JP4645400B2 - Dither matrix generation - Google Patents

Description

  The present invention relates to a technique for printing an image by forming dots on a print medium.

  2. Description of the Related Art Printing apparatuses that form dots on a printing medium and print images are widely used as output apparatuses for images created by computers and images taken by digital cameras. In such a printing apparatus, since the gradation value of dots that can be formed with respect to the input gradation value is small, gradation expression is performed by halftone processing. As one of the halftone processes, a systematic dither method using a dither matrix is widely used. Since the systematic dither method greatly affects the image quality depending on the contents of the dither matrix, for example, as disclosed in Patent Document 1, analysis such as simulated annealing or genetic algorithm using an evaluation function taking human vision into consideration The dither matrix has been optimized by techniques.

JP-A-7-177351 JP-A-7-81190 Japanese Patent Laid-Open No. 10-329381

  However, in such dither matrix optimization processing, ink dots are formed by scanning a common area on a printing medium a plurality of times, and thus image quality deterioration due to printing an image is not taken into consideration. .

  The present invention has been made to solve the above-described problems in the prior art, and forms an ink dot by scanning a common area on a print medium a plurality of times, thereby printing an image. An object of the present invention is to provide a technique for suppressing the deterioration in image quality caused by the above.

In order to solve at least a part of the above-described problems, the present invention forms dots on each print pixel of a print image to be formed on a print medium in order to perform halftone processing on input image data. A method for generating a dither matrix for determining a state, comprising:
The print image is formed by combining print pixels belonging to each of a plurality of pixel groups assumed to be physically different in the formation of the dots in a common print region,
The halftone processing is performed by a connected matrix in which a plurality of the dither matrices are connected according to a certain rule,
The dither matrix generation method includes:
Of the plurality of threshold values to be stored in each element of the dither matrix, the focus is determined as the focus threshold value, which is the threshold value for which the element to be stored is undetermined and in which dot formation is most likely to be turned on. A threshold determination step;
It is considered that a dot is formed in a pixel corresponding to a determined element whose threshold to be stored is a determined element, and the attention is paid to any one of the blank elements whose thresholds are not yet determined. A storage element determination step for determining a comprehensive evaluation value for each of cases where a threshold value is assumed to be stored, and determining an element for storing the target threshold value according to the comprehensive evaluation value;
For all of the plurality of thresholds, a repetition step of repeating each of the focus threshold value determination step and the storage element determination step;
With
The storage element determination step includes:
A connected matrix generating step of generating a connected matrix by connecting a plurality of surrounding matrices having the same elements as the center matrix around the center matrix, which is a dither matrix to be evaluated, according to the predetermined rule;
An overall evaluation value determining step for evaluating the formation state of dots corresponding to all groups of the determined elements of the connection matrix and determining an overall evaluation value;
Among the determined elements of the connected matrix, a group evaluation value is obtained by evaluating a dot formation state corresponding only to an element belonging to an element group corresponding to a rating pixel group selected as a rating from the plurality of pixel groups. A group evaluation value determination process to be determined;
In accordance with the overall evaluation value and the group evaluation value, a comprehensive evaluation value determining step for determining the comprehensive evaluation value;
Including
The element group includes a rating group that is an element group corresponding to the rating pixel group in the central matrix, and a related group that is an element group corresponding to the rating pixel group in the plurality of surrounding matrices. It is characterized by including.

  In the dither matrix generation method of the present invention, attention is paid to the dot pattern in the dot formation process, and the dither matrix is generated while evaluating each of a plurality of pixel groups assumed to be physically different. In the generation of the dither matrix, it is assumed that a plurality of dither matrices are connected according to a certain rule, and the dither matrix is optimized based on such assumptions, and supports various dither matrix connection methods. It has also been realized. Here, the “physical difference” includes a positional or temporal difference such as a shift in dot formation position or a shift in dot formation timing due to a main scanning feed error.

  In the dither matrix generation method, the plurality of surrounding matrices may be shifted in at least one of a main scanning direction and a sub-scanning direction with respect to the center matrix. By doing so, it is possible to suppress low-frequency noise caused by, for example, the dot formation position error caused by the mechanical error of the nozzle that ejects ink being repeatedly generated at the cycle of the connected dither matrix.

  The present invention is directed to causing a computer to implement various forms such as a dither matrix, a dither matrix generation apparatus, a printing apparatus and a printing method using the dither matrix, and a printed material generation method, or the functions of these methods or apparatuses. The present invention can be realized in various forms such as a computer program, a recording medium that records the computer program, and a data signal that includes the computer program and is embodied in a carrier wave.

  In addition, the use of a dither matrix in the printing device, printing method, and printed material generation method is based on whether or not dots are formed for each pixel by comparing the threshold value set in the dither matrix and the gradation value of the image data for each pixel However, for example, the presence or absence of dot formation may be determined by comparing the sum of the threshold value and the gradation value with a fixed value. Furthermore, the presence / absence of dot formation may be determined according to the data generated in advance based on the threshold value and the gradation value without directly using the threshold value. In general, the halftone processing of the present invention may be any method that determines the presence or absence of dot formation according to the gradation value of each pixel and the threshold value set at the corresponding pixel position of the dither matrix.

