JP2706401B2 - Concentration processing method - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a density processing method for an image forming apparatus such as a copying machine.

[0002]

2. Description of the Related Art In a digital color copying machine, first, in a scanner section, an original is irradiated by an exposure lamp, and the reflected light is detected by a CCD, and is sequentially converted into an electric signal. At this time, the image of the original is color-separated and pixel-separated by the CCD, and is separated into electric signals corresponding to the density of each pixel. This electric signal is sent to the image processing unit.

[0003] In the image processing section, after the output of the CCD is converted to digital, the shading correction section performs processing for each color (B,
The dispersion of the CCD, the exposure lamp, etc. is corrected for each of the G, R) signals. And the signal of each color (BGR signal)
Toner density signal (YMC signal) in BGR / YMC conversion unit
Is converted to Further, a BK signal is generated from the YMC signal by the BK generation unit.

After that, the color correction unit corrects the density level of each color of the YMC signal and the BK signal according to the characteristics of the filter and the toner. Further, the color conversion unit performs designated color conversion, trimming, masking processing, and the like.
Thereafter, in the density processing unit, the level of the sent digital density signal is converted according to the developing color, the copy density specified by the operation unit, the document image type specified by the operation unit, and the like. Thereafter, the digital density signal is sent to the printing unit via the scaling / moving processing unit for performing scaling / moving processing in the main scanning direction of the image, and recording on the recording paper is performed.

In digital copiers and digital color copiers, density processing is generally performed by a dither method in a density processing section in order to obtain a halftone image. As a digital color copying machine already developed by the present applicant, using a dither matrix having 2 × 2 pixels as one block, data of the gradation of the recording pixel with respect to the gradation of the read pixel (hereinafter, input gradation−output) Tone data) is created in advance and stored in the storage device,
There is a method in which gradation data (output gradation data) of a recording pixel with respect to gradation data (input gradation data) of an input read pixel is obtained based on input gradation-output gradation data. The gray scale of the read pixel is 256 gray scales, and the gray scale of the recording pixel is 64 gray scales. FIG. 1 shows a density processing circuit of this copying machine.

This density processing circuit includes a CPU 10, a data ROM 11, a table memory 13, and an address generation circuit 12. In the data ROM 11, the input gradation-output gradation data created in advance is stored in the developing colors (M, C,
Y, BK), a plurality of types are stored according to the copy density specified by the operation unit and the document image type specified by the operation unit.

The CPU 10 outputs one type of input tone-output tone data according to the development color, the copy density designated by the operation unit, and the type of the original image from the plurality of types of input tone-output tone data. The data is transferred from the data ROM 11 to the table memory 13. The address generation circuit 12 includes image data (input gradation data) representing the density of the read pixel and a signal (line signal HSYN) (not shown) indicating the position.
C and the dot signal CLK) are sent. The address generation circuit 12 stores the output gradation data corresponding to the gradation and pixel position (pixel of the dither matrix corresponding to the read pixel) of the transmitted image data among the addresses of the table memory 13. Output the specified signal. As a result, the gradation data stored at each specified address is output from the table memory 13 as output gradation data.

[0008]

In such a copying machine, the gray scale of the read pixels is 256 gray scales, and the number of pixels in one block of the dither matrix is four. The number of output gradation data is 256 × 4
= 1024. Such input tone-output tone data is obtained for each developed color (M, C, Y, BK), for each copy density specified by the operation unit, and for the original image type specified by the operation unit. Different for each. 4 kinds of developing colors
If the copy density specified by the operation unit is fifteen levels and the document image type specified by the operation unit is three types: text, photo, and mixed text / photo mode,
When the data ROM 11 is an 8-bit memory, the capacity of the data ROM 11 is 1024 × 15 × 3 × 4 = 1.
84320 bytes are required. Therefore, in such a copying machine, there is a problem that a large-capacity storage device is required for storing the input gradation-output gradation data.

In order to reduce the capacity of the data ROM 11, the applicant designates the input gradation-output gradation data with each of the developed colors (M, C, Y, BK) and the operation unit. It has been drafted that the output gradation adjustment created based on the copy density specified by the operation unit, which is created for each document image type, is performed based on the created input gradation-output gradation data. More specifically, the output gradation for the input gradation is obtained based on the input gradation-output gradation data corresponding to the developed color and the original image type specified by the operation unit, and then the obtained output gradation is obtained. Is increased or decreased by the gradation corresponding to the copy density specified by the operation unit to obtain the output gradation. That is, the offset value is simply added or subtracted from the output gradation obtained from the input gradation-output gradation data.

FIG. 13 shows input gradation-output gradation data corresponding to a development color and a document image type designated by the operation unit. FIG. 14 shows an example in which the output gradation is increased by a predetermined gradation by the above method. FIG. 9 shows output gradation data after correction with respect to input image data when the correction is performed.

It is preferable that the relationship between the input gradation data and the actually printed density be a linear relationship. However, the relationship between the actual density and the output tone data is not a linear relationship due to the unique characteristics of the copying machine. Therefore, if the relationship between the output tone data and the input tone data is linear, the input tone data Is no longer linear. In consideration of the relationship between the input tone data and the output tone data specific to the copying machine, the relationship between the actual tone and the output tone data is considered to be linear. Created to be.
For this reason, the characteristics of the generated input gradation-output gradation data become non-linear characteristics and characteristics unique to the output gradation.

In the method of simply adding or subtracting an offset value to or from the output gradation obtained from the input gradation-output gradation data, the original input gradation-output gradation characteristic shown in FIG. The input gradation obtained by this method shown in FIG.
Compared with the output gradation characteristics, the magnitude of the output gradation for each corresponding point on the characteristic curve changes. -The output gradation characteristics cannot be maintained. For this reason, there is a problem that the relationship between the actually printed density and the input gradation data does not become a linear relationship. In addition, the method of simply adding or subtracting an offset value to the output gradation obtained from the input gradation-output gradation data has a problem that the range of the output gradation becomes extremely narrow.

According to the present invention, it is possible to reduce the capacity of a storage device for storing output gray scale data with respect to input gray scale, and to simply offset the output gray scale obtained from input gray scale-output gray scale data. Compared with a method of obtaining an output gradation corresponding to the copy density by adding or subtracting a value, the range of the output gradation is not extremely reduced, and the relationship between the density actually printed and the input gradation data is determined. It is an object of the present invention to provide a density processing method capable of achieving a linear relationship.

[0014]

According to a first density processing method of the present invention, output gradation data for an input gradation is prepared in advance using a dither matrix and stored in a first storage means. The output gradation data corresponding to the gradation is transferred to the second storage means, and the address of the second storage means is specified based on the input gradation data, whereby the specified address in the second storage means is stored. A density processing method for outputting the stored output gradation data, wherein a transfer destination address of the output gradation data with respect to the input gradation to the second storage means is converted in accordance with the designated copy density; In the portion where the address of the data storage area of the output gradation with respect to the input gradation of the storage means is small, there is a blank transfer destination address which no longer exists in the converted transfer destination address. In this case, the pattern of the output gradation data for those blank transfer destination addresses is obtained based on the output gradation data for the original input gradation, and the calculated pattern and the minimum value of the converted transfer destination address are calculated. Obtaining output grayscale data for a blank transfer destination address based on the corresponding output grayscale data; and, from the above steps, a new output grayscale data corresponding to the entire output grayscale data storage area for the input grayscale of the second storage means. Generating output gradation data and transferring the data to the second storage means.

