BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a schematic diagram for explaining INDEX patterning processing;
FIG. 2 is a schematic diagram for explaining INDEX patterns in an ink jet printing apparatus that forms an image by using large and small dots;
FIGS. 3A and 3B are diagrams each showing a dot alignment state in a case of continuously printing data of a level 2 on a certain range of area;
FIGS. 4A and 4B show magnified diagrams when focusing on boundary parts in a sub scan direction in FIGS. 3A and 3B, respectively;
FIG. 5A is a table showing the number of printed small dots, the number of printed large dots, the total number of the small and large dots and the ink application amount in one pixel of 600 dpi with respect to each inputted level, in a case of using the INDEX patterns shown in FIG. 2.
FIG. 5B is a graph showing an ink amount applied to one pixel of the image resolution, and the average value of the number of dots printed to apply the ink amount;
FIG. 6 is a view of a schematic configuration of a main part of an ink jet printing apparatus to which an embodiment is applied;
FIG. 7 is a perspective view for explaining a configuration of an ink jet printing cartridge applicable to this embodiment;
FIG. 8 is a schematic block diagram for explaining a configuration of a control system in the printing apparatus of this embodiment;
FIG. 9 is a block diagram for explaining a series of image processing steps performed by the printing apparatus of this embodiment, and a host apparatus that provides image data to the printing apparatus;
FIG. 10 is a diagram for explaining INDEX patterns used in Example 1 by comparing the conventional one shown in FIG. 2;
FIGS. 11A and 11B are diagrams showing dot alignment states in a case of printing data of level 2 in Example 1 on a certain range of area in comparison with FIGS. 3A and 3B;
FIGS. 12A and 12B are diagrams showing dot alignment states in cases of having the ink application amounts per unit area substantially equal to those of FIGS. 3A and 3B;
FIG. 13A is a table showing the number of printed small dots, the number of printed large dots, the total number of the small and large dots and the ink application amount in one pixel of 600 dpi with respect to each inputted level, in a case of using the INDEX patterns shown in FIG. 10;
FIG. 13B is a graph showing an ink amount applied to one pixel of the image resolution, and the average value of the number of dots printed to apply the ink amount, together with the curve shown in FIG. 5B;
FIG. 14 is a diagram showing another example of the INDEX pattern applicable to Example 1;
FIG. 15 is a diagram for explaining INDEX patterns used in Example 2;
FIG. 16 is a schematic diagram for explaining nozzle arrays of a print head used in Example 3;
FIG. 17 is a diagram for explaining a dot alignment state printed by an ejection port array for small dots and an ejection port array for middle dots shown in FIG. 16;
FIG. 18 is a schematic diagram for explaining INDEX patterns of Example 3;
FIGS. 19A and 19B are diagrams each for explaining a dot alignment state in a case of continuously printing data of the level 2 in a certain range of area, in comparison with FIG. 3;
FIG. 20 is a schematic diagram for explaining nozzle arrays of a print head used in Example 4;
FIG. 21 is a diagram for explaining displacement of print positions attributable to an inclination of a print head;
FIG. 22 is a schematic diagram for explaining INDEX patterns of Example 4 in comparison with the INDEX patterns in FIG. 18; and
FIG. 23 is a diagram showing another example of INDEX patterns applicable to Example 4.
DESCRIPTION OF THE EMBODIMENTS
Hereinafter, an exemplary embodiment of the present invention will be described with reference to the drawings. Incidentally, although an ink jet printing apparatus will be cited as an applied example in the following embodiment, the present invention is not limited to this. The present invention can be applied to any printing apparatus as long as the apparatus is capable of forming an image with a dot alignment using pulse-surface-area modulation.
FIG. 6 is a view of a schematic configuration of a main part of an ink jet printing apparatus F102 to which this embodiment is applied. A chassis M3019 housed in an external package member of the printing apparatus F102 is composed of a plurality of plate-shape metal members each having a predetermined stiffness to form a frame of the printing apparatus, and includes each of the following mechanisms. An automatic feeding unit M3022 automatically feeds sheets (printing media) to the inside of a main body of the apparatus. A conveying unit M3029 guides sheets fed one by one from the automatic feeding unit M3022 to a predetermined print position, and then guides the sheets from the print position to a discharging unit M3030. An arrow Y is a conveying direction of sheets (a sub scan direction). A printing unit makes a print as desired on a sheet conveyed to the print position. A recovery unit M5000 performs a recovery process on this printing unit. Reference numerals M2015 and M3006 denote a paper-to-paper gap adjusting lever and a bearing of the conveyance roller M3001, respectively.
