JP2018207501A - Image processing apparatus, record device, image processing method, and program - Google Patents

Abstract

To provide an image processing apparatus and an image processing method capable of outputting a uniform and smooth image by suppressing graininess in the entire image in an ink jet recording system in which an image is recorded by using a plurality of types of ink each having different dot power.SOLUTION: An image processing apparatus sets reference data for determining a threshold matrix and a threshold value for each of a plurality of coloring materials so that graininess of dot patterns of the plurality of coloring materials gets smaller than the graininess of a mix dot pattern which is obtained by mixing the dot patterns of the plurality of coloring materials respectively.SELECTED DRAWING: Figure 7

Description

  The present invention relates to an image processing apparatus, an image processing method, a program, and a storage medium for performing quantization processing to form an image on a recording medium.

  When an image is recorded using the pseudo gradation method, it is necessary to quantize multivalued image data. As a quantization method used at this time, an error diffusion method or a dither method is known. In particular, a dither method that determines dot recording or non-recording by comparing a threshold value stored in advance and a gradation value of multi-value data has a smaller processing load than an error diffusion method, and many image processing apparatuses. Has been useful in. In such a dither method, the dispersibility of dots particularly in a low gradation region becomes a problem. For example, Patent Document 1 discloses a threshold matrix having blue noise characteristics as a threshold matrix for obtaining suitable dot dispersibility. A method of using it has been proposed.

  FIGS. 18A to 18C are diagrams for explaining dither processing using a threshold matrix having a blue noise characteristic. FIG. 18A shows an example of image data input to a 10 pixel × 10 pixel region. Here, a state where the gradation value “36” is input to all the pixels is shown. FIG. 18B shows a threshold value matrix prepared corresponding to the 10 pixel × 10 pixel region. Each pixel is associated with any threshold value from 0 to 254. In the case of the dither method, when the gradation value indicated by the multivalued image data is larger than the threshold value, the pixel is designated as dot recording “1”. On the other hand, when the gradation value indicated by the multivalued image data is equal to or smaller than the threshold value, the pixel is designated as “0” for non-recording of dots. FIG. 18C shows the result of quantization by the dither method. Pixels indicating recording “1” are indicated in gray, and pixels indicating non-recording “0” are indicated in white. The distribution of recorded “1” pixels as seen in FIG. 18C depends on the arrangement of threshold values in the threshold matrix. When a threshold value matrix having a blue noise characteristic as shown in FIG. 18B is used, even when multi-value data equal to a predetermined area is input as shown in FIG. 18A, as shown in FIG. 18C. The recording “1” pixel is arranged in a highly dispersive state.

  FIGS. 19A and 19B are diagrams showing blue noise characteristics and human visual characteristics (VTF) at a clear visual distance of 300 mm. In both figures, the horizontal axis is the frequency (cycles / mm), and indicates that the lower the graph is, the lower the frequency is and the higher the frequency is the right. On the other hand, the vertical axis indicates the intensity (power) corresponding to each frequency.

Referring to FIG. 19A, the blue noise characteristic has characteristics such as a low frequency component being suppressed, a sharp rise, and a high frequency component being flat. A frequency fg corresponding to a peak with a sudden rise is referred to as a principal frequency. On the other hand, for the human visual characteristic (VTF) shown in FIG. 19B, the following Dooley approximation is used as an example. Here, l is an observation distance, and f is a frequency.
VTF = 5.05 × exp (−0.138 × πlf / 180)
× (1-exp (0.1 × πlf / 180)) (Equation 1)

  As can be seen from FIG. 19B, the human visual characteristics have high sensitivity in the low frequency region and low sensitivity in the high frequency region. That is, the low frequency component is easily noticeable, but the high frequency component is not easily noticeable. The blue noise characteristics are based on such visual characteristics. In the visual characteristics, the low frequency region with high sensitivity (easy to see) has almost no power, and the high frequency with low sensitivity (hard to see). Has power in the area. For this reason, when a human visually observes an image that has been subjected to quantization processing using a threshold matrix having blue noise characteristics, it is difficult to perceive dot deviation and periodicity, and the image is recognized as a comfortable image.

  On the other hand, Patent Document 2 discloses that even when preferable dispersibility is obtained with individual color materials (that is, a single color), the dispersibility is impaired when an image is recorded with a plurality of color materials (that is, mixed colors), and the graininess is noticeable. A dither method is disclosed to eliminate the situation. Specifically, a method is disclosed in which one common dither matrix having suitable dispersibility as shown in FIG. 18B is prepared, and quantization processing is performed while shifting each other's threshold value between a plurality of colors. Yes. Hereinafter, the quantization method disclosed in Patent Document 2 is referred to as intercolor processing in this specification. According to the inter-color processing, dots of different colors are recorded in a state where they are mutually exclusive and highly dispersible in the low gradation part, so that it is possible to realize a suitable image quality even in a mixed color image.

US Pat. No. 5,113,310 US Pat. No. 6,867,884

  However, in the inter-color processing, the graininess can be made inconspicuous in a dot pattern in which a plurality of colors of ink dots are mixed, but the dispersibility of specific ink dots is sometimes noticeable. According to Patent Document 2, priority is given to increasing the dispersibility of black ink having the strongest dot power among a plurality of color inks, and a threshold value is set without applying an offset among a plurality of channels for a common threshold value matrix. Black is set for the channel. However, for example, when a full color image is expressed using cyan, magenta, and yellow without using black, setting the lowest threshold area channel to one of cyan and magenta having the same dot power results in the other graininess. May be noticeable. This will be specifically described below.

  FIG. 20 is a diagram illustrating a dot recording state when the inter-color processing is performed by assigning each of the three colors of ink to the first channel to the third channel. While using an equal threshold matrix having blue noise characteristics, the first color data addressed to the first channel is set without offset, and the second color data is based on the first color data. Set the offset threshold. Furthermore, an offset threshold value is set for the third color data based on the first color data and the second color data. For this reason, in the sum 1940 of the dot pattern 1910 of the first color and the dot patterns of the first color to the third color, the dots are suitably dispersed and the graininess is also suppressed. On the other hand, the second color dot pattern 1920 and the third color dot pattern 1930 are inferior in both dispersibility and graininess.

  21A to 21C are diagrams that quantitatively show the granularity of the dot pattern shown in FIG. In FIG. 21A, the horizontal axis indicates the spatial frequency, and the vertical axis indicates the average intensity (power) corresponding to the spatial frequency. It can be seen that for the first color and mixed color dot patterns, the power in the low frequency region is sufficiently suppressed, and the peak is located in the vicinity of the principal frequency fg. That is, it has the characteristic of blue noise. On the other hand, for the second color and the third color, there is a certain amount of power in the low frequency region, there is no sharp peak, and the power has already decreased in the vicinity of the principal frequency fg. That is, it has no blue noise characteristics.

  FIG. 21B is a diagram showing a response value obtained by multiplying the frequency characteristic shown in FIG. 21A by the human visual characteristic (VTF) shown in FIG. 18B. FIG. 21C shows the integrated value of FIG. It means that the larger the response value or its integral value, the more visually perceived the granularity of the dot pattern. In the case of this example, the response values and integration values of the second color and the third color are larger than the response values and integration values of the first color and the mixed dot pattern, and the graininess is easily sensed. That is, even if the patent document 2 is adopted to suppress the graininess in the mixed color image, the granularity of the dot pattern of a specific ink color is conspicuous in the same mixed color image, which may impair the image quality.

  The present invention has been made to solve the above problems. Therefore, an object of the present invention is to provide an image processing apparatus and an image processing method capable of suppressing graininess in the entire image when recording a color image by a pseudo gradation method using a plurality of color materials. Is to provide.

  Therefore, the present invention is an image processing apparatus for recording an image on a recording medium using a plurality of color materials, and data for acquiring multivalued data corresponding to each of the plurality of color materials for a target pixel. One threshold value matrix is set for each of the plurality of color materials from among a plurality of threshold value matrices configured by an acquisition unit and a plurality of threshold values arranged, and corresponds to the target pixel from the set threshold value matrix. Threshold acquisition means for acquiring a first threshold and reference data to be referred to for performing predetermined processing on the first threshold acquired by the threshold acquisition means for each of the plurality of color materials. The reference color setting means set from the multi-value data of the color material and the reference color setting means for each of the plurality of color materials set by the reference color setting means with respect to the first threshold value By calculating the second threshold value by performing the predetermined processing based on the reference data, and comparing the multi-value data and the second threshold value for each of the plurality of color materials, Generating means for generating quantized data for recording dots, and the threshold value acquiring means and the reference color setting means have a granularity of dot patterns of each of the plurality of color materials determined by the quantized data, The threshold matrix and the reference data are set for each of the plurality of color materials so as to be smaller than the graininess of the mixed dot pattern obtained by mixing the dot patterns of the plurality of color materials. To do.