In a second density processing method according to the present invention, output gradation data for an input gradation is created in advance using a dither matrix and stored in a first storage means.
The data of the output gradation with respect to the input gradation is transferred to the second storage means, and the address of the second storage means is designated based on the inputted input gradation data, whereby the designated address in the second storage means is designated. In the density processing method in which the output gradation data stored in the memory is output, the transfer destination address of the output gradation data corresponding to the input gradation to the second storage means according to the designated copy density; If there is a blank transfer destination address that no longer exists in the converted transfer destination address in a portion where the address of the output gradation data storage area with respect to the input gradation of the second storage means is large, the blank transfer destination address The output grayscale data pattern is obtained based on the output grayscale data with respect to the original input grayscale, and the obtained pattern and the converted destination address are converted. Determining output grayscale data for a blank transfer destination address based on output grayscale data corresponding to the maximum value of the above. Generating corresponding new output gradation data and transferring the data to the second storage means.

In a third density processing method according to the present invention, output gradation data for an input gradation is prepared in advance using a dither matrix and stored in a first storage means.
The data of the output gradation with respect to the input gradation is transferred to the second storage means, and the address of the second storage means is designated based on the inputted input gradation data, whereby the designated address in the second storage means is designated. In the density processing method in which the output gradation data stored in the memory is output, the transfer destination address of the output gradation data corresponding to the input gradation to the second storage means according to the designated copy density; If there is a blank transfer destination address that no longer exists in the converted transfer destination address in the portion where the address of the output gradation data storage area with respect to the input gradation of the 2 storage means is small, the blank transfer destination address corresponding to the blank transfer destination address The output grayscale data pattern is obtained based on the output grayscale data with respect to the original input grayscale, and the obtained pattern and the converted destination address are converted. Calculating output grayscale data for a blank transfer destination address based on output grayscale data corresponding to the minimum value of the output grayscale data. When there is a blank transfer destination address that no longer exists in the converted transfer destination address, the pattern of the output gradation data corresponding to the blank transfer destination address is determined based on the output gradation data corresponding to the original input gradation. Ask for
Obtaining output grayscale data for a blank destination address based on the obtained pattern and output grayscale data corresponding to the maximum value of the converted destination address; Generating new output gradation data corresponding to all the data storage areas of the output gradations for the tones and transferring the data to the second storage means.

[0017]

According to the first density processing method of the present invention, the transfer destination address of the output gradation data to the second storage means with respect to the input gradation is converted according to the designated copy density. If there is a blank transfer destination address that no longer exists in the converted transfer destination address in a portion where the address of the output gradation data storage area with respect to the input gradation of the second storage means is small, those blank transfer destination addresses are used. Is obtained based on the output grayscale data corresponding to the original input grayscale, based on the obtained pattern and the output grayscale data corresponding to the minimum value of the transfer destination address after conversion. Thus, output gradation data for the blank transfer destination address is obtained. In this way, new output gradation data corresponding to the entire data storage area of the output gradation with respect to the input gradation of the second storage means is created and transferred to the second storage means.

In the second density processing method according to the present invention, the transfer destination address of the output gradation data with respect to the input gradation to the second storage means is converted according to the designated copy density. If there is a blank transfer destination address that no longer exists in the converted transfer destination address in a portion where the address of the output gradation data storage area is larger than the input gradation of the second storage means, the blank transfer destination address is used. Is obtained based on the output gradation data corresponding to the original input gradation, and based on the obtained pattern and the output gradation data corresponding to the maximum value of the transfer destination address after conversion. Thus, output gradation data for the blank transfer destination address is obtained. In this way, new output gradation data corresponding to the entire data storage area of the output gradation with respect to the input gradation of the second storage means is created and transferred to the second storage means.

In the first density processing method according to the present invention, the transfer destination address of the output gradation data with respect to the input gradation to the second storage means is converted according to the designated copy density. If there is a blank transfer destination address that no longer exists in the converted transfer destination address in a portion where the address of the output gradation data storage area with respect to the input gradation of the second storage means is small, those blank transfer destination addresses are used. Is obtained based on the output grayscale data corresponding to the original input grayscale, based on the obtained pattern and the output grayscale data corresponding to the minimum value of the transfer destination address after conversion. Thus, output gradation data for the blank transfer destination address is obtained. If there is a blank transfer destination address that no longer exists in the converted transfer destination address in a portion where the address of the output gradation data storage area is larger than the input gradation of the second storage means, the blank transfer destination address is used. Is obtained based on the output gradation data corresponding to the original input gradation, and based on the obtained pattern and the output gradation data corresponding to the maximum value of the transfer destination address after conversion. Thus, output gradation data for the blank transfer destination address is obtained. In this way, new output gradation data corresponding to the entire data storage area of the output gradation with respect to the input gradation of the second storage means is created and transferred to the second storage means.

[0020]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment in which the present invention is applied to a digital color copying machine will be described below with reference to the drawings.

FIG. 1 shows a density processing circuit of a digital color copying machine.

This density processing circuit includes a CPU 10, a data ROM 11, a table memory 13, and an address generation circuit 12. The CPU 10 has a RAM 14 for storing necessary data. In the data ROM 11,
Data representing the gradation of the recording pixel with respect to the gradation of the read pixel (hereinafter referred to as input gradation-output gradation data) is a document image designated by each developing color (M, C, Y, BK) and the operation unit. A plurality of types are stored according to types. There are three types of original image types: text, photo, and text / photo mixture. The input gradation-output gradation data is created in advance using a dither matrix in which 2 × 2 pixels G ₀ , G ₁ , G ₂ , and G _{3 shown} in FIG. The gray scale of the read pixel is 256 gray scales, and the gray scale of the recording pixel is 64 gray scales.

In the table memory 13, the input gradation-output gradation data corresponding to the development color and the original image type among the plurality of types of input gradation-output gradation data stored in the ROM 11 are processed by the CPU 10. After being transferred. The address generation circuit 12 receives image data (input gradation data) representing the density of the read pixel and signals (not shown) (line signal HSYNC and dot signal CLK) indicating the position of the pixel.

The address generation circuit 12 provides an address designation signal represented by a predetermined 10-bit binary number in accordance with the gradation of the input image data and the position signal of the read pixel (the pixel of the dither matrix corresponding to the read pixel). Is output. The lower 8 bits of the address designation signal correspond to the input gradation 0 to 255, and the upper 2 bits are the pixels G ₀ ,
It corresponds to G ₁ , G ₂ and G ₃ . Pixels G ₀ , G ₁ , G ₂ ,
Upper two bits of the address signal corresponding to G ₃ are, "00", "01", "10", "11", and becomes. When an address designation signal is output from the address generation circuit 12, data stored at a designated address in the table memory 13 is output from the table memory 13 as output gradation data.

Table 1 shows an example of input gradation-output gradation data. In Table 1, the reference address indicates the value of the lower 8 bits of the designated address as the input gradation data 0 to 25.
5 shows an address when a one-to-one correspondence is made. The total gradation is the sum of output gradation data for input image data having the same input gradation among input image data corresponding to each pixel of the dither matrix.