Below, in order to demonstrate the effect | action and effect of this invention more clearly, embodiment of this invention is described in the following orders.
A. Print image generation performed while performing main and sub-scans:
B. Dither matrix generation method in the first embodiment:
C. Dither matrix generation method in the second embodiment:
D. Variations:

A. Print image generation performed while performing main and sub-scans:
FIG. 1 is an explanatory diagram showing a state in which a print image is generated on a print medium with ink dots while performing main scanning and sub-scanning in the first embodiment of the present invention. The main scanning means an operation of moving the print head 10 relative to the print medium in the main scanning direction. The sub scanning means an operation of moving the print head 10 relative to the print medium in the sub scanning direction. The print head 10 is configured to eject ink droplets onto a print medium to form ink dots. The print head 10 is equipped with ten nozzles (not shown) at intervals of twice the pixel pitch k.

  The print image is generated as follows while performing main scanning and sub-scanning. In the main scan of pass 1, the pixel position number is 1, 3, 5, out of 10 main scan lines with raster numbers 1, 3, 5, 7, 9, 11, 13, 15, 17, 19 Ink dots are formed on 7 pixels. The main scanning line means a line formed by pixels that are continuous in the main scanning direction. Each circle indicates a dot formation position. The numbers in each circle indicate a pixel group composed of a plurality of pixels on which ink dots are formed simultaneously. In pass 1, dots are formed in the print pixels belonging to the first pixel group.

  When the pass 1 main scan is completed, the sub-scan feed is performed in the sub-scan direction with a movement amount L that is three times the pixel pitch. In general, the sub-scan feed is performed by moving the print medium. However, in this embodiment, the print head 10 is moved in the sub-scan direction in order to make the explanation easy to understand. When the sub-scan feed is completed, the main scan of pass 2 is performed.

  In the pass 2 main scan, among the 10 main scan lines with raster numbers 6, 8, 10, 12, 14, 16, 18, 20, 22, 24, the pixel position numbers are 1, 3, 5, Ink dots are formed on 7 pixels. In this way, in pass 2, dots are formed in the print pixels belonging to the second pixel group. The two main scanning lines with raster numbers 22 and 24 are not shown. When the pass 2 main scan is completed, the sub-scan feed similar to that described above is performed, and then the pass 3 main scan is performed.

  In the main scan of pass 3, ink is applied to pixels having pixel position numbers 2, 4, 6, and 8 out of 10 main scan lines including the main scan lines having raster numbers 11, 13, 15, 17, and 19. Dots are formed. In the pass 4 main scan, ink dots are applied to pixels having pixel position numbers 2, 4, 6, and 8 out of 10 main scan lines including three main scan lines with raster numbers 16, 18, and 20. Is formed. In this way, it can be seen that ink dots can be formed without gaps in the sub-scanning positions with raster numbers of 15 and later. In pass 3 and pass 4, dots are formed on the print pixels belonging to the third and fourth pixel groups, respectively.

  Observing the generation of such a printed image by paying attention to a certain area, it can be seen that the following is performed. For example, if an area having raster numbers 15 to 19 and pixel position numbers 1 to 8 is set as a target area, it can be seen that a print image is formed in the target area as follows.

  In pass 1, it can be seen that the same dot pattern as the ink dots formed at the pixel positions with the raster numbers 1 to 5 and the pixel position numbers 1 to 8 is formed in the region of interest. This dot pattern is formed of dots formed on pixels belonging to the first pixel group. That is, in pass 1, dots are formed in the pixels belonging to the first pixel group in the region of interest.

  In pass 2, dots are formed in the pixels belonging to the second pixel group in the region of interest. In pass 3, dots are formed in the pixels belonging to the third pixel group in the region of interest. In pass 4, dots are formed in the pixels belonging to the fourth pixel group in the region of interest.

  As described above, in this embodiment, it is understood that the print pixels belonging to each of the first to fourth pixel groups are formed by being combined with each other in the common print region.

  FIG. 2 is an explanatory diagram showing a state in which a print image is generated on a print medium by combining print pixels belonging to each of a plurality of pixel groups in a common print area in the first embodiment of the present invention. It is. In the example of FIG. 2, the print image is a print image having a predetermined intermediate tone (monochrome). The dot patterns DP1 and DP1a indicate dot patterns formed on a plurality of pixels belonging to the first pixel group. The dot patterns DP2 and DP2a indicate dot patterns formed on a plurality of pixels belonging to the first and third pixel groups. The dot patterns DP3 and DP3a indicate dot patterns formed on a plurality of pixels belonging to the first to third pixel groups. The dot patterns DP4 and DP4a indicate dot patterns formed on a plurality of pixels belonging to all pixel groups.

  The dot patterns DP1, DP2, DP3, and DP4 are dot patterns when a conventional dither matrix is used. The dot patterns DP1a, DP2a, DP3a, and DP4a are dot patterns when the dither matrix of the present invention is used. As can be seen from FIG. 2, when the dither matrix of the present invention is used, the dot dispersibility is more uniform in the dot patterns DP1a and DP2a with less dot pattern overlap than when the conventional dither matrix is used. It is.

