JPH0669211B2 - Image processing device - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial field of application] The present invention relates to a digital copying apparatus, a revision camera,
The present invention relates to an image processing apparatus used in an electronic still camera or the like, and particularly to an image processing apparatus capable of obtaining optimum corrected density data for input density data regardless of the characteristics of an input image.

[Conventional technology]

In the field of handling image information, various density corrections are often performed, such as gamma correction when the input-output density characteristic is non-linear. In particular, in a digital copying apparatus for obtaining a color copy, it is necessary to correct the input density information based on a curve corresponding to human visual characteristics.

Therefore, in a digital copying apparatus that processes image density information with digital data, a ROM (read-only memory) in which corrected density data is written is prepared as a density conversion table in order to speed up correction processing. A technique has been proposed in which a luminance signal is given as an address of the ROM and the corresponding corrected density data is output to a printer (Japanese Patent Publication No. 60-23541).

[Problems to be solved by the invention]

However, in such a copying machine, the correction density data is 1
Since only the type is given, when copying various original papers with different background densities, or when the originals are dirty, the background (fogging) may occur or the necessary parts may not be copied. Or In order to avoid this, it is necessary to give an appropriate correction amount according to each original.

However, assuming all cases, in order to store the respective correction data in the memory in advance, it was necessary to require a very large memory capacity.

The present invention has been made based on the above-described conventional drawbacks, and provides an image processing apparatus capable of obtaining an output image of optimum density for various types of input pixels without increasing the memory capacity. The purpose is to provide.

[Means for solving problems]

According to the present invention, in an image processing apparatus having a density conversion table for correcting density data of an input image and converting the density data into corrected density data of an output image, the density conversion table is composed of a rewritable memory. With
The image forming apparatus may further include a generation unit that generates the correction density data according to the type of the input image and writes the correction density data in the memory.

[Action]

Since the density conversion table is a rewritable memory, the corrected density data can be freely rewritten. Then, the generation means generates the optimum correction density data according to the type of the input image (normal manuscript, photograph, illustration, etc.) and writes it in the memory to correspond to the type of the input image. The optimum correction can be made.

Further, since the correction density data is generated according to the type of the input image, the capacity of the memory is sufficient to store one kind of correction density data.

〔Example〕

 Examples of the present invention will be described below.

FIG. 1 is a diagram showing an embodiment in which the present invention is applied to a digital color copying machine.

The scanner 1 scans an original document (not shown), photoelectrically converts reflected light from the original document through three color filters, and further A / D-converts these signals to obtain a digital image signal Si ′. is there. Here, i represents the color of the color filter, and i = {red (r), green (g),
Blue (b)}, {yellow (y), green (g), cyan (c)} etc. are usually used. This digital image signal Si ′ is input to the shading correction circuit 2. The shading correction circuit 2 normalizes the signal value obtained by scanning the white reference plate as "1" and outputs the signal value obtained by scanning the black reference plate as "0", for example. When the reflectance of the white reference plate is Rwi and the reflectance of the black reference plate is Rbi, the signal value Si when the reflectance of the document is Ri is expressed by the following equation.

Si = (Ri-Rbi) / (Rwi-Rbi) (1) The color signal Si standardized by the white and black reference plates is corrected for uneven sensitivity of the sensor, illumination light, and optical unevenness such as color filters. It has become a signal.

Further, even when the sensor is a CCD sensor having a plurality of chips, it is possible to obtain a signal in which unevenness such as variation in gain of a plurality of amplifiers connected to each sensor chip is corrected.

The color signal Si is input to the matrix circuit 3 and separated into a luminance signal I and color difference signals C ₁ and C ₂ . At this time,
The magnitudes of the color difference signals C ₁ and C ₂ change according to the operation switch that controls the density characteristic. Details of this circuit will be described later.

The luminance signal I is a gamma conversion circuit 4 which is the gist of the present invention.
In step 2, density conversion and background processing are performed. The corrected density data for the correction is obtained by processing the content of the data stored in the ROM 5 by the CPU 7 based on the input from the operation panel 6 as described later.