[0026]

[Table 1]

FIG. 3 shows the contents of the data ROM 11.

In this copying machine, the input gradation-output gradation data is composed of four types of colors (M, C, Y, BK) and characters designated by the operation unit, photographs, and mixed characters / photos.
One document image type is created in advance. That is, twelve types of input gradation-output gradation data are stored in the areas RE _{0 to} RE 11 of the data ROM ₁₁ , respectively. In this example, since the data ROM 11 is an 8-bit memory, the number of bytes in each of the areas RE _{0 to} RE ₁₁ is:
(Number of gradations of input gradation data) × (Number of pixels of dither matrix), which is 1024 bytes. Therefore, 1
The number of bytes in all two areas is 1024 × 12 = 12
This is 288 bytes.

FIG. 4 shows the inside of the table memory 13.

The table memory 13 is an 8-bit memory and has an area TE ₀ (addresses _{0 to} 255) for storing output gradation data for input gradation data (256 gradations) for the pixel G ₀ of the dither matrix. Area TE ₁ (addresses 256 to 511) for storing output gray scale data corresponding to input gray scale data for pixel G ₁ , area for storing output gray scale data corresponding to input gray scale data for pixel G ₂ TE ₂ (512~767 address), area TE ₃ for storing the output gray-scale data for the input gray level data for the pixel G ₃ (seven hundred sixty-eight to one
023).

In the density processing circuit of this copying machine, the ROM 1
Based on the twelve types of input gradation-output gradation data stored in the unit 1, output gradations corresponding to 15 levels of copy density specified by the operation unit are output.
There are roughly four methods for this purpose.
Hereinafter, each of the first, second, third, and fourth methods will be described.

(1) First Method In the first method, as shown in FIG. 6 or FIG. 8, the input gradation is plotted on the horizontal axis and the output gradation is plotted on the vertical axis, and the original input gradation-output gradation data is plotted. Is represented by a graph line a,
A characteristic (graph line b2 or c2) is obtained by shifting the graph line a in the horizontal direction according to the designated copy density.
The input gradation-output gradation characteristic is converted in accordance with the designated copy density, and the output gradation is obtained by a predetermined operation for the input gradation in which the output gradation data does not exist as a result of the shift.

(1-1) The concept of the first method will be described first taking an example where the designated copy density is higher than the reference copy density.

FIGS. 5 and 6 show the characteristics of the output gray scale with respect to the input gray scale, where the horizontal axis indicates the input gray scale and the vertical axis indicates the output gray scale. Graph line a in FIGS. 5 and 6
Indicates an original input gradation-output gradation characteristic. When the designated copy density is higher than the reference copy density, as shown by a graph line b1 in FIG. 5, the characteristic is obtained by shifting the graph line a indicating the original input gradation-output gradation characteristic to the left. Then, the input gradation-output gradation characteristics are converted. This shift amount is a value corresponding to the designated copy density. If the graph line a indicating the original input gradation-output gradation characteristic is simply shifted to the left, there is no output gradation data for a high portion of the input gradation data. As shown by the graph line b20 in FIG.
The characteristic is represented by a straight line of degrees. In this way, the input gradation is converted based on the output gradation data b2 for the obtained input gradation.

(1-1-1) The case where the designated copy density is higher than the reference copy density will be described based on the concept of converting the designated address. Table 2 shows the original input gradation-output gradation data shown by the graph line a in FIGS.
Shown in

[0036]

[Table 2]

In Table 2, the dither step indicates the order of the threshold value of the dither matrix used when creating the input gradation-output gradation data.
ayer) type, spiral type, halftone type and the like. In this example, the dither step has four stages of 0, 1, 2, and 3. The dither step is performed for each pixel G ₀ , G ₁ , G ₂ , G of the dither matrix.
0, 1, 2, and 3 are sequentially and repeatedly assigned in ascending order of the input gradation of the input image data corresponding to ₃ .

The dither thickening order is output gradation data for input image data corresponding to each pixel G ₀ , G ₁ , G ₂ , G ₃ of the dither matrix and having the same input gradation. Any pixel G ₀ , G ₁ , G ₂ , G
_This indicates whether the output gradation data for the input image data corresponding to ₃ is higher than the output gradation data for the input gradation lower by one step. The output gray-scale data for one step lower input gradation, when the data of the pixel G ₀ becomes large, a dither fat how rank is zero. The output gray-scale data for one step lower input gradation, when the data of the pixel G ₁ is increased, the dither fat how rank is one. The output gray-scale data for one step lower input gradation, when the data of the pixel G ₂ becomes large, dither fat how rank is two. The output gray-scale data for one step lower input gradation, when the data of the pixel G ₃ becomes large, dither fat how rank is three.

The characteristic b1 obtained by shifting the original input gradation-output gradation data a of FIG. 5 to the left can be obtained by converting the reference address for the input gradation data by the following conversion formula.

[0040]

## EQU1 ## Sadr = Oadr + Sft Sadr: designated address after conversion Oadr: reference address for input gradation Sft: number of shifts

The value of Sft corresponding to the characteristic b1 in FIG.
When 5, reference address 0 to 255 with respect to gradation 0 to 255 of the input image data corresponding to the pixel G ₀ is converted to the specified address 15 to 270. Base address 256 to 511 for gradation 0 to 255 of the input image data corresponding to the pixel G ₁ is converted to the specified address 271-526. Tone 0 of the input image data corresponding to the pixel G ₂
The reference addresses 512 to 767 for 255 are converted into designated addresses 527 to 782. Reference address 768-1023 for gradation 0 to 255 of the input image data corresponding to the pixel G ₃ are designated address 783-1038
Is converted to

As described above, when the designated address is converted by setting the value of Sft at +15, the gradations 241 to 241 of the input image data corresponding to each pixel G ₀ , G ₁ , G ₂ , G ₃ of the dither matrix are obtained. The designated address after the conversion for 255 is the maximum value 255, 511, and 7 of the reference address for the input image data of the corresponding pixels G ₀ , G ₁ , G ₂ , and G _3.
67, 1023. That is, there is no output gradation data for the input image data whose input gradation is 241 to 255.

In such a case, if the input gradation is 241-2.
As the designated address corresponding to the input image data of 55, the maximum value of the reference address for the input image data of the corresponding pixels G ₀ , G ₁ , G ₂ , and G ₃ may be used as the designated address. By doing so, the output gradation for the input image data having the input gradation of 241 to 255 is the output gradation data for the input image data having the same pixel of the corresponding dither matrix and the input gradation of 240. . In this case, the designated address and the output gradation data for the input gradation data are as shown in Table 3. 5 is a graph showing output grayscale data corresponding to input grayscale data 241-255.
Indicated at 10.

[0044]

[Table 3]

In this embodiment, the designated address after conversion of the input image data for each pixel of the dither matrix becomes larger than the maximum value of the reference address for the input image data for each pixel of the corresponding dither matrix. The output gradation for (gradations 241 to 255) is obtained as follows.