  Since the dither matrix of the prior art does not have the concept of a pixel group, optimization is performed by paying attention only to the dispersibility of dots in the finally formed printed image (dot pattern DP4 in the example of FIG. 2). . In other words, since the dispersibility of the dots formed in the pixels belonging to each pixel group is not considered, the dispersibility of the dots formed in the pixels belonging to each pixel group is not good, and the dot density is sparse and dense. .

  The dither matrix of the present invention is formed on the pixels belonging to each pixel group because the dispersibility of the dots formed on the pixels belonging to each pixel group is considered in addition to the dispersibility of the dots in the printed image. The dispersibility of both the dot dispersibility and the dot dispersibility in the printed image is improved.

  The dither matrix of the present invention has been optimized by focusing on not only the finally formed dot pattern but also the dot pattern in the dot formation process. Such a focus has not existed in the past. This is because, conventionally, it has been common technical knowledge that even if the dot pattern dispersion in the dot formation process is poor, the image quality is good if the dispersibility of the finally formed dot pattern is good.

  However, the inventor of the present application dared to analyze the image quality of the printed image by paying attention to the dot pattern in the dot formation process. As a result of this analysis, it has been found that image unevenness occurs due to the density of the dot pattern in the dot formation process. The inventors of the present application have also found out that the unevenness of the image is perceived remarkably by human eyes due to the physical phenomenon of ink such as unevenness of ink aggregation, unevenness of gloss and bronze phenomenon. The bronze phenomenon is a phenomenon in which the state of light reflected on the surface of the printing paper changes, for example, the printing surface becomes a bronze color depending on the viewing angle due to the aggregation of dyes in ink droplets.

  For example, ink aggregation and bronzing can occur even when a printed image is formed in a single pass. However, even if ink agglomeration or the like occurs uniformly on the entire surface of the printed image, it is unlikely to be close to the human eye. This is because, since it occurs uniformly, ink non-aggregation or the like does not occur as non-uniform “unevenness” including low-frequency components.

  However, in a dot pattern formed in a pixel group in which ink dots are formed almost simultaneously in the same main scan, if the unevenness occurs in a low frequency region that is easily recognized by the human eye due to ink aggregation or the like, significant image quality degradation occurs. Will become apparent. In this way, when forming a print image by forming ink dots, it is possible to optimize the dither matrix by focusing on the dot pattern formed in the pixel group in which the ink dots are formed almost simultaneously. It was first discovered by the inventor that it leads to

  In addition, since the dither matrix of the prior art is optimized on the premise that the mutual positional relationship of each pixel group is assumed in advance, if the mutual positional relationship is shifted As a result, the optimality is not guaranteed and the image quality is significantly deteriorated. However, according to the dither matrix of the present invention, since the dispersibility of the dots is ensured even in the dot pattern of each pixel group, it is possible to secure a high robustness against the displacement of the mutual positional relationship. First confirmed by experiment.

  Furthermore, the inventor has also found out that the technical idea of the present invention is increasing in importance as the printing speed increases. This is because the increase in printing speed leads to the formation of dots in the next pixel group before sufficient time is taken for ink absorption. The present invention is configured as follows based on such completely new knowledge.

B. Dither matrix generation method in the first embodiment:
FIG. 3 is a flowchart showing the processing routine of the dither matrix generation method in the first embodiment of the present invention. This dither matrix generation method is configured to be optimized in consideration of the dispersibility of dots formed almost simultaneously in the print image formation process. In this example, a small dither matrix of 8 rows and 8 columns is generated for easy understanding.

  In step S100, a grouping process is performed. In the present embodiment, the grouping process is a process of dividing the dither matrix for each element corresponding to a plurality of pixel groups in which dots are formed almost simultaneously in the print image forming process.

  FIG. 4 is an explanatory diagram showing the dither matrix M after performing the grouping process in the first embodiment of the present invention. In this grouping process, the pixel group is divided into four pixel groups in FIG. The numbers described in each element of the dither matrix M indicate the pixel group to which each element belongs. For example, the element of 1 row and 1 column belongs to the first pixel group (FIG. 1), and the element of 1 row and 2 columns belongs to the second pixel group.

  FIG. 5 is an explanatory diagram showing four divided matrices M0 to M3 in the first embodiment of the present invention. The division matrix M0 includes a plurality of elements corresponding to pixels belonging to the first pixel group among elements of the dither matrix M and blank elements that are blank elements. The blank element is an element in which dots are not always formed regardless of the input gradation value. Each of the divided matrices M1 to M3 includes a plurality of elements corresponding to pixels belonging to the second to fourth pixel groups among the elements of the dither matrix M and blank elements.

  In this way, when the grouping process (FIG. 3) in step S100 is completed, the process proceeds to the threshold value determination process (step S200).

  In step S200, a target threshold value determination process is performed. The target threshold value determination process is a process for determining a threshold value to be a storage element determination target. In this embodiment, the threshold value is determined by selecting in order from a threshold value having a relatively small value, that is, a threshold value having a value at which dots are easily formed. The reason for this will be described later.

  In step S300, a dither matrix evaluation process is performed. The dither matrix evaluation process is a process for digitizing the optimality of the dither matrix based on a preset evaluation function. In this embodiment, the evaluation function is the uniformity of dot recording density. In other words, whether or not a plurality of dots formed on pixels corresponding to each element of the matrix are uniformly formed at each gradation value is a criterion for evaluation. However, in this embodiment, the evaluation is performed not only considering the dither matrix M but also considering each of the four divided matrices M0 to M3.

