JPH05260289A - Density processing method - Google Patents

Abstract

PURPOSE:To provide a density processing method where the capacity of a storage device for output gradation data as against an input gradation is reduced, the range of an output gradation does not decrease excessively and also the relation of actually printed density with input gradation data becomes linear relation. CONSTITUTION:The transfer destination address of output gradation data to a second storing means (table memory) 13 as against the input gradation is converted by an address generating circuit 12 in accordance with designation copy density. When null transfer destination addresses which do not exist in the transfer destination address after conversion exist in the part of a data storage area in the second storing means 13, where the addresses are small, the pattern of output gradation data for them is obtained based on output gradation data as against the input gradation being an original so that output multilevel data for the null transfer destination address is obtained based on the obtained pattern and output gradation data corresponding to the min. value of the transfer destination address after conversion. Thus, new output gradation data corresponding to all is generated so as to be transferred to the second storing means 13.

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 in 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 illuminated by an exposure lamp, and its reflected light is detected by a CCD and sequentially converted into an electric signal. At this time, the image of the original is color-separated and pixel-separated by the CCD, and decomposed into an electric signal corresponding to the density of each pixel. This electric signal is sent to the image processing unit.

In the image processing section, after the CCD output is digitally converted, in the shading correction section, each color (B,
Variations in the CCD, exposure lamp, etc. are corrected for each of the G, R) signals. Then, the signal of each color (BGR signal) is
Toner density signal (YMC signal) in BGR / YMC converter
Is converted to. Further, the BK generation unit generates a BK signal from the YMC signal.

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

By the way, in a digital copying machine and a digital color copying machine, a density processing section generally performs density processing by a dither method in order to obtain a halftone image. As a digital color copying machine already developed by the applicant, a dither matrix having 2 × 2 pixels as one block is used, and gradation data of a recording pixel with respect to a gradation of a reading pixel (hereinafter referred to as input gradation-output Gradation data) is created in advance and stored in a storage device,
There is a method in which the gradation data (output gradation data) of a recording pixel with respect to the gradation data (input gradation data) of an input read pixel is obtained based on the input gradation-output gradation data. The gradation of the reading pixel is 256 gradations, and the gradation of the recording pixel is 64 gradations. The density processing circuit of this copying machine is shown in FIG.

This density processing circuit comprises 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 used for developing colors (M, C,
(Y, BK), a plurality of types are stored according to the copy density designated by the operation unit and the document image type designated by the operation unit.

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

[0008]

In such a copying machine, since the gradation of the read pixel is 256 and the number of pixels of the dither matrix of one block is 4, one kind of input gradation- The number of output gradation data is 256 × 4
= 1024. Such input gradation-output gradation data is used for each developing 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. It is different for each. Development color type is 4
Assuming that there are 15 types of copy densities specified by the operation unit and the document image types specified by the operation unit are character, photo, and character / photo mixed mode,
When the data ROM 11 is an 8-bit memory, the capacity of the data ROM 11 is 1024 × 15 × 3 × 4 = 1.
It requires 84320 bytes. 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.

Therefore, in order to reduce the capacity of the data ROM 11, the present applicant designates the input gradation-output gradation data in each developing color (M, C, Y, BK) and the operating section. It was drafted that the output gradation adjustment based on the copy density specified by the operation unit and created for each document image type is performed based on the created input gradation-output gradation data. Specifically, after obtaining the output tone for the input tone based on the input tone-the output tone data corresponding to the development color and the original image type designated by the operation unit, the obtained output tone Is a method of obtaining an output gradation by increasing or decreasing by a gradation corresponding to the copy density designated by the operation unit. That is, the offset value is simply added to or subtracted from the output gradation obtained from the input gradation-output gradation data.

FIG. 13 shows the input gradation-output gradation data corresponding to the developing color and the original image type designated by the operating section. FIG. 14 shows that the output gradation is increased by a predetermined gradation by the above method. The output gradation data after correction with respect to the input image data when corrected is shown.

It is preferable that the relationship between the input gradation data and the density actually printed is a linear relationship. However, the actual density relationship with the output gradation data does not have a linear relationship due to the characteristics peculiar to the copying machine. Therefore, when the relationship between the input gradation data and the output gradation data is linear, the input gradation data The relationship of the density actually printed with respect to is no longer linear. Therefore, the input grayscale-output grayscale data has a linear relationship between the input grayscale data and the actually printed density in consideration of the actual density relation to the output grayscale data unique to the copying machine. Will be created.
For this reason, the characteristics of the created input gradation-output gradation data are non-linear characteristics and are unique to the output gradation.

In the method of simply adding or subtracting the offset value to the output gradation obtained from the input gradation-output gradation data, the original input gradation-output gradation characteristic shown in FIG. Input gradation obtained by this method shown
Since the size of the output gradation for each corresponding point on the characteristic curve changes when compared with the output gradation characteristics, the relationship between the density actually printed and the output gradation data is calculated using the original input gradation. -The output gradation characteristic cannot be maintained as it is. Therefore, there is a problem in that the relationship between the input gradation data and the density actually printed is not a linear relationship. Further, in the method of simply adding or subtracting the offset value to the output gradation obtained from the input gradation-output gradation data, there is a problem that the range of the output gradation becomes extremely narrow.