First, a dither thickening order for a dither step is obtained based on output tone data for four consecutive input tone levels. For example, the designated address after conversion of the input image data for each pixel of the dither matrix corresponds to the input image data (gradation 240) having the maximum reference address for the input image data for each pixel of the corresponding dither matrix. The input image data (gradation 23) having a value smaller than the maximum value of the reference address for the input image data for each pixel of the dither matrix by 1
9) input image data (gradation 238) having a value smaller than the maximum value of the reference address for the input image data for each pixel of the corresponding dither matrix (gradation 238), reference address for the input image data for each pixel of the corresponding dither matrix Based on the output gradation data corresponding to the input image data (gradation 237) having a value smaller by 3 than the maximum value of
Obtain the dither fattening order for the dither step.

These input image data (gradations 237 to 2)
The dither step of the output gradation data corresponding to (40) is
The designated address corresponding to these input image data (gradations 237 to 240) is divided by 4 and obtained from the remainder.
0, 1, 2, and 3. Further, the dither thickening order of the output grayscale data corresponding to the input image data (grayscales 237 to 240) is, for example, compared with the output grayscale data of the immediately preceding stage by any one of the pixels G ₀ , G ₁ ,
It is determined by checking whether the data corresponding to G ₂ and G ₃ is large, and is 3, 2, 1, 0. That is, the dither steps 0, 1, 2, and 3 correspond to the dither fattening ranks 3, 2, 1, 0.

Next, the designated address after conversion of the input image data for each pixel of the dither matrix becomes larger than the maximum value of the reference address for the input image data for each pixel of the corresponding dither matrix. The dither steps of the output gradation for the tones 241 to 255) are obtained, and these input image data (gradations 241 to 255) are obtained.
２５255) to obtain the dither thickening order of the output gradation. Then, the designated address after the conversion for the input image data for each pixel of the dither matrix becomes the maximum value of the reference address for the input image data for each pixel of the corresponding dither matrix (gradation 24
Using the dither thickening order obtained as described above from the output gradation data for 0), the designated address after conversion of the input image data for each pixel of the dither matrix is calculated for each pixel of the corresponding dither matrix. Output gradation data is obtained for input image data (gradations 241 to 255) that are larger than the maximum value of the reference address for the input image data.

The thus determined gradations 241 to 241-2
Table 4 shows output gradations for 55 input image data.
When the output gradation data for the input image data of gradations 241 to 255 is corrected in this manner, the relationship between the input gradation data and the output gradation data is represented by a graph line b2 in FIG. The graph line b2 shifts the graph a of FIG. 6 to the left by a predetermined amount, and the characteristic (shown by a straight line portion b20) of the output gradation data with respect to the input gradations 241 to 255 is the output gradation with respect to the input gradation data 240. The graph is a straight line that passes through the key point and has a gradient θ of 45 degrees.

[0050]

[Table 4]

(1-1-2) Next, the processing actually performed will be described. In the actual processing, the address generation circuit 12 does not convert the designated address. That is, the address generation circuit 12 outputs a reference address corresponding to the pixel of the dither matrix corresponding to the input gradation data and the pixel position designation signal. The CPU 10 converts input gradation-output gradation data corresponding to the developed color and the designated copy density into data RO based on the developed color and the designated copy density.
Read from M11 and transfer to RAM14.

The destination address of the input gradation-output gradation data transferred to the RAM 14 to the table memory 13 is converted. In this transfer destination address conversion, the designated address Sadr in the above formula 1 is converted to the reference transfer destination address OTa.
dr is obtained by replacing the reference address Oadr of the above equation 1 with the new transfer destination address NTadr, and is represented by the following equation. Here, the reference transfer destination address OTadr
"One-to-one" means that 1024 pieces of data of input gradation-output gradation data are stored in addresses 0 to 1023 of the table memory 13 in a one-to-one correspondence.
This is the address of the table memory 13 corresponding to each data when it is made to correspond.

[0053]

## EQU2 ## NTadr = OTadr-Sft

The value of Sft for the designated copy density is predetermined and stored in the data ROM 11 or another ROM. The reference transfer destination address OTadr of the input gradation-output gradation data to be transferred is converted from the value of Sft corresponding to the designated copy density into a new transfer destination address NTadr based on the above equation (2).

[0055] designated copy density is higher than the reference copy level, if Sft value in the above equation 2 is +15, the input gray level data 0 to the pixel G ₀ of the dither matrix
Reference transfer destination address 0 of output gradation data for 255
255 are converted to new transfer destination addresses-15 to 240. Reference destination address of the output gray-scale data for the input gray level data 0 to 255 for pixel G ₁ two hundred fifty-six to fifty-one
1 is converted to new transfer destination addresses 241 to 496.
Reference destination address 512-767 of output gray-scale data for the input gray level data 0 to 255 for the pixel G ₂ is,
It is converted to a new transfer destination address 497 to 752. Pixel G
The reference transfer destination addresses 768 to 1023 of the output gradation data corresponding to the input gradation data 0 to 255 for ₃ are converted into new transfer destination addresses 753 to 1008.

Then, each pixel G _{0 of} the dither matrix,
G _1, G _2, new destination address of data for G ₃ is, each pixel _{G 0, G 1, G 2} , the minimum value of the reference destination address of data corresponding to G ₃ 0,256,512,76
Data corresponding to data smaller than 7 is deleted.

By the transfer destination address conversion, each storage area 0 to 25 of the table memory 13 for data corresponding to each pixel G ₀ , G ₁ , G ₂ , G ₃ of the dither matrix.
5, 256-511, 512-767, 768-102
The blank transfer destination addresses 241 to 255 and 49 in which the transfer destination address no longer exists in the portion where the address value of No. 3 is large
7-511, 753-767, 1009-1023. Therefore, the output grayscale data for these blank transfer destination addresses is obtained by the same method as that for obtaining the output grayscale data for the input grayscale data 241 to 255 in Table 3 in (1-1-1). Can be Therefore, the blank transfer destination addresses 241-255, 497-511, 7
Output grayscale data corresponding to 53 to 767 and 1009 to 1023 (output grayscale data corresponding to input grayscale data 241 to 255) are input grayscale data 241 to 255 in Table 3.
Becomes the same as the output grayscale data corresponding to.

In this manner, each of the storage areas 0 to 255, 256 to 256 of the table memory 13 for data corresponding to each pixel G ₀ , G ₁ , G ₂ , G ₃ of the dither matrix.
When output gradation data for all of 511, 512 to 767, and 768 to 1023 is created in the RAM 14, the data is transferred from the RAM 14 to the table memory 13. Thereafter, when a reference address corresponding to the input gradation and the pixel position signal is output from the address generation circuit 12, data is output from the corresponding address in the table memory 13.

(1-2) A case where the designated copy density is lower than the reference copy density will be described. 7 and 8 show the characteristics of the output gray scale with respect to the input gray scale, with the horizontal axis indicating the input gray scale and the vertical axis indicating the output gray scale. The graph line a in FIGS. 7 and 8 shows the original input gradation-output gradation characteristics. If the designated copy density is lower than the reference copy density, a graph line c in FIG.
As shown by 1, the input gradation-output gradation characteristic is converted so that the graph line a indicating the original input gradation-output gradation characteristic is shifted to the right. This shift amount is a value corresponding to the designated copy density. If the graph line a indicating the original input gray scale-output gray scale characteristic is simply shifted rightward, output gray scale data for a low portion of input gray scale data will not be present, so that the input gray scale for that portion will not be present. The output gradation data is represented by a graph line c in FIG.
As shown by 20, the characteristic is such that the gradient θ is represented by a straight line of 45 degrees. In this way, the input gradation is converted based on the output gradation data c2 for the obtained input gradation.