The luminance signal I ′ subjected to density conversion and background processing and the color difference signals C ₁ and C ₂ are input to the color conversion circuit 8. The color conversion circuit 8 is composed of, for example, a ROM, and converts a color represented by a combination of the values of the luminance signal I ′ and the color difference signals C ₁ and C ₂ into an ink amount necessary when outputting the color by a printer. The values are tabulated. When the signals I ', C ₁ and C ₂ are input to the color conversion circuit 8, an output converted into the ink amount necessary for printing is obtained. This color conversion table may be created by the method described by the masking equation or the Neugebauer equation. Each signal converted into this ink amount, for example, each signal for yellow, magenta, cyan, and black in a four-color printer is input to the dither circuit 9. The dither circuit 9 is a circuit that performs binarization by the dither method so that halftone recording can be performed even in a color printer that can only store binary data. The signal binarized by the dither circuit 9 is output to the printer 10. The printer 10 prints ink of each color according to an input signal to generate a color copy.

Next, details of the density correction will be described centering on the gamma conversion circuit 4.

A document has different characteristics of density and saturation depending on its type. Therefore, when trying to get various copies,
It is desirable that the correction value can be changed according to the original.
It is also necessary to be able to freely change the output value depending on the purpose of copying. The former is, for example, an ordinary original, an original with high overall density such as a photograph and low saturation,
This is a case where it is necessary to distinguish and set the modes, such as a document that is required to have a high saturation like an illustration. The latter is a case where it is desired to change the density of an output image depending on the purpose, such as a case where a picture is desired to be copied particularly beautifully even in an ordinary manuscript, and a case where both pictures and characters are normally copied. In addition, if the background of the document is dark, or if a part of the document is dirty and you want to remove it, set a certain threshold value and give a saturation level uniformly to luminance signals that exceed this value. Processing is also required. In order to meet such requirements, the operation panel 6 of FIG. 1 is constructed as shown in FIG. 2 in this embodiment. That is, the operation panel 6 includes a mode setting switch 11 for selecting one of five modes, that is, an automatic mode, a picture mode, a character mode, a photo mode, and an illustration mode, and density adjustment for each of these modes. In order to make it possible to change the density characteristics, for example, a density setting switch 12 for setting the density characteristics in five steps, and a background density setting lever 13 capable of independently performing nine levels of background processing for each density characteristic are provided. It is a thing.

FIG. 3 shows how 25 kinds of density characteristics can be set by combining the mode setting switch 11 and the density setting switch 12. However, in this case, since the automatic mode, the picture mode, and the character mode are all modes for ordinary documents, the same density characteristics are given. As an example of the density characteristics described here, I00 to I04 are shown in FIG. This concentration characteristic is expressed by the following equation.

I ′ = Iα (2) where I is the density data, that is, the luminance signal value input to the gamma correction circuit 4, and I ′ is the corrected density data.
That is, each represents a luminance signal value output from the gamma correction circuit 4, and α represents an appropriate positive real value. The conversion formula shown in the formula (2) is used for the following reason. That is, it is necessary for the density conversion to ensure naturalness for the human eye, but as described above, the human sense is said to be approximately proportional to the logarithm of the strength of the stimulus. Then, if the logarithm is taken on both sides of the equation (2), the following equation is obtained.

log I ′ = α log I (3) That is, the luminance signal I ′ obtained by multiplying the luminance signal I by α.
Will appear to be α times the original luminance signal I. The luminance signals I and I'will appear to change while maintaining this relationship.

Further, the conversion according to the equation (2) is convenient, if the luminance signals I and I ′ are, for example, 8-bit numbers, I = 0.
Since I '= 0 when I = 255 and I' = 255 when I = 255, there is crushing in the high density area, the density does not reach the maximum density level, or the area that is not printed in the low density area. Does not occur, nor does the density of fog that causes background stain occur. The value of α is, for example, 0.5,
0.67, 0.8, 1.0, 1.25, 1.5 etc. can be used. By varying α in this way, various concentration characteristics as shown in FIG. 4 can be obtained. When there are only a few α values of the density characteristics selected in the ROM 5 as in the present embodiment, Iij in all cases (here, i = 0 to 4, j = 1)
2) are stored, and each density data is directly obtained from this ROM5. However, if α can take a value of several tens to several hundreds, ROM5 has, for example, a typical density characteristic of I02 (α =
1) is stored. Then, each corrected density data can be obtained by processing it.