In the printing unit, a carriage M4001 is supported by a carriage shaft M4021 so as to be movable in main scan directions shown by an arrow X. An ink jet print head cartridge H1000 capable of ejecting ink is detachably mounted on the carriage M4001.
FIG. 7 is a perspective view for explaining a configuration of an ink jet printing cartridge applicable to this embodiment. A print head cartridge H1001 (hereinafter, also simply referred to as a print head) is composed of an ink tank holder and a print head portion having printing elements for ejection. Each of ink tanks H1900 is attachable to and detachable from the print head cartridge H1001 as shown in FIG. 7, and supplies an ink to a corresponding printing element array. In this embodiment, the print head H1001 is configured to use four color inks of black, cyan, magenta and yellow, and to be capable of ejecting each color ink of amounts at multiple levels.
The printing element of the print head H1001 in this embodiment has a mechanism that causes film boiling in ink by applying a voltage to a heater provided inside an ink path, and that causes a predetermined amount of ink to be ejected as an ink droplet. Ejection ports for ejecting ink of the same color and of the same amount are arranged in the sub scan direction at predetermined pitches, and ejection port arrays for respectively ejecting different amounts of ink are arranged side by side in a main scan direction.
Here, again, refer to FIG. 6. The carriage M4001 is provided with a carriage cover M4002 for guiding the print head H1001 to a predetermined mounting position on the carriage M4001. Moreover, the carriage M4001 is provided with a head set lever M4007 that sets the print head H1001 at a predetermined mounting position while being engaged with the tank holder of the print head H1001. The head set lever M4007 is provided so as to be rotatable about a head set lever shaft located at an upper portion of the carriage M4001. An engagement portion of the head set lever M4007 that is engaged with the print head H1001 is provided with a head set plate (not illustrated) biased by a spring. While pressing the print head H1001 with a force of the spring, the head set lever M4007 mounts the print head H1001 on the carriage M4001. The print head H1001 mounted on the carriage H4001 obtains head drive signals needed for printing from a main substrate E0001 through a flexible cable E0012.
The recovery unit M5000 is provided with a cap (not illustrated) for capping a surface of the print head cartridge H1001 having ink ejection ports formed thereon. A suction pump capable of introducing negative pressure into the inside of the suction pump may be connected to this cap. In this case, ink is sucked and discharged from the ink ejection ports by introducing negative pressure into the inside of the cap covering the ink ejection ports in the print head cartridge H1001. By the use of this, it is possible to perform recovery processing (also called “suction recovery processing”) for maintaining the print head H1001 in good conditions for ink ejection. In addition, another type of recovery processing (also called “ejection recovery processing” or “preliminary ejection”) for maintaining the print head H1001 in good conditions for ink ejection can be performed by causing the ink, which does not contribute to image printing, to be ejected to the inside of the cap.
FIG. 8 is a schematic block diagram for explaining a configuration of a control system in the printing apparatus F102 of this embodiment. A CPU B100 executes control of operations of the entire printing apparatus F102, image data processing and the like. In a ROM B101, stored are programs needed for the CPU B100 to perform control, and data necessary for printing INDEX patterns specific to the present invention. The CPU B100 executes various types of processing by referring the programs and data stored in the ROM B101 as needed, and by using a RAM B102 as a work area. Besides such a work area, a receiving buffer F115 for temporarily storing received image data, a print buffer F118 for storing print data for driving the print head H1001 and the like are reserved in the RAM B102.