  According to the present invention, in a color image recorded using a plurality of color materials, graininess can be suppressed in the entire image.

It is a block diagram which shows the structure of control of an inkjet recording system. 1 is a schematic perspective view of a recording apparatus that can be used in the present invention. FIG. 3 is a block diagram for explaining a control configuration of a recording apparatus. 2 is a block diagram showing a configuration of an ASIC 3001. FIG. It is a flowchart for demonstrating the process of image data. It is a block diagram for demonstrating the detail of a quantization process. (A) And (b) is the block diagram and flowchart for demonstrating the process in an intercolor processing part. It is a figure which shows the threshold value range judged as recording (1) in the general intercolor process. (A) And (b) is the figure which showed the granularity of each dot pattern quantitatively. It is a figure which shows the threshold value range judged as recording (1) in 1st Embodiment. (A) And (b) is the figure which showed the granularity of each dot pattern quantitatively. (A) And (b) is a figure which compares the granularity of this embodiment and the conventional method. It is a figure which shows the threshold value range judged as recording (1) in 2nd Embodiment. It is a figure which shows the conversion state of the gray line in the color conversion process of 3rd Embodiment. It is a figure which shows the threshold value range judged as recording (1) in 3rd Embodiment. It is a figure which shows the conversion state of the gray line in the color conversion process of 4th Embodiment. It is a figure which shows the threshold value range judged as recording (1) in 4th Embodiment. (A)-(c) is a figure for demonstrating a dither process. (A) And (b) is a figure which shows a blue noise characteristic and visual characteristics. It is a figure which shows the recording state of a dot at the time of performing an intercolor process by 3 colors. (A)-(c) is a figure which shows the granularity of a dot pattern quantitatively. It is a figure which shows the example of a 1st threshold value matrix and a 2nd threshold value matrix.

(First embodiment)
FIG. 1 is a block diagram showing a control configuration of an ink jet recording system applicable to the present invention. The ink jet recording system according to the present embodiment includes an image supply device 3, an image processing apparatus 2, and an ink jet recording apparatus 1 (hereinafter also simply referred to as a recording apparatus). Image data supplied from the image supply device 3 is subjected to predetermined image processing by the image processing apparatus 2, then sent to the recording apparatus 1, and recorded using ink as a color material.

  In the recording apparatus 1, the recording apparatus main control unit 101 is for controlling the entire recording apparatus 1 and includes a CPU, a ROM, a RAM, and the like. The recording buffer 102 can store the image data before being transferred to the recording head 103 as raster data. The recording head 103 is an ink jet recording head having a plurality of recording elements that can eject ink as droplets, and ejects ink from each recording element in accordance with image data stored in the recording buffer 102. In the present embodiment, it is assumed that recording element arrays for three colors of cyan, magenta, and yellow are arranged on the recording head 103.

  The paper supply / discharge motor control unit 104 controls the conveyance of the recording medium and the paper supply / discharge. A recording apparatus interface (I / F) 105 exchanges data signals with the image processing apparatus 2. The I / F signal line 114 connects both. As the type of the I / F signal line 114, for example, a specification of Centronics can be applied. The data buffer 106 temporarily stores the image data received from the image processing apparatus 2. A system bus 107 connects the functions of the recording apparatus 1.

  On the other hand, in the image processing apparatus 2, the image processing apparatus main control unit 108 performs various processes on the image supplied from the image supply device 3 to generate image data that can be recorded by the recording apparatus 1. Yes, it includes a CPU, ROM, RAM and the like. The characteristic configuration of the present invention shown in FIG. 6 and FIG. 7A described later is also provided in the image processing apparatus main control unit 108, and the flowchart described in FIG. 5 and FIG. The CPU of the main control unit 108 executes. An image processing apparatus interface (I / F) 109 exchanges data signals with the recording apparatus 1. The external connection interface (I / F) 113 exchanges image data and the like with the image supply device 3 connected to the outside. The display unit 110 displays various information to the user, and for example, an LCD can be applied. The operation unit 111 is a mechanism for a user to perform a command operation. For example, a keyboard or a mouse can be applied. The system bus 112 connects the image processing apparatus main control unit 108 to each function.

  As for the recording device 1, in addition to the image data supplied from the image processing device 2, image data stored in a storage medium such as a memory card or image data from a digital camera may be directly received and recorded. I can do it.

  FIG. 2 is a schematic perspective view of the recording apparatus 1 used in the present embodiment. The recording apparatus 1 prints image data received from a digital camera and a function as a normal PC printer that receives data from the image processing apparatus 2 and prints, and image data recorded on a storage medium such as a memory card. It has a function.

  The main body that forms the outer shell of the recording apparatus 1 includes exterior members such as a lower case 1001, an upper case 1002, an access cover 1003, a paper feed tray 1007, and a discharge tray 1004. The lower case 1001 forms a substantially lower half portion of the apparatus 1 and the upper case 1002 forms a substantially upper half portion of the main body, and a storage space for storing each mechanism is formed by a combination of both cases.

  The paper feed tray 1007 can stack and hold a plurality of recording media. When a feed recording command is input, the uppermost sheet is automatically fed into the apparatus. . On the other hand, one end of the discharge tray 1004 is rotatably held by the lower case 1001, and the opening formed in the front surface of the lower case 1001 can be opened and closed by the rotation. When the recording operation is executed, the discharge tray 1004 is rotated to the front side to open the opening, so that the recording sheets can be discharged from here and the discharged recording sheets can be sequentially stacked. It is like that. The paper discharge tray 1004 stores two auxiliary trays 1004a and 1004b, and the support area of the recording medium can be expanded and reduced in three stages by pulling each tray forward as necessary. It has become.

  In the space inside the apparatus, a recording head 103 for recording an image on a recording medium, a carriage mounted with the recording head 103 and an ink tank and movable in the X direction in the figure, and the recording medium by a predetermined amount in the Y direction A transport mechanism for transporting to the center is provided.

  When a recording command is input, the recording medium carried into the apparatus from the paper feed tray 1007 is conveyed to a recordable area by the recording head 103. When one recording scan is performed by the recording head 103, the transport mechanism transports the recording medium in the Y direction by a distance corresponding to the recording width D. By repeating the recording scan by the recording head 103 and the conveying operation of the recording medium as described above, an image is formed stepwise on the recording medium. The recording medium on which recording has been completed is discharged to a paper discharge tray 1004.

  One end of the access cover 1003 is rotatably held by the upper case 1002 so that an opening formed on the upper surface can be opened and closed. By opening the access cover 1003, it is possible to replace the recording head 103, the ink tank, and the like housed in the main body. Although not shown here, a protrusion is provided on the back surface of the access cover 1003 for detection by a microswitch installed in the main body when the access cover 1003 is closed. That is, the open / closed state of the access cover 1003 can be detected based on the detection result of the protrusion by the microswitch.

  A power key 1005 is provided on the upper surface of the upper case 1002 so that it can be pressed. An operation panel 1010 including a liquid crystal display unit 110 and various key switches is provided on the upper surface of the upper case 1002.

  A paper gap selection lever 1008 is a lever for adjusting the interval between the ink ejection surface of the recording head 103 and the surface of the recording medium. The card slot 1009 is an opening for receiving an adapter in which a memory card can be mounted. The image data stored in the memory card is sent to the control unit 3000 of the recording apparatus via an adapter inserted in the card slot 1009, and after being subjected to predetermined processing, is recorded on the recording medium. Examples of the memory card (PC) include a compact flash (registered trademark) memory, smart media, and a memory stick. A viewer (liquid crystal display unit) 1011 displays an image for each frame, an index image, and the like when searching for an image to be printed from images stored in a memory card. In this embodiment, the viewer 1011 is detachable from the main body of the recording apparatus 1. A terminal 1012 is a terminal for connecting a digital camera, and a terminal 1013 is a USB bus connector for connecting a personal computer (PC).

  FIG. 3 is a block diagram for explaining a control configuration of the recording apparatus 1. In the control unit 3000 (control board), a DSP 3002 (digital signal processor) has a CPU therein, and performs various image processing and control of the entire recording apparatus. The memory 3003 includes a program memory 3003a that stores a program to be executed by the CPU of the DSP 3002, a RAM area that stores a program at the time of execution, and a memory area that functions as a work memory that stores image data and the like. The printer engine 3004 is equipped with a printer engine for recording a color image using a plurality of color inks.

