JP2004088363A - Image processing apparatus, image output system, and image processing method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To obtain an image output with higher quality when expressing gradation by a dot pattern. <P>SOLUTION: In converting input image data into recording data expressed by a pattern comprising dots wherein each pixel is recorded in a prescribed area and outputting the result, first pixel data with a first gradation value are converted into second pixel data with a second gradation value lower than the first gradation value, and when a plurality of dot patterns are present with respect to any of the second gradation values, a pattern assigned with respect to the second pixel data having the second gradation values wherein a plurality of the patterns exist is decided in the orders of a plurality of the patterns A to D in the same raster and decided at random from a plurality of the patterns in the same column. <P>COPYRIGHT: (C)2004,JPO

Description

[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to an image processing apparatus, an image output system, and an image processing method, and more particularly, to image processing for converting input image data into recording data represented by a plurality of dot patterns in which each pixel is recorded in a predetermined area. About.
[0002]
[Prior art]
2. Description of the Related Art As an information output device in a word processor, a personal computer, a facsimile, or the like, a printer that records desired information such as characters and images on a sheet-like recording medium such as paper or film is widely used.
[0003]
Various methods are known as recording methods for printers. Ink jet methods have recently been used because non-contact recording is possible on recording media such as paper, colorization is easy, and quietness is high. In particular, as a configuration, a recording head that ejects ink according to desired recording information is mounted on a carriage, and recording is performed while reciprocating scanning in a direction intersecting with a feeding direction of a recording medium such as paper. The serial recording method is generally widely used because of its low cost and easy downsizing.
[0004]
In general, an ink-jet printer expresses an image with a binary value based on the presence or absence of ink. However, as an image to be recorded by the printer, natural images such as photographic data captured from an image input device such as a digital camera are increasing. ing. Since such a natural image is generally represented by multi-valued pixels, when a natural image is recorded by an ink jet printer, an image for converting a multi-valued image into a binary image is used. Processing is performed.
[0005]
As a method of converting a multivalued image into a binary image, a method called a density pattern method has been conventionally known. According to this method, in order to express multi-gradation using an image output system in which each pixel can be expressed only in binary, a multi-level image is expressed by a density pattern (dot pattern) represented by a pixel block in which a plurality of binary pixels are collected. This is a technique for expressing the key. However, this density pattern method has a problem that the resolution is reduced when the size of the pixel block is increased, and the gradation is reduced when the size is reduced.
[0006]
One method of solving the problem of the density pattern method is to store an error between a tone value represented by a pixel block and an input tone value, add the error to a non-input pixel value, and simulate continuous tone. There is known an image processing method that combines an error diffusion processing method for expressing an image and a density pattern method. This is called a multi-level error diffusion processing method, and according to this method, it is possible to output an image with both resolution and gradation.
[0007]
In addition, Japanese Patent Application Laid-Open No. 2001-63016 by the present inventors discloses that, as shown in FIG. 7, two ejection opening arrays arranged side by side in the main scanning direction (x direction) at a pitch px are arranged in the sub scanning direction. In an ink jet recording apparatus in which the resolution in the sub-scanning direction is improved by using recording heads which are provided to be shifted from each other by half of the pitch py of each ejection port row in the (y direction), two rasters adjacent to each other in the ejection port row are used. A method for suppressing a decrease in image quality due to a shift in dot formation position between rasters when printing is performed is disclosed.
[0008]
This deterioration in image quality is caused by the fact that the ejection direction in the scanning direction of the droplets ejected from the ejection ports (nozzles) of the even and odd rows of the recording head shown in FIG. If the direction is different from the intended direction, it will affect the dot arrangement of the error diffusion process, which is a pseudo halftone process, and a noise component having a spatial frequency component that is not visually pleasing to humans will be generated. It is caused by
[0009]
In this case, when the multi-level error diffusion processing method combining the density pattern method and the error diffusion processing is used, the influence of the displacement in the ejection direction between the two rows of ejection ports tends to be reduced. When a pattern is used, there is a problem that density fluctuation is likely to occur.