The printing apparatus F102 receives image data through an interface (I/F) F114 from a host apparatus F101 connected to the outside. The CPU B100 temporarily stores the received image data in the receiving buffer F115 in the RAM B102, and performs image processing on the received image data by using various parameters stored in the ROM B101. The resultant image data after a series of image processing are stored in the print buffer F118 in the RAM B102, and then are sequentially transferred to a head driver H1001A with progress of a printing operation of the print head H1001. The head driver H1001A drives the print head H1001 according to received print signals. The CPU B100 provides the head driver H1001A with drive data (print data) and drive control signals (heat pulse signals) for driving the electrothermal elements and the like, thereby causing the print head H1001 to ejects ink. The CPU B100 causes the carriage M4001 to scan at a predetermined speed by driving a carriage motor B103 with a carriage motor driver B103A, while causing the print head H1001 to eject the ink. In this way, one main scan for printing is executed. Upon completion of one main scan for printing, the CPU B100 causes a printing medium to be conveyed (sub scan) by a predetermined amount by driving a conveyance motor B104 with a conveyance motor driver B104A. An image received from the host apparatus F101 can be printed on a printing medium by repeating the main scan for printing and the sub scan alternately.
FIG. 9 is a block diagram for explaining a series of image processing steps performed by the printing apparatus F102 of this embodiment, and the host apparatus F101 that provides image data to the printing apparatus F102. In this embodiment, the host apparatus F101 firstly converts luminance data F110 of multiple values (8 bits (256 values)) of RGB into density data of multiple values (8 bits (256 values)) of CMYK corresponding to ink colors included in the printing apparatus. Here, the density data have an image resolution of 600 dpi. Subsequently, at n-valued processing, the host apparatus F101 converts the multiple-valued density data of each ink color into data of n values (n is an integer satisfying 3≦n≦256). In this embodiment, the host device quantizes 256 values into 5 values (6 values in Example 3) without changing the resolution by using a multi-level error diffusion method. Moreover, at print coding F113 the n-valued image data of 600 dpi is converted into command codes that the ink jet printing apparatus F102 can recognize. The 5-valued (or 6-valued) density data thus coded are transferred to the printing apparatus F102 through the interface F114.
The printing apparatus F102 temporarily stores the received image data in the receiving buffer F115, and then analyzes the codes stored in the receiving buffer F115 by code analyzing F116. The image data thus analyzed are expressed with 5 values (or 6 values) of 600 dpi. At print data expanding F117 INDEX expansion processing on these data is performed. Specifically, according to a density level of 1 pixel (1 pixel of 600 dpi) corresponding to an area represented with density at n (n is an integer at least 3) density levels (n tones), an INDEX pattern for printing the pixel is determined. Thus, the 5-valued (or 6-valued) density data of each color are converted into print data containing 2 values of each dot size of each color. The print data of each dot size of each color are individually expanded in the print buffer F118. As such, the print data expanding F117 is equivalent to determination step for determining dots to be used for printing the pixel. The print data expanded in the print buffer F118 are transferred to the print head driver H1001A. Then, the print head driver H1001A drives the printing elements of each size of each color in the print head H1001 according to the print data. Thereby, a color image is printed on a printing medium. Incidentally, hereinafter, the “levels” of the density data expressed with n values (5 values, 6 values or the like) are also referred to as the “tone levels” or “density levels.”
Specific examples will be described below by using the ink jet printing apparatus described above.
EXAMPLE 1
FIG. 10 is a diagram for explaining INDEX patterns used in Example 1 by comparing the conventional patterns shown in FIG. 2. FIG. 10 shows the patterns each specifying whether or not to print large and small dots in each printing pixel of a printing resolution of 600 dpi (vertical)×1200 dpi (horizontal), corresponding to density data having 5-valued levels of an image resolution of 600 dpi. In Example 1, the diameters of large and small dots are also 60 μm and 35 μm, respectively. In comparison with the patterns in FIG. 2, one large dot and one small dot are printed in 1 pixel of 600 dpi at the tone level 2 in Example 1. A characteristic of Example 1 is that there is no level at which two of only small dots are printed side by side in a main scan direction like the level 2 shown in FIG. 2.
FIGS. 11A and 11B are diagrams each showing a dot alignment state in a case of continuously printing data at the level 2 of Example 1 on a certain range of area in comparison with FIGS. 3A and 3B. In Example 1, the multi-pass printing method is employed, and multiple dots in the area are printed in multiple main scans with multiple sub scans each performed between the main scans. FIG. 11A shows a state printed without variation in the multiple sub scans, and FIG. 11B shows a state printed with variations therein to the same extent as in the case of FIG. 3B.
