INKJET PRINTING APPARATUS, PRINTING CONTROL METHOD FOR INKJET PRINTING APPARATUS, PROGRAM, AND STORAGE MEDIUM

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A and 1B are schematic views showing an example of the arrangement of the nozzles (discharge apertures) of an inkjet printhead which can be used in an embodiment of the present invention;

FIG. 2 is a perspective view showing the arrangement of the main part of a printing apparatus which can be used in an embodiment of the present invention and prints on printing paper by using a printhead;

FIGS. 3A and 3B are views for explaining the number of ink dots to cover a printing area based on a droplet size in an embodiment of the present invention;

FIGS. 4A and 4B are schematic views showing a printed state which causes deterioration in graininess and an improved printed state in the present invention;

FIG. 5 is a view schematically showing a printed state when printing is performed by high-speed ink discharging operation in a conventional 1-pass printing mode;

FIGS. 6A to 6D are views showing the state of a printing area in a case wherein a solid image with the same tone and different printing densities is printed by an arbitrary scan in the 1-pass printing mode, and graphs showing the relationship between changes in the printing position of the printhead, head temperature, and ink discharge amount in an inkjet printing apparatus;

FIG. 7 is a block diagram showing a control arrangement for an inkjet printing apparatus in an embodiment of the present invention;

FIG. 8 is a graph showing the temperature dependence of ink discharge amount in a case wherein conditions for driving pulses are fixed;

FIG. 9 is a graph for explaining image conversion which is used in an embodiment of the present invention when an input image is printed with large and small ink discharge amounts;

FIGS. 10A and 10B are views for explaining conversion of multilevel image data expressed by four bits (16 tones) into binary image data expressed by one bit (two tones) as an intermediate process;

FIG. 11 is a view showing the state of printed dots;

FIG. 12 is a view for schematically explaining the concept of count areas for counting the numbers of dots printed and printing areas in which the numbers of dots of printing data are modified, which are used in an embodiment of the present invention;

FIG. 13 is a view for schematically explaining the divided state of count areas for counting the numbers of dots printed and printing areas in which the numbers of dots of printing data are modified;

FIG. 14 is a flowchart showing processing for each ink discharge amount which is performed for each scan in an embodiment of the present invention;

FIG. 15 is a flowchart showing the processing of counting the number of dots of printing data and the processing of correcting printing data on the basis of a count result;

FIG. 16 is a view schematically showing multilevel input image data equivalent to data in an arbitrary area of printing data in an embodiment of the present invention;

FIG. 17 is a view showing a threshold matrix used for the correction of printing data in an embodiment of the present invention;

FIG. 18 is a view schematically showing the processing of modifying the tone values of input image data in accordance with the numerical order of a threshold matrix in an embodiment of the present invention;

FIG. 19 is a view schematically explaining the divided state of count areas and printing areas in which the numbers of dots of printing data are modified in a case wherein a method of setting different division numbers and sizes for the respective scans is used as a count area dividing method in an embodiment of the present invention;

FIG. 20 is a view schematically explaining the divided state of count areas and printing areas in which the numbers of dots of printing data are modified in a case wherein a method of setting different division sizes within a scan is used as a count area dividing method in an embodiment of the present invention;

FIG. 21 is a view schematically explaining the divided state of count areas and printing areas in which the numbers of dots of printing data are modified in a case wherein a method of shifting the boundary position between a count area and a printing area for each scan is used as a count area dividing method in an embodiment of the present invention;

FIG. 22 is a view schematically explaining the divided state of count areas and printing areas in which the numbers of dots of printing data are modified in a case wherein count areas have a size different from that of printing areas in an embodiment of the present invention;

FIGS. 23A and 23B are views schematically showing other threshold matrices used for the correction of printing data in an embodiment of the present invention;

FIG. 24 is a view schematically showing another threshold matrix used for the correction of printing data in an embodiment of the present invention;

FIG. 25 is a view schematically showing the processing of modifying the tone values of input image data in accordance with the numerical order of a threshold matrix in an embodiment of the present invention; and

FIGS. 26A and 26B are views for explaining a printhead used in another embodiment of the present invention.

DESCRIPTION OF THE EMBODIMENTS

FIG. 7 is a block diagram showing the control arrangement of an inkjet printing apparatus according to an embodiment of the present invention. Note that the arrangement of the mechanical part of the inkjet printing apparatus according to this embodiment has a structure similar to that shown in FIG. 2.

Referring to FIG. 7, a CPU 700 executes control and data processing in each unit of the apparatus through a main bus line 705. That is, the CPU 700 performs control and data processing in each unit (to be described later) including a head drive control circuit 715 and a carriage drive control circuit 716 in accordance with the programs stored in a ROM 701. A RAM 702 is also used as a work area for data processing and the like performed by the CPU 700.