  A USB bus connector 3005 is a port for connecting the digital camera 3012. A connector 3006 connects the viewer 1011. A USB bus hub 3008 (USB HUB) is a line concentrator for USB transfer to the printer engine 3004. When image data that has already been subjected to predetermined image processing is received from the externally connected image processing apparatus 2 (PC), the USB bus hub 3008 transmits the image data as it is to the printer engine. As a result, the connected PC 2 can directly exchange data and signals with the printer engine 3004 (that is, functions as a general PC printer).

  The power connector 3009 inputs a DC voltage converted from commercial AC by the power source 3013 into the apparatus. Note that signal exchange between the control unit 3000 and the printer engine 3004 is performed via the USB bus 3021 or the IEEE1284 bus 3022.

  FIG. 4 is a block diagram showing the configuration of the ASIC 3001. The PC card interface unit 4001 reads image data stored in the attached PC card 3011 and writes data to the PC card 3011. An IEEE 1284 interface unit 4002 exchanges data with the printer engine 3004. The IEEE 1284 interface unit is a bus used when recording image data stored in the digital camera 3012 or the PC card 3011. The USB interface unit 4003 exchanges data with the PC 2. The USB host interface unit 4004 exchanges data with the digital camera 3012. The operation panel / interface unit 4005 inputs various operation signals from the operation panel 1010 and outputs display data to the display unit 1006. A viewer interface unit 4006 controls display of image data on the viewer 1011. An interface 4007 is an interface unit that controls various switches, the LED 4009, and the like. The CPU interface unit 4008 controls data exchange with the DSP 3002. These units are connected by an internal bus (ASIC bus) 4010. These control programs are configured in a multitask format in which each functional module is made into a task.

  FIG. 5 is a flowchart for explaining the processing of the image data performed by the image processing apparatus main control unit 108 of the present embodiment. This processing is executed by a CPU provided in the image processing apparatus main control unit 108 according to a program stored in the ROM. In FIG. 5, when image data of the target pixel is input from the image supply device 3 (step S200), the image processing apparatus main control unit 108 first performs color correction in step S201. The image data received by the image processing apparatus 2 from the image supply apparatus 3 is 8-bit luminance data of R (red), G (green), and B (blue) for expressing a standardized color space such as sRGB. is there. In step S201, the luminance data is converted into RGB 12-bit luminance data corresponding to the color space unique to the recording apparatus. As a method for converting the signal value, a known method such as referring to a look-up table (LUT) stored in advance in a ROM or the like can be employed.

  In step S202, the image processing apparatus main control unit 108 converts the converted RGB data into 16-bit gradation data (density) of C (cyan), M (magenta), and Y (yellow), which are ink colors of the recording apparatus. Data). At this stage, a 16-bit gray image is generated for three channels (three colors). Also in the ink color separation process, a look-up table (LUT) stored in advance in a ROM or the like can be referred to as in the color correction process.

  In step S <b> 203, the image processing apparatus main control unit 108 performs a predetermined quantization process on 16-bit gradation data corresponding to each ink color, and converts the data into several bits of quantization data. For example, in the case of quantization to ternary values, 16-bit gradation data is converted into 2-bit data of level 0 to level 2. The quantization process will be described in detail later.

  In subsequent step S204, the image processing apparatus main control unit 108 performs index expansion processing. Specifically, one dot arrangement pattern is selected in association with the level obtained in step S203 from among a plurality of dot arrangement patterns that define the number and positions of dots to be recorded in individual pixels. At this time, the dot arrangement table may have a form in which the number of dots to be recorded in an area corresponding to each pixel varies depending on the level value, or a form in which the dot size varies depending on the level value. May be.

  When such index expansion processing is completed, the dot data is output as binary data (step S205). This process is completed.

  Each process shown in each step of FIG. 5 is processed in the ink jet recording system of the present embodiment. Up to which process is performed by the image processing apparatus 2, and after which process is performed by the recording apparatus 1. There is no specific categorization of whether to do it. For example, when the image processing apparatus 2 performs quantization, the quantized data is transferred to the recording apparatus 1, and the recording apparatus main control unit 101 uses the index pattern stored in the data buffer 106 in step S204. Index development may be performed to control the recording operation. Further, depending on the performance of the recording apparatus 1, it is possible to directly receive multi-value RGB image data and perform all steps S201 to S204.

  FIG. 6 is a block diagram for explaining the details of the quantization processing executed in step S203 of FIG. In the quantization process of the present embodiment, first, a process related to an input value is performed, then a process related to a threshold value is performed, and finally a quantization process using a dither method is performed. These series of processes are performed in parallel for each color (each channel). Hereinafter, each process will be described in detail with reference to FIG.

  The image data acquisition unit 301 acquires 16-bit gradation data indicating the density of each pixel. It is assumed that the image data acquisition unit 301 of this embodiment can receive a maximum 16-bit signal for eight colors. The figure shows a state in which 16-bit data for each of the first to third colors is input.

  The noise addition processing unit 302 adds predetermined noise to the 16-bit gradation data. By adding noise, even when gradation data of the same level is continuously input, it is possible to avoid a state in which the same pattern is continuously arranged and to relieve streaks and textures. In the noise addition processing unit 302, noise is generated for each pixel and added to the input value by multiplying a predetermined random table, a fixed intensity, and a fluctuation intensity according to the input value. Here, the random table is a table for setting positive / negative of noise, and positive, zero, or negative is set for each pixel position. The random table of the present embodiment can have a maximum of eight surfaces, and the size of each table can be arbitrarily set. The fixed strength indicates the strength of the noise amount, and the magnitude of the noise is determined by the magnitude. In the present embodiment, it is possible to appropriately adjust the amount of noise by setting an optimal random table and fixed strength for each print mode in accordance with the granularity of the image, the level of streaks, texture, and the like. .

  The normalization processing unit 303 normalizes the range of each level to 12 bits after associating the gradation value of each pixel represented by 16 bits with the level value at which index expansion is possible in step S204. This will be specifically described below. When the index expansion processing in step S204 is processing corresponding to n values of level 0 to level (n−1), the normalization processing unit 303 equally divides 65535 gradations represented by 16 bits into (n−1). . Furthermore, the range corresponding to each level is normalized to 12 bits (4096 gradations). Thereby, 12-bit data in a state associated with any one of level 0 to level (n−1) is obtained for each pixel.

  For example, when the index expansion processing corresponds to three values of level 0, level 1, and level 2, the normalization processing unit 303 divides the 65535 gradation represented by 16 bits into two equal parts. Then, the gradation values 0 to 32767 and the gradation values 32768 to 65535, which are the respective ranges, are normalized to 12 bits (0 to 4095 gradations). Pixels with input gradation values 0 to 32767 in the first range are output level 0 or level 1 by the subsequent quantization process, and pixels with input gradation values 32768 to 65535 in the second range are quantized in the latter stage. Level 1 or level 2 is output by the conversion processing. With the above control, the subsequent quantization process can be performed by the same process regardless of the number of quantizations (n).

  The processes of the image data acquisition unit 301 to the normalization unit 303 described above are performed in parallel for the gradation data of each color. That is, in this embodiment, 12-bit data for cyan, magenta, and yellow is generated and input to the dither processing unit 311.

  In the dither processing unit 311, 12-bit data (processing target data) to be quantized is transmitted to the quantization processing unit 306 as it is. On the other hand, 12-bit data of colors other than the processing target data is input to the inter-color processing unit 304 as reference data. The inter-color processing unit 304 performs a predetermined process on the threshold acquired by the threshold acquisition unit 305 based on the reference data to determine a final threshold, and transmits this to the quantization processing unit 306. The quantization processing unit 306 determines recording (1) or non-recording (0) by comparing the processing target data with the threshold value input from the inter-color processing unit 304.

  The threshold acquisition unit 305 selects a corresponding threshold matrix from a plurality of dither patterns 310 stored in a memory such as a ROM, and acquires a threshold corresponding to the pixel position of the processing target data. In the present embodiment, the dither pattern 310 is a threshold matrix formed so that threshold values of 0 to 4095 have blue noise characteristics, and various patterns such as 512 × 512 pixels, 256 × 256 pixels, 512 × 256 pixels, and the like. It can exhibit various sizes and shapes. That is, a plurality of threshold matrixes having different sizes and shapes are stored in advance in the memory, and the threshold storage unit 305 selects a threshold matrix corresponding to the print mode and ink color from these. Then, a threshold corresponding to the pixel position (x, y) of the processing target data is provided to the inter-color processing unit from the plurality of thresholds arranged in the selected threshold matrix.