According to the present invention, the capacity of the storage device for storing the output gradation data with respect to the input gradation can be reduced, and the output gradation obtained by the input gradation-output gradation data can be simply offset. Compared with the method of obtaining the output gradation according to the copy density by adding or subtracting the value, the range of the output gradation is not extremely reduced, and the relationship of the density actually printed to the input gradation data is shown. It is an object of the present invention to provide a concentration processing method that can make a linear relationship.

[0014]

According to a first density processing method of the present invention, data of output gradation with respect to input gradation is created in advance using a dither matrix and stored in the first storage means. The data of the output gradation with respect to the gradation is transferred to the second storage means, and the address of the second storage means is specified based on the input input gradation data, so that the specified address in the second storage means is obtained. In the density processing method of outputting the stored output gradation data, a step of converting a transfer destination address of the output gradation data with respect to the input gradation to the second storage means according to the designated copy density, A blank transfer destination address that no longer exists in the converted transfer destination address exists in a portion where the address of the output gradation data storage area with respect to the input gradation of the storage means is small. In this case, the pattern of the output grayscale data for those blank transfer destination addresses is calculated based on the output grayscale data for the original input grayscale, and the calculated pattern and the minimum value of the converted transfer destination address are set. From the step of obtaining the output grayscale data for the blank transfer destination address based on the corresponding output grayscale data, and from the above steps, the new grayscale data corresponding to all the output grayscale data storage areas for the input grayscale of the second storage means is obtained. Creating output gradation data and transferring it to the second storage means.

In the second density processing method according to the present invention, the output gradation data with respect to the input gradation is created in advance using the dither matrix and stored in the first storage means.
The output gradation data for the input gradation is transferred to the second storage means, and the address of the second storage means is specified based on the input input gradation data, whereby the specified address in the second storage means. In the density processing method for outputting the output grayscale data stored in, the step of converting the transfer destination address of the output grayscale data with respect to the input grayscale to the second storage means according to the designated copy density, (2) When there is a blank transfer destination address that is no longer present 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 two storage means is large, the blank transfer destination address is deleted. The output gradation data pattern is obtained based on the output gradation data for the original input gradation, and the obtained pattern and the transfer destination address after conversion are obtained. The step of obtaining output gradation data for the blank transfer destination address based on the output gradation data corresponding to the maximum value of, and from the above steps to all the output gradation data storage areas for the input gradation of the second storage means. Creating a corresponding new output gradation data and transferring it to the second storage means.

In the third density processing method according to the present invention, output gradation data with respect to input gradation is created in advance using a dither matrix and stored in the first storage means.
The output gradation data for the input gradation is transferred to the second storage means, and the address of the second storage means is specified based on the input input gradation data, whereby the specified address in the second storage means. In the density processing method for outputting the output grayscale data stored in, the step of converting the transfer destination address of the output grayscale data with respect to the input grayscale to the second storage means according to the designated copy density, (2) When there is a blank transfer destination address that is no longer present 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 two storage means is small, the blank transfer destination address is deleted. The output gradation data pattern is obtained based on the output gradation data for the original input gradation, and the obtained pattern and the transfer destination address after conversion are obtained. The step of obtaining output gradation data for the blank transfer destination address based on the output gradation data corresponding to the minimum value of , When there are blank transfer destination addresses that no longer exist in the converted transfer destination address, the pattern of the output gradation data for those blank transfer destination addresses is used as the basis for the output gradation data for the input gradation. Asked,
The step of obtaining the output gradation data for the blank transfer destination address based on the obtained pattern and the output gradation data corresponding to the maximum value of the transfer destination address after conversion, and from the above steps, the input floor of the second storage means A step of creating new output grayscale data corresponding to all the data storage areas of the output grayscale for the key and transferring it to the second storage means.

[0017]

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. When there is a blank transfer destination address that is no longer present 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 present, these blank transfer destination addresses are present. Is obtained based on the output grayscale data for the input grayscale that is the original input grayscale data, and based on the obtained pattern and the output grayscale data corresponding to the minimum value of the transfer destination address after conversion. Output grayscale data for the blank transfer destination address is obtained. In this way, new output grayscale data corresponding to all the output grayscale data storage areas with respect to the input grayscale 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. When there is a blank transfer destination address that does not exist 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, these blank transfer destination addresses are present. Is obtained based on the output grayscale data for the input grayscale that is the original input grayscale data, and based on the obtained pattern and the output grayscale data corresponding to the maximum value of the transfer destination address after conversion. Output grayscale data for the blank transfer destination address is obtained. In this way, new output grayscale data corresponding to all the output grayscale data storage areas with respect to the input grayscale 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. When there is a blank transfer destination address that is no longer present 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 present, these blank transfer destination addresses are present. Is obtained based on the output grayscale data for the input grayscale that is the original input grayscale data, and based on the obtained pattern and the output grayscale data corresponding to the minimum value of the transfer destination address after conversion. Output grayscale data for the blank transfer destination address is obtained. When there is a blank transfer destination address that does not exist 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, these blank transfer destination addresses are present. Is obtained based on the output grayscale data for the input grayscale that is the original input grayscale data, and based on the obtained pattern and the output grayscale data corresponding to the maximum value of the transfer destination address after conversion. Output grayscale data for the blank transfer destination address is obtained. In this way, new output grayscale data corresponding to all output grayscale data storage areas with respect to the input grayscale 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.