In Example 1, since larger dots than a pitch of the image resolution in the sub scan direction are printed at the level 2, lines in the main scan direction, which are shown in FIG. 3A, are not observed. The coverage on the printing medium is also more than 100%.
Even under the condition that there are variations in the sub scans, a portion where dots overlap with each other serves as a margin to prevent the coverage from changing, because the diameter of the large dot is greater than the pitch of the image resolution. As a result, as indicated in FIG. 11B, the coverage hardly changes, and the coverage remains in a state of 100%. It has been pointed out in this description that a difference in lightness between such two states leads to a banding problem described in the section of “DESCRIPTION OF THE RELATED ART.” In other words, in the state at the level 2 of Example 1, a difference in lightness as observed at the level 2 of the conventional example is not observed, and thus banding attributable to the difference does not appear.
In this way, in Example 1, the dot alignment is determined so that only one dot smaller than one pixel size would be arranged in 1 pixel (1 pixel of 600 dpi) corresponding to an area represented with density at n (n is an integer at least 3) density levels (n tones). Precisely, in order to represent the density at the level (the tone level 2) that is next higher than the level (the tone level 1) at which one small dot is used, the dot alignment is determined so that a large dot would be used instead of using two small dots. This dot alignment makes it possible to reduce the coverage change, and thereby to reduce the banding problem caused by the coverage change.
However, even though an image at the level 2 is in good condition, this does not necessarily ensure the obtaining of images in good condition at all tone levels.
FIGS. 12A and 12B are diagrams, in comparison with FIGS. 3A and 3B, showing dot alignment states that are respectively printed so as to have the ink application amounts per unit area substantially equal to those of FIGS. 3A and 3B. At the level 2 of the INDEX patterns shown in FIG. 2, two small dots, that is, a total ink amount of 2 pl×2=4 pl is applied onto 1 pixel of 600 dpi. Here, consider a case where a larger area is printed by combining the INDEX patterns shown in FIG. 10 in order to obtain the ink application amount same as in the case of printing with the INDEX pattern at the level 2 in FIG. 2. When the same ink amount is obtained, the patterns at the level 1 and the level 2 in FIG. 10 are distributed at a ratio of 6:4.
When there is no variation in sub scans as shown in FIG. 12A, dots each being larger than a pitch of the image resolution in the sub scan direction are distributedly printed, and thereby lines, like those shown in FIG. 3A, in a main scan direction are not observed. The coverage on a printing medium is not 100%, and white background portions are distributed in places.
On the other hand, even when there are variations in sub scans as shown in FIG. 12B, the variations influence the coverage and lightness of an entire image to a small extent, that is, white background portions appear at positions being a little bit different from those of FIG. 12A, because the image contain dots each being larger than the pitch of the image resolution in the sub scan direction.
As described above, an image uniformly printed with banding reduced can be obtained by preparing the INDEX patterns causing large dots to be printed more preferentially as in Example 1 even at a tone level, at which only small dots are conventionally used for printing.
Hereinafter, descriptions will be provided for an effect in controlling a temperature rise in a head in a case of using the INDEX patterns in Example 1. The descriptions for the effect in controlling the temperature rise in the head in Example 1 will be described below by comparing the case (Example 1) of using the INDEX patterns in FIG. 10 with the case (the conventional example) of using the INDEX patterns in FIG. 2.
FIG. 5A is a table showing the number of printed small dots, the number of printed large dots, the total number of the small and large dots and the ink application amount in one pixel of 600 dpi with respect to each inputted level, in a case of using the INDEX patterns shown in FIG. 2.
In addition, FIG. 5B is a graph showing an ink amount applied to one pixel of the image resolution, and the average value of the number of dots printed to apply the ink amount. The horizontal axis indicates the amount of ink (pl) applied on average to one pixel of 600 dpi when a uniform image is printed in a certain range of area at various density levels. The vertical axis indicates the average value of the total number of large and small dots printed in each pixel to apply each of the amounts of ink thereto.