  FIG. 6 is a flowchart showing the processing routine of the dither matrix evaluation process. In the dither matrix evaluation process, a comprehensive evaluation value is calculated. The comprehensive evaluation value is calculated based on the overall evaluation value that is the evaluation value of the dither matrix M and the group evaluation value that is the evaluation value of each of the four divided matrices M0 to M3. In this processing routine, first, an overall evaluation value and a group evaluation value are calculated in order, and a total evaluation value is calculated by multiplying these evaluation values by weighting and adding.

  In step S310, a plurality of dither matrices are connected. This connection is performed by the same method as the connection state performed in the halftone process. Such connection is performed in order to increase the optimality in accordance with the state in which the dither matrix is used in the halftone process.

  FIG. 7 is an explanatory diagram showing a connection state of nine dither matrices in the first embodiment of the present invention. In this connected state, eight dither matrices are connected around the center matrix with respect to the center matrix, which is a dither matrix to be directly evaluated. The eight dither matrices have the same elements as the center matrix. In the present specification, these eight dither matrices are referred to as a surrounding matrix. Note that the numbers of the rows and columns of the center matrix are written outside the nine connected dither matrices.

  In the present embodiment, the nine dither matrices are connected according to a certain rule. Specifically, the two surrounding matrices arranged on the left and right of the center matrix are shifted by one pixel only in the main scanning direction. In this shift, the relatively right dither matrix is shifted upward by one pixel from the adjacent relatively left dither matrix.

  Such a dither matrix shift is performed in order to improve the image quality in the halftone process. Specifically, this shift can contribute to the suppression of low-frequency noise (or unevenness) generated, for example, in the arrangement cycle of the dither matrix. Low-frequency noise is generated in the dither matrix array period because dots are formed at the same position in the dither matrix when halftone processing is performed for the same gradation value using the same dither matrix. It is.

  In step S320, the corresponding threshold threshold dot is turned on. The determined threshold means a threshold at which the storage element is determined. In this embodiment, since the threshold value is selected in order from the value at which dots are likely to be formed as described above, when a dot is formed as the threshold value of interest, the pixel corresponding to the element storing the determined threshold value is not used. Dots are always formed. Conversely, at the smallest input tone value at which dots are formed at the threshold value of interest, no dots are formed at pixels corresponding to elements other than the element storing the determined threshold value.

  FIG. 8 is an explanatory diagram showing a state in which dots are formed in each of the eight pixels corresponding to the elements storing the thresholds at which dots are likely to be formed first to eighth in the dither matrix M. This dot pattern is used to determine in which pixel the ninth dot is to be formed. That is, it is used to determine the storage element of the target threshold value at which the ninth dot is most likely to be formed. In this embodiment, the storage element is determined so that the threshold value is stored in the element corresponding to the pixel in which the dot formation is sparse. This is because whether or not a plurality of dots formed in pixels corresponding to each element of the matrix are uniformly formed in each gradation value is a criterion for evaluation. As can be seen from FIG. 7, the dot pattern for each dither matrix is also shifted in the main scanning direction in the same manner as the dither matrix.

  FIG. 9 is an explanatory diagram showing a matrix obtained by quantifying the state in which dots are formed in each of the eight pixels in FIG. 8, that is, a dot density matrix that quantitatively represents the dot density. The number 0 means that no dot is formed, and the number 1 means that a dot is formed.

  In step S330, an overall evaluation value is calculated. The overall evaluation value is calculated by performing a low pass filter process on the dot density matrix.

  FIG. 10 is an explanatory diagram showing the low-pass filter in the first embodiment of the present invention. In the present embodiment, since the filtered result is used only for dot density magnitude comparison, the low pass filter is not normalized. The low-pass filter process is performed only for the elements of the center matrix. However, the calculation of the peripheral elements of the central matrix also uses data of eight peripheral matrices connected around the central matrix.

  FIG. 11 is an explanatory diagram showing the result of low-pass filter processing of the dot density matrix. The number in each element represents the overall evaluation value. The overall evaluation value means an evaluation value of each element when it is assumed that the ninth dot is formed in the dither matrix M in which the storage elements of eight threshold values are determined. A large number means that the dot density is high, and a small number means that the dot density is low, that is, the dots are sparse.

  In step S340, a rating matrix is selected. The rating matrix means one matrix to be evaluated at the time of determining the storage element of the target threshold value among the four divided matrices M0 to M3.

  In this embodiment, the rating matrix is selected in order along with the target threshold value. Specifically, the division rating matrix M0 is selected in the first target threshold value, the division matrix M1 is selected in the second target threshold value, and the division rating matrix is sequentially selected. The focus threshold value is stored in one of the elements belonging to the rating matrix.

  Here, an example in which the division matrix M0 is selected as the rating matrix will be described. The division matrix M0 includes a plurality of elements corresponding to pixels belonging to the first pixel group among elements of the dither matrix M and blank elements that are blank elements.

  In step S350, an association matrix is determined. The association matrix means a group of elements corresponding to the pixel group corresponding to the rating matrix among the elements of the surrounding matrix.