In addition, a hard disk or the like sometimes is connected, as another storage device, to the CPU 700 via the main bus line 705, as needed. An image input unit 703 includes an interface with a host apparatus, and temporarily stores image data input from the host apparatus. An image signal processing unit 704 comprises a data converting unit 718 which performs color conversion, binarization, a mask process, and the like, and a data correcting unit 719 which executes various kinds of data processing. An operation unit 706 comprises keys, a mouse, and the like, which allow the user to perform control input and the like. A recovery system control circuit 707 controls recovery operation such as preliminary discharge in accordance with the recovery process program stored in the RAM 702. A recovery system motor 708 drives a printhead 101 (identical to that shown in FIGS. 1A and 1B), a blade 709 which faces or separates from the printhead 101, a cap 710, a pump 711 for suction, and the like. The head drive control circuit 715 controls the driving of electrothermal transducers for ink discharge in the printhead 101 to cause the printhead 101 to discharge ink for preliminary discharge or printing. In addition, the carriage drive control circuit 716 and a paper feed control circuit 717 respectively control the movement of a carriage 206 and paper feed in accordance with control programs. A board on which the ink discharging electrothermal transducers of the printhead 101 are mounted includes heat insulation heaters, and can raise and adjust the ink temperature in the printhead 101 to a desired set temperature. A thermister 712 is also mounted on the board and is used to measure the actual temperature of ink in the printhead 101. The thermister 712 may also be provided outside the board or near the printhead 101.

The operation of each embodiment of the present invention will be described in detail below on the basis of the above apparatus arrangement.

First Embodiment

A printhead 101 is identical to the printhead 101 shown in FIGS. 1A and 1B, and comprises a combination of a nozzle array which discharges ink in a large discharge amount and a nozzle array which discharges ink in a small discharge amount. The internal arrangement of each nozzle which includes an ink flow path and the like is the same as that described above. In addition, the apparatus arrangement is the same as the arrangement shown in FIG. 2, and is configured to perform 1-pass printing operation of completing an image by performing one main printing/scanning operation with respect to the same printing area. This point is also the same as that described in the prior art.

The ink temperature of an ink discharge unit (the temperature of the ink printhead 101 in some case) is a factor which determines the ink discharge amount of the printhead 101. FIG. 8 is a graph showing the temperature dependence of ink discharge amount in a case wherein a driving pulse condition is fixed. As indicated by a solid line A in FIG. 8, an ink discharge amount Vd linearly increases with an increase in a printhead temperature T_H(which is equal to the ink temperature of the discharge unit because this graph represents a static temperature characteristic). If the slope of this straight line is defined as a temperature dependence coefficient KT, the temperature dependence coefficient KT is given by

KT=≢VdT/ΔT
_H(pl/° C·dots)

The temperature dependence coefficient KT is determined by the physical properties of the printhead 101 and ink and the like regardless of driving conditions. Referring to FIG. 8, a broken line B and a chain line C indicate temperature dependences of other printheads.

This embodiment controls a variation in ink discharge amount due to a fluctuation in the above ink temperature by using image correcting operation of modifying the total number of dots of image data to be printed so as to make the printing density on printing paper P constant. In this case, a method of modifying multilevel data is used as a correction method for image data.

That is, a data converting unit 718 in an image signal processing unit 704 generates a binary pattern corresponding to a tone value “K” indicated by each pixel of input multilevel image data with respect to the image data input from the host apparatus and held in an image input unit 703. If, for example, the image input unit 703 receives multilevel image data represented by four bits (16 tones), the data converting unit 718 converts the input image data into binary data. The following will exemplify a case wherein a conversion process based on area coverage modulation is used as a binarization process for the input image data.

The following will exemplify a case wherein an external host apparatus sends image data each pixel of which has a size of ( 1/300) inch square (a resolution of 300×300 dpi) as indicated by pixel data 1000 in FIG. 10A. This embodiment will exemplify a case wherein the printhead 101 shown in FIGS. 1A and 1B performs printing with a combination of ink LC with a large ink discharge amount, ink SC with a small ink discharge amount, ink LM with a large ink discharge amount, ink SM with a small ink discharge amount, ink LY with a large ink discharge amount, and ink LBK with a large ink discharge amount. In this case, assume that the host apparatus sends 4-bit (16-tone) image data for each color with each ink discharge amount.

The following is a case wherein input image data is converted into printing data for each of the nozzle arrays with different ink discharge amounts since nozzle arrays with different ink discharge amounts are used for cyan C and magenta M. As shown in FIG. 9, this apparatus converts input image data into printing data based on small dots and printing data based on large dots by referring to a LUT (Look Up Table) in accordance with the input values of the image data. The apparatus binarizes each data by a binarization method to be described later. For example, at the tone indicated by the arrow in FIG. 9, the apparatus prints in a state wherein large and small dots are coexistent, and reproduces, on the printing paper P, a density corresponding to the tone of the input image data.

For this reason, this apparatus performs a pseudo-halftone process and resolution converting process for each input image data of each color with each ink discharge amount. More specifically, the apparatus assigns 4×4 pixels to one pixel of input image data vertically and horizontally, and replaces tone values ranging from 0 to 15 in each unit matrix of 4×4 pixels with 0 to 15 representing the numbers of discharged “1” dots in each unit matrix.