  The present invention is characterized in the inter-color processing method in the inter-color processing unit 304. Before explaining the characteristic inter-color processing method, first, general inter-color processing as disclosed in Patent Document 2 will be described.

  FIGS. 7A and 7B are a block diagram and a flowchart for explaining the configuration and steps of the process in the intercolor processing unit 304. FIG. The inter-color processing unit 304 uses 12-bit data corresponding to colors other than the processing target data as reference data, performs predetermined processing on the threshold acquired by the threshold acquisition unit 305 using these reference data, and quantizes the processing target data A threshold for calculating the threshold is calculated. For example, when the processing target data is cyan 12-bit data, the reference data is magenta and yellow 12-bit data. In FIG. 7A, the processing target data is shown as In1 (x, y), and the reference data is shown as In2 (x, y) and In3 (x, y). Here, (x, y) indicates a pixel position, which is a coordinate parameter for the threshold value acquisition unit 305 to select a threshold value corresponding to the pixel position of the processing target data from the threshold value matrix.

Referring to FIG. 7A, the reference data In2 (x, y) and In3 (x, y) input to the intercolor processing unit 304 are first input to the threshold offset amount calculation unit 308 (step S401). Then, the threshold value offset amount calculation unit 308 calculates the threshold value offset Ofs_1 (x, y) for the processing target data In1 (x, y) using these reference data (step S402). In the present embodiment, the threshold offset value Ofs_1 (x, y) is calculated by (Expression 2).
Ofs_1 (x, y) = Σi [Ini (x, y)] (Expression 2)

  Here, i is the reference data (hereinafter referred to as actual reference data) used for obtaining a threshold for the processing target data In1 among the reference data In2 (x, y) and In3 (x, y). It is a parameter to show. The number and type of such actual reference data are designated in advance for each processing target data.

In this example, there is no actual reference data when the processing target data is In1 (x, y), and the actual reference data when the processing target data is In2 (x, y) is In1 (x, y). y). Further, the actual reference data when the processing target data is In3 (x, y) is In1 (x, y) and In2 (x, y). Therefore, the offsets Ofs_1 (x, y) to Ofs_3 (x, y) with respect to the individual processing target data In1 (x, y) to In3 (x, y) can be expressed as follows from (Equation 2). .
Ofs_1 (x, y) = Σi [In (x, y)]
= 0 (Formula 2-1)
Ofs_2 (x, y) = Σi [In (x, y)]
= In1 (x, y) (Formula 2-2)
Ofs — 3 (x, y) = Σi [In (x, y)]
= In1 (x, y) + In2 (x, y) (Formula 2-3)

  As described above, when the threshold value offset values Ofs_1 (x, y) to Ofs_3 (x, y) are calculated, these are input to the threshold value offset amount adding unit 309. The threshold value offset amount adding unit 309 acquires the threshold value Dth corresponding to the coordinates (x, y) of the processing target data In (x, y) from the threshold value acquiring unit 305 (step S403).

In step S404, the threshold offset amount addition unit 309 subtracts the threshold offset value Ofs_1 (x, y) input from the threshold offset amount calculation unit 308 from the threshold Dth (x, y) input from the threshold acquisition unit 305. Then, the quantization threshold value Dth ′ (x, y) is obtained.
Dth ′ (x, y) = Dth (x, y) −Ofs — 1 (x, y) (Formula 3)

At this time, if Dth ′ (x, y) becomes a negative value, Dth_max (the maximum threshold value of the dither pattern) is added to obtain a quantization threshold Dth ′ (x, y). Thereby, the quantization threshold Dth ′ is always Dth ′ = 0 to Dth_max.
That is,
When Dth ′ (x, y) <0, Dth ′ (x, y) = Dth ′ (x, y) + Dth_max (Expression 4)
And

  When the quantization threshold Dth ′ (x, y) is obtained by (Expression 3) or (Expression 4), the quantization processing unit 306, the processing target data In1 (x, y) and the quantization threshold Dth ′ (x, y) y) is compared, and dot recording (1) or non-recording (0) for the pixel position (x, y) is determined. This process is completed.

  After that, as described in the flowchart of FIG. 5, the index expansion process is performed on the quantized data Out1 (x, y) represented by several bits, and the dot pattern to be recorded at the pixel position (x, y) It is determined. At this time, the number (size) of dots recorded at the pixel position (x, y) is, for example, 1 dot (or small dot) when the level value is 1 and 2 dots (or when the level value is 2). The number is set to a number (size) corresponding to the level value.

  FIG. 8 illustrates a case where the first to third multi-value data (In1 to In3) are input for each of the first to third colors, among the plurality of thresholds 0 to Dth_max arranged in the threshold matrix. It is a figure which shows the range of the threshold value judged as recording (1). The horizontal axis represents threshold values Dth0 to 4094, and 1710 represents Dth_max (the maximum threshold value of the dither pattern). Each line indicates a threshold range in which dots are arranged. In the case of this example, For the first color, Ofs_1 = 0 from (Equation 2-1). Therefore, a pixel position corresponding to a threshold value of 0 to In1 (1702 to 1703) among 0 to Dth_max is set in recording (1).

  For the second color, Ofs_2 = In1 from (Equation 2-2). Therefore, when quantization is performed with the threshold value Dth ′ obtained according to (Equation 3) and (Equation 4), among the threshold values 0 to Dth_max arranged in the dither pattern 310, the threshold values of In1 to In1 + In2 (1705 to 1706) are recorded in (1) Is set.

  For the third color, Ofs_3 = In1 + In2 from (Equation 2-3). Therefore, when quantization is performed using the threshold value Dth ′ obtained according to (Expression 3) and (Expression 4), among the threshold values 0 to Dth_max arranged in the threshold value matrix, In1 + In2 to In1 + In2 + In3 (1708 to 1709) are set in the recording (1). Will be. However, in this example, it is assumed that In1 + In2 + In3 exceeds Dth_max. In this case, for an area exceeding Dth_max, an area corresponding to the remainder obtained by dividing (In1 + In2 + In3) by Dth_max, that is, a threshold value of 0 to In1 + In2 + In3-Dth_max is set to recording (1). That is, the threshold ranges determined as recording (1) are In1 + In2-Dth_max (1708-1710) and 0-In1 + In2 + In3-Dth_max (1707-1711).

  As described above, in the general inter-color processing, while using a common threshold value matrix, a unique quantization threshold value Dth ′ is obtained for each color by using each input value as an offset value. Then, by using the newly obtained quantization threshold Dth ′ in the quantization process, it is possible to arrange dots so that a dot recording pattern in which a plurality of colors are mixed has a blue noise characteristic.

  However, as already described, when a color having a strong dot power exists in a mixed dot pattern of a plurality of colors, the graininess of the dot pattern of that color may stand out and the image quality may be impaired. Here, the dot power corresponds to the conspicuousness of one dot, and depends on, for example, the brightness of the dot when one dot is recorded on the recording medium. A dot with lower lightness (higher density) has stronger dot power, and even if a dot pattern having blue noise characteristics is formed together with dots of other colors, the dot pattern of that color has blue noise characteristics. Otherwise, the graininess will be noticeable.

FIGS. 9A and 9B show the amount of granularity of each dot pattern when the first color is cyan, the second color is magenta, and the third color is yellow in the above-described general intercolor processing. FIG. FIG. 9A shows the response values of the dot pattern and mixed dot pattern for each ink color, and FIG. 9B shows the integral value. The response value is a result of multiplying the frequency characteristic of each dot pattern by the human visual characteristic (VTF) and the dot power coefficient. For visual characteristics (VTF), the Dooley approximation already shown as (Equation 1) is used. Further, the dot power coefficient is a value according to the strength of the dot power, and increases as the brightness decreases. As an example, the dot power coefficient of each color is set from cyan lightness L ^* = 55, magenta lightness L ^* = 55, and yellow lightness L ^* = 85 in the CIEL ^* a ^* b ^* color space.

  Since the mixed dot pattern and the cyan dot pattern of the first color have blue noise characteristics, both the response value and the integrated value thereof are relatively low. In addition, the yellow dot pattern, which is the third color, does not have blue noise characteristics, but has a lower dot power than cyan and magenta, so that both the response value and its integrated value are sufficiently low. On the other hand, the magenta dot pattern, which is the second color, does not have a blue noise characteristic and has a strong dot power. Therefore, both the response value and the integrated value thereof are relatively high.