  FIG. 12 is an explanatory diagram showing a divided matrix corresponding to the first pixel group in the first embodiment of the present invention. As can be seen from FIG. 12, in the central matrix, the divided matrix M0 corresponds to the first pixel group, while in the surrounding matrix, the divided matrix M1 corresponds to the first pixel group. Thus, since the surrounding matrix is shifted and connected by one pixel with respect to the center matrix, a pixel group different from the center matrix corresponds to the same pixel group.

  In step S360, the corresponding threshold threshold dot is turned on. However, here, the corresponding dot of the determined threshold is turned on only in the rating matrix in the center matrix and the related matrix in the surrounding matrix. As a result, the dot corresponding to the determined threshold value is turned on in the element corresponding to the first pixel group.

  FIG. 13 is an explanatory diagram showing a dot density matrix including elements corresponding to the first pixel group. As can be seen from FIG. 13, a dot pattern different from the central matrix is formed in the surrounding matrix.

  In step S370, a group evaluation value (FIG. 14) is calculated. The group evaluation value is calculated by performing filter processing using a low-pass filter (FIG. 10) on the dot density matrix of FIG. The group evaluation value means an evaluation value of each element when it is assumed that the third dot is formed in the divided matrix M0 in which two threshold storage elements are determined.

  The overall evaluation value and group evaluation value calculated in this way are used to determine the overall evaluation value.

  In step S380, comprehensive evaluation value determination processing is performed. The comprehensive evaluation value determination process is determined by adding a predetermined weight to the overall evaluation value and the group evaluation value. In this embodiment, as an example, the weights of the overall evaluation value and the group evaluation value are “4” and “1”, respectively.

  FIG. 15 is an explanatory diagram showing a matrix that stores overall evaluation values, group evaluation values, and overall evaluation values calculated from these evaluation values. For example, the overall evaluation value is determined to be “26” for an element of one row and one column. This value is obtained by multiplying the value of “6”, which is the overall evaluation value stored in the 1 × 1 element of the matrix for storing the overall evaluation value, by “4”, which is the weighting value, and also stores the group evaluation value. This is determined by adding “2” which is the value of the group evaluation value stored in the element of the first row and the first column of the matrix.

  There are 16 elements belonging to the divided matrix M0, and 2 elements among the 16 elements have already been determined as 2 threshold value storage elements. The two threshold value storage elements are indicated as “completed”.

  In step S400 (FIG. 3), a storage element determination process is performed. The storage element determination process is a process of determining a storage element of a target threshold value (in this example, a threshold value at which dots are most easily formed in the eighth example). In the present embodiment, the storage element is determined from elements having the smallest overall evaluation value. In this example, since the elements in the 7th row and the 1st column and the 5th row and the 7th column have the same overall evaluation value “16”, they become storage element candidates. A method for selecting from the two storage element candidates may be based on the knowledge of a skilled engineer or may be a method described later.

  When such processing is performed for all the threshold values from the threshold at which dots are most likely to be formed to the threshold at which dots are hardly formed, the dither matrix generation processing is completed (step S500).

  Thus, in the dither matrix generation method of the present embodiment, it is assumed that a plurality of dither matrices are connected in a shifted state, and the dither matrix is optimized based on such assumptions. For example, print pixels belonging to each of the first to fourth pixel groups (FIGS. 1 and 2) formed almost simultaneously in each main scan while suppressing low-frequency noise generated in the arrangement cycle of the dither matrix. A dither matrix optimized for a printing apparatus configured to be formed by being combined with each other in a common printing region can be generated.

  In the above-described embodiment, the division matrix corresponding to the first pixel group is described. However, the division matrixes corresponding to the second to fourth pixel groups are evaluated as shown in FIGS. Based on the matrix and the association matrix, it can be calculated as in the case of the first pixel group.

C. Dither matrix generation method in the second embodiment:
FIG. 22 is a flowchart showing the processing routine of the dither matrix evaluation method in the second embodiment of the present invention. The generation method of the second embodiment differs from the generation method of the first embodiment in the dither matrix evaluation method. That is, the generation method according to the second embodiment assumes that dots are not formed as threshold storage elements, that is, dots are formed in any of a plurality of pixels corresponding to a plurality of undetermined candidate elements. It differs from the generation method of the first embodiment in that the storage element is determined based on the RMS granularity of the dot pattern formed based on this assumption.

  In the generation method of the second embodiment, the steps S320, S330, S360, and S370 are modified to be steps S320a, S330a, S360a, and S370a, and the step S380 is performed. Is added to the processing routine of the second embodiment.

  Steps S320a and S360a differ from Steps S320 and S360 of the first embodiment in that not only the dots corresponding to the elements for which the determined threshold value is stored, but also the dots of the pixels corresponding to the element of interest are turned on. . The element of interest is one element selected from a plurality of candidate elements.

  Step S330a and step S370a differ from step S330 and step S370 of the first embodiment in that RMS granularity calculation processing is also performed. The RMS granularity calculation process is a process for calculating a standard deviation after low-pass filtering the dot density matrix. The standard deviation can be calculated using the calculation formula of FIG. The standard deviation is not necessarily calculated for the dot pattern corresponding to all elements of the dither matrix M. In order to reduce the amount of calculation, the standard deviation is calculated for pixels belonging to a predetermined window (for example, a 5 × 5 partial matrix). You may make it carry out using only a dot density. Such processing is performed for all the target pixels (step S380).