Assume that one dot has a size of ( 1/1200) inch square (a resolution of 1200×1200 dpi) in both the main scanning direction and the sub-scanning direction. This apparatus then generates printing data 1001 (1002 to 1017) corresponding to the original 1-pixel data 1000, with each pixel being represented by one bit (two tones) for each color ink of LC, SC, LM, SM, LY, and LBK with each ink discharge amount, and prints on the basis of the printing data 1001. FIG. 10B shows the conversion of pixel data 1020 (a resolution of 300×300 dpi) of one pixel (four bits) whose pixel value is “9h” (h represents a hexadecimal number), which is sent from the host apparatus. That is, FIG. 10B shows a case wherein the pixel data 1020 is converted into 9-dot printing data 1021 comprising printing data (a dot resolution of 1200×1200 dpi) each pixel of which is represented by one bit.

As pseudo-halftone process and resolution converting process techniques, various techniques have been proposed. In this case, the apparatus converts printing data (a resolution of 300×300 dpi) each pixel of which comprises four bits and which is sent from the host apparatus into 16 tones by using printing data (a resolution of 1200×1200 dpi) each dot of which is represented by one bit and which is prepared in advance, in accordance with the value of each pixel. The apparatus then simultaneously performs a pseudo-halftone process and resolution converting process by the technique of referring to a LUT (Look Up Table).

In this case, when using the LUT for separating input image data into large and small dots, the apparatus uses a resolution converting process based on area coverage modulation as a binarization process for input image data. However, the present invention is not limited to this. The present invention may generate printing data for large and small dots, and can perform a pseudo-halftone process by using an arbitrary processing method such as an average density storage method or a dither matrix method.

This embodiment counts large and small dots corresponding to one color, which are formed by the 128 nozzles of a nozzle array for large dots and the 128 nozzles of a nozzle array for small dots which are used for cyan and magenta, in a predetermined number of divided areas, thereby correcting the image data.

FIG. 12 is a view for schematically explaining the concept of count areas for counting the numbers of dots to be printed and printing areas in which the numbers of dots of printing data are modified. Referring to FIG. 12, for the sake of easy understanding, the size of each count area is set to be the same as that of each printing area. This setting applies to the following description of all the embodiments of the present invention. As will be described later, however, it is not essential for the present invention to set the size of each count area to be the same as that of each printing area. That is, it suffices to set the size of each count area to be different from that of each printing area.

This embodiment will exemplify a case wherein an entire printing area is printed by 1=L scans. FIG. 13 shows printing data corresponding to one main scan of the printhead 101 in the first scanning operation, that is, input image data equivalent to image data corresponding to (number of nozzles) x (number of pixels printed on one line in scanning direction). That is, the input image data is divided into N portions in the vertical direction (nozzle array direction)×M portions in the horizontal direction (scanning direction), that is, N (in the vertical direction)×M (in the horizontal direction) count areas and printing areas, each including the same number of pixels having multilevel information.

More specifically, assume that when the printhead 101 having a total of 256 large and small nozzles is to be used, the resolution is set to 300×300 dpi. In this case, the input image data is divided upon setting 64 pixels (= 256/4) in the vertical direction (nozzle array direction) to N=1 and a printing width of eight inches in the horizontal direction (scanning direction) to M=20. The printing width is 2,400 pixels (=8×300). Therefore, the size of each of the count and printing areas is set to 64 (vertical)×120 (horizontal). Assume that the data of tone values 0 to 15 correspond to the respective pixels in the count and printing areas.

Assume that the number of droplets discharged in the count area corresponding to the mth row and the nth column which is obtained by accumulating multilevel data tone values in the area as a count result in each count area is represented by a dot count value Et (m, n). Assume also that the accumulated number of droplets discharged (in the horizontal direction) of the numbers of droplets discharged in the respective nth count areas in the vertical direction is represented by a count value Smt(m, n). Similarly, assume that the total number of the accumulated numbers of droplets discharged in printing operation from the start of printing to an immediately preceding scan is represented by a total dot count value Sat(1−l), and a correction amount calculated for each printing area is represented by Ht(m, n).

More specifically, referring to FIG. 13, the value represented by Smt(3, 1) is the sum of Et(1, 1) to Et(3, 1) which are the number of droplets discharged in the first count area in the vertical direction to the third count area in the horizontal direction. In addition, Sat(1) represents the same value as Smt(20, 1) which is the accumulated number of droplets discharged in all the count areas in the horizontal direction in each scan. Furthermore, Ht(1, 1) represents a printing data correction amount (the number of dots) in the printing area corresponding to the first row and the first column.

FIG. 14 is a flowchart showing processing in the first embodiment. FIG. 15 is a flowchart showing the processing of counting the number of dots of pixel data of each image data corresponding to each discharge amount in each scan in the first embodiment, the processing of correcting the pixel data on the basis of the count result, and the like.

As shown in FIG. 14, in each printing/scanning operation, first of all, in step S1400, this apparatus converts the pixel values of multilevel image data corresponding to large dots by determining modified pixel positions at which image correction is sequentially performed for the data by using a threshold matrix (to be described later). In step S1401, when starting correcting multi-tone image data corresponding to small dots, the apparatus modifies the correction start position of the small dots so as to designate pixel positions at which correction is performed for the small dots after the value of the threshold matrix by which conversion has already been executed by using large dots. Thereafter, in step S1402, the apparatus executes an image correction process for the multi-tone image data for small dots.