The density processing circuit includes a CPU 10, a data ROM 11, a table memory 13 and an address generation circuit 12. The CPU 10 includes a RAM 14 that stores 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 an original image specified by each developing color (M, C, Y, BK) and the operation unit. Multiple types are stored according to the type. There are three types of manuscript image types: character, photo, and mixed character / photo. 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. 2 are used as one block. The gradation of the reading pixel is 256 gradations, and the gradation of the recording pixel is 64 gradations.

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

The address generation circuit 12 is an address designation signal represented by a predetermined 10-bit binary number according to the gradation of the input image data and the position signal of the read pixel (pixel of the dither matrix corresponding to the read pixel). Is output. The lower 8 bits of the addressing signal correspond to the input gradations 0 to 255, and the upper 2 bits are the pixel G _{0 of the} dither matrix,
It corresponds to G ₁ , G ₂ , and G ₃ . Pixels G ₀ , G ₁ , G ₂ ,
The values of the upper 2 bits of the addressing signal corresponding to G ₃ are “00”, “01”, “10”, “11”. When the address designation signal is output from the address generation circuit 12, the data stored at the designated address in the table memory 13 is output from the table memory 13 as output grayscale data.

Table 1 shows an example of input gradation-output gradation data. In Table 1, for the reference address, the value of the lower 8 bits of the specified address is the input gradation data 0 to 25.
5 shows an address in the case of one-to-one correspondence with 5. The total gradation is the sum of output gradation data for input image data having the same input gradation among the 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 three characters, which are characters designated by the operation unit, a photograph, and a mixture of characters and photographs.
It is created in advance for each document image type. That is, 12 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 area 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
It becomes 288 bytes.

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

The table memory 13 is an 8-bit memory and is 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. , An area TE ₁ (addresses 256 to 511) for storing output grayscale data corresponding to the input grayscale data for the pixel G ₁ , and an area for storing output grayscale data corresponding to the input grayscale data for the pixel G _2. TE ₂ (addresses 512 to 767), area TE ₃ (768 to 1) for storing output gradation data with respect to input gradation data for the pixel G _3.
No. 023).

In the density processing circuit of this copying machine, the ROM 1
Based on the 12 types of input gradation-output gradation data stored in 1, the output gradation according to the 15-step copy density designated by the operation unit is output.
There are roughly four methods for that purpose.
Each of the first, second, third and fourth methods will be described below.

(1) First Method In the first method, as shown in FIG. 6 or 8, the horizontal axis represents the input gradation and the vertical axis represents the output gradation. Is represented by the graph line a,
In order to obtain a characteristic (graph line b2 or c2) obtained by shifting the graph line a in the left-right direction according to the designated copy density,
The input grayscale-output grayscale characteristic is converted according to the designated copy density, and the output grayscale is obtained by a predetermined calculation with respect to the input grayscale in which there is no output grayscale data as a result of the shift.

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

5 and 6 show the characteristics of the output gradation with respect to the input gradation, with the horizontal axis representing the input gradation and the vertical axis representing the output gradation. Graph line a in FIGS. 5 and 6
Shows the original input gradation-output gradation characteristics. When the designated copy density is higher than the reference copy density, the characteristic is obtained by shifting the graph line a showing the original input gradation-output gradation characteristic to the left as shown by the graph line b1 in FIG. Then, the input gradation-output gradation characteristics are converted. This shift amount is a value corresponding to the designated copy density. If the original graph line a showing the input grayscale-output grayscale characteristic is simply shifted to the left, the output grayscale data for the portion with high input grayscale data does not exist. As shown by the graph line b20 in FIG. 6, the output gradation data has a gradient θ of 45.
The characteristics are 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) A case where the designated copy density is higher than the reference copy density will be described with reference to 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 threshold values of the dither matrix used when creating the input gradation-output gradation data.
It corresponds to a grace scale pattern in an ayer type, a spiral type, a halftone type and the like. In this example, the dither step consists of four stages of 0, 1, 2, 3. The dither step is performed for each pixel G ₀ , G ₁ , G ₂ , G of the dither matrix.
0, 1, 2, 3 are sequentially repeated and assigned in order from the input image data having the lowest input gradation corresponding to ₃ .

The dither fatting 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 has the same input gradation. Which pixel G ₀ , G ₁ , G ₂ , G
_It shows whether the output gradation data for the input image data corresponding to ₃ is higher than the output gradation data for the input gradation one step lower. When the data of the pixel G ₀ becomes larger than the output gradation data for the input gradation that is one step lower, the dithering thickening rank becomes 0. When the data of the pixel G ₁ is larger than the output gradation data for the input gradation that is one step lower, the dither fattening rank is 1. When the data of the pixel G ₂ is larger than the output grayscale data for the input grayscale which is one step lower, the dither fattening order is 2. When the data of the pixel G ₃ is larger than the output grayscale data for the input grayscale which is one step lower, the dithering fattening rank is 3.