In each printing element, more drive energy is necessary for ejecting a large amount of ink than that for ejecting a small amount of ink, and thereby the heating value inside the ink path is also larger. There is almost no difference, however, between a large dot and a small dot in terms of a degree of temperature rise inside the print head, because a larger amount of heated ink is ejected for printing the large dot than that for printing the small dot. The present inventors examined and found out that the degree of temperature rise in a print head does not depend on the amount of ejected ink, but mainly depends on the number of ejections.
To be more precise, in the case of FIGS. 5A and 5B, the temperature of the print head is more likely to rise at the levels 1, 2 and 4 than in the case of using a large dot. In contrast, the temperature thereof is less likely to rise at the level 5. However, tone levels frequently used for printing general images are not as high as the level 5, and a majority thereof is at the level 2 or below in the case of Example 1. Accordingly, in the conventional ink jet printing apparatus which uses a print head capable of printing large and small dots, and in which the INDEX patterns shown in FIG. 2 are introduced, the temperature of the print head is likely to rise, which may easily lead to a reduction in a printing speed.
On the other hand, FIG. 13A is a table showing the number of printed small dots, the number of printed large dots, the total number of the small and large dots and the ink application amount in one pixel of 600 dpi corresponding to each inputted level, in a case of using the INDEX patterns shown in FIG. 10. FIG. 13A also shows an average number of dots or an average ink application amount for obtaining an ink application amount corresponding to each of the inputted levels (7 values) of the INDEX patterns shown in FIG. 2.
Moreover, FIG. 13B is a graph showing an ink amount applied to 1 pixel of the image resolution, and the average value of the number of dots printed to apply the ink amount, together with the curve shown in FIG. 5B. As is clear from FIGS. 13A and 13B, the number of ejections can be reduced by using the INDEX patterns of Example 1, even in cases of obtaining the ink application amounts equivalent to those of the levels 2 and 4 in the INDEX patterns shown in FIG. 2. More specifically, the number of ejections can be reduced down to 70% of the conventional number at the level 2, and can be reduced down to 80% thereof at the level 4. As a result, the temperature rise in the print head is reduced more than in the conventional case, thereby avoiding a reduction in the printing speed with temperature rise.
In Example 1, the descriptions have been provided for the example of printing an image at the image resolution of 600 dpi by using the two levels of dot sizes of 5 pl and 2 pl. Such a combination of parameters, however, does not place limitations on the effect of the present invention. It suffices to use at least two kinds of dots including a dot smaller and a dot larger than a pitch of a resolution in a sub scan direction. For example, in a case where an image resolution is 1200 dpi, it suffices that dot sizes include a combination of a dot with the diameter lager and a dot with the diameter smaller than 21 μm which is a pitch of the resolution.
Furthermore, the INDEX patterns shown in FIG. 10 do not also place limitations on Example 1.
FIG. 14 is a diagram showing another example of the INDEX pattern applicable to Example 1. In FIG. 14, as a pattern corresponding to the level 1, prepared are two kinds of patterns, one of which causes one small dot to be printed on the left side of a printing pixel, and the other of which causes one small dot to be printed on the right side of a printing pixel. Since these two patterns are different only in the position of the small dot, the density in an image is not largely changed regardless of the use of any one of these patterns for the level 1. However, these two patterns may be changed column by column, or raster by raster, may be changed whenever a print data piece appears, or may be changed randomly, in order to render less noticeable harmful effects on an image that are attributable to variations in carriage scans and various errors included in the apparatus main body.
According to Example 1 described above, even with a print head capable of printing multiple sizes of dots, it is possible to perform printing with banding and the temperature rise of the print head reduced, by using INDEX patterns preferentially allowing a large dot to be printed in a low tone area.
EXAMPLE 2
Hereinafter, Example 2 of the present invention will be described. A print head used in Example 2 is capable of ejecting each color ink of amounts of three levels. For a large dot, the ejection amount is 15 pl, and the diameter is 80 μm. For a middle dot, the ejection amount is 5 pl, and the diameter is 60 μm. In addition, for a small dot, the ejection amount is 2 pl, and the diameter is 35 μm. The middle dot is equivalent in size to the large dot in Example 1.