  The value calculated by such processing corresponds to the overall evaluation value or group evaluation value of the first embodiment. The second embodiment can generate an optimum dither matrix by performing an evaluation based on the RMS granularity by handling the calculated overall evaluation values and group evaluation values in the same manner as the first embodiment.

  The evaluation method of the second embodiment can be combined with the evaluation method of the first embodiment. That is, the candidate elements of the second embodiment may be narrowed down by the evaluation method of the first embodiment, and the storage elements may be determined from the narrowed candidate elements based on the RMS granularity. For example, in the example shown in the first embodiment, the evaluation values of the two elements are the same, but these two elements can be used as candidate elements in the second embodiment. Further, an element within a predetermined evaluation value range (for example, an evaluation value difference within 5) may be configured as a candidate element.

D. Variations:
As mentioned above, although several embodiment of this invention was described, this invention is not limited to such embodiment at all, and implementation in a various aspect is possible within the range which does not deviate from the summary. It is. For example, the present invention can optimize a dither matrix for the following modifications.

D-1. In the above-described embodiment, the plurality of surrounding matrices are connected with being shifted in the main scanning direction with respect to the center matrix. However, the present invention can also be applied when there is no such shift. Specifically, for example, as shown in the first modification (FIG. 24), the present invention is applicable and effective even when there are six pixel groups.

  In such a case, the number of groups in the main scanning direction is three, while the dither matrix is configured as 8 rows and 8 columns, so the number of rows in the dither matrix is not divisible by the number of groups in the main scanning direction. As a result, the division matrix corresponding to the same pixel group differs between the central matrix and the matrix connected in the main scanning direction. However, according to the present invention, when the rating matrix is an element of the first pixel group (described as “0” in the element), as in the above-described embodiment, the related matrix is changed according to the position of the surrounding matrix. It can be understood that the processing can be performed by using the element of the third pixel group (described as “2” in the element) and the element of the fifth pixel group (described as “0” in the element).

  Further, as shown in the second modification (FIG. 25), the dither matrix shift as in the first embodiment is applied to the first modification in which there are six pixel groups. Is also possible. In addition, as shown in the third modification (FIG. 26), it is also possible to apply a shift in the main scanning direction of the dither matrix to the first modification in which there are six pixel groups. It is.

  Thus, the present invention can be widely applied to halftone processing performed by a connected matrix in which a plurality of the dither matrices are connected according to a certain rule.

D-2. In the above-described embodiments, it is assumed that a print image is formed by four main scans. For example, a configuration in which a print image is formed by two main scans in both the forward direction and the bidirectional direction or a plurality of print heads. The present invention can be applied to various configurations such as a configuration in which is used. The present invention is generally applied to printing in which a print image is formed by combining print pixels belonging to each of a plurality of pixel groups assumed to have a physical difference in dot formation in a common print region. can do.

D-3. In the above-described embodiment, whether or not a plurality of dots formed in pixels corresponding to each element of the entire matrix are uniformly formed in each gradation value is a criterion for evaluation. You may comprise so that it may evaluate based only on the some dot formed in the pixel corresponding to each element of a part of matrix.

D-4. In the above-described embodiment, low-pass filter processing is performed and the optimality of the dither matrix is evaluated based on dot density uniformity and RMS granularity. For example, a Fourier transform is performed on a dot pattern and a VTF function is calculated. It may be used to evaluate the optimality of the dither matrix. Specifically, the evaluation scale (Grainess scale: GS value) used by Dooley et al. Of Xerox may be applied to the dot pattern, and the optimality of the dither matrix may be evaluated based on the GS value. Here, the GS value is a granularity that can be obtained by performing a predetermined process including a two-dimensional Fourier transform on a dot pattern and digitizing the dot pattern, and integrating after cascading with a visual spatial frequency characteristic VTF. Evaluation values (reference: fine imaging and hard copy, Corona, Japan Photographic Society, Japanese Imaging Society Joint Publishing Committee, P534). However, the former has an advantage that complicated calculation such as Fourier transform is unnecessary. The GS value is also called a graininess index.

D-5. In the above-described embodiments and modifications, the presence or absence of dot formation is determined for each pixel by comparing the threshold value set in the dither matrix and the gradation value of the image data for each pixel. Further, the sum of the gradation values may be compared with a fixed value to determine the presence or absence of dot formation. Furthermore, the presence / absence of dot formation may be determined according to the data generated in advance based on the threshold value and the gradation value without directly using the threshold value. In general, the halftone processing of the present invention may be any method that determines the presence or absence of dot formation according to the gradation value of each pixel and the threshold value set at the corresponding pixel position of the dither matrix.