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

[0040]

[Formula 1] Sadr = Oadr + Sft Sadr: Designated address after conversion Oadr: Reference address for input gradation Sft: Number of shifts

The value of Sft according to the characteristic b1 in FIG. 5 is +1.
5, the reference addresses 0 to 255 corresponding to the gradations 0 to 255 of the input image data corresponding to the pixel G ₀ are converted into the designated addresses 15 to 270. The reference addresses 256 to 511 corresponding to the gradations 0 to 255 of the input image data corresponding to the pixel G ₁ are converted into the designated addresses 271 to 526. Gradation 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. The reference addresses 768 to 1023 for the gradations 0 to 255 of the input image data corresponding to the pixel G ₃ are designated addresses 783 to 1038.
Is converted to.

As described above, when the value of Sft is set to +15 and the conversion of the designated address is performed, the gradations 241 to 241 of the input image data corresponding to the respective pixels G ₀ , G ₁ , G ₂ and G ₃ of the dither matrix. The converted designated address for 255 is the maximum value 255, 511, 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, the input gradation is 241-2.
As the designated address corresponding to the input image data of 55, it is conceivable to use 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 as the designated address. By doing so, the output gradation for the input image data having the input gradations of 241 to 255 becomes the output gradation data for the input image data having the same pixels of the corresponding dither matrix and having the input gradation of 240. .. Table 3 shows the designated address and the output gradation data for the input gradation data in this case. Further, when the output grayscale data corresponding to the input grayscale data 241 to 255 is shown in a graph, the straight line portion b in FIG.
Indicated by 10.

[0044]

[Table 3]

In this embodiment, the designated address after conversion for the input image data for each pixel of the dither matrix is 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 241-255) is obtained as follows.

First, the dither fatting order for the dither step is obtained based on the data of the output gradation for four consecutive input gradations. For example, the designated address after conversion for the input image data for each pixel of the dither matrix corresponds to the input image data (gradation 240) in which the reference address is the maximum value for the input image data for each pixel of the corresponding dither matrix. The input image data 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 (gradation 23
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), the 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) that is a value smaller than the maximum value of
Obtain the dithering weight ranking for the dithering 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 (gradation 237 to 240) is divided by 4, and the remainder is obtained,
It becomes 0, 1, 2, 3. Further, the dithering fattening order of the output gradation data corresponding to these input image data (gradation 237 to 240) is, for example, in which pixel G ₀ , G ₁ ,
It is obtained by checking whether the data corresponding to G ₂ and G ₃ is large, and becomes 3, 2, 1, 0. That is, the dither steps 0, 1, 2, 3 correspond to the dither fattening rank order 3, 2, 1, 0.

Next, the designated address after conversion for 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 floor). Tones 241 to 255) are used to obtain the dither step of the output gradation, and these input image data (gradation 241) are calculated.
Up to 255), the dithering fattening rank order of the output gradation is obtained. Then, the designated address after 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
0), the designated address after conversion of the input image data of each pixel of the dither matrix is converted into the corresponding address of each pixel of the corresponding dither matrix using the dither thickening order obtained as described above. Output tone data for the input image data (tones 241 to 255) that is larger than the maximum value of the reference address for the input image data is obtained.

Gradations 241-2 obtained in this way
Table 4 shows the output gradations of the 55 input image data.
Further, the relationship between the input gradation data and the output gradation data when the output gradation data with respect to the input image data of the gradations 241-255 is corrected in this way becomes a graph line b2 in FIG. This graph line b2 is obtained by shifting the graph a of FIG. 6 to the left by a predetermined amount, and the characteristic of the output gradation data for the input gradations 241-255 (shown by the straight line portion b20) is the output floor for the input gradation data 240. The graph becomes a straight line that passes through the key points and the gradient θ is 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 the reference address corresponding to the input gradation data and the pixel of the dither matrix corresponding to the pixel position designation signal. Based on the development color and the designated copy density, the CPU 10 sets the input gradation-output gradation data corresponding to the development color and the designated copy density as data RO.
It is read from M11 and transferred to RAM14.

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 formula 1 is used as the reference transfer destination address OTa.
It is obtained by replacing the reference address Oadr in the above formula 1 with the new transfer destination address NTadr, and is represented by the following formula. Here, the standard transfer destination address OTadr
Means that 1024 pieces of data of input gradation-output gradation data are in a one-to-one correspondence with addresses 0 to 1023 of the table memory 13.
It is an address of the table memory 13 corresponding to each data in the case of correspondence.

[0053]

## EQU00002 ## 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 to the new transfer destination address NTadr based on the above equation 2.

When the designated copy density is higher than the reference copy level and the Sft value in the above equation 2 is +15, the input gradation data 0 to the pixel G ₀ of the dither matrix are ₀ to 0.
Reference transfer destination address 0 of output grayscale data for 255
To 255 are converted to new transfer destination addresses -15 to 240. Reference transfer destination addresses 256 to 51 of output gradation data for input gradation data 0 to 255 for the pixel G ₁ .
1 is converted into new transfer destination addresses 241 to 496.
The reference transfer destination addresses 512 to 767 of the output grayscale data corresponding to the input grayscale data 0 to 255 for the pixel G ₂ are
Converted to new transfer destination addresses 497-752. Pixel G
The reference transfer destination addresses 768 to 1023 of the input gradation data 0 to 255 corresponding to ₃ are converted into new transfer destination addresses 753 to 1008.