Further, in this embodiment, the threshold adjustment can be performed by the background adjustment lever so that the background contamination of the original document and the background contamination of the copy due to the background density of the paper itself do not occur. When the threshold value is Th, the density characteristic obtained by this processing is as shown in the following equation.

That is, as shown in FIG. 4, the input density data I
When the value exceeds the threshold Th, the corrected density data I ', which is the output, is output as a white level, whereby the background stain is cut.

Such corrected density data I ′ is processed by the CPU 7. Generation of correction data to RAM (gamma conversion circuit 4)
FIG. 5 shows the flow of processing up to writing to the. First, when the mode and density characteristics are input (21,22), the CPU
7 selects a combination of correction data (23). The threshold value Th for background processing is input (24). I is cleared (25). When I is smaller than Th (26), the corrected density data I'of the selected density characteristic is processed or RO
Read the correction data from M (27) and store the data I'in RAM.
Write (28). If I is greater than or equal to Th, the correction data is set to saturation level data (29) and written in RAM (28). Count up I by 1 (30), I is 25
Repeat this until 5 is reached (31). RAM by this
Corrected density data is stored inside the.

The gamma correction circuit 4 can be composed of, for example, four buffers and a rewritable memory (RAM) as shown in FIG. The brightness signal I obtained from the matrix circuit 3 is stored in the buffer 40. On the other hand, the writing of the correction data is performed as follows. The 8-bit data 0-255 generated by the CPU 7 is stored as it is in the buffer 41 as an address, and the corrected density data I'corresponding to 0-255 is stored in the buffer 42. The corrected density data I'which is a power value of the address corresponding to the above address is written in the RAM 43. At this time, buffer 4
1 is separated from RAM43. The luminance signal I stored in the buffer 40 is given a signal value as an address to the RAM 43, and the corrected density data stored at the address is stored as a corrected luminance signal I'in the buffer 45. To go. In this way, the density is corrected.

By the way, when such density conversion is performed, the chromatic color may become pale or cloudy and may appear to deteriorate again. Therefore, it is necessary to increase the saturation by the amount of movement of the density. This saturation correction is performed by enlarging the value of the color difference signal. When the enlargement ratio of the color difference signal is P, the conversion from the signal Si input to the matrix circuit 3 to the luminance signal I and the color difference signals C ₁ and C ₂ output from the matrix circuit 3 is expressed by the following equation. .

Here, when P = 1, the 3 × 3 matrix (hereinafter, referred to as M) of the expression (5) is a basic matrix when the saturation does not change. In this way, in order to adjust the saturation according to each mode and each density, a plurality of types of matrix M to be used are set as shown in FIG. This combination of matrices corresponds to the combination of FIG. According to an experiment conducted by the present inventors, the enlargement ratio P of the color difference signal values C ₁ and C ₂ is
The appropriate range for moving was 1.0 to 1.3. This is because when P is 1.3, the saturation of the chromatic color reaches the saturation level, and even if the value of P is further increased, the saturation does not rise. Explaining with reference to an experimental example, for example, when α = 0.8 in the density conversion formula, the density becomes light as a whole. At this time, if P = 1.0 and the saturation is left in the standard state, the apparent saturation is reduced by the amount by which the chromatic color becomes lighter. Therefore, by making P larger than 1, the value of the color difference signal is enlarged to increase the apparent saturation. Note that here, the image looked more natural when P = 1.1. By performing such density conversion, background processing, and saturation correction, density conversion that looks natural to the human eye can be performed, and background stains or background stains on the original even when the original has density on the paper itself are copied. It is possible to obtain a preferable copy without causing the above-mentioned phenomenon and preventing the apparent saturation reduction that is likely to occur as a result of the density conversion.

As described above, according to the present embodiment, it is possible to obtain 25 different density characteristics.
By combining threshold values of various types, a total of 225 density corrections can be performed, and the required memory amount is 1/225 compared to the case where all data is stored in ROM. .