FIG. 15 is a flowchart showing, in detail, operation in a correction process for each multi-tone image data. This flowchart will be described in detail below. This apparatus starts this process for each scan. First of all, in step S1500, the apparatus sets a count area Et( m, n) of interest with m=1, n=1, and l=1, and initializes a memory area such as a register which stores the values of Smt(1, 1) to Smt(M, N) and Sat(0) to Sa(L).

In step S1501, the apparatus matches the data start position of input image data which corresponds to the first printing data with the count start position of a count area. In step S1502, the apparatus counts the sum of tone values in the count area of the interest specified by all the values of m=1 and n=1 of the first count area in the horizontal direction, and temporarily stores the dot count value of the count area as Et(1, 1) in the memory area.

In step S1503, the apparatus determines whether the count area of interest is the first area in the horizontal direction, that is, is located at the start position in the scanning direction. If the result is YES in step S1503, the process advances to step S1504, otherwise, the process advances to step S1505.

In step S1504, the apparatus performs processing on the basis of the count data of a total dot count value Sat(0) (0 at this point because the first printing/scanning operation is performed, that is, l=1, and hence no discharge is performed immediately before this operation) representing the total number of dots discharged from the start of printing to printing in the immediately preceding scanning operation. That is, the apparatus computes a predicted value of increasing density based on an increase in ink discharge amount, and calculates a correction amount Ht(1, 1) of the printing data of the area specified by the values of m=1 and n=1 of the first printing area in the horizontal direction. The apparatus then computes a count value, as a new value Et(1, 1), from Et(1, 1) which is the same value as the count value in this printing area and the calculated correction amount Ht(1, 1).

In step S1506, the apparatus adds Et(1, 1) obtained by the above operation to an accumulated dot count value Smt(m, n) in the horizontal direction, and stores the resultant value as the value of a new count value Smt(1, 1) in a corresponding memory area.

In step S1507, the apparatus executes correction equivalent to the above correction amount Ht(l, 1) with respect to the printing data in the first printing area in the horizontal direction, and modifies the number of printing data in the printing area. The apparatus modifies the printing data by modifying the tone values of the pixels in the printing area by a numerical value equivalent to the correction amount on the basis of the sequence of the threshold matrix.

In step S1508, the apparatus determines whether m>M (=20). If m<M, the process advances to step S1509 to increment the value of m by one to shift the count area by one pixel in the printing/scanning direction. The process then returns to step S1502.

The apparatus then repeats the processing from step S1502 to step S1509 with m=2 to execute correction for the printing data in the printing area. First of all, in step S1502, the apparatus sets the dot count value of the second count area of interest in the horizontal direction to Et(2, 1), and temporarily stores it in the memory area. In step S1503, since the count area of interest is the second area in the horizontal direction, the process advances to step S1505.

The apparatus performs the processing in step S1505 in the following manner. That is, the apparatus performs the processing on the basis of three kinds of count data, i.e., the two kinds of count values Et(1, 1), and Smt(1, 1) and the total dot count value Sat(0) (0 because l=1, which indicates there is no discharge immediately before this operation) representing the total number of dots discharged from the start of printing to printing in the immediately preceding scanning operation. That is, the apparatus computes a predicted value of increasing density based on an increase in ink discharge amount, and calculates a correction amount Ht(2, 1) of the printing data of the area specified by the values of m=2 and n=1 of the second printing area in the horizontal direction. The apparatus then computes a count value, as a new value Et(2, 1), from Et(2, 1) which is the same value as the count value in this printing area and the calculated correction amount Ht(2, 1).

In step S1501, the apparatus adds Et(2, 1) obtained by the above operation to an accumulated dot count value Smt(1, 1) in the horizontal direction, and stores the resultant value as the value of a new accumulated dot count value Smt(2, 1) in the horizontal direction in a corresponding memory area.

In step S1507, the apparatus executes correction equivalent to the above correction amount Ht(2, 1) with respect to the printing data in the second printing area in the horizontal direction, and modifies the number of printing data in the printing area. In step S1508, the apparatus determines whether m>M (=20). If m<M, the process advances to step S1509 to increment the value of m by one to shift the count area by one pixel in the printing/scanning direction. The process then returns to step S1502.

Subsequently, the apparatus repeats the processing from steps S1502 to S1509 with respect to all values of m (1 to M) to complete correction for the printing data in the corresponding printing area.

In step S1510, the apparatus sets the value of the accumulated dot count value Smt(m, n) in the horizontal direction (which matches Smt(20, 1)at this point) as a new total dot count value Sat(l) (l=1 at this point), and stores it in the memory area. The data converting unit 718 in the image signal processing unit 704 in FIG. 7 further processes the corrected data. That is, the data converting unit 718 executes a resolution converting process of converting 1-pixel (4-bit) pixel data (a resolution of 300×300 dpi) into data (at a resolution of 1200×1200 dpi) each pixel of which is presented by one bit. The apparatus further transfers the converted printing data to the printhead 101 to execute printing with a printing width corresponding to one main scan. The apparatus starts a correction process for printing data corresponding to a main scan in the next printing operation concurrently with this printing operation.