Then, each pixel G _{0 of} the dither matrix,
The new transfer destination address of data for G ₁ , G ₂ , and G ₃ is ₀ , 256, 512, and 76 which is the minimum value of the reference transfer destination address of data corresponding to each pixel G ₀ , G ₁ , G ₂ , and G _3.
Data corresponding to those smaller than 7 are deleted.

By the transfer destination address conversion, the storage areas 0 to 25 of the table memory 13 for the data corresponding to the pixels G ₀ , G ₁ , G ₂ and G ₃ of the dither matrix.
5, 256-511, 512-767, 768-102
Blank transfer destination addresses 241-255, 49 in which the transfer destination address no longer exists in the portion having a large address value of 3
7-511, 753-767, 1009-1023 occur. Therefore, the output grayscale data for these blank transfer destination addresses are obtained by the same method as the method for obtaining the output grayscale data for the input grayscale data 241 to 255 in Table 3 in (1-1-1) above. Be done. Therefore, the blank transfer destination addresses 241-255, 497-511, 7
The output grayscale data corresponding to 53 to 767 and 1009 to 1023 (the output grayscale data corresponding to the input grayscale data 241 to 255) are the input grayscale data 241 to 255 of Table 3.
Is the same as the output gradation data corresponding to.

In this way, each storage area 0-255, 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 is described.
When the 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. After that, when the reference address corresponding to the input gradation and the pixel position signal is output from the address generation circuit 12, the data is output from the corresponding address of 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 gradation with respect to the input gradation, with the horizontal axis representing the input gradation and the vertical axis representing the output gradation. The graph line a in FIGS. 7 and 8 shows the original input gradation-output gradation characteristics. When the designated copy density is lower than the reference copy density, the graph line c in FIG.
As shown by 1, the input gradation-output gradation characteristics are converted so that the original graph curve a showing the input gradation-output gradation characteristics is shifted to the right. This shift amount is a value corresponding to the designated copy density. If the original graph line a showing the input grayscale-output grayscale characteristic is simply shifted to the right, the output grayscale data for a portion having low input grayscale data does not exist. The output gradation data is represented by the 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) A case where the designated copy density is lower than the reference copy density will be described with reference to the concept of converting the designated address.

If the designated copy density is lower than the reference copy density and the value of Sft in the above equation 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 addresses 256 to 511 corresponding to gradations 0 to 255 of the input image data corresponding to the pixel G _1.
Are converted into designated addresses 231 to 486. Pixel G
_The reference address 51 corresponding to the reference addresses 256 to 511 for the gradations 0 to 255 of the input image data corresponding to ₂
2 to 767 are converted into designated addresses 487 to 742. Input image data gradation 0 to 25 corresponding to the pixel G ₃
The reference addresses 768 to 1023 for 5 are converted into designated addresses 743 to 998.

In this way, when the value of Sft is set to -25 and the conversion of the designated address is performed, 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. The converted designated addresses for .about.24 are the minimum values 0, 256, 512, 768 of the reference addresses for the input image data of the corresponding pixels G ₀ , G ₁ , G ₂ , G _3.
Will be 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:
It is possible to use 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 as the designated address. With this setting, 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 in which the corresponding pixels of the dither matrix are the same and the input gradation is 25. When the output grayscale data corresponding to the input grayscale data 0 to 24 in this case is shown in a graph, the straight line portion c1 in FIG.
It is indicated by 0.

In this embodiment, 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 output gradation for (gradation 0 to 24) is
It is calculated as follows.

First, based on the data of the output gradations for four consecutive input gradations, the dither fatting order for the dither step is obtained. Next, the input address data (gradation 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 output gradation dithering step for 24) is calculated, and the dithering thickening order of the output gradation for these input image data (gradation 0 to 24) is calculated.

Then, the designated address after conversion for 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). Using the dithering fattening order obtained as described above from the output grayscale data for, the designated address after conversion for the input image data for each pixel of the dither matrix is the input image for each pixel of the corresponding dither matrix. Output gradation data for input image data (gradation 0 to 24) that is smaller than the minimum value of the reference address for the data is obtained.

The relationship between the input grayscale data and the output grayscale data when the output grayscale data for the input image data of the grayscales 0 to 24 thus obtained is corrected is shown by the graph line c2 in FIG. Become. The graph line c2 is obtained by shifting the graph a of FIG. 8 to the right by a predetermined amount and the characteristics of the output gradation data with respect to the input gradations 0 to 24 (shown by the straight line portion c20).
Is a graph that passes through the points of the output gradation with respect to the input gradation data 25 and has a slope 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 the 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 the input gradation-output gradation data corresponding to the development color and the designated copy density from the data ROM 11 based on the development color and the designated copy density, and transfers it to the RAM 14. And RAM1
4 is a transfer destination address of each data of the input gradation-output gradation data transferred to the table 4 to the table memory 13.
Convert based on.

If the designated copy density is lower than the reference copy density and the value of Sft in the above equation 2 is -25,
Input gradation data 0 for pixel G ₀ of the dither matrix
The standard transfer destination addresses 0 to 255 of the output grayscale data corresponding to ~ 255 are converted to the new transfer destination addresses 25 to 280. Reference transfer destination addresses 256 to 51 of output gradation data for input gradation data 0 to 255 for the pixel G ₁ .
1 is converted into new transfer destination addresses 281 to 536.
The reference transfer destination addresses 512 to 767 of the output grayscale data corresponding to the input grayscale data 0 to 255 for the pixel G ₂ are
The new transfer destination addresses 537 to 792 are converted. 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 the 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,
The data corresponding to the data larger than 1023 is deleted.