Incidentally, in Example 2, the diameter of the small dot is smaller than a pitch of an image resolution in a sub scan direction, and the diameters of the middle and large dots are larger than the pitch of the image resolution in the vertical direction.
FIG. 15 is a diagram for explaining INDEX patterns used in Example 2 in comparison with INDEX patterns in FIG. 2 or 10. In Example 2, density data containing 5-valued levels with an image resolution of 600 dpi are to be printed by using patterns each specifying the numbers of large, middle and small dots to be printed in each printing pixel with the same resolution of 600 dpi.
In the case of Example 2, the numbers of small and middle (corresponding to large of Example 1) dots to be printed in one pixel of 600 dpi at the levels 1 to 3 are the same as in the case of Example 1. However, in Example 2, the printing resolution is also 600 dpi that is equal to that of the image resolution, and accordingly all the printed dots are each arranged at a substantially center of a pixel of 600 dpi. Incidentally, at the level 3, two middle dots are arranged off the center for the purpose of showing that two dots are printed in one pixel.
At the level 4, two large dots (corresponding to middle dots of Example 2) and two small dots are assigned in Example 1, while one middle dot and one large dot are assigned in Example 2. The amount of ink applied to one pixel is 2 pl×2+5 pl×2=14 pl in Example 1, while the amount thereof is 15 pl+5 pl=20 pl in Example 2. Consequently, in the case of Example 2, the ink application amount at the level 4 is lager, and thereby the maximum value of density that can be represented is higher than in the case of Example 1.
In the configuration in which dot sizes of multiple levels are prepared as described above, the larger the ejection amount of the largest dot is set, the higher the maximum value of density that can be represented can be set. However, the tone may jump as in the case of Example 2 where the ejection amount (20 pl) of the large dot is four times larger than that (5 pl) of the medium dot that is one size smaller than the large dot. To be more precise, the ink application amount at the level 2 of Example 2 is 5 pl+2 pl=7 pl, while the application amount at the level 3 is 20 pl even only by employing one large dot. This amount is approximately three times larger than that of the level 2. The density represented at all the levels may not always be of linearity. However, when the density difference between two successive levels is extremely large the gradation of an image is likely to be damaged.
For this reason, two middle dots are arranged at the level 3 in Example 2. With this arrangement, the tone continuity between the level 2 and the level 4, at which a large dot is printed, can be maintained preferable. In Example 2, the effect can be obtained as long as at least one dot larger than the pitch of the resolution in the sub scan direction is arranged in an area at a level higher than a density level (level 1) at which only one dot (small dot) smaller than the pitch of the resolution in the sub scan direction is arranged. When this condition is satisfied, variations in sub scans are less likely to appear on a printed image, which is an object achieved by the present invention. Accordingly, as long as at least one dot larger than a pitch of a resolution in a sub scan direction is arranged in a pixel, the present invention does not place limitations on a combination of dots, and two middle dots can be arranged in one pixel as is the case with Example 2.
EXAMPLE 3
Example 3 will be described below. In Example 3, the n-valued processing to be described by referring to FIG. 9 quantizes multiple-valued density data into 6-valued density data (levels 0 to 5).
FIG. 16 is a schematic diagram for explaining nozzle arrays of a print head used in Example 3. In FIG. 16, S denotes a nozzle ejecting an ink droplet of 1 pl, and printing a small dot with the diameter of approximately 25 μm; M denotes a nozzle ejecting an ink droplet of 2 pl, and printing a middle dot with the diameter of approximately 35 μm; and L denotes a nozzle ejecting an ink droplet of 5 pl, and printing a large dot with the diameter of approximately 60 μm. In each of ejection port arrays for small and middle dots, ejection ports are arranged with density of 600 dpi in a sub scan direction. These two arrays are arranged to be shifted from each other in the sub scan direction by one pixel of 1200 dpi. On the other hand, an ejection port array for large dots includes two ejection port arrays, and the two arrays are arranged to be shifted from each other as similar to the arrangement of the small and middle dots.