(1-2-1) The case where the designated copy density is lower than the reference copy density will be described based on the concept of converting the designated address.

Assuming that the designated copy density is lower than the reference copy density and the value of Sft in Expression 1 is -25, the pixel G
₀ base address 0 to 255 with respect to gradation 0 to 255 of the input image data corresponding to the designated address -25～23
Converted to 0. Reference address for gradation 0 to 255 of the input image data corresponding to the pixel G ₁ 256 ... 511
Are converted into designated addresses 231 to 486. Pixel G
Reference address 51 for reference addresses 256 to 511 for gradations 0 to 255 of input image data corresponding to ₂
2 to 767 are converted into designated addresses 487 to 742. The gray level of the input image data corresponding to the pixel G ₃ 0 to 25
The reference addresses 768 to 1023 for 5 are converted into designated addresses 743 to 998.

As described above, when the designated address is converted by setting the value of Sft to −25, the gradation 0 of the input image data corresponding to each pixel G ₀ , G ₁ , G ₂ , G ₃ of the dither matrix is obtained. specified address after conversion for 24, the minimum value of the reference address for the input image data of the corresponding pixel _{G 0, G 1, G 2} , G 3 0,256,512,768
Smaller than. That is, there is no output gradation data for the input image data whose input gradation is 0 to 24.

In such a case, the designated address corresponding to the input image data whose input gradation is 0 to 24 is as follows.
It is conceivable that the minimum value of the reference address for the input image data of the corresponding pixels G ₀ , G ₁ , G ₂ , G ₃ is the designated address. In this case, the input gradation is 0 to 2
The output gradation for the input image data of 4 is the output gradation data for the input image data having the same pixel of the corresponding dither matrix and the input gradation of 25. The output grayscale data corresponding to the input grayscale data 0 to 24 in this case is shown by a graph.
Indicated by 0.

In this embodiment, the designated address after conversion of the input image data for each pixel of the dither matrix is smaller than the minimum value of the reference address for the input image data for each pixel of the corresponding dither matrix. The output gradation for (gradations 0 to 24) is
It is determined as follows.

First, a dither thickening order for a dither step is obtained based on output gradation data for four consecutive input gradations. Next, input image data (gray level 0 to 0) in which the designated address after conversion for the input image data for each pixel of the dither matrix is smaller than the minimum value of the reference address for the input image data for each pixel of the corresponding dither matrix. The dither step of the output gradation with respect to 24) is obtained, and the dither thickening order of the output gradation with respect to these input image data (gradations 0 to 24) is obtained.

Then, the designated address after conversion of the input image data for each pixel of the dither matrix becomes the minimum value of the reference address for the input image data for each pixel of the corresponding dither matrix (gradation 25). From the output gradation data for the input image data for each pixel of the dither matrix using the dither fattening order determined as described above, the input address for each pixel of the corresponding dither matrix is Output gradation data is obtained for input image data (gradations 0 to 24) smaller than the minimum value of the reference address for the data.

The relationship between the input gradation data and the output gradation data when the output gradation data for the input image data of gradations 0 to 24 obtained in this manner is corrected is represented by a graph line c2 in FIG. Become. The graph line c2 shifts the graph a in FIG. 8 to the right by a predetermined amount, and furthermore, characteristics of the output gradation data with respect to the input gradations 0 to 24 (shown by a straight line portion c20).
Is a graph that passes through the point of the output gradation with respect to the input gradation data 25 and is a straight line having a gradient θ of 45 degrees.

(1-2-2) The processing method actually performed when the designated copy density is lower than the reference copy density will be described.

In the actual processing, the address generation circuit 12 outputs a reference address corresponding to the input gradation data and the pixel of the dither matrix corresponding to the pixel position designation signal. The CPU 10 reads input gradation-output gradation data corresponding to the developed color and the designated copy density from the data ROM 11 based on the developed color and the designated copy density, and transfers the data to the RAM 14. And RAM1
The transfer destination address of each data of the input gradation-output gradation data transferred to the table memory 13 to the table memory 13 is expressed by the above equation (2).
Convert based on

If the designated copy density is lower than the reference copy density and the value of Sft in equation (2) is -25,
Input gradation data 0 for pixel G ₀ of the dither matrix
The reference transfer destination addresses 0 to 255 of the output gradation data corresponding to .about.255 are converted to new transfer destination addresses 25 to 280. Reference destination address of the output gray-scale data for the input gray level data 0 to 255 for pixel G ₁ two hundred fifty-six to fifty-one
1 is converted into new transfer destination addresses 281 to 536.
Reference destination address 512-767 of output gray-scale data for the input gray level data 0 to 255 for the pixel G ₂ is,
It is converted to the new transfer destination address 537-792. Pixel G
The reference transfer destination addresses 768 to 1023 of the output gradation data corresponding to the input gradation data 0 to 255 for ₃ are converted to new transfer destination addresses 793 to 1048.

Then, each pixel G _{0 of} the dither matrix,
G _1, G _2, new destination address of data for G ₃ is the maximum value of the reference destination address of data corresponding to each pixel _{G 0, G 1, G 2} , G 3 255,511,767,
Data corresponding to data larger than 1023 is deleted.

By the transfer destination address conversion, each storage area 0 to 25 of the table memory 13 for the data corresponding to each pixel G ₀ , G ₁ , G ₂ , G ₃ of the dither matrix.
5, 256-511, 512-767, 768-102
The blank transfer destination addresses 0 to 24, 256 to 2 in which the transfer destination address no longer exists in the small portion of the address value 3
80, 512-536, 768-792. Therefore, output grayscale data for these blank transfer destination addresses is obtained by the same method as that for obtaining output grayscale data for input grayscale data 0 to 24 in (1-2-1).

As described above, each of the storage areas 0 to 255, 256 to 256 of the table memory 13 for the data corresponding to each pixel G ₀ , G ₁ , G ₂ , G ₃ of the dither matrix.
When output gradation data for all of 511, 512 to 767, and 768 to 1023 is created in the RAM 14, the data is transferred from the RAM 14 to the table memory 13. Thereafter, when a reference address corresponding to the input gradation and the pixel position signal is output from the address generation circuit 12, data is output from the corresponding address in the table memory 13.

The original input gray scale-output gray scale characteristic shown by the graph line a in FIG. 6 or FIG. 8 and the input gray scale obtained by the first method shown by the graph line b2 or c2 in FIG. When compared with the output tone characteristics, the magnitude of the output tone for each corresponding point on the characteristic curve does not change. The gradation characteristics can be maintained. Therefore, the relationship between the actually printed density and the input gradation data can be made a linear relationship.
In addition, the range of the output gradation is not extremely narrow as compared with a method in which the offset value is simply added or subtracted from the output gradation obtained by the original input gradation-output gradation data.

(2) Second Method In the second method, as shown in FIG. 10, the input gradation is plotted on the horizontal axis and the output gradation is plotted on the vertical axis, and the original input gradation-output gradation data is represented by a graph line a. In this case, the characteristic is such that the graph is rotated clockwise or counterclockwise by a required angle around the point having the largest output gradation on the graph according to the designated copy density (for example, graph line c1). ,
The input tone-output tone characteristic is converted according to the designated copy density, and as a result of the conversion, the output tone is obtained by a predetermined operation on the input tone data for which the output tone data no longer exists. is there.