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

In this way, the storage areas 0 to 255, 256 to 256 of the table memory 13 for the data corresponding to the pixels G ₀ , G ₁ , G ₂ and G ₃ of the dither matrix.
When the 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. After that, when the reference address corresponding to the input gradation and the pixel position signal is output from the address generation circuit 12, the data is output from the corresponding address of the table memory 13.

The original input gradation-output gradation characteristics shown by the graph line a in FIG. 6 or 8 and the input gradation obtained by the first method shown by the graph line b2 or c2 in FIG. 6 or FIG. Since the size of the output gradation for each corresponding point on the characteristic curve does not change when compared with the output gradation characteristic, the relationship between the output gradation data and the density actually printed is calculated from the original input gradation-output. It is possible to maintain the gradation characteristics. Therefore, the relationship between the input gradation data and the density actually printed can be made linear.
Further, the range of the output gradation is not extremely narrowed as compared with the method of simply adding or subtracting the offset value to the output gradation obtained from 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. In the case of, the characteristic is obtained by rotating the graph clockwise or counterclockwise by a required angle according to the designated copy density with the point having the largest output gradation on the graph as the center (for example, graph line c1). ,
The input gradation-output gradation characteristics are converted according to the designated copy density, and as a result of the conversion, the output gradation is obtained by a predetermined calculation with respect to the input gradation data where the output gradation data does not exist. is there.

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

9 and 10 show the characteristics of the output gradation with respect to the input gradation with the horizontal axis representing the input gradation and the vertical axis representing the output gradation. Graph line a in FIGS. 9 and 10
Shows the original input gradation-output gradation characteristics. When the designated copy density is lower than the reference copy density, as shown by the graph line c1 in FIG. 9, the graph line a showing the original input gradation-output gradation characteristics is centered on the maximum point of the output gradation. The input gradation-output gradation characteristics are converted so that the characteristics are obtained by rotating the counterclockwise direction by a predetermined angle.
This rotation angle amount is a value corresponding to the designated copy density. If the original graph line a showing the input grayscale-output grayscale characteristic is simply rotated counterclockwise, the output grayscale data does not exist for the portion where the input grayscale data is low. 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 of 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 with reference to the concept of converting the designated address. The characteristic c1 obtained by rotating the original graph line a showing the input grayscale-output grayscale data in FIG. 9 by a predetermined amount in the counterclockwise direction converts the reference address for the input grayscale data by the following conversion formula. It is obtained by

[0079]

## EQU00003 ## Sadr = 255- {Gain (255-Oadr) + Offset} Sadr: Specified address Oadr: Reference address for input gradation Gain: Gain Offset: Offset

Here, Offset is always 0.
The value of Gain is set to a value smaller than 1 when the designated copy density is higher than the reference copy density, and becomes smaller as the designated copy density becomes higher. Further, 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 is determined to be larger as the designated copy density is lower.

When the gain value corresponding to the characteristic c1 in FIG. 9 is set to 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 conversion for the gradation 0 to (xL-1) of the data (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)
There is no output grayscale 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), it is conceivable that the minimum value of the reference addresses for the input image data of the corresponding pixels G ₀ , G ₁ , G ₂ , and G ₃ is set as the designated address. In this way, the output gradation for the input image data whose input gradation is 0 to (xL-1) is the output floor for the input image data whose pixels of the corresponding dither matrix are the same and whose input gradation is xL. It becomes key data. When the output grayscale data corresponding to the input grayscale data 0 to (xL-1) in this case is shown in a graph, it is shown by a straight line portion c10 in FIG.

In this embodiment, 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 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 input gradation-output gradation characteristic thus obtained is represented by the graph line c2 in FIG. That is, the graph line c2 is obtained by rotating the graph a, which shows the relationship between the original input gradation and the output gradation data, by a predetermined angle in the counterclockwise direction around the maximum point of the output gradation and at the input gradation 0. To (xL-1) the characteristics of the output gradation (shown by the straight line portion c20) are the input gradation data xL
The graph is such that a straight line that passes through the points of the output gradation with respect to and the gradient θ is 45 degrees.

(2-1-2) Next, the processing actually performed will be described. In the actual processing, the address generation circuit 12 outputs the 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 development color and the designated copy density from the data ROM 11 based on the development color and the designated copy density, and transfers it 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 expression 3 is converted into the reference transfer destination address OTadr.
Is obtained by replacing the reference address Oadr in the above Expression 3 with the new transfer destination address NTadr, and is expressed by the following expression.

[0086]

## EQU00004 ## 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 the new transfer destination address NTad based on the value of Gain corresponding to the designated copy density, based on the above equation 4.
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 conversion.
Storage areas 0 to 255, 256 to 511, 512 to 767 for the data corresponding to
A blank transfer destination address in which the transfer destination address no longer exists is generated in a portion having a small address value of 768 to 1023. Therefore, output gradation data for these blank transfer destination addresses is created.