[0010]
For example, when the input resolution is 600 × 600 dpi, the output resolution is 1200 × 1200 dpi, and the density pattern to be used is a 2 × 2 dot pattern, the gradation level of each density pattern has five values from 0 to 4. At this time, since the patterns of the gradation levels 0 and 4 are solid white and black, there is only one type of pattern. Since there are various types of patterns, it is necessary to make sure that the nozzles used are not biased.
[0011]
FIG. 9 shows an example in which recording is performed using two types of patterns for each of the gradation levels 1 to 3. (A) corresponds to gradation level 1, (b) corresponds to gradation level 2, and (c) corresponds to gradation level 3. When printing is performed using two types of patterns as described above, if the ejection direction in the scanning direction of the two ejection port arrays is different from the intended direction, the dot formation position shifts and the density fluctuates.
[0012]
FIG. 10 is a diagram showing a shift amount of a dot formation position recorded by two ejection opening arrays and a pattern to be formed when the pattern of gradation level 1 shown in FIG. 9A is printed. It is. 10A shows a case where there is no shift in the dot formation position, FIG. 10B shows a case where there is a shift by one pixel, FIG. 10C shows a case where there is a shift by two pixels, and FIG. Respectively are shown. As shown in the drawing, when the dot formation position is shifted, the coverage changes, and the density of the pattern to be printed fluctuates.
[0013]
FIG. 11 is a graph showing the relationship between the amount of dot formation position deviation and the coverage. In FIG. 11, two types of gradation levels 1 shown in FIG. This is the case where the following pattern is used. As is clear from FIGS. 10 and 11, in this case, if the dot formation position is shifted by two pixels, the patterns almost overlap in a straight line as shown in FIG. To fall.
[0014]
In such a range that the displacement of the ejection direction can actually occur, if such a drastic decrease of the coverage occurs, the image quality is remarkably deteriorated. To prevent this, it has been proposed to increase the number of patterns used for recording each gradation.
[0015]
FIG. 12 shows an example in which printing is performed using four types of patterns for each of the gradation levels 1 to 3. (A) corresponds to gradation level 1, (b) corresponds to gradation level 2, and (c) corresponds to gradation level 3. FIGS. 13 and 14 show the amount of displacement of the dot formation position recorded by the two ejection opening arrays and the pattern formed when the pattern of gradation level 1 shown in FIG. 12A is printed. FIG. 13A shows a case where there is no shift in the dot formation position, FIG. 13B shows a case where there is a shift by one pixel, FIG. 13C shows a case where there is a shift by two pixels, and FIG. FIGS. 14A and 14B respectively show patterns that are shifted by 4 pixels, FIG. 14B shows a case where the pixels are shifted by 5 pixels, FIG. 14C shows a case where the pixels are shifted by 6 pixels, and FIG. Each pattern formed in the case of deviation is shown.
[0016]
As is clear from FIGS. 13 and 14, when four types of patterns are used for each gradation level, even if a dot formation position shift occurs, if the amount of shift is about ± 2 pixels, the coverage ratio is reduced. Little change. Further, in the graph showing the relationship between the amount of dot formation position shift and the coverage in FIG. 11, the case where recording is performed using four types of patterns for gradation level 1 is indicated by a solid line as pattern 2. From this graph, it can also be seen that when four types of patterns are used for each gradation level, the change in coverage is smaller than in Pattern 1 using two types of patterns.
[0017]
[Problems to be solved by the invention]
However, in the above-described conventional example, the pattern used for recording at the same gradation level is changed only in the main scanning direction, and the same pattern is continuously recorded in the sub-scanning direction.
[0018]
For this reason, if recording is performed at the same gradation level in an area of a certain size or more, it is likely to be recognized as a streak in the vertical direction (sub-scanning direction), and there is a problem that image quality is degraded.