In step S1511, the apparatus determines whether 1>L. If 1>L, the apparatus finishes the process. If l<L, the process advances to step S1512 to increment the value of 1 by one to shift the count area by one in the vertical direction. In step S1513, the apparatus initializes the memory to 0, which temporarily stores the count value Et(m, n) in the count area of interest and the accumulated dot count value Smt(m, n) of the values counted in the previous scan. Subsequently, the apparatus repeats the processing from step S1501 to step S1513 to complete an image by discharging ink from the printhead 101 on the basis of the corrected printing data while sequentially performing counting and a correction process for the printing data. Note that this embodiment uses a method of changing the tone value level of multilevel data as a method of correcting image data.

FIG. 16 schematically shows input multilevel image data in an arbitrary area of printing data. At this time, the total number of dots of the printing data matches the accumulated tone value of the input image data.

First of all, with regard to multilevel image data for large dots, in steps S1504 and S1505 in the flowchart of FIG. 15, the apparatus calculates, as a correction amount, a value which is subtracted from the accumulated data tone value of the input image data. At this time, the apparatus decreases an arbitrary pixel tone value of the input image data by one level and repeats this until the tone value becomes equal to the correction amount. At this time, the apparatus uses a method of preparing a threshold matrix mask with the same size as that of a count area and selecting a pixel as a method of selecting a pixel as a target whose tone value is-to be decreased. This threshold matrix is a matrix mask 1701 in which 0 to 7,679 (=64×120) are assigned to the respective pixels within the mask size (64×120 pixels) and which is shown in FIG. 17. The numerical values corresponding to the pixels at this time are sequenced such that the variance of the positions of all numerical values at arbitrary numbers becomes high.

The apparatus then performs the processing of decreasing the tone value of each corresponding pixel on the input image data by one level in accordance with the numerical order of the threshold matrix 1701. The apparatus sequentially subtracts the tone values of the pixels of the input image data in accordance with the sequence of the threshold matrix until the total number of pixels whose tone values are subtracted reach the calculated correction value. FIG. 18 is a schematic view for explaining the processing at this time in detail.

Consider a case wherein there is input image data 1801 in a given count area, and an accumulated dot count value of 3,000 and a value of 300 to be subtracted as a correction amount are calculated. In this case, the apparatus decreases the tone values of the input image, of all 7,680 pixels, which correspond to pixel positions 0 to 299 in a threshold matrix 1802 by one. The modified image data becomes like data 1803. In data 1803, the numerical values in the parentheses represent data-modified pixels.

Assume that as shown in FIG. 25, the number of pixels of input image data 2501 for large dots is 7,680 (=64×120) (300 dpi), the accumulated value of the tone values is 40,000, and a value to be subtracted as a correction amount is calculated as 9,000. In this case, since the subtraction value is larger than 7,680, the tone values of all the 7,680 pixels are decreased by one level. In addition, this apparatus further corrects the remaining 1,320 (=9000−7680) pixels by decreasing, by one, the tone values of the input image data which correspond to pixel positions 0 to 1319 in the threshold matrix. Reference numeral 2503 denotes data after the modification at this time. The tone values in the parentheses are obtained by decreasing their original values by two levels.

After determining pixel positions at which multilevel image data for large dots are to be corrected, the apparatus sequentially determines pixel positions at which correction for small dots is to be performed. Consider a case wherein the number of pixels of input image data 2501 for small dots is 7,680 (=64×120) (300 dpi), the accumulated value of the tone values is 30,000, and a value to be subtracted as a correction amount is calculated as 2,000. In this case, the apparatus decreases, by one, the tone values of the input image data which correspond to pixel positions 1320 to 3319 in the threshold matrix. This makes it possible to distribute the positions at which the tone values for large dots are decreased by two levels and the positions at which the tone values for small dots are decreased by one level with a high variance without making them overlap each other. Since the tone values for the large dots have already been decreased by one level as a whole, the correction for the large dots does not interfere with that for the small dots.

As described above, the apparatus executes correction for large dots at the positions corresponding to pixel positions 0 to 299, and then starts correction for small dots from pixel position 300. That is, upon calculating a correction amount for the corresponding count area for small dots as 700, the apparatus decreases, by one, the tone values of the input image data which correspond to pixel positions 300 to 999 in the threshold matrix. Since the number arrangement of the threshold matrix is designed with a high variance, the apparatus can determine correction positions for small dots with a high variance while exclusively avoiding correction positions for large dots. This embodiment need only use one kind of threshold matrix instead of preparing different threshold matrices for large and small dots. This also makes it possible to save memory space.

As described above, this embodiment divides printing data for large dots in the printing/scanning direction into a plurality of areas, counts the number of discharged data for each area, and modifies the number of dots to be discharged in a printing area in accordance with the duty cycle of the counted discharge data. Only modifying correction start positions in a threshold matrix makes it possible to determine correction positions for small dots exclusively from pixel positions at which correction is performed for large dots. Printing with the printing data corrected in this manner will perform correction for large dots with a high variance when reducing density irregularity due to an increase in ink discharge amount in the main scanning direction. As shown in FIG. 4B, the apparatus can also perform correction for small dots with a high variance without any deterioration in graininess due to interference between the correction positions for large dots with those for small dots.

This embodiment has exemplified the case wherein 20 count areas are set in the horizontal direction. However, it suffices to set the number of count areas to an arbitrary division number in accordance with the temperature rise characteristic of the printhead 101 or the droplet size. For example, as shown in FIG. 19, the division number and size of count areas may be changed for each scan. Alternatively, as shown in FIG. 20, count areas may have different sizes.

