human input I/F 710 are converted into bit numbers. The configuration is configured to maintain the value corresponding to an achromatic color even if the value is limited. The masking converter has 6 bits a◆,b!
The flaw signal and the 8-bit signal are converted into 8-bit C, M, and Y signals and input to the printer 711.

In each of the above embodiments, the case where the number of bits of each signal is limited from 8 to 6 has been described, but the number of bits is not limited to this value. Furthermore, various types of color signals such as other primary color systems, luminance/color difference systems, etc. can be applied.

As explained above, masking conversion is performed to match the characteristics between a color image input device and a color image output device, and YIQ, L*, which is a luminance (brightness)/color difference signal system, is used.
When converting color signals such as mutual conversion between a*b medium signals and RGB and CMY signals, if the number of bits of the human input signal to the conversion circuit is limited due to the characteristics of the conversion or constraints on the circuit scale, the image By treating the signal areas that have a large effect on the quality of the signal independently so as not to cause image quality deterioration even when limiting the number of effective bits of the signal, compared to simply ignoring the lower bits, Image quality deterioration can be prevented.

〔Effect of the invention〕

As described above, according to the present invention, color signals can be processed without deteriorating image quality.

[Brief explanation of the drawing]

FIG. 1 is a block diagram of a color conversion circuit in a first embodiment of the present invention, FIG. 2 is a block diagram of a color conversion circuit in a conventional example, and FIG. 3 is a block diagram of a color conversion circuit in another conventional example. FIG. 4 is a block diagram of a color conversion circuit according to another conventional example, FIG. 5 is a block diagram of a color conversion circuit according to a second embodiment of the present invention, and FIG. 6 is a block diagram of a color conversion circuit according to a third embodiment of the present invention. Block Diagram of Color Conversion Circuit FIG. 7 is a block diagram of a color conversion circuit in a fourth embodiment of the present invention.

Claims (1)

[Claims] Bit number conversion that converts the number of bits of an input color information signal into a color information signal having a smaller number of bits than the number of bits, and retains specific input color information. and means for converting an input color information signal into a color information signal of another form using the converted color information signal obtained by the bit number conversion means. .