FIG. 3 is an explanatory diagram illustrating a state where a print image is generated on a print medium while performing main scanning and sub scanning in the first embodiment of the present invention. FIG. 3 is an explanatory diagram illustrating a state in which a print image is generated on a print medium by combining print pixels belonging to each of a plurality of pixel groups in a common print area in the first embodiment of the present invention. The flowchart which shows the processing routine of the production | generation method of the dither matrix in 1st Example of this invention. Explanatory drawing which shows the dither matrix M in which the grouping process in 1st Example of this invention was performed. Explanatory drawing which shows four division | segmentation matrices M0-M3 in 1st Example of this invention. The flowchart which shows the process routine of a dither matrix evaluation process. Explanatory drawing which shows the connection state of nine dither matrices in 1st Example of this invention. FIG. 9 is an explanatory diagram showing a state in which dots are formed in each of eight pixels corresponding to elements storing threshold values at which dots are likely to be formed first to eighth in the dither matrix M; Explanatory drawing which shows the matrix which digitized the state in which the dot was formed in each of eight pixels, ie, the dot density matrix which expressed the dot density quantitatively. Explanatory drawing which shows the low-pass filter in 1st Example of this invention. Explanatory drawing which shows the result of carrying out the low pass filter process of the dot density matrix. Explanatory drawing which shows the division | segmentation matrix corresponding to a 1st pixel group in 1st Example of this invention. Explanatory drawing which shows the dot density matrix comprised from the element corresponding to a 1st pixel group. Explanatory drawing which shows the matrix which stores a group evaluation value. Explanatory drawing which shows the matrix which stores the total evaluation value, the group evaluation value, and the comprehensive evaluation value calculated from these evaluation values. Explanatory drawing which shows the division | segmentation matrix corresponding to a 2nd pixel group. Explanatory drawing which shows the dot density matrix comprised from the element corresponding to a 2nd pixel group. Explanatory drawing which shows the division | segmentation matrix corresponding to a 3rd pixel group. Explanatory drawing which shows the dot density matrix comprised from the element corresponding to a 3rd pixel group. Explanatory drawing which shows the division | segmentation matrix corresponding to a 4th pixel group. Explanatory drawing which shows the dot density matrix comprised from the element corresponding to a 4th pixel group. The flowchart which shows the processing routine of the evaluation method of the dither matrix in 2nd Example of this invention. Explanatory drawing which shows the calculation formula used for a RMS granularity calculation process. Explanatory drawing which shows the connection state of nine dither matrices in the 1st modification of this invention. Explanatory drawing which shows the connection state of nine dither matrices in the 2nd modification of this invention. Explanatory drawing which shows the connection state of nine dither matrices in the 3rd modification of this invention.

Explanation of symbols

M ... Dither matrix M0 ... Divided matrix M1 ... Divided matrix M2 ... Dither matrix M3 ... Divided matrix M1 ... Divided matrix 10 ... Print head DP1, DP1a ... Dot pattern DP2, DP2a ... Dot pattern DP3, DP3a ... Dot pattern DP4, DP4a ... Dot pattern

Claims (7)