  As a result of intensive studies, the present inventors have considered that the granularity tends to be perceived visually when the response value of the dot pattern of a specific color exceeds the response value of the mixed dot pattern in view of the above. It was judged. Then, setting the threshold matrix and the reference color in the inter-color processing so that the response value of the dot pattern of each ink color is smaller than the response value of the mixed dot pattern suppresses the graininess of the entire image. Therefore, they have come to the knowledge that they are effective. Hereinafter, characteristic inter-color processing of the present embodiment will be described.

Referring to FIGS. 7A and 7B again, in this embodiment, when the processing target data is cyan data In1 (x, y), the threshold value acquisition unit 305 has a first threshold value having a blue noise characteristic. Select a matrix. Then, the threshold offset amount calculation unit 308 sets the threshold offset value, that is, reference data to null.
Ofs_1 (x, y) = 0

When the processing target data is magenta data In2 (x, y), the threshold acquisition unit 305 selects a second threshold matrix having blue noise characteristics but different from the first threshold matrix. The threshold offset amount calculation unit 308 sets the threshold offset value, that is, reference data to null.
Ofs_2 (x, y) = 0

When the processing target data is yellow data In3 (x, y), the threshold acquisition unit 305 selects the second threshold matrix in the same manner as In2 (x, y). The threshold offset amount calculation unit 308 sets a threshold offset value, that is, reference data to In2 (x, y).
Ofs_3 (x, y) = In2 (x, y)

  The threshold value offset amount adding unit 309 subtracts the threshold value offset value Ofs_1 (x, y) input from the threshold value offset amount calculating unit 308 from the threshold value Dth (x, y) in the threshold value matrix selected by the threshold value acquiring unit 305. The quantization threshold Dth ′ (x, y) is obtained. Thereafter, the same processing as the normal intercolor processing already described is performed.

  FIG. 22 is an example of a first threshold matrix and a second threshold matrix that can be employed in the present embodiment. Each threshold matrix has a blue noise characteristic, but the threshold distributions are different from each other. These two matrices are not particularly limited as long as they have blue noise characteristics, and may or may not have a correlation. However, since the blue dot formed by overlapping cyan dots and magenta dots has a stronger dot power than cyan and magenta, the threshold values are set so that such blue dots are not generated as much as possible. It is preferable. Note that the first threshold value matrix and the second threshold value matrix may be, for example, two matrices that have the same threshold value matrix shifted in the vertical direction or the horizontal direction.

  FIG. 10 is a diagram showing a range of threshold values determined for recording (1) in the threshold value matrix for each color in the present embodiment. In the present embodiment, the threshold offset value is Ofs_1 = 0 for cyan, which is the first color. Therefore, the pixel position corresponding to 0 to In1 (902 to 903) among the threshold values 0 to Dth_max of the first threshold value matrix is set to recording (1).

  Also for magenta, which is the second color, the threshold offset value is Ofs_2 = 0. Therefore, a pixel position corresponding to a threshold value of 0 to In2 (905 to 906) among 0 to Dth_max of the second threshold value matrix is set to recording (1).

  For yellow, which is the third color, the threshold offset value is Ofs_3 = In2. Therefore, among the thresholds 0 to Dth_max of the second threshold matrix, In2 to In2 + In3 (908 to 909) is set to recording (1).

  According to the present embodiment, blue noise characteristics can be obtained with a cyan dot pattern and a magenta dot pattern having a relatively strong dot power, and a mixed dot pattern of magenta and yellow.

  FIGS. 11A and 11B are views showing the granularity of each dot pattern in this embodiment in the same manner as the conventional general intercolor processing shown in FIG. 9. FIG. 11A shows the response values of the dot patterns and mixed dot patterns for each ink color, and FIG. 11B shows the integrated values thereof. The cyan dot pattern which is the first color and the magenta dot pattern which is the second color have blue noise characteristics, and both the response value and the integrated value thereof are relatively low. In addition, the yellow dot pattern, which is the third color, does not have blue noise characteristics, but its dot power is very small compared to cyan and magenta, so both the response value and its integrated value are sufficiently low. . On the other hand, the mixed dot pattern of cyan, magenta, and yellow is composed of two overlapping dot patterns each having a blue noise characteristic, so that the responsiveness and integrated value are slightly different from those of conventional inter-color processing. Get higher. However, compared to a state where no inter-color processing is employed, the responsiveness and integral value, that is, graininess can be suppressed sufficiently low.

  FIGS. 12A and 12B show a case where a threshold matrix having no correlation is used for each of cyan, magenta, and yellow without using intercolor processing, and a case where this embodiment is used. It is a figure which compares the granularity in a mixed dot pattern. It can be seen that the mixed dot pattern in the present embodiment has both a response value and an integrated value thereof kept low compared to the mixed dot pattern obtained by the conventional method.

  In other words, according to the present embodiment, while sufficiently suppressing the graininess of the mixed dot pattern of cyan, magenta, and yellow, the response value of the dot pattern for each color is further lower than the response value of the mixed dot pattern. Can be suppressed. As a result, the granularity of the entire image can be suppressed as compared with the conventional case, and a smooth image can be output.

As long as even overlapping of dots between different colors is suppressed, the graininess can be suppressed to some extent regardless of the frequency characteristics of the mixed dot pattern. For this reason, regarding the second threshold value matrix shared by two colors, the graininess of the entire image can be kept lower than before even if it does not necessarily have a blue noise characteristic. Further, in order to avoid dot overlap, it is not always necessary to perform inter-color processing between two colors, and the threshold setting method has been devised so that mutually exclusive threshold regions are set as much as possible. It ’s fine. For example, when the two colors are magenta and yellow, the overlap of magenta and yellow dots can be suppressed as much as possible by obtaining the yellow threshold value using the following equation.
Y threshold = maximum threshold in threshold matrix−M threshold

In the above, the response value for quantitatively comparing the graininess of the mixed dot pattern and the dot pattern for each color is obtained from the frequency characteristics of the dot pattern, the human visual characteristics (VTF), and the dot power coefficient. However, the response value is not limited to this. For the visual characteristics, an expression other than the Dooley approximate expression can be adopted, and for the dot power coefficient, not the lightness L ^* but also the optical density when recorded on the recording medium can be adopted. Further, the response value may be set by adopting an evaluation value such as a known RMS granularity or Wiener spectrum.

  Hereinafter, a case where various combinations of ink colors used for expressing a color image will be described as another embodiment.

(Second Embodiment)
In this embodiment, as in the first embodiment, cyan, magenta, and yellow ink systems are used. However, in the present embodiment, inter-color processing is performed using the first threshold value matrix for cyan and yellow, and the second threshold value matrix is used for magenta.

In this embodiment, the block diagram and the flowchart shown in FIGS. 7A and 7B can also be used. In the present embodiment, when the processing target data is cyan data In1 (x, y), the threshold acquisition unit 305 selects a first threshold matrix having blue noise characteristics. The threshold offset amount calculation unit 308 sets the threshold offset value to null.
Ofs_1 (x, y) = 0

When the processing target data is magenta data In2 (x, y), the threshold acquisition unit 305 selects a second threshold matrix having blue noise characteristics but different from the first threshold matrix. The threshold offset amount calculation unit 308 sets the threshold offset value to null.
Ofs_2 (x, y) = 0

When the processing target data is yellow data In3 (x, y), the threshold acquisition unit 305 selects the first threshold matrix in the same manner as In1 (x, y). The threshold offset amount calculation unit 308 sets the threshold offset value to In1 (x, y).
Ofs_3 (x, y) = In1 (x, y)

  As the first threshold value matrix and the second threshold value matrix, for example, the one shown in FIG. 22 can be used as in the first embodiment.

  FIG. 13 is a diagram illustrating a range of threshold values determined for recording (1) in the set threshold value matrix for each color in the present embodiment. In the present embodiment, the threshold offset value of cyan, which is the first color, is Ofs_1 = 0. Therefore, among the threshold values 0 to Dth_max of the first threshold matrix, the pixel position corresponding to 0 to In1 (1202 to 1203) is set to recording (1).

  Also for magenta, which is the second color, the threshold offset value is Ofs_2 = 0. Therefore, pixel positions corresponding to 0 to In2 (1208 to 1209) among the threshold values 0 to Dth_max of the second threshold value matrix are set to recording (1).