[0019]
In an ink jet recording apparatus using a recording head having two ejection port arrays as described above, such a decrease in image quality is remarkable. It is known that even if the printing apparatus adopts another printing method, the image quality deteriorates when the density pattern method of expressing the gradation by the dot pattern is adopted.
[0020]
For example, it is known that when the same pattern is used in the sub-scanning direction, a portion where the density is uneven occurs in the area of the same gradation level of the recorded image.
[0021]
The present invention has been made in view of the above circumstances, and when expressing gradation by using a dot pattern, an image processing apparatus, an image output system, and an image capable of obtaining higher-quality image output. It is intended to provide a processing method.
[0022]
[Means for Solving the Problems]
In order to achieve the above object, an image processing apparatus according to the present invention converts input image data into recording data in which each pixel is represented by a plurality of dot patterns recorded in a predetermined area and outputs the recording data. And
Gradation conversion means for converting first pixel data having a first gradation value into second pixel data having a second gradation value lower than the first gradation value;
When there are a plurality of dot patterns for any of the second gradation values, a pattern to be assigned to the second pixel data having a second gradation value in which the plurality of patterns exist, Pattern determining means for periodically determining a plurality of patterns in a predetermined order in the same raster and randomly determining the plurality of patterns in the same column.
[0023]
Further, the above object is also achieved by an image output system including the image processing device described above and an image output device that records recording data output from the image processing device on a recording medium.
[0024]
Further, the image processing method of the present invention for achieving the above object is a method for converting input image data into recording data in which each pixel is represented by a plurality of dot patterns recorded in a predetermined area and outputting the recording data. And
A gradation conversion step of converting first pixel data having a first gradation value into second pixel data having a second gradation value lower than the first gradation value;
When there are a plurality of dot patterns for any of the second gradation values, a pattern to be assigned to the second pixel data having a second gradation value in which the plurality of patterns exist, A pattern determination step of periodically determining a plurality of patterns in the same raster in a predetermined order and randomly determining the plurality of patterns in the same column.
[0025]
That is, in the present invention, when performing image processing of converting input image data into recording data in which each pixel is represented by a plurality of dot patterns recorded in a predetermined area and outputting the data, the first gradation is used. The first pixel data having a second gradation value is converted into the second pixel data having a second gradation value lower than the first gradation value, and a dot pattern is formed for one of the second gradation values. When there are a plurality of patterns, a pattern to be assigned to the second pixel data having the second gradation value in which the plurality of patterns exist is determined on the same raster by periodically determining a plurality of patterns in a predetermined order, In the same column, it is randomly determined from a plurality of patterns.
[0026]
In this way, even when data of the same gradation value is continuous at the same column position, different dot patterns are used and recorded, so that the recorded image has the same column position (vertical) direction. It is possible to suppress the occurrence of streaks and unevenly distributed portions.
[0027]
Therefore, when expressing the gradation by the dot pattern, a higher quality image output can be obtained.
[0028]
BEST MODE FOR CARRYING OUT THE INVENTION
Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the accompanying drawings.
[0029]
In the present specification, “recording” (sometimes referred to as “printing”) refers not only to forming significant information such as characters and figures, but also to meaningless or insignificant human perception. Regardless of whether or not the image is exposed, a case where an image, a pattern, a pattern, or the like is widely formed on a recording medium or a case where the medium is processed is also described.
[0030]
In addition, the term “recording medium” refers to not only paper used in general recording devices, but also a wide range of materials that can accept ink, such as cloth, plastic films, metal plates, glass, ceramics, wood, and leather. Shall be.
[0031]
Further, “ink” (sometimes referred to as “liquid”) is to be interpreted broadly as in the definition of “recording (printing)”, and when applied on a recording medium, an image or pattern , A liquid that can be used for forming a pattern or the like, processing a recording medium, or treating ink (for example, coagulation or insolubilization of a colorant in ink applied to a recording medium).