A method of generating a dither matrix for determining a dot formation state on each print pixel of a print image to be formed on a print medium in order to perform halftone processing on input image data,
The print image is formed by combining print pixels belonging to each of a plurality of pixel groups assumed to be physically different in the formation of the dots in a common print region,
The halftone processing is performed by a matrix in which a plurality of the dither matrices are connected according to a certain rule.
The dither matrix generation method includes:
Of the plurality of threshold values to be stored in each element of the dither matrix, the focus is determined as the focus threshold value, which is the threshold value for which the element to be stored is undetermined and in which dot formation is most likely to be turned on. A threshold determination step;
It is considered that a dot is formed in a pixel corresponding to a determined element whose threshold to be stored is a determined element, and the attention is paid to any one of the blank elements whose thresholds are not yet determined. A storage element determination step for determining a comprehensive evaluation value for each of cases where a threshold value is assumed to be stored, and determining an element for storing the target threshold value according to the comprehensive evaluation value;
For all of the plurality of thresholds, a repetition step of repeating each of the focus threshold value determination step and the storage element determination step;
With
The storage element determination step includes:
A connected matrix generating step of generating a connected matrix by connecting a plurality of surrounding matrices having the same elements as the center matrix around the center matrix, which is a dither matrix to be evaluated, according to the predetermined rule;
An overall evaluation value determining step for evaluating the formation state of dots corresponding to all groups of the determined elements of the connection matrix and determining an overall evaluation value;
Among the determined elements of the connected matrix, a group evaluation value is obtained by evaluating a dot formation state corresponding only to an element belonging to an element group corresponding to a rating pixel group selected as a rating from the plurality of pixel groups. A group evaluation value determination process to be determined;
In accordance with the overall evaluation value and the group evaluation value, a comprehensive evaluation value determining step for determining the comprehensive evaluation value;
Including
The element group includes a rating group that is an element group corresponding to the rating pixel group in the central matrix, and a related group that is an element group corresponding to the rating pixel group in the plurality of surrounding matrices. A dither matrix generation method comprising: The dither matrix generation method according to claim 1,
The dither matrix generation method, wherein the plurality of surrounding matrices are shifted in at least one of a main scanning direction and a sub-scanning direction with respect to the center matrix. A printing method for printing on a print medium,
Dots representing the dot formation state on each print pixel of the print image to be formed on the print medium by performing halftone processing on the image data representing the gradation value of each pixel constituting the original image Dot data generation process for generating data;
In accordance with the dot data, a print image generation step for generating a print image by forming dots in the respective print pixels;
With
The print image is formed by combining print pixels belonging to each of a plurality of pixel groups assumed to be physically different in a common print region in the formation of dots by the print image generation unit,
The printing method according to claim 1, wherein the halftone processing is performed by a connection matrix in which the plurality of dither matrices according to claim 1 or 2 are connected according to the predetermined rule. A printing device for printing on a print medium,
Dots representing the dot formation state on each print pixel of the print image to be formed on the print medium by performing halftone processing on the image data representing the gradation value of each pixel constituting the original image A dot data generator for generating data;
According to the dot data, a print image generation unit that generates a print image by forming dots in the print pixels;
With
The print image is formed by combining print pixels belonging to each of a plurality of pixel groups assumed to be physically different in a common print region in the formation of dots by the print image generation unit,
The printing apparatus according to claim 1, wherein the halftone process is performed by a connection matrix in which the plurality of dither matrices according to claim 1 or 2 are connected according to the predetermined rule. A method for generating a printed material for forming a print image on a print medium,
Dots representing the dot formation state on each print pixel of the print image to be formed on the print medium by performing halftone processing on the image data representing the gradation value of each pixel constituting the original image Dot data generation process for generating data;
In accordance with the dot data, a print image generation step for generating a print image by forming dots in the respective print pixels;
With
The print image is formed by combining print pixels belonging to each of a plurality of pixel groups assumed to be physically different in a common print region in the formation of dots by the print image generation unit,
3. The printed material generation method according to claim 1, wherein the halftone processing is performed by a connection matrix in which the plurality of dither matrices according to claim 1 or 2 are connected according to the predetermined rule. An apparatus for generating a dither matrix for determining a dot formation state on each print pixel of a print image to be formed on a print medium in order to perform halftone processing on input image data,
The print image is formed by combining print pixels belonging to each of a plurality of pixel groups assumed to be physically different in the formation of the dots in a common print region,
The halftone processing is performed by a matrix in which a plurality of the dither matrices are connected according to a certain rule.
The dither matrix generation device includes:
Of the plurality of threshold values to be stored in each element of the dither matrix, the focus is determined as the focus threshold value, which is the threshold value for which the element to be stored is undetermined and in which dot formation is most likely to be turned on. A threshold determination unit;
It is considered that a dot is formed in a pixel corresponding to a determined element whose threshold to be stored is a determined element, and the attention is paid to any one of the blank elements whose thresholds are not yet determined. A storage element determination unit that determines a comprehensive evaluation value for each of cases where a threshold value is assumed to be stored, and determines an element for storing the target threshold value according to the comprehensive evaluation value;
With
The storage element determination unit
A connected matrix generation unit that generates a connected matrix by connecting a plurality of surrounding matrices having the same elements as the center matrix around the center matrix that is the dither matrix to be evaluated according to the predetermined rule;
An overall evaluation value determination unit that evaluates the formation state of dots corresponding to all groups of the determined elements of the connection matrix and determines an overall evaluation value;
Among the determined elements of the connected matrix, a group evaluation value is obtained by evaluating a dot formation state corresponding only to an element belonging to an element group corresponding to a rating pixel group selected as a rating from the plurality of pixel groups. A group evaluation value determination unit to be determined;
In accordance with the overall evaluation value and the group evaluation value, a comprehensive evaluation value determining unit that determines the comprehensive evaluation value;
Including
The element group includes a rating group that is an element group corresponding to the rating pixel group in the central matrix, and a related group that is an element group corresponding to the rating pixel group in the plurality of surrounding matrices. A dither matrix generation device comprising: A computer program for generating a dither matrix for determining a dot formation state on each print pixel of a print image to be formed on a print medium in order to perform halftone processing on input image data. And
The print image is formed by combining print pixels belonging to each of a plurality of pixel groups assumed to be physically different in the formation of the dots in a common print region,
The halftone processing is performed by a matrix in which a plurality of the dither matrices are connected according to a certain rule.
The computer program is
Of the plurality of threshold values to be stored in each element of the dither matrix, the focus is determined as the focus threshold value, which is the threshold value for which the element to be stored is undetermined and in which dot formation is most likely to be turned on. A threshold determination function;
It is considered that a dot is formed in a pixel corresponding to a determined element whose threshold to be stored is a determined element, and the attention is paid to any one of the blank elements whose thresholds are not yet determined. A storage element determination function that determines a comprehensive evaluation value for each of cases where a threshold value is assumed to be stored, and determines an element for storing the target threshold value according to the comprehensive evaluation value;
Comprising a program for causing the computer to realize
The storage element determination function includes:
A connected matrix generation function for generating a connected matrix by connecting a plurality of surrounding matrices having the same elements as the central matrix around the center matrix, which is a dither matrix to be evaluated, according to the predetermined rule;
An overall evaluation value determination function for evaluating the formation state of dots corresponding to all groups of the determined elements of the connection matrix and determining an overall evaluation value;
Among the determined elements of the connected matrix, a group evaluation value is obtained by evaluating a dot formation state corresponding only to an element belonging to an element group corresponding to a rating pixel group selected as a rating from the plurality of pixel groups. A group evaluation value determination function to be determined;
In accordance with the overall evaluation value and the group evaluation value, a comprehensive evaluation value determining function for determining the comprehensive evaluation value;
Including
The element group includes a rating group that is an element group corresponding to the rating pixel group in the central matrix, and a related group that is an element group corresponding to the rating pixel group in the plurality of surrounding matrices. A computer program characterized by comprising.

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