In addition, as shown in FIG. 21, even if count areas have the same size, it is possible to set a total size larger than the maximum image size and set the count areas such that the boundary position shifts for each scan.

This embodiment has also exemplified the case wherein the size of each count area is equal to that of a corresponding printing area. However, as shown, for example, in FIG. 22, the present invention can be satisfactorily applied even to a case wherein the size of each count area differs from that of a corresponding printing area. In this case, when correcting the image data in a printing area on the basis of the data counted in a count area, the apparatus redundantly performs a correction process for one of the areas. This can improve the correction effect at boundary portions.

Although a technique of modifying the tone value of input multilevel image data as a method for correcting image data has been described in this embodiment, techniques for correcting input multilevel image data of this invention are not limited to this embodiment.

For example, another technique using a threshold matrix as in this embodiment will be described with reference to FIGS. 23A and 23B. Assume that the size of a threshold matrix 2301 for the large nozzles is the same as that for a count area (i.e., 64×64 pixels). A threshold matrix comprises an aggregation of sub-matrices each comprising, for example, 8×8 pixels. Numerical values 0 to 63 are given to each sub-matrix in the threshold matrix, and numerical values 0 to 63 are given to the respective positions in each sub-matrix. This also applies to a threshold matrix 2302 for small nozzles. Referring to FIG. 23, the numerical values in the threshold matrix for large nozzles are reversed with respect to those in the threshold matrix for small nozzles.

In this case, the method of modifying the tone values of input image data performs the processing of decreasing the tone values of input image data corresponding to the respective positions in sub-matrices in accordance with a correction amount in the numerical order of sub-matrices and the numerical order of the respective positions in the sub-matrices. This method sequentially repeats this processing up to a value corresponding to the correction amount. Using such sub-matrices makes it possible to decrease the threshold matrix size itself. This method is therefore effective as a technique for a case wherein importance is attached to the apparatus cost.

In addition, in this embodiment, the numbers of the threshold matrix are allocated such that the variance of all allocations at arbitrary numbers becomes high. As shown in FIG. 24, however, numbers may be allocated such that smaller numbers appear on the forward side in the scanning direction, and larger numbers appear on the backward side in the scanning direction. This technique is effective in reducing abrupt density changes at the boundary portions of count areas especially when the head has a very high tendency toward a rise in temperature.

As another technique of modifying multilevel image data, this embodiment can use, for example, a method of preferentially reducing higher tone values within an area.

Using the correction process for multilevel image data is very effective for photographic image printing which demands high image quality. This embodiment has exemplified the method of calculating a correction amount for image data, which calculates a correction amount on the basis of three kinds of count results, that is, a dot count in a target count area, an accumulated dot count in the horizontal direction, and a total dot count from the start of printing to printing in the immediately preceding scan. However, the apparatus preferably selects an optimal method of calculating a correction amount in consideration of the accuracy of calculation and the apparatus cost.

With regard to the timing of correction, this embodiment has exemplified the method of starting printing operation after executing correction for all printing data corresponding to one main scan. Obviously, the present invention is not limited to this. For example, it is possible to use a method of always processing data corresponding to a plurality of scans in advance or a method of printing in real time by transferring printing data to the head at the same time as the end of correction. That is, it is preferable to use an optimal technique in accordance with conditions such as a printing speed, count area size, and the number of nozzles at the time of printing.

In addition, this embodiment has exemplified the case wherein the printhead 101 prints in the 1-pass printing mode of completing an image by repeatedly performing printing with the total head width and paper feeding. However, the present invention can be applied to the multi-pass printing mode as another printing mode. When the present invention is applied to this multi-pass printing mode, it is preferable to print in the same printing area by overprinting a plurality of times or by calculating a correction amount for printing data in each pass in consideration of conditions such as when the number of dots printed per scan is small.

In addition, the present invention can be applied to an inkjet printing system, comprising a unit (e.g., an electrothermal transducer, laser beam generator, or the like) which generates heat energy as energy utilized for ink discharge. That is, the present invention is very effective for an inkjet printhead and inkjet printing apparatus based on a system which causes a change in the state of ink using heat energy.

As the typical arrangement and principle of the inkjet printing system, one practiced by use of the basic principle disclosed in, for example, U.S. Pat. Nos. 4,723,129 and 4,740,796 is preferable. This system is applicable to either a so-called on-demand type or a continuous type. Particularly, in the case of the on-demand type, the system is effective because it gives a rapid temperature rise exceeding nuclear boiling to each of the electrothermal transducers arranged in correspondence with a sheet or fluid channels holding a liquid (ink), while heat energy is generated by the electrothermal transducer to effect film boiling on the heat acting surface of the printhead; and consequently, a bubble can be formed in the liquid (ink) in one-to-one correspondence with one or more applied driving signals which corresponds to printing information. By discharging the liquid (ink) through a nozzle by growth and shrinkage of the bubble, at least one droplet is formed. If the driving signal is applied as a pulse signal, growth and shrinkage of the bubble can be attained instantly and adequately to achieve discharge of the liquid (ink) with the particularly high responsiveness. As the pulse driving signal, signals disclosed in U.S. Pat. Nos. 4,463,359 and 4,345,262 are suitable. Note that still better printing can be performed by using the conditions described in U.S. Pat. No. 4,313,124, the invention of which relates to the temperature rise rate of the heat acting surface.