[0032]
[First Embodiment]
FIG. 1 schematically shows a schematic configuration of an image input / output system for implementing an image processing method according to the present invention.
[0033]
In the figure, reference numeral 10 denotes an image input unit, which receives image data from an image input device such as a scanner or a digital camera, or outputs multi-valued image data from image data stored in various recording media such as a hard disk. Entered as data.
[0034]
Reference numeral 11 denotes an image processing unit which performs image processing according to the present invention on multi-valued image data input from the image input unit 10 and converts the data into binary image data.
[0035]
An image output unit 12 receives the binary image data converted by the image processing unit 11 and forms an actual image on an output medium. In the following, an example in which the image output unit 12 records and outputs using a recording medium will be described, but the present invention can also be applied to an image output unit 12 that outputs in another format.
[0036]
FIG. 2 is a diagram illustrating the configuration of the image processing unit 11 of FIG. 1 in detail.
[0037]
Reference numeral 200 denotes an input data correction unit, which receives multi-valued image data input from the preceding image input unit 10, for example, image data of 256 gradations in which each pixel is 8-bit, The input data is corrected by adding the error data generated in the pixel to the image data of the pixel currently being processed. At this time, since 9-bit error data of -255 to 255 is added to 8 bits of input data 0 to 255, the result is 10-bit data of -255 to 510, but the data is limited to the range of 0 to 255. , 8-bit data.
[0038]
Reference numeral 201 denotes a quantization unit that quantizes the multi-valued image data corrected by the input data correction unit 200 to N gradation values. This N value is determined by the relationship between the input resolution and the output resolution. That is, when the resolution of input image data is A and the output resolution is B (here, the vertical and horizontal resolutions of the image are assumed to be the same for simplicity), the number of dots used for one input pixel is Since there are ^two (B / A) dots, when the (B / A) ² dots constitute one block, the gradation value that can be expressed in one block is (B / A) ² +1.
[0039]
As an example, when the input resolution is 600 dpi and the output resolution is 1200 dpi, one block represented by 2 × 2 4 dots is output for one input pixel, and the number of gradations that can be represented by one block is five values. It becomes. Therefore, the quantizer 201 outputs values quantized into five values of 0, 64, 128, 192, and 255.
[0040]
Reference numeral 202 denotes an error calculation unit that calculates and outputs an error between the gradation value quantized by the quantization unit 201 and the multi-valued image data corrected by the input data correction unit 200. In the above example, since the quantization unit 201 quantizes the data into five values, the gradation values are input to the error calculation unit 202 as 8-bit data of 0, 64, 128, 192, and 255. Since 8-bit data of 0 to 255 is input from the input data correction unit 200, the error data is 9-bit data of -255 to 255.
[0041]
Reference numeral 203 denotes an error memory, which distributes an error generated by the error calculation unit 202 to surrounding pixels at a ratio as shown in FIG. Therefore, when the error memory is distributed to surrounding pixels at the ratio shown in FIG. 3, a memory for at least two lines is required.
[0042]
Next, the five-level gradation data quantized by the quantization unit 201 is input to the pattern conversion unit 204, and a 2 × 2 pattern is selected from the gradation data and output. Here, a pattern to be output with respect to the gradation data of each input pixel is determined by the pattern table storage unit 206 and the address generation unit 205.
[0043]
First, a specific example of pattern conversion will be described. FIG. 4 is a diagram illustrating an example of a pattern table stored in the pattern table storage unit 206, showing a correspondence between a quantization value output from the quantization unit 201 and a dot pattern. For the five quantized values 0, 64, 128, 192, and 255, the tone code 0, 1, 2 indicating the number of dots (= tone level) in the dot pattern inside the pattern conversion unit 204. , And 3, respectively. In the present embodiment, four types of patterns arranged in the order of A, B, C, and D shown in FIG. It has as a pattern arrangement group.