Each pixel G ₀ , G ₁ , G of the dither matrix
₂ , table memory 13 for data corresponding to G ₃
When a blank transfer destination address in which the transfer destination address does not exist is generated in a portion having a small address value in each storage area of, the same method as the case described in (1-2-2) of the first method. Then, the output gradation for the blank transfer destination address is obtained. In this way, each storage area 0-255, 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 is
When the 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. After that, when the reference address corresponding to the input gradation and the pixel position signal is output from the address generation circuit 12, the data is output from the corresponding address of the table memory 13.

(2-2) When the designated copy density is higher than the reference copy density, the graph line a showing the original input gradation-output gradation characteristics shown in FIG. 9 or FIG. The input gradation-output gradation characteristics are converted so that the characteristics are rotated clockwise by a predetermined angle. Then, the input gradation is converted based on the output gradation data for the converted input gradation.

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

Comparing the original input gradation-output gradation characteristics with the input gradation-output gradation characteristics obtained by the second method, the output gradation for each corresponding point on the characteristic curve in the low density part However, there is an advantage that the range of reproducible input gradation is not narrowed as in 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 plotted on the graph line a. Graph line a
Input gradation-output floor so that the graph has a characteristic (for example, graph line b) obtained by rotating the graph clockwise or counterclockwise by a required angle in accordance with the designated copy density with the point having the smallest output gradation as the center. The tone characteristic is converted according to the designated copy density, and as a result of the above conversion, the output tone is obtained by a predetermined calculation with respect to the input tone where the output tone data does not exist.

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

11 and 12 show the characteristics of the output gradation with respect to the input gradation with the horizontal axis representing the input gradation and the vertical axis representing the output gradation. 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 the graph line b1 in FIG. 11, the graph line a showing the original input gradation-output gradation characteristics is centered on the minimum point of the output gradation. The input gradation-output gradation characteristics are converted so that the characteristics are obtained by rotating the counterclockwise direction by a predetermined angle. This rotation angle amount is a value corresponding to the designated copy density. If the original graph line a showing the input grayscale-output grayscale characteristic is simply rotated counterclockwise, the output grayscale data for the portion with high input grayscale data does not exist, so the input grayscale of that portion does not exist. The output gradation data for the gradient θ is 4 as shown by the graph line c20 in FIG.
The characteristics are 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) A case where the designated copy density is higher than the reference copy density will be described with reference to the concept of converting the designated address. A characteristic b1 obtained by rotating the original input grayscale-output grayscale data a in the counterclockwise direction by a predetermined amount in FIG. 11 is that the reference address for the input grayscale data is converted by the following conversion equation, which is the same as the mathematical expression 3 above. It is obtained by doing.

[0097]

## EQU00005 ## Sadr = 255- {Gain (255-Oadr) + Offset}

When the designated copy density is higher than the reference copy density, the value of Gain is set to a value larger than 1, and the higher the designated copy density, the larger the value. 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 of FIG. 11, each pixel G ₀ , G ₁ , G ₂ , G of the dither matrix is converted. The designated address after conversion for the gradation (xH + 1) to 255 (see FIGS. 11 and 12) of the input image data corresponding to ₃ is the corresponding pixel G ₀ , G
_It becomes larger than the maximum values 255, 511, 767, and 1023 of the reference address 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, the input gradation is (xH +
As the designated address corresponding to the input image data 1) to 255, 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 set as the designated address. . In this way, the output grayscale data for the input image data whose input grayscale is (xH + 1) to 255 is the output grayscale data for the input image data whose pixels of the corresponding dither matrix are the same and whose input grayscale is xH. Becomes When the output grayscale data for the input grayscale data (xH + 1) to 255 in this case is shown in a graph, it is shown by a straight line portion b10 in FIG.

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

(3-1-2) Next, the processing actually performed will be described. In the actual processing, the address generation circuit 12 outputs the 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 development color and the designated copy density from the data ROM 11 based on the development color and the designated copy density, and transfers it 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 expressed by the following equation, which is the same as the above equation 4.

[0103]

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

Gain and O for the designated 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 the specified copy density
Based on the above equation 4 from the value of
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 conversion.
Storage areas 0 to 255, 256 to 511, 512 to 767 for the data corresponding to
A blank transfer destination address in which the transfer destination address no longer exists occurs in a portion having a large address value of 768 to 1023. Therefore, output gradation data for these blank transfer destination addresses is created.

Each pixel G ₀ , G ₁ , G of the dither matrix
₂ , table memory 13 for data corresponding to G ₃
If a blank transfer destination address in which the transfer destination address does not exist is generated in a portion having a large address value in each storage area of 1), the same method as the case described in (1-1-2) of the first method Then, the output gradation for the blank transfer destination address is obtained. In this way, each storage area 0-255, 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 is
When the 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. After that, when the reference address according to the input gradation and the pixel position signal is output from the address generation circuit 12, the data is output from the corresponding address of the table memory 13.

(3-2) When the designated copy density is lower than the reference copy density, the straight line a showing the original input gradation-output gradation characteristics shown in FIG. 11 or 12 is set to the minimum output gradation point. The input gradation-output gradation characteristics are converted so that the characteristics are rotated clockwise by a predetermined angle with respect to 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 transfer destination address is converted by the above equation 6 and the original input gradation-output gradation data is transferred to the table memory. It However, in this case, each pixel G ₀ , G ₁ , G of the dither matrix is
₂ , New transfer destination address NTadr of data for each G ₃
May become 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 , and G ₃ , but such a new transfer destination address NT
The data corresponding to adr is not transferred.