(2-1) The concept of the second method will be described first by taking as an example a case where the designated copy density is lower than the reference copy density.

FIGS. 9 and 10 show the characteristics of the output gray scale with respect to the input gray scale, where the horizontal axis indicates the input gray scale and the vertical axis indicates the output gray scale. Graph line a in FIGS. 9 and 10
Indicates an original input gradation-output gradation characteristic. When the designated copy density is lower than the reference copy density, as shown by a graph line c1 in FIG. 9, a graph line a indicating the original input gradation-output gradation characteristic is drawn centering on the maximum point of the output gradation. The input gradation-output gradation characteristic is converted so that the characteristic is rotated by a predetermined angle in the counterclockwise direction.
This rotation angle amount is a value corresponding to the designated copy density. If the graph line a indicating the original input gradation-output gradation characteristic is simply rotated in the counterclockwise direction, there is no output gradation data for a low portion of the input gradation data. The output gradation data for
As shown by the graph line c20 in FIG. 10, the characteristic is such that the gradient θ is represented by a straight line with 45 degrees. In this way,
The input gradation is converted based on the output gradation data c2 for the obtained input gradation.

(2-1-1) A case where the designated copy density is lower than the reference copy density will be described based on the concept of converting the designated address. A characteristic c1 obtained by rotating the graph line a indicating the original input gradation-output gradation data of FIG. 9 counterclockwise by a predetermined amount converts a reference address for the input gradation data by the following conversion formula. It can be obtained by:

[0079]

Sadr = 255− {Gain (255−Oadr) + Offset} Sadr: Designated address Odr: Reference address for input gradation Gain: Gain Offset: Offset

In this case, Offset is always 0.
When the designated copy density is higher than the reference copy density, the value of Gain is set to a value smaller than 1 and becomes smaller as the designated copy density becomes higher. When the designated copy density is lower than the reference copy density, the value of Gain is set to a value larger than 1 and the value becomes larger as the designated copy density becomes lower.

Assuming that the value of Gain corresponding to the characteristic c1 in FIG. 9 is 1.1 and the designated address is converted, the input image corresponding to each pixel G ₀ , G ₁ , G ₂ , G ₃ of the dither matrix is obtained. The designated address after the conversion for the data gradation 0 to (xL-1) (see FIGS. 9 and 10) is the minimum of the reference address for the input image data of the corresponding pixels G ₀ , G ₁ , G ₂ , and G _3. It is smaller than the values 0, 256, 512, 768. That is, the input gradation is 0 to (xL-1)
No output gradation data for the input image data.

In such a case, the input gradation is 0 to (xL
As the designated address corresponding to the input image data of -1), the minimum value of the reference address for the input image data of the corresponding pixels G ₀ , G ₁ , G ₂ , and G ₃ may be used as the designated address. By doing so, the output gradation for the input image data whose input gradation is 0 to (xL-1) is the output gradation for the input image data whose pixels of the corresponding dither matrix are the same and whose input gradation is xL. Key data. In this case, when the output gradation data with respect to the input gradation data 0 to (xL-1) is shown in a graph, it is indicated by a straight line portion c10 in FIG.

In this embodiment, the designated address after conversion of the input image data for each pixel of the dither matrix is smaller than the minimum value of the reference address for the input image data for each pixel of the corresponding dither matrix. The output gradation for (gradation 0 to (xL-1)) is obtained in the same manner as described in (1-2-1) of the first method. The thus obtained input gradation-output gradation characteristic is represented by a graph line c2 in FIG. That is, the graph line c2 rotates the graph a indicating the relationship between the original input gray scale and the output gray scale data by a predetermined angle counterclockwise around the maximum point of the output gray scale, and the input gray scale 0 The characteristics of the output gradation (shown by a straight line portion c20) with respect to .about. (XL-1) are the input gradation data xL.
Is a straight line passing through the point of the output gradation with respect to and having a gradient θ of 45 degrees.

(2-1-2) Next, the processing actually performed will be described. In the actual processing, the address generation circuit 12 outputs a reference address corresponding to the input gradation data and the pixel position designation signal. CPU10
Reads the input gradation-output gradation data corresponding to the developed color and the designated copy density from the data ROM 11 based on the developed color and the designated copy density, and transfers the data to the RAM 14. Then, the transfer destination address of the input gradation-output gradation data transferred to the RAM 14 to the table memory 13 is converted.

In this transfer destination address conversion, the designated address Sadr in the above equation (3) is changed to the reference transfer destination address OTadr.
Is obtained by replacing the reference address Oadr in Equation 3 with the new transfer destination address NTadr, and is expressed by the following equation.

[0086]

[Equation 4] NTadr = {OTadr + 255 (Gain-1) + Offset} {Gain

However, Offset is 0. The Gain value for the designated copy density is predetermined and stored in the data ROM 11 or another ROM. The reference transfer destination address OTadr of the input gradation-output gradation data to be transferred is obtained from the value of Gain corresponding to the designated copy density based on the above formula 4 based on the new transfer destination address NTad.
is converted to r.

When the designated density level is lower than the reference density level (Gain <1), each pixel G ₀ , G ₁ , G ₂ , G _{3 of the} dither matrix is converted by the transfer destination address.
, Each of the storage areas 0 to 255, 256 to 511, 512 to 767 of the table memory 13 for the data corresponding to
A blank transfer destination address in which the transfer destination address no longer exists occurs in a portion where the address value of 768 to 1023 is small. Thus, output gradation data for these blank transfer destination addresses is created.

Each pixel G ₀ , G ₁ , G of the dither matrix
_2, the table memory 13 for the data corresponding to the G ₃
In the case where a blank transfer destination address in which the transfer destination address no longer exists occurs in a portion where the address value of each storage area is small, a method similar to that described in the first method (1-2-2) is used. Thus, the output gradation for the blank transfer destination address is obtained. In this manner, each of the storage areas 0 to 255, 256 to 256 of the table memory 13 for the data corresponding to each pixel G ₀ , G ₁ , G ₂ , G ₃ of the dither matrix.
When output gradation data for all of 511, 512 to 767, and 768 to 1023 is created in the RAM 14, the data is transferred from the RAM 14 to the table memory 13. Thereafter, when a reference address corresponding to the input gradation and the pixel position signal is output from the address generation circuit 12, data is output from the corresponding address in the table memory 13.

(2-2) When the designated copy density is higher than the reference copy density, a graph line a showing the original input tone-output tone characteristic shown in FIG. 9 or FIG. Is converted from an input gradation to an output gradation characteristic so that the characteristic is rotated clockwise by a predetermined angle with respect to. Then, the input gradation is converted based on the output gradation data corresponding to the converted input gradation.

When the designated density level is higher than the reference density level (Gain <1), the destination address is converted by the above equation (4), and the original input gradation-output gradation data is transferred to the table memory. You. However, in this case, the specified address after the conversion for the input image data for each pixel of the dither matrix may be smaller than the minimum value of the reference address for the input image data for each pixel of the corresponding dither matrix. Data corresponding to such a new transfer destination address is not transferred.