In the above embodiment, the correction parameters for the density characteristic, background processing, and saturation are manually given by using the operation switch.
However, it is possible to do this automatically. In this case, obtain the degree of saturation correction from the density distribution of the document obtained by the scanner, estimate the background processing threshold value from the density and stains of the paper itself, and process the signal. Good. By doing so, it is possible to save the user the trouble of selecting the operation switch.

Further, in the above embodiment, both the density correction and the background processing are performed by the gamma correction circuit 4, but as shown in FIG. 8, the gamma correction circuit 51 and the background processing circuit 52 are provided separately and two You may make it process individually. In this case, the gamma correction circuit 51 performs density conversion using a curve of I '= I ^α as shown in FIG. 9 (a), and the background processing circuit 52 further performs the density conversion as shown in FIG. 9 (b). The threshold processing may be performed by performing such conversion. In this case, there is an advantage that only the threshold value can be easily changed. It is also possible to set _two thresholds, Th ₁ and Th _2, to remove the background in the low density region and adjust the saturation density in the high density region. The thresholds Th ₁ and Th ₂ may be freely changed by a switch. Of course, the processing using these two threshold values is also possible in the previous embodiment.

Further, in the above embodiment, the saturation is corrected by the matrix circuit, but it can be corrected by correcting the gamma characteristic of the color difference signal. In this case, the tenth
As shown in the figure, the color difference correction circuit 53 may be provided. By giving the color difference signal a characteristic with a high gamma value as shown in FIG. 11 by the color difference correction circuit 53, the apparent saturation can be increased.

Furthermore, when the input value of the color difference signal is Ci and the output value is Ci ′, it is possible to increase the saturation using the conversion formula shown below.

However, i = 1,2 and β are real numbers of 1 or more. As described in the case of the luminance signal correction, since this is proportional to the logarithm of the strength of the human stimulus, this relationship also holds for the saturation. Therefore, the apparent saturation is multiplied by β by the equation (6). In addition, the correction using the expression (6) does not result in a solid image having a saturation of a certain level or more as in the case where the gamma of the color difference signals C ₁ and C ₂ is changed. While maintaining the abundance of
It is convenient because the saturation can be copied higher.

In the above, an example in which the image processing apparatus of the present invention is mainly applied to a digital copying machine has been described, but the present invention is not limited to the copying machine, and various types of devices such as a color television camera, an electronic still camera and the like can be used. It goes without saying that it can be applied to various purposes.

〔The invention's effect〕

As described above, according to the present invention, it is possible to provide an image processing apparatus with high adaptability that performs optimum density correction according to the type of an input image without increasing the memory capacity required for the density conversion table. You can

[Brief description of drawings]

1 to 7 are views for explaining a digital copying machine according to an embodiment of the present invention. FIG. 1 is a block diagram of the copying machine, FIG. 2 is a view showing an operation panel, and FIG. FIG. 3 is a diagram showing density correction determined by mode selection and density selection,
FIG. 4 is a diagram showing a corrected density data curve, FIG. 5 is a flow chart for explaining the operation of the copying machine, FIG. 6 is a block diagram showing further details of the gamma correction circuit, and FIG. FIG. 8 is a diagram showing a matrix determined by density selection,
FIG. 11 to FIG. 11 are views for explaining other embodiments of the present invention. 1 ... Scanner, 2 ... Shading correction circuit, 3 ...
... Matrix circuit, 4,51 ... Gamma conversion circuit, 5 ... RO
M, 6 ... Operation panel, 7 ... CPU, 8 ... Color conversion circuit,
9 ... Dither circuit, 10 ... Printer, 52 ... Background processing circuit, 53 ... Color difference correction circuit.

Claims (3)

[Claims] 1. An image processing apparatus having a density conversion table for correcting density data of an input image and converting the density data into corrected density data of an output image, wherein the density conversion table is composed of a rewritable memory. Along with
An image processing apparatus comprising: a generation unit that generates the corrected density data according to the type of the input image and writes the corrected density data in the memory. 2. An image signal of the input image is separated into a luminance signal and a color difference signal, the luminance signal is given as density data to the density conversion table, and the color difference signal is corrected corresponding to the corrected density data. The image processing apparatus according to claim 1, wherein 3. The value of the color difference signal is expanded regardless of whether the density characteristic is lightened or darkened with respect to the luminance signal.
The image processing device according to the item.