[0044]
This causes a dot formation position shift (hereinafter referred to as an even-odd registration shift) between the ejection rows of the recording head having two ejection port arrays of the even-numbered row and the odd-numbered row shown in FIG. Occasionally, the pattern arrangement has little density fluctuation.
[0045]
In the printing in the main scanning direction, the order of A, B, C, and D in the pattern array group is not changed and is constant. In a certain raster, the pattern of A is n columns (the unit of the column here is main scanning). Assuming that the pattern B is used for printing at the 600th resolution (input resolution in the direction), the pattern B is used for printing n + 1 columns, the pattern C is used for n + 2 columns, and the pattern D is used for printing n + 3 columns. Therefore, for printing in the main scanning direction at the same density level, the pattern is repeatedly used in the order of A, B, C, and D.
[0046]
Next, how the pattern array group is arranged in the raster direction will be described with reference to FIG. Here, for the sake of simplicity, the description will be made on the assumption that the data of the gradation code 1 is always transmitted continuously.
[0047]
First, in the Nth raster (here, the unit of raster is 600 dpi which is the input resolution in the sub-scanning direction), an address indicating which pattern of the pattern array group is to be arranged is determined by the address generation unit 205 in FIG. . In the example shown in FIG. 5, the address of A is specified in the Nth raster. At this time, the pattern for the gradation code 1 is determined in order from the 0th column in the column direction based on the address A specified by the address generation unit 205 according to the pattern table of FIG. Specifically, the patterns used for the N rasters are in the order of A, B, C, D, A, B, C, D,... In the column direction.
[0048]
Similarly, since the address D is specified by the address generation unit 205 for the N + 1 raster, patterns of D, A, B, C, D, A, B, C,... Are used in order from the 0th column. . Similarly, for the rasters after N + 2, the pattern to be used for printing in the main scanning direction is determined in the order according to the pattern table of FIG. 4 according to the start address of each raster specified by the address generation unit 205.
[0049]
The address generation unit 205 of this embodiment randomly generates a designated address for each raster. As a method of generating the designated address, a method of randomly generating any of the addresses A, B, C, and D using a random table having a predetermined capacity can be considered. Since the random table only needs to generate a random value once for one raster, it is sufficient for the random number to correspond to the number of rasters required for the image output device in this embodiment to record one page.
[0050]
For example, in the case of an image output apparatus that prints on an A4 (210 × 297 mm) size recording medium at 1200 × 1200 dpi, 297 ÷ 25.4 × 600 = 7015 rasters for a width of 297 mm in the sub-scanning direction ( Since the input resolution is 600 dpi, the number of rasters is inch width × 600). Therefore, it is sufficient if there is a table having 7015 random values.
[0051]
As described above, according to the present embodiment, in the conventional example, vertical stripes are generated by using the same pattern continuously in the sub-scanning direction. Is used, so that vertical streaks do not occur.
[0052]
In addition, the same effect as that of the related art can be obtained with respect to the density fluctuation due to the even-odd registration deviation between the even-numbered row and the odd-numbered row of ejection opening rows.
[0053]
[Second embodiment]
Hereinafter, a second embodiment according to the present invention will be described. In the following description, the description of the same parts as those in the first embodiment will be omitted, and the description will focus on the characteristic parts of the present embodiment.
[0054]
In the first embodiment, the pattern used for each raster is determined at random, but in the present embodiment, the pattern used is determined for each of a plurality of rasters. In the example described below, the same pattern is used for two rasters, the Nth raster and the (N + 1) th raster.
[0055]
FIG. 6 is a diagram illustrating an example of a pattern array used for each raster in the present embodiment. On the basis of the N rasters, the address A is designated by the address generation unit 205 in FIG. 2 for two consecutive rasters of the Nth raster and the (N + 1) th raster. Therefore, the same pattern is continuously used in the sub-scanning direction for the Nth raster and the (N + 1) th raster. Next, an address D is designated for the (N + 2) th raster and the (N + 3) th raster, and based on the designation, a pattern to be used in the order of D, A, B, C, D, A, B, C,. It is determined.