When the original input tone-output tone characteristics are compared with the input tone-output tone characteristics obtained by the second method, the output tone for each corresponding point on the characteristic curve in the low density portion is obtained. Is slightly changed, but there is an advantage that the range of reproducible input gradation is not narrowed unlike the first method.

(3) Third Method In the third method, as shown in FIG. 12, the input gradation is plotted on the horizontal axis and the output gradation is plotted on the vertical axis, and the original input gradation-output gradation data is represented by a graph line a. When represented by the graph line a
The input gradation-output gradation is such that the graph is rotated clockwise or counterclockwise by a required angle in the clockwise or counterclockwise direction according to the designated copy density centering on the point where the upper output gradation is the minimum. The tone characteristic is converted in accordance with the designated copy density, and as a result of the conversion, an output tone is obtained by a predetermined operation with respect to an input tone at which no output tone data exists.

(3-1) The concept of the third method will be described first taking an example where the designated copy density is higher than the reference copy density.

FIGS. 11 and 12 show the characteristics of the output gray scale with respect to the input gray scale with the input gray scale on the horizontal axis and the output gray scale on the vertical axis. The graph line a in FIGS. 11 and 12 shows the original input gradation-output gradation characteristics. When the designated copy density is higher than the reference copy density, as shown by a graph line b1 in FIG. 11, a graph line a indicating the original input gradation-output gradation characteristic is drawn centering on the minimum point of the output gradation. The input gradation-output gradation characteristic is converted so that the characteristic is rotated by a predetermined angle in the counterclockwise direction. This rotation angle amount is a value corresponding to the designated copy density. If the graph line a indicating the original input gradation-output gradation characteristic is simply rotated in the counterclockwise direction, the output gradation data corresponding to the high portion of the input gradation data does not exist. Output gradation data corresponding to the slope θ is 4 as shown by the graph line c20 in FIG.
The characteristic is represented by a straight line of 5 degrees. In this way, the input gradation is converted based on the output gradation data c2 for the obtained input gradation.

(3-1-1) The case where the designated copy density is higher than the reference copy density will be described based on the concept of converting the designated address. A characteristic b1 obtained by rotating the original input gradation data-output gradation data a of FIG. 11 in the counterclockwise direction by a predetermined amount is obtained by converting the reference address for the input gradation data by the same conversion equation It is obtained by doing.

[0097]

Sadr = 255- {Gain (255-Odr) + Offset}

When the designated copy density is higher than the reference copy density, the value of Gain is determined to be a value greater than 1 and to increase as the designated copy density increases. Further, when the designated copy density is lower than the reference copy density, the value of Gain is set to a value smaller than 1 and the value becomes smaller as the designated copy density becomes lower. Here, $ 255 · Gain + Off
The value of Offset is adjusted so that the value of set $ becomes 255.

When the designated address is converted using the value of Gain (> 1) and the value of Offset according to the characteristic b1 in FIG. 11, each pixel G ₀ , G ₁ , G ₂ , G of the dither matrix is converted. The designated addresses after conversion for the gradations (xH + 1) to 255 (see FIGS. 11 and 12) of the input image data corresponding to ₃ are the corresponding pixels G ₀ , G
_It becomes larger than the maximum values 255, 511, 767, and 1023 of the reference addresses for the input image data of ₁ , G ₂ , and G ₃ . That is, there is no output gradation data for the input image data whose input gradation is (xH + 1) to 255.

In such a case, when the input gradation is (xH +
As the designated address corresponding to the input image data of 1) to 255, it is conceivable that the maximum value of the reference address for the input image data of the corresponding pixels G ₀ , G ₁ , G ₂ , G ₃ is the designated address. . In this case, the output gradation for the input image data whose input gradation is (xH + 1) to 255 is the output gradation data for the input image data having the same pixel of the corresponding dither matrix and the input gradation of xH. Becomes In this case, when the output gradation data corresponding to the input gradation data (xH + 1) to 255 is shown in a graph, it is indicated by a straight line portion b10 in FIG.

In this embodiment, the designated address after conversion of the input image data for each pixel of the dither matrix becomes larger than the maximum value of the reference address for the input image data for each pixel of the corresponding dither matrix. The output gradation for (gradation (xH + 1) to 255) is obtained in the same manner as described in (1-1-1) of the first method. The thus obtained input gradation-output gradation characteristic is represented by a graph line b2 in FIG. That is, the graph line b2 represents the graph a indicating the relationship between the original input gradation and the output gradation data.
The output gradation characteristic (indicated by the straight line portion b20) for the input gradations (xH + 1) to 255 is rotated by a predetermined angle counterclockwise around the minimum point of the output gradation, and the output gradation for the input gradation data xH. The graph is a straight line that passes through the key point and has a gradient θ of 45 degrees.

(3-1-2) Next, the processing actually performed will be described. In the actual processing, the address generation circuit 12 outputs a reference address corresponding to the input gradation data and the pixel position designation signal. CPU10
Reads the input gradation-output gradation data corresponding to the developed color and the designated copy density from the data ROM 11 based on the developed color and the designated copy density, and transfers the data to the RAM 14. Then, the transfer destination address of the input gradation-output gradation data transferred to the RAM 14 to the table memory 13 is converted. This transfer destination address conversion is represented by the following equation, which is the same as Equation (4).

[0103]

[Equation 6] NTadr = {OTadr + 255 (Gain-1) + Offset} {Gain

Gain and O for specified copy density
The value of ffset is predetermined and stored in the data ROM 11 or another ROM. The reference transfer destination address OTadr of the input gradation-output gradation data to be transferred is
Gain and Offset corresponding to specified copy density
, Based on the above equation 4, the new transfer destination address NT
Converted to adr.

When the designated density level is higher than the reference density level (Gain> 1), each pixel G ₀ , G ₁ , G ₂ , G _{3 of the} dither matrix is converted by the transfer destination address.
, Each of the storage areas 0 to 255, 256 to 511, 512 to 767 of the table memory 13 for the data corresponding to
A blank transfer destination address in which the transfer destination address no longer exists occurs in a portion where the address value of 768 to 1023 is large. Thus, output gradation data for these blank transfer destination addresses is created.

Each pixel G ₀ , G ₁ , G of the dither matrix
_2, the table memory 13 for the data corresponding to the G ₃
In the case where a blank transfer destination address in which the transfer destination address no longer exists occurs in a portion where the address value of each storage area is large, a method similar to that described in (1-1-2) of the first method is used. Thus, the output gradation for the blank transfer destination address is obtained. In this manner, each of the storage areas 0 to 255, 256 to 256 of the table memory 13 for the data corresponding to each pixel G ₀ , G ₁ , G ₂ , G ₃ of the dither matrix.
When output gradation data for all of 511, 512 to 767, and 768 to 1023 is created in the RAM 14, the data is transferred from the RAM 14 to the table memory 13. Thereafter, when a reference address corresponding to the input gradation and the pixel position signal is output from the address generation circuit 12, data is output from the corresponding address in the table memory 13.

(3-2) When the designated copy density is lower than the reference copy density, a straight line a indicating the original input tone-output tone characteristic shown in FIG. 11 or FIG. The input gradation-output gradation characteristic is converted so as to have a characteristic rotated clockwise by a predetermined angle as the center. Then, the input gradation is converted based on the output gradation data corresponding to the converted input gradation data.