  The threshold matrix of the third color yellow is Ofs_3 = In1. Therefore, among the threshold values 0 to Dth_max of the first threshold value matrix, the pixel position corresponding to In1 to In1 + In3 (1205 to 1206) is set to recording (1).

  According to the present embodiment, blue noise characteristics can be obtained with a cyan dot pattern and a magenta dot pattern, and a mixed dot pattern of cyan and yellow with relatively strong dot power. As in the first embodiment, while the graininess of the mixed dot pattern of cyan, magenta, and yellow is sufficiently suppressed, the response value of the dot pattern for each color is set lower than the response value of the mixed dot pattern. Can be suppressed. As a result, the granularity of the entire image can be suppressed lower than before.

(Third embodiment)
In the present embodiment, an ink system in which black is added to cyan, magenta, and yellow is used. The black ink has a lower lightness than the other inks, and here, the lightness is L ^* = 10. For this reason, the dot power of black is the largest among the four inks. In such an ink system, in this embodiment, inter-color processing is performed using the first threshold matrix for black and cyan, and inter-color processing is performed using the second threshold matrix for magenta and yellow.

  In the present embodiment, in step S202 of FIG. 5, the image processing apparatus main control unit 108 converts RGB data into 16-bit gradation data of C (cyan), M (magenta), Y (yellow), and K (black). Decompose into (density data). In step S203, quantization processing is performed on each of these four colors.

  FIG. 14 shows a gray line conversion state of the look-up table referred to by the image processing apparatus main control unit 108 in the color conversion process of step S202. The abscissa indicates each grid point of gradation that shifts from white to black, and the ordinate indicates output signal values corresponding to the respective colors in 256 gradations. The output signal value of each color corresponds to input data In0 to In3 for the quantization processing unit.

  As can be seen from the drawing, in the highlight to medium density region from white to gray, a black image is not used, and a gray image is expressed using only cyan, magenta, and yellow. About black, it is used in a medium density-high density area. In a high density area where black ink is used, since many dots of other colors are already recorded, the recording density of ink dots is sufficiently high, and the granularity of black dots is rarely regarded as a problem. In the present embodiment, in consideration of such a situation, combinations of threshold matrixes and reference colors used for each ink are set as follows.

First, when the processing target data is black data In0 (x, y), the threshold acquisition unit 305 selects a first threshold matrix having blue noise characteristics. The threshold offset amount calculation unit 308 sets the threshold offset value to null.
Ofs_0 (x, y) = 0

When the processing target data is cyan data In1 (x, y), the threshold acquisition unit 305 selects the first threshold matrix in the same manner as In0 (x, y). The threshold offset amount calculation unit 308 sets the threshold offset value to In0 (x, y).
Ofs_1 (x, y) = In0 (x, y)

When the processing target data is magenta data In2 (x, y), the threshold acquisition unit 305 selects a second threshold matrix having blue noise characteristics but different from the first threshold matrix. The threshold offset amount calculation unit 308 sets the threshold offset value to null.
Ofs_2 (x, y) = 0

When the processing target data is yellow data In3 (x, y), the threshold acquisition unit 305 selects the second threshold matrix in the same manner as In2 (x, y). The threshold value offset amount calculation unit 308 sets the threshold value offset value to In2 (x, y).
Ofs_3 (x, y) = In2 (x, y)

  In the present embodiment as well, the first threshold value matrix and the second threshold value matrix shown in FIG. 22 can be used.

  FIG. 15 is a diagram showing threshold ranges determined as recording (1) in the set threshold matrix in the present embodiment. Here, a case where a signal having a gray intermediate density (128) is input is shown. In this case, the black ink signal value In0 (x, y) is 0, as described with reference to FIG. Therefore, there is no threshold area determined as recording (1).

  For cyan, the threshold offset value is Ofs_1 = In0 (x, y). However, as described above, In0 (x, y) = 0. Therefore, pixel positions corresponding to 0 to In1 (1404 to 1405) among the threshold values 0 to Dth_max of the first threshold value matrix are set to recording (1).

  For magenta, the threshold offset value is Ofs_2 = 0. Therefore, among the threshold values 0 to Dth_max of the second threshold matrix, pixel positions corresponding to 0 to In2 (1407 to 1408) are set to recording (1).

  For yellow, the threshold offset value is Ofs_3 = In2. Therefore, among the thresholds 0 to Dth_max of the second threshold matrix, In2 to In2 + In3 (1410 to 1411) is set to recording (1).

  According to the present embodiment, since black dots are not recorded in the highlight to medium density region where graininess is an issue, the inter-color processing is effectively performed as in the first and second embodiments. A dot pattern similar to that treated as the first color can be obtained. That is, blue noise characteristics can be obtained with a cyan dot pattern, a magenta dot pattern, and a mixed dot pattern of magenta and yellow. As a result, as in the above embodiment, while suppressing the granularity of the mixed dot pattern sufficiently, the response value of the dot pattern of each color is suppressed lower than the response value of the mixed dot pattern, and the granularity of the entire image is suppressed. can do.

  In the third embodiment, the same threshold matrix is used for cyan and black, and the same threshold matrix is used for magenta and yellow. However, as in the relationship between the first embodiment and the second embodiment, cyan and magenta are used. Can also be replaced. That is, the combination of the threshold matrix and the reference color used for each ink can be set as follows.

When the processing target data is the black data In0 (x, y), the threshold acquisition unit 305 selects a first threshold matrix having blue noise characteristics. The threshold offset amount calculation unit 308 sets the threshold offset value to null.
Ofs_0 (x, y) = 0

When the processing target data is cyan data In1 (x, y), the threshold acquisition unit 305 selects a second threshold matrix that has blue noise characteristics but is different from the first threshold matrix. The threshold offset amount calculation unit 308 sets the threshold offset value to null.
Ofs_1 (x, y) = 0

When the processing target data is magenta data In2 (x, y), the threshold acquisition unit 305 selects the first threshold matrix in the same manner as In0 (x, y). The threshold offset amount calculation unit 308 sets the threshold offset value to In0 (x, y).
Ofs_2 (x, y) = In0 (x, y)

When the processing target data is yellow data In3 (x, y), the threshold acquisition unit 305 selects the second threshold matrix in the same manner as In1 (x, y). The threshold offset amount calculation unit 308 sets the threshold offset value to In1 (x, y).
Ofs_3 (x, y) = In1 (x, y)

  Even in such a case, a dot pattern similar to the case where magenta is handled as the first color of the intercolor processing can be obtained without recording black dots in the highlight to medium density region. . That is, blue noise characteristics can be obtained with a cyan dot pattern, a magenta dot pattern, and a mixed dot pattern of cyan and yellow, and the graininess of the entire image can be made lower than before.

(Fourth embodiment)
In the present embodiment, an ink system in which gray is further added to cyan, magenta, yellow, and black is used. Gray ink has higher brightness than cyan, magenta, and black. Here, the brightness is L ^* = 70. For this reason, the gray dot power is the second smallest after yellow. In such an ink system, in this embodiment, inter-color processing is performed using the first threshold value matrix for black, cyan, and gray, and inter-color processing is performed using the second threshold value matrix for magenta and yellow.

  In the present embodiment, in step S202 of FIG. 5, the image processing apparatus main control unit 108 converts RGB data into C (cyan), M (magenta), Y (yellow), K (black), and Gray (gray). To 16-bit gradation data (density data). In step S203, quantization processing is performed on each of these five colors.

  FIG. 16 shows the gray line conversion state of the lookup table referred to by the image processing apparatus main control unit 108 in the color conversion step of step S202. The horizontal axis indicates each grid point of gradation from white to black, and the vertical axis indicates the output signal value corresponding to each ink color in 256 gradations. As can be seen from the drawing, in the medium density region from highlight to gray, black ink is not used, and a gray image is expressed using cyan, magenta, yellow, and a relatively large number of grays. It can be seen that the signal values of cyan and magenta are lower than in FIG. 14 that does not use the gray ink described in the third embodiment. That is, according to the present embodiment, by introducing gray ink, the number of dots of cyan and magenta, which have higher dot power than gray, can be reduced.

  On the other hand, as in the third embodiment, black is not used in the highlight to medium density region, but is used in the medium to high density region. In a high density region where black ink is used, the recording density of ink dots is sufficiently high, and the granularity of black dots is unlikely to be a problem. In the present embodiment, in consideration of such a situation, combinations of threshold matrixes and reference colors used for each ink are set as follows.