[0056]
In the conventional recording method, the vertical stripe generated by the continuation of the same pattern in the sub-scanning direction is recognized by the human eye when the length of the portion recorded with the same pattern is 5 mm or more. That turns out to be the case. Therefore, even if two rasters are continuous in 600 dpi units as in the present embodiment, human eyes are not recognized at all, and it is possible to suppress the occurrence of vertical streaks that occur in the conventional method.
[0057]
Further, according to the present embodiment, the capacity of the random table required to randomly generate the address for the raster is reduced by half as compared with the case where the address is generated for each raster as in the first embodiment. Therefore, the effect that the ROM capacity for storing the table can be reduced can be obtained.
[0058]
[Other embodiments]
In the above-described embodiment, when an ink jet recording apparatus having a recording head having two ejection port arrays is employed as the image output unit, the effect of significantly improving the quality of the recorded image is obtained. Even if the head has only one ejection port array, or a recording apparatus (image output apparatus) employing a recording apparatus other than the ink jet method, if the gradation is expressed by the density pattern method based on the dot pattern, the recording is performed. The image quality is improved.
[0059]
This is because the use of the same pattern in the sub-scanning direction in the conventional method makes it possible to suppress the occurrence of a portion where the density is uneven in the area of the same gradation level of the recorded image.
[0060]
The present invention can be applied to a system including a plurality of devices (for example, a host computer, an interface device, a reader, a printer, etc.), but may be applied to an apparatus (for example, a copying machine, a facsimile machine, etc.) including one device. May be applied.
[0061]
Further, an object of the present invention is to provide a storage medium storing a program code of software for realizing the functions of the above-described embodiments to a system or an apparatus, and a computer (or CPU or MPU) of the system or apparatus to store the storage medium. It is needless to say that the present invention can also be achieved by reading and executing the program code stored in the program.
[0062]
In this case, the program code itself read from the storage medium realizes the function of the above-described embodiment, and the storage medium storing the program code constitutes the present invention.
[0063]
As a storage medium for supplying the program code, for example, a floppy disk, a hard disk, an optical disk, a magneto-optical disk, a CD-ROM, a CD-R, a magnetic tape, a nonvolatile memory card, a ROM, and the like can be used.
[0064]
When the computer executes the readout program code, not only the functions of the above-described embodiments are realized, but also an OS (Operating System) running on the computer based on the instruction of the program code. It goes without saying that a part or all of the actual processing is performed and the functions of the above-described embodiments are realized by the processing.
[0065]
Further, after the program code read from the storage medium is written into a memory provided in a function expansion board inserted into the computer or a function expansion unit connected to the computer, the function expansion is performed based on the instruction of the program code. It goes without saying that a CPU or the like provided in the board or the function expansion unit performs part or all of the actual processing, and the processing realizes the functions of the above-described embodiments.
[0066]
【The invention's effect】
As described above, according to the present invention, even when data of the same gradation value is continuous at the same column position, printing is performed using a different dot pattern, so that the same column position It is possible to suppress the generation of a streak or a portion where the density is uneven in the (vertical) direction.
[0067]
Therefore, when expressing the gradation by the dot pattern, a higher quality image output can be obtained.
[Brief description of the drawings]
FIG. 1 is a diagram showing a schematic configuration of an image input / output system according to the present invention.
FIG. 2 is a block diagram illustrating a configuration of an image processing unit in FIG. 1 in detail.
FIG. 3 is a diagram illustrating a diffusion coefficient in an error diffusion process.
FIG. 4 is a diagram illustrating a relationship between a gradation level and a pattern according to the first embodiment of the present invention.
FIG. 5 is a diagram illustrating an example of a specific dot pattern arrangement according to the first embodiment of the present invention.
FIG. 6 is a diagram illustrating an example of a specific dot pattern arrangement according to the second embodiment of the present invention.
FIG. 7 is a diagram showing an arrangement of two ejection port arrays provided in a recording head.