As an arrangement of the printhead, in addition to the arrangement as a combination of discharge apertures (nozzles), fluid channels, and electrothermal transducers (linear fluid channels or right angle fluid channels) as disclosed in the above specifications, the arrangement using U.S. Pat. Nos. 4,558,333 and 4,459,600, which has a heat acting portion arranged in a flexed region is also incorporated in the present invention. In addition, the present invention can be effectively applied to an arrangement based on Japanese Patent Laid-Open No. 59-123670 which discloses the arrangement using a slit common to a plurality of electrothermal transducers as a discharge portion of the electrothermal transducers, or Japanese Patent Laid-Open No. 59-138461 which discloses the arrangement having an opening for absorbing a pressure wave of heat energy in correspondence with a discharge portion. That is, the present invention can print reliably and efficiently regardless of the form of a printhead.

Furthermore, the present invention can also be effectively applied to a printhead of the full line type having a length corresponding to the maximum width of a printing medium on which the printing apparatus can print. Such a printhead may have either an arrangement which satisfies its length by a combination of a plurality of printheads or an arrangement as a single printhead which is integrally formed.

The present invention can be applied to a serial type like that described above, a printhead fixed to the apparatus body, or an exchangeable chip type printhead which can be electrically connected to the apparatus body and can receive ink from the apparatus body after being mounted on the apparatus body. In addition, the present invention can be effectively applied to a cartridge type printhead in which an ink tank is integrally provided on the printhead itself. It is preferable to add a recovery unit for the printhead, a preliminary auxiliary unit, and the like provided as components of the printing apparatus of the present invention because the effects of the printing operation can be further stabilized. More specifically, examples of such units include, for the printhead, a capping unit, a cleaning unit, a pressurization or suction unit, a pre-heating unit using electrothermal transducers, another heating element, or a combination thereof, and a preliminary discharge unit which performs discharge independently from printing.

Consider also the type and number of printheads to be mounted. For example, the printing apparatus may have only one printhead in correspondence with monochrome ink or a plurality of printheads in correspondence with a plurality of kinds of inks having different printing colors or densities. In addition to a printing mode which prints images in only a primary color such as blacks the printing apparatus may have either a printing mode using an integral printhead or a printing mode which uses a combination of printheads. Furthermore, the present invention is very effective for an apparatus comprising at least one of a printing mode which prints images in different colors and a printing mode which prints images in full-color as a mixture of colors.

Furthermore, although ink has been described as a liquid in the above embodiment of the present invention, the apparatus may use an ink which solidifies at room temperature or below, and as well as that which softens or liquefies at room temperature. Alternatively, the apparatus may use an ink which liquefies when the print signal is supplied because the inkjet system is generally configured to control the temperature of ink itself within the range from 30° C. or higher to 70° C. or lower so as to make the viscosity of the ink fall within a stable discharge range.

Furthermore, the apparatus may use an ink which solidifies when it is caused to stand, and liquefies when being heated, in order to prevent a temperature rise caused by heat energy by utilizing the temperature rise as energy to cause a state transition from the solid state to the liquid state or to prevent ink evaporation. In any case, the present invention can use an ink which liquefies only after heat energy is applied, for example, an ink which liquefies in accordance with a print signal as a heat energy source so as to be discharged in the form of liquid ink or an ink which begins to solidify when it reaches a printing medium.

In the above case, the ink may be of a type which is held as liquid or solid material in a recess of a porous sheet or a through hole at a position to face the electrothermal transducer as disclosed in Japanese Patent Laid-Open No. 54-56847 or Japanese Patent Laid-Open No. 60-71260. In the present invention, the above film boiling system is most effective for each type of ink described above.

In addition, the inkjet printing apparatus of the present invention is used in the form of an image output terminal of an information processing device such as a computer. In addition, this apparatus may be used in the form of a copying machine combined with a reader, and the like, or a facsimile apparatus having a transmission/reception function.

Second Embodiment

The same conditions as those used in the first embodiment are used for a printhead 101, inkjet printing apparatus, and inkjet printing method used in the second embodiment. As threshold matrix masks for large and small dots, this embodiment uses masks comprising main matrices and sub-matrices like those shown in FIGS. 23A and 23B. The printhead 101 comprises two pairs of nozzle arrays LC, SC, LM, and SM, each nozzle array of which comprises 256 large and small nozzles alternately arrayed at a pitch of 200 dpi (about 21.2 μm), with 128 nozzles being used for a large ink discharge amount and 128 nozzles being used for a small ink discharge amount. Nozzle arrays LY and LBK comprise nozzle arrays for a large ink discharge amount. A printing method based on the 1-pass printing mode and the arrangement of the printing apparatus are the same as those in the first embodiment. In addition, the volume of each ink droplet and a printing material for ink containing a coloring material are the same as those in the first embodiment.