First, when the processing target data is black data In0 (x, y), the threshold acquisition unit 305 selects a first threshold matrix having blue noise characteristics. The threshold offset amount calculation unit 308 sets the threshold offset value to null.
Ofs_0 (x, y) = 0

When the processing target data is cyan data In1 (x, y), the threshold acquisition unit 305 selects the first threshold matrix in the same manner as In0 (x, y). The threshold offset amount calculation unit 308 sets the threshold offset value to In0 (x, y).
Ofs_1 (x, y) = In0 (x, y)

When the processing target data is magenta data In2 (x, y), the threshold acquisition unit 305 selects a second threshold matrix having blue noise characteristics but different from the first threshold matrix. The threshold offset amount calculation unit 308 sets the threshold offset value to null.
Ofs_2 (x, y) = 0

When the processing target data is yellow data In3 (x, y), the threshold acquisition unit 305 selects the second threshold matrix in the same manner as In2 (x, y). The threshold value offset amount calculation unit 308 sets the threshold value offset value to In2 (x, y).
Ofs_3 (x, y) = In2 (x, y)

When the processing target data is gray data In4 (x, y), the threshold acquisition unit 305 selects the first threshold matrix in the same manner as In0 (x, y) and In1 (x, y). The threshold value offset amount calculation unit 308 sets the threshold value offset value to In0 (x, y) + In1 (x, y).
Ofs — 4 (x, y) = In0 (x, y) + In1 (x, y)

  As for the first threshold matrix and the second threshold matrix, those shown in FIG. 22 can be used as in the above embodiment.

  FIG. 17 is a diagram showing a range of threshold values determined as recording (1) in the threshold value matrix for each color in the present embodiment. Here, a case where a signal (128) having a gray intermediate density is input is shown. In the case of this density, the black ink signal value In0 (x, y) is 0, as described in FIG. Therefore, there is no threshold area determined as recording (1).

  For cyan, the threshold offset amount is Ofs_1 = In0 (x, y). However, as described above, In0 (x, y) = 0. Therefore, among the threshold values 0 to Dth_max of the first threshold value matrix, the pixel position corresponding to 0 to In1 (1604 to 1605) is set to recording (1).

  For gray, the threshold offset amount is Ofs — 4 = In0 + In1. Therefore, among the threshold values 0 to Dth_max of the first threshold value matrix, In0 + In1 to In0 + In1 + In4, that is, In1 to In1 + In4 (1607 to 1608) is substantially set to the recording (1).

  For magenta, the threshold offset value is Ofs_2 = 0. Therefore, among the threshold values 0 to Dth_max of the second threshold matrix, the pixel position corresponding to 0 to In2 (1610 to 1611) is set to recording (1).

  For yellow, the threshold offset value is Ofs_3 = In2. Therefore, among the thresholds 0 to Dth_max of the second threshold matrix, In2 to In2 + In3 (1613 to 1614) is set to the recording (1).

  According to the present embodiment, since black dots are not recorded in the highlight to medium density region where graininess is an issue, the inter-color processing is effectively performed as in the first and second embodiments. A dot pattern similar to that treated as the first color can be obtained. That is, blue noise characteristics can be obtained with a cyan dot pattern, a magenta dot pattern, a mixed dot pattern of cyan and gray, and a mixed dot pattern of magenta and yellow. On the other hand, blue noise characteristics cannot be obtained with a yellow dot pattern and a gray dot pattern. However, since gray and yellow have a much lower dot power than cyan and magenta, the response values and their integrated values as described in the first embodiment are sufficiently low.

  As described above, according to the present embodiment, cyan and magenta with relatively high dot power are set to the first color in effect of intercolor processing, and yellow and gray with relatively low dot power are processed with intercolor processing. The second and subsequent colors are set. Thus, as in the above embodiment, while sufficiently suppressing the granularity of the mixed dot pattern, the response value of the individual dot pattern for each color is suppressed lower than the response value of the mixed dot pattern, thereby reducing the granularity of the entire image. It can be suppressed more than before.

  Note that the effect of the present embodiment of using gray ink can be obtained without using black ink in combination. When black ink is not used, the signal values of cyan, magenta, and yellow increase in place of black in the medium density to high density region in the graph shown in FIG. Even in such a form, it is the same that the gray signal value becomes higher than the cyan and magenta signal values in the highlight to intermediate density region, and the effect of suppressing the graininess can be obtained.

  Also in the fourth embodiment, the relationship between cyan and magenta can be exchanged, and the combination of threshold matrix and reference color used for each ink can be set as follows.

When the processing target data is the black data In0 (x, y), the threshold acquisition unit 305 selects a first threshold matrix having blue noise characteristics. The threshold offset amount calculation unit 308 sets the threshold offset value to null.
Ofs_0 (x, y) = 0

When the processing target data is cyan data In1 (x, y), the threshold acquisition unit 305 selects a second threshold matrix that has blue noise characteristics but is different from the first threshold matrix. The threshold offset amount calculation unit 308 sets the threshold offset value to null.
Ofs_1 (x, y) = 0

When the processing target data is magenta data In2 (x, y), the threshold acquisition unit 305 selects the first threshold matrix in the same manner as In0 (x, y). The threshold offset amount calculation unit 308 sets the threshold offset value to In0 (x, y).
Ofs_2 (x, y) = In0 (x, y)

When the processing target data is yellow data In3 (x, y), the threshold acquisition unit 305 selects the second threshold matrix in the same manner as In1 (x, y). The threshold offset amount calculation unit 308 sets the threshold offset value to In1 (x, y).
Ofs_3 (x, y) = In1 (x, y)

When the processing target data is gray data In4 (x, y), the threshold acquisition unit 305 selects the first threshold matrix in the same manner as In0 (x, y) and In2 (x, y). The threshold value offset amount calculation unit 308 sets the threshold value offset value to In0 (x, y) + In2 (x, y).
Ofs — 4 (x, y) = In0 (x, y) + In2 (x, y)

  Even in such a case, cyan and magenta are treated as the first color between colors without recording black dots in the highlight to medium density region, and yellow and gray are treated as the second and subsequent colors. A dot pattern similar to that handled can be obtained. That is, blue noise characteristics can be obtained with a cyan dot pattern, a magenta dot pattern, a cyan and yellow mixed dot pattern, and a magenta and gray mixed dot pattern, and the graininess of the entire image can be made lower than before. it can.

  As described above, according to the present invention, the combination of the threshold value matrix and the reference color in the inter-color processing is set so that the color having a relatively large dot power becomes the actual first color of the inter-color processing. . As a result, while the graininess of the mixed dot pattern is sufficiently suppressed, the response value of the dot pattern for each color can be kept lower than the response value of the mixed dot pattern. Can be kept low.

  In the above, in the four embodiments, the case of using cyan, magenta, yellow, black, and gray inks has been described as an example. However, the present invention can be applied to systems using various types of ink. For example, light cyan ink or light magenta ink can be used instead of gray as light-colored light color ink, and special color inks such as red, green, and blue can be used in combination. In any case, while preparing a plurality of threshold matrixes, the threshold matrix and the reference data in the inter-color processing are set so that the color having a relatively large dot power becomes the actual first color of the inter-color processing. If the combination is set, the above-described effects of the present invention can be obtained.

  In the above description, the 16-bit data is quantized to several levels by the quantization process and then binarized by the index expansion process. However, the quantization process executed in step S203 is a multi-level quantization process. Not necessarily. That is, in the quantization process in step S203, 16-bit gradation data may be directly converted into 1-bit binary data by dithering. In this case, the index expansion process shown in step S204 is omitted, and the binary data obtained in step S203 is output to the recording apparatus 1 as it is. Of course, the number of input / output data bits in other steps in FIG. 2 is not limited to the above-described embodiment. In order to maintain accuracy, the number of output bits may be larger than the number of input bits, and the number of bits may be variously adjusted depending on the application and situation.

  Furthermore, although the above embodiment has been described using the serial type recording apparatus shown in FIG. 2, the present invention can also be applied to a full line type recording apparatus.

  Furthermore, a program that realizes one or more functions of the above-described embodiment is supplied to a system or apparatus via a network or a storage medium, and one or more processors in a computer of the system or apparatus read and execute the program. The present invention is feasible. It can also be realized by a circuit (for example, ASIC) that realizes one or more functions.