Comparing the original input gradation-output gradation characteristics with the input gradation-output gradation characteristics obtained by the second method, the output gradation for each corresponding point on the characteristic curve in the low density part However, there is an advantage that the range of reproducible input gradation is not narrowed as in 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 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, the original input gradation-output gradation data is represented by the graph a in FIG. 12, the input gradation-output gradation characteristic is represented by the graph b2. To be the characteristics of the third
Use the method. When the designated copy density is lower than the reference density, when the original input gradation-output gradation data is represented by the graph a in FIG. 10, the input gradation-output gradation characteristic becomes the graph c2. The second method is used. At this time, as a transfer destination address conversion expression, Expression 4 common to the second and third methods is used.

A table memory having a capacity capable of storing four kinds of input gradation-output gradation data for developing colors is used, and among the plurality of kinds of input gradation-output gradation data stored in the ROM 11. Alternatively, the input gradation-output gradation data for four developing colors corresponding to the document image type may be transferred. In this case, the 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 address designation signal containing 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. In addition, the range of the output gradation is much more extreme than the method of obtaining the output gradation corresponding to the copy density by simply adding or subtracting the offset value to the output gradation obtained from the input gradation-output gradation data. It is possible to make the relationship of the density actually printed with respect to the input gradation data into a linear relationship.

[Brief description of drawings]

FIG. 1 is an electrical 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 an output gradation-output gradation characteristic for explaining the first method when the designated copy density is lower than the reference copy density.

FIG. 6 is a graph showing an input gradation-output gradation characteristic obtained by the first method when the designated copy density is lower than the 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 input gradation-output gradation characteristics 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 is an input gradation obtained when the second method is used.
It is a graph which shows an output gradation characteristic.

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

FIG. 12 is an input gradation obtained when the third method is used.
It is a graph which shows an output gradation characteristic.

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

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]

 10 CPU 11 Data ROM 12 Address Generation Circuit 13 Table Memory 14 RAM

Claims (3)

[Claims] 1. Output gradation data for an input gradation is created in advance using a dither matrix and stored in a first storage means, and output gradation data for an input gradation is transferred to a second storage means. The density processing method in which the output gradation data stored at the specified address in the second storage means is output by specifying the address of the second storage means based on the input input gradation data In the step of converting the transfer destination address of the output gradation data with respect to the input gradation to the second storage means according to the designated copy density, and the step of converting the output gradation data storage area with respect to the input gradation of the second storage means. If there are blank transfer destination addresses that do not exist in the converted transfer destination addresses in the part where the address is small, the output floor for those blank transfer destination addresses is output. A data transfer pattern is obtained based on output gradation data for the original input gradation, and a blank transfer destination address is obtained based on the calculated pattern and output gradation data corresponding to the minimum value of the transfer destination address after conversion. From the step of obtaining the output grayscale data corresponding to the input grayscale of the second storage means, and new output grayscale data corresponding to all the output grayscale data storage areas corresponding to the input grayscale of the second storage means are created in the second storage means. And a step of transferring the density. 2. Output gradation data corresponding to the input gradation is created in advance using a dither matrix and stored in the first storage means, and output gradation data corresponding to the input gradation is transferred to the second storage means. The density processing method in which the output gradation data stored at the specified address in the second storage unit is output by specifying the address of the second storage unit based on the input input gradation data In the step of converting the transfer destination address of the output gradation data with respect to the input gradation to the second storage means according to the designated copy density, and the step of converting the output gradation data storage area with respect to the input gradation of the second storage means. If there is a blank transfer destination address that does not exist in the converted transfer destination address in the part where the address is large, the output floor for those blank transfer destination addresses is output. A data transfer pattern is obtained based on the output grayscale data for the original input grayscale, and the blank transfer destination address is obtained based on the obtained pattern and the output grayscale data corresponding to the maximum value of the transfer destination address after conversion. From the step of obtaining the output grayscale data corresponding to the input grayscale of the second storage means, and new output grayscale data corresponding to all the data storage areas of the output grayscale with respect to the input grayscale of the second storage means are created in the second storage means. And a step of transferring the density. 3. Output gradation data for the input gradation is created in advance using a dither matrix and stored in the first storage means, and output gradation data for the input gradation is transferred to the second storage means. The density processing method in which the output gradation data stored at the specified address in the second storage means is output by specifying the address of the second storage means based on the input input gradation data In the step of converting the transfer destination address of the output gradation data with respect to the input gradation to the second storage means according to the designated copy density, and the step of converting the output gradation data storage area with respect to the input gradation of the second storage means. If there are blank transfer destination addresses that do not exist in the converted transfer destination addresses in the part where the address is small, the output floor for those blank transfer destination addresses is output. A data transfer pattern is obtained based on output gradation data for the original input gradation, and a blank transfer destination address is obtained based on the calculated pattern and output gradation data corresponding to the minimum value of the transfer destination address after conversion. And a blank transfer destination address which is no longer present 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 second storage means is large. If present, 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 set. The step of obtaining output gradation data for the blank transfer destination address based on the corresponding output gradation data, and the second step from the above steps A step of creating new output gradation data corresponding to all data storage areas of output gradation with respect to the input gradation of the storage means, and transferring the new output gradation data to the second storage means. ..