As a method of correcting printing data, this embodiment uses a method of modifying the level of the tone value of multilevel data as in the first embodiment. As shown in FIGS. 23A and 23B, the numerical values in the threshold matrix for large nozzles are reversed with respect to those in the threshold matrix for small nozzles. Therefore, the embodiment designates positions at which image data for large and small nozzles are exclusively and sequentially reduced.

In the second embodiment, a method of modifying the tone values of input image data decreasing, at each level, the tone values of input image data corresponding to the respective positions in sub-matrices in accordance with a correction amount in the numerical order of sub-matrices and the numerical order of the respective positions in the sub-matrices. The method repeats this processing until the tone value becomes equal to the correction amount.

A numerical order in each sub-matrix is determined such that the sequence of reducing multilevel image data for large dots becomes exclusive with respect to the sequence of reducing multilevel image data for small dots. Using a sub-matrix arrangement allows the apparatus to have a simpler arrangement, although the number of types of matrices increases.

Image 1 formed by the above arrangement using the same processing steps as in the first embodiment is free from density differences like those visually recognized on the entire image or density irregularity near the two ends. In addition, it is possible to obtain good image quality without any deterioration in graininess due to local reductions in multilevel image data for large and small dots.

Third Embodiment

The third embodiment can be implemented by using the same arrangement and the same inkjet printing apparatus as those in the second embodiment. The third embodiment reduces dots with respect to image data instead of calculating a correction amount by counting multilevel image data for each ink discharge amount. This makes it possible to reduce image printing data for a small ink discharge amount on the basis of the correction ratio calculated by counting printing data to be printed with nozzles with a large ink discharge amount which causes a large amount of change in density. This allows a reduction in the processing load as compared with a case wherein multilevel image data are counted for all discharge amounts, because printing data for only certain ink discharge amounts are counted. Correcting recording data for a large ink discharge amount allows sufficient correction. In addition, setting the correction ratio of a large ink discharge amount to be equal to the correction ratio of a small ink discharge amount makes it possible to perform correction to some extent. Although inferior to the first embodiment in terms of effect, the third embodiment can obtain good image quality without any density difference which is visually recognizable on an entire image, any density irregularity near the two ends, and any deterioration in graininess due to local reductions in multilevel image data for large and small dots.

Fourth Embodiment

This embodiment prepares inks of similar colors with different densities in place of inks of large and small ink discharge amounts, and uses heads with the same ink discharge amount shown in FIG. 26A and 26B for all the types of inks. A printhead 2601 comprises six pairs of nozzle arrays 2602 each having an array of 256 nozzles. The embodiment sequentially supplies dark cyan (with low lightness), light cyan (with high lightness), yellow, black, light magenta, and dark magenta inks from the left in FIG. 26A. Other arrangements are the same as those of the printhead 101 shown in FIG. 1A. That is, each nozzle array 2602 has nozzles 2603 arrayed on both sides of an ink supply path 2605. Each nozzle can print a dot on printing paper P. Ink is supplied from the ink supply path 2605 through ink flow paths 2064 provided in correspondence with the nozzles 2603.

This embodiment can print a color image on the basis of image data. With regard to the processing in the first embodiment, the fourth embodiment performs processing for printing data with ink having low lightness instead of processing for printing data with a large ink discharge amount, and performs processing for printing data with ink having high lightness instead of processing for printing data with a small ink discharge amount. The fourth embodiment can also obtain good image quality without any density difference which is visually recognizable on an entire image, any density irregularity near the two ends, and any deterioration in graininess due to local reductions in multilevel image data for dots with low lightness and dots with high lightness.

The object of the present invention can be achieved even by supplying a storage medium storing software program codes for implementing the functions of the above embodiments to a system or apparatus. That is, it is obvious that the object of the present invention can be achieved by causing the computer (or a CPU or an MPU) of the system or apparatus to read out and execute the program codes stored in the storage medium. In this case, the program codes themselves read out from the storage medium implement the functions of the above embodiments, and the storage medium storing the program codes constitutes the present invention.

As a storage medium for supplying the program codes, for example, a flexible disk, hard disk, optical disk, magnetooptical disk, CD-ROM, CD-R, magnetic tape, nonvolatile semiconductor memory card, ROM, or the like can be used. In addition, the functions of the above embodiments may be implemented by causing a computer to execute readout programs.

Obviously, the functions of the above embodiments are implemented when the OS (Operating System) or the like running on the computer performs part or all of actual processing on the basis of the instructions of the program codes.

In addition, the program codes read out from the storage medium may be written in the memory of a function expansion board inserted into the computer or a function expansion unit connected to the computer. Obviously, the present invention incorporates a case wherein the functions of the above embodiments are implemented by causing the CPU of the function expansion board or function expansion unit to perform part or all of actual processing on the basis of the instructions of the program codes.

While the present invention has been described with reference to exemplary embodiments, it is to be understood that the invention is not limited to the disclosed exemplary embodiments. The scope of the following claims is to be accorded the broadest interpretation so as to encompass all such modifications and equivalent structures and functions.

This application claims the benefit of Japanese Patent Application No. 2006-169380, filed Jun. 19, 2006, which is hereby incorporated by reference herein in its entirety.

INKJET PRINTING APPARATUS, PRINTING CONTROL METHOD FOR INKJET PRINTING APPARATUS, PROGRAM, AND STORAGE MEDIUM

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