1 Recording device
2 Image processing device
108 Image processing apparatus main controller
301 Image data acquisition unit
305 Threshold acquisition unit
306 Quantization processing unit
308 Threshold offset calculation unit
309 Threshold offset addition unit
310 Dither Pattern

  Therefore, the present invention provides first multi-value data corresponding to a first color, second multi-value data corresponding to a second color different from the first color, and the first color and the An acquisition means for acquiring third multi-value data corresponding to a third color having a lightness higher than that of the second color; a second threshold value matrix; and a second threshold value array different from the first threshold value matrix. A storage unit that stores a threshold value matrix, and a threshold value corresponding to a position of a pixel to be processed among a plurality of threshold values arranged in the first threshold value matrix and the second threshold value matrix stored in the storage unit. And performing the quantization process using the first quantized data corresponding to the first color, the second quantized data corresponding to the second color, and the third corresponding to the third color. Generating means for generating quantized data of Based on the first quantized data, the second quantized data, and the third quantized data, the recording means for applying the recording material and the recording medium are moved relative to each other while the recording means is moved. Image processing for performing data processing for recording an image on a recording medium by applying the recording material of the first color, the recording material of the second color, and the recording material of the third color to the recording medium by In the apparatus, the generation unit generates the first quantized data by comparing the first multi-value data and a threshold value arranged in the first threshold matrix, and generates the second multi-value data. The second quantized data is generated by comparing the value data and the threshold value arranged in the second threshold value matrix, the second multi-value data and the third multi-value data, and the second The second multi-value data arranged in the threshold matrix of Based on the compared is a threshold value and generates said third quantized data.

Claims (21)

An image processing apparatus for recording an image on a recording medium using a plurality of color materials,
Data acquisition means for acquiring multi-value data corresponding to each of the plurality of color materials for the pixel of interest;
One threshold matrix is set for each of the plurality of color materials from among a plurality of threshold matrices configured by arranging a plurality of thresholds, and a first threshold corresponding to the target pixel is set from the set threshold matrix. Threshold acquisition means for acquiring
For each of the plurality of color materials, a reference color for setting reference data to be referred to for performing a predetermined process on the first threshold acquired by the threshold acquisition means from the multi-value data of the plurality of color materials Setting means;
For each of the plurality of color materials, calculation means for calculating a second threshold value by performing the predetermined processing on the first threshold value based on the reference data set by the reference color setting means When,
Generating means for generating quantized data for recording dots by comparing the multi-value data and the second threshold for each of the plurality of color materials;
The threshold value acquisition unit and the reference color setting unit are mixed dot patterns obtained by mixing the dot patterns of the plurality of color materials with the graininess of the dot patterns of the plurality of color materials determined by the quantized data. An image processing apparatus, wherein the threshold value matrix and the reference data are set for each of the plurality of color materials so as to be smaller than a graininess.   The predetermined processing is performed so that the quantized data generated by the generating unit based on the second threshold is exclusive with the quantized data according to the reference data set by the reference color setting unit. The image processing apparatus according to claim 1, wherein the image processing apparatus is a process.   The image processing apparatus according to claim 2, wherein the predetermined process is a process of applying an offset to the first threshold based on the reference data. The threshold acquisition means sets different threshold matrices for a plurality of color materials having relatively low brightness among the plurality of color materials,
The image processing apparatus according to claim 1, wherein the reference color setting unit sets the reference data to null for the plurality of color materials. The plurality of color materials include cyan, magenta and yellow,
The threshold acquisition means sets a first threshold matrix for cyan and a second threshold matrix different from the first threshold matrix for magenta and yellow;
5. The reference color setting unit according to claim 1, wherein the reference data is set to null for cyan and magenta, and the reference data is set to multi-value data for magenta for yellow. The image processing apparatus according to item. The plurality of color materials include cyan, magenta and yellow,
The threshold acquisition means sets a first threshold matrix for cyan and yellow, and a second threshold matrix different from the first threshold matrix for magenta,
5. The reference color setting means sets the reference data to null for cyan and magenta, and sets the reference data to cyan multi-value data for yellow. The image processing apparatus according to item. The plurality of color materials include cyan, magenta, yellow and black,
The threshold acquisition means sets a first threshold matrix for black and cyan, and a second threshold matrix different from the first threshold matrix for magenta and yellow,
The reference color setting means sets the reference data for black and magenta to null, the reference data for cyan to black multi-value data, and the reference data for yellow to magenta multi-value data. The image processing apparatus according to claim 1. The plurality of color materials include cyan, magenta, yellow and black,
The threshold acquisition means sets a first threshold matrix for black and magenta, and a second threshold matrix different from the first threshold matrix for cyan and yellow,
The reference color setting means sets the reference data for black and cyan to null, the reference data for magenta to multi-value data for black, and the reference data for yellow to multi-value data for cyan. The image processing apparatus according to claim 1. The plurality of color materials include cyan, magenta, yellow, and light color,
The threshold acquisition means sets a first threshold matrix for cyan and light colors, and sets a second threshold matrix different from the first threshold matrix for magenta and yellow,
The reference color setting means sets the reference data for magenta to null, the reference data for light color to cyan multi-value data, and the reference data for yellow to magenta multi-value data. The image processing apparatus according to any one of claims 1 to 4. The plurality of color materials include cyan, magenta, yellow, and light color,
The threshold acquisition means sets a first threshold matrix for magenta and light colors, and a second threshold matrix different from the first threshold matrix for cyan and yellow,
The reference color setting means sets the reference data for cyan to null, the reference data for light color to magenta multi-value data, and the reference data for yellow to cyan multi-value data. The image processing apparatus according to any one of 1 to 4. The plurality of color materials include cyan, magenta, yellow, black, and light colors,
The threshold acquisition means sets a first threshold matrix for black, cyan and light colors, and sets a second threshold matrix different from the first threshold matrix for magenta and yellow,
The reference color setting means sets the reference data for black and magenta to null, the reference data for cyan to multi-value data for black, the reference data for light color to multi-value data for black and cyan, and the reference for yellow. The image processing apparatus according to claim 1, wherein the data is set to magenta multi-value data. The plurality of color materials include cyan, magenta, yellow, black, and light colors,
The threshold acquisition means sets a first threshold matrix for black, magenta and light colors, and sets a second threshold matrix different from the first threshold matrix for cyan and yellow,
The reference color setting means sets the reference data for black and cyan to null, multi-value data for reference to magenta to black multi-value data, the reference data for light color to black and magenta multi-value data, and yellow for 5. The image processing apparatus according to claim 1, wherein the reference data is set to cyan multi-value data.   13. The image processing apparatus according to claim 7, wherein black dots are not recorded in a region from highlight to medium density.   The image processing apparatus according to claim 9, wherein the light color is gray having a higher brightness than cyan and magenta.   The image processing apparatus according to any one of claims 9 to 12, wherein the light color is light cyan or light magenta having a higher brightness than cyan and magenta.   The image processing apparatus according to claim 1, wherein the threshold value matrix has a blue noise characteristic.   The graininess is a value set based on a frequency characteristic of a dot pattern, a human visual characteristic (VTF), and a brightness of each color material, according to any one of claims 1 to 16. The image processing apparatus described.   The image processing apparatus according to claim 1, further comprising: a unit that records each of the plurality of color materials on the recording medium according to the quantized data. An image processing method for recording an image on a recording medium using a plurality of color materials,
A data acquisition step for acquiring multi-value data corresponding to each of the plurality of color materials for the target pixel;
One threshold matrix is set for each of the plurality of color materials from among a plurality of threshold matrices configured by arranging a plurality of thresholds, and a first threshold corresponding to the target pixel is set from the set threshold matrix. A threshold acquisition step of acquiring
For each of the plurality of color materials, a reference color that sets reference data to be referred to for performing a predetermined process on the first threshold acquired by the threshold acquisition step from the multi-value data of the plurality of color materials A setting process;
For each of the plurality of color materials, a calculation step of calculating a second threshold value by performing the predetermined process on the first threshold value based on the reference data set by the reference color setting step When,
For each of the plurality of color materials, the method includes a step of generating quantized data for recording dots by comparing the multi-value data and the second threshold value,
In the threshold value acquisition step and the reference color setting step, a mixed dot pattern obtained by mixing the dot patterns of the plurality of color materials with the graininess of the dot patterns of the plurality of color materials determined by the quantized data. The threshold value matrix and the reference data are set for each of the plurality of color materials so as to be smaller than the graininess of the image.   The program for functioning a computer as each means of the image processing apparatus of any one of Claims 1 thru | or 18.   A storage medium for storing a program for causing a computer to function as each unit of the image processing apparatus according to claim 1.