FIG. 8 is a diagram illustrating a state in which discharge directions from two discharge port arrays are open.
FIG. 9 is a diagram showing an example in which gradation levels 1 to 3 are recorded using two patterns.
FIG. 10 is a diagram showing a shift in the ejection direction of two ejection port arrays and a printing result of gradation level 1 using two patterns.
FIG. 11 is a graph showing the relationship between the amount of dot formation position deviation and the coverage for gradation level 1.
FIG. 12 is a diagram illustrating an example in which recording is performed using four types of patterns for each of the 1 to 3 gradation levels.
FIG. 13 is a diagram illustrating a shift in the ejection direction of two ejection port arrays and a printing result of gradation level 1 using four patterns.
FIG. 14 is a diagram illustrating a shift in the ejection direction of two ejection port arrays and a printing result of gradation level 1 using four patterns.

Claims (13)

An image processing apparatus that converts input image data into recording data represented by a plurality of dot patterns in which each pixel is recorded in a predetermined area, and outputs the recording data.
Gradation conversion means for converting first pixel data having a first gradation value into second pixel data having a second gradation value lower than the first gradation value;
When there are a plurality of dot patterns for any of the second gradation values, a pattern to be assigned to the second pixel data having a second gradation value in which the plurality of patterns exist, An image processing apparatus comprising: pattern determination means for periodically determining a plurality of patterns in a predetermined order in the same raster and randomly determining the plurality of patterns in the same column. The pattern determination means includes a pattern storage unit that stores a pattern for each of the second gradation values in a predetermined order, and an address generation unit that randomly outputs an address value indicating a pattern for pixel data in a first column of a raster. The image processing apparatus according to claim 1, comprising: 3. The image processing apparatus according to claim 2, wherein the address generator outputs the same address value to a plurality of rasters. 4. The image processing apparatus according to claim 1, wherein the gradation conversion unit includes an error diffusion processing unit that performs an error diffusion process of distributing an error generated by the conversion to surrounding pixels in a predetermined pattern. An image processing apparatus according to claim 1. An image output system comprising: the image processing device according to claim 1; and an image output device that records recording data output from the image processing device on a recording medium. The image output apparatus moves an ink jet recording head that performs recording by discharging ink by main scanning for recording the same raster and sub-scanning for moving the recording medium in a direction intersecting the main scanning direction. The image output system according to claim 5, wherein recording is performed on the recording medium. The inkjet recording head includes two orifice rows in which a plurality of orifices are arranged at predetermined intervals in the sub-scanning direction in parallel in the main scanning direction. 7. The image output system according to claim 6, wherein the image is shifted by half of the interval in the sub-scanning direction. An image processing method for converting input image data into recording data represented by a plurality of dot patterns in which each pixel is recorded in a predetermined area and outputting the recording data,
A gradation conversion step of converting first pixel data having a first gradation value into second pixel data having a second gradation value lower than the first gradation value;
When there are a plurality of dot patterns for any of the second gradation values, a pattern to be assigned to the second pixel data having a second gradation value in which the plurality of patterns exist, A pattern determination step of periodically determining a plurality of patterns in a predetermined order in the same raster and randomly determining the plurality of patterns in the same column. The pattern determining step includes a pattern storing step of storing a pattern for each of the second gradation values in a predetermined order in a storage unit, and randomly outputting an address value indicating the pattern for the pixel data in the first column of the raster. 9. The image processing method according to claim 8, further comprising an address generation step. 10. The image processing method according to claim 9, wherein the same address value is output to a plurality of rasters in the address generation step. 11. The method according to claim 8, wherein the gradation conversion step includes an error diffusion processing step of performing an error diffusion processing for distributing an error generated by the conversion to surrounding pixels in a predetermined pattern. The image processing method according to 1. A computer program comprising a program code for causing a computer to execute each step of the image processing method according to any one of claims 8 to 11. A storage medium storing the computer program according to claim 12.

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