When the designated density level is lower than the reference density level (Gain <1), the destination address is converted by the above equation (6), and the original input gradation-output gradation data is transferred to the table memory. You. However, in this case, each pixel G ₀ , G ₁ , G
_2, the new transfer destination address of the data of each G ₃ NTadr
May be larger than the maximum value of the reference transfer destination address OTadr of the data for each of the corresponding pixels G ₀ , G ₁ , G ₂ , G ₃ , but such a new transfer destination address NT
Data corresponding to adr is not transferred.

When the original input tone-output tone characteristics are compared with the input tone-output tone characteristics obtained by the second method, the output tone for each corresponding point on the characteristic curve in the low density portion is obtained. Is slightly changed, but there is an advantage that the range of reproducible input gradation is not narrowed unlike the first method.

(4) Fourth Method The fourth method uses the third method when the copy density specified by the operation unit is higher than the reference density, and uses the third method when the copy density specified by the operation unit is lower than the reference density. This is a method using the second method.

That is, when the designated copy density is higher than the reference density, in FIG. 12, when the original input tone-output tone data is represented by a graph a, the input tone-output tone characteristic is represented by a graph b2. The third
Method. When the designated copy density is lower than the reference density, in FIG. 10, when the original input tone-output tone data is represented by a graph a, the input tone-output tone characteristic becomes a graph c2. The second method is used. At this time, as the conversion formula of the transfer destination address, Expression 4 common to the second and third methods is used.

As the table memory, a table memory having a capacity capable of storing four types of input gradation-output gradation data for the developed colors is used. Of the plurality of types of input gradation-output gradation data stored in the ROM 11, Alternatively, input gradation-output gradation data for four development colors according to the type of the document image may be transferred. In this case, a development color signal is sent from the CPU 10 to the address generation circuit 12, and the address generation circuit 1
2 outputs a 12-bit addressing signal including the development color.

[0113]

According to the present invention, it is possible to reduce the capacity of the storage device for storing the data of the output gradation with respect to the input gradation. Further, the output gradation range is extremely large compared to a method of obtaining an output gradation corresponding to the copy density by simply adding or subtracting an offset value to or from the output gradation obtained from the input gradation-output gradation data. And the relationship between the actually printed density and the input gradation data can be made a linear relationship.

[Brief description of the drawings]

FIG. 1 is an electric block diagram showing a density processing circuit.

FIG. 2 is a schematic diagram showing four pixels of a dither matrix.

FIG. 3 is a schematic diagram showing the contents of a data ROM 11;

FIG. 4 is a schematic diagram showing the inside of a table memory 13;

FIG. 5 is a graph showing output gradation-output gradation characteristics for explaining a first method when a designated copy density is lower than a reference copy density.

FIG. 6 is a graph showing an input tone-output tone characteristic obtained by the first method when a designated copy density is lower than a reference copy density.

FIG. 7 is a graph showing output gradation-output gradation characteristics for explaining the first method when the designated copy density is higher than the reference copy density.

FIG. 8 is a graph showing an input tone-output tone characteristic obtained by the first method when the designated copy density is higher than the reference copy density.

FIG. 9 is a graph showing output gradation-output gradation characteristics for explaining the second method.

FIG. 10 shows an input gradation obtained when the second method is used.
6 is a graph showing output gradation characteristics.

FIG. 11 is a graph showing output gradation-output gradation characteristics for explaining a third method.

FIG. 12 shows an input gradation obtained when the third method is used.
6 is a graph showing output gradation characteristics.

FIG. 13 is a graph showing an example of an original input gradation-output gradation characteristic.

FIG. 14 is a graph showing an input gradation-output gradation characteristic obtained by shifting the original input gradation-output gradation characteristic of FIG. 9 upward.

[Explanation of symbols]

 Reference Signs List 10 CPU 11 Data ROM 12 Address generation circuit 13 Table memory 14 RAM

Claims (3)

(57) [Claims] 1. An output gradation data corresponding to an input gradation is prepared in advance using a dither matrix and stored in a first storage unit, and an output gradation data corresponding to an input gradation is transferred to a second storage unit. A density processing method in which the address of the second storage means is designated based on the input gradation data, whereby the output gradation data stored at the designated address in the second storage means is output. Converting the transfer destination address of the data of the output gradation with respect to the input gradation to the second storage means in accordance with the designated copy density; and the step of converting the data storage area of the output gradation with respect to the input gradation of the second storage means. If there is a blank destination address that no longer exists in the converted destination address in the small address portion, the output floor for those blank destination addresses will be output. A data pattern is obtained based on output grayscale data corresponding to the original input grayscale, and a blank transfer destination address is calculated based on the obtained pattern and output grayscale data corresponding to the minimum value of the converted transfer destination address. Calculating output grayscale data for the output grayscale data, and generating new output grayscale data corresponding to all of the output grayscale data storage areas for the input grayscales of the second storage means from the above steps. Transferring the image data. 2. An output gradation data corresponding to an input gradation is created in advance using a dither matrix and stored in a first storage unit, and an output gradation data corresponding to an input gradation is transferred to a second storage unit. A density processing method in which the address of the second storage means is designated based on the input gradation data, whereby the output gradation data stored at the designated address in the second storage means is output. Converting the transfer destination address of the data of the output gradation with respect to the input gradation to the second storage means in accordance with the designated copy density; and the step of converting the data storage area of the output gradation with respect to the input gradation of the second storage means. If there is a blank destination address that no longer exists in the converted destination address in the portion where the address is large, the output floor for those blank destination addresses is output. A data pattern is obtained based on output grayscale data with respect to the original input grayscale, and a blank transfer destination address is calculated based on the obtained pattern and output grayscale data corresponding to the maximum value of the converted transfer destination address. Calculating output grayscale data for the output grayscale data, and generating new output grayscale data corresponding to all of the output grayscale data storage areas for the input grayscales of the second storage means from the above steps. Transferring the image data. 3. The output gradation data corresponding to the input gradation is created in advance using a dither matrix and stored in the first storage means, and the output gradation data corresponding to the input gradation is transferred to the second storage means. A density processing method in which the address of the second storage means is designated based on the input gradation data, whereby the output gradation data stored at the designated address in the second storage means is output. Converting the transfer destination address of the data of the output gradation with respect to the input gradation to the second storage means in accordance with the designated copy density; and the step of converting the data storage area of the output gradation with respect to the input gradation of the second storage means. If there is a blank destination address that no longer exists in the converted destination address in the small address portion, the output floor for those blank destination addresses will be output. A data pattern is obtained based on output grayscale data corresponding to the original input grayscale, and a blank transfer destination address is calculated based on the obtained pattern and output grayscale data corresponding to the minimum value of the converted transfer destination address. Calculating the output grayscale data for the input grayscale, and in the portion where the address of the data storage area of the output grayscale corresponding to the input grayscale of the second storage means is large, the blank transfer destination address which no longer exists in the converted transfer destination address is used. If it exists, the pattern of the output gradation data for those blank transfer destination addresses is obtained based on the output gradation data for the original input gradation, and the obtained pattern and the maximum value of the converted transfer destination address are calculated. Obtaining output grayscale data for the blank transfer destination address based on the corresponding output grayscale data; Generating new output gradation data corresponding to all of the output gradation data storage areas for the input gradation of the storage means and transferring the data to the second storage means. .