FIG. 17 is a diagram for explaining a dot alignment state printed by an ejection port array 1602 for small dots and an ejection port array 1603 for middle dots. Each of the ejection port arrays makes a print with 600 dpi in the sub scan direction in a single print scan. Thereby, a print at 1200 dpi in the sub scan direction can be made by combining small and middle dots. The two arrays of the ejection port for large dots are capable of making a print at 1200 dpi in the sub scan direction, although FIG. 17 does not show. In Example 3, density data of an image resolution of 600 dpi are handled by using a print head that achieves a printing resolution of 1200 dpi by combining large, middle and small dots.
FIG. 18 is a schematic diagram for explaining INDEX patterns of Example 3 in comparison with the INDEX patterns in FIGS. 2, 10 and 15. FIG. 18 shows patterns each specifying whether or not to print large, middle and small dots in each printing pixel of 1200 dpi (vertical)×1200 dpi (horizontal) for density data having an image resolution of 600 dpi, and including 6-valued levels (levels 0 to 5). At the level 1, one small dot is printed in one pixel of 600 dpi. The small dot in Example 3 has a diameter smaller than those of Examples 1 and 2, thus leading to a further reduction of granularity at a highlight area.
At the level 2, a middle dot is added to the pattern at a position adjacent to the small dot printed at the level 1 in the sub scan direction. A characteristic of Example 3 is to preferentially arrange a dot at such an adjacent position in the sub scan direction as described above. Unlike Examples 1 and 2, Example 3 has the printing resolution of 1200 dpi also in the sub scan direction. Accordingly, the same effect as in aforementioned Examples 1 and 2 can be obtained by continuously arranging dots in the sub scan direction, as long as the dots are larger than one pixel width (21 μm) of the printing resolution (1200 dpi) even though being smaller than one pixel width (42 μm) of the image resolution (600 dpi).
FIGS. 19A and 19B are diagrams each for explaining a dot alignment state in a case of continuously printing data of the level 2 in a certain range of area, in comparison with FIG. 3.
In Example 3, dots adjacent in the sub scan direction are overlapped and connected with each other, and coverage in the sub scan direction is 100% or more. For this reason, even when there are variations in sub scans (see FIG. 19B), areas where dots overlap with each other increase or decrease only to a small extent, and thus white background portions do not change in size as in FIG. 3B. In other words, even under influence of the variations in the sub scan amount, the coverage on a printing medium does not change largely, which makes the lightness of an image stable.
In Example 3, as is the case with aforementioned Examples 1 and 2, there is no particular limitation on a combination of dots in dot patterns at the levels 3 and higher, as long as the combination satisfies a condition that the coverage in the sub scan direction exceeds 100%.
Incidentally, at the level 2 in Example 3, one middle dot is added to the pattern at the level 1 at which one small dot is printed, but an added dot is not limited to the middle dot. The three kinds of dots used in Example 3 each have the diameter larger than one pixel area of the printing resolution. Accordingly, whichever of these dots are printed, the condition that “the coverage in the sub scan direction exceeds 100%” is satisfied. As a result, the effect of the present invention can be obtained. For example, two small dots may be printed adjacently in a sub scan direction at the level 2 by changing the entire ejection port array for middle dots, described by referring to FIG. 16, to another ejection port array for small dots.
EXAMPLE 4
Example 4 will be described below. In Example 4, the n-valued processing process described by referring to FIG. 9 quantizes multiple-valued density data into 5-valued density data (levels 0 to 4) as similar to Examples 1 and 2.
FIG. 20 is a schematic diagram for explaining nozzle arrays of a print head used in Example 4. In FIG. 20, S, M and L respectively denote ejection port arrays for small, middle and large dots which eject the same amounts of ink and print dots with the same diameters as those in Example 3. The positional relationship between the ejection port arrays for small and middle dots is also the same as in Example 3. In addition, it is also the same as Example 3 that a print of 1200 dpi is made in a sub scan direction by combining these two arrays. However, in Example 4, these two arrays are arranged at a longer distance in a main scan direction than in the configuration of Example 3. When the print head is inclined in a main scan direction, the distance between these two arrays appears as displacement in a sub scan direction of print positions.
FIG. 21 is a diagram for explaining displacement of print positions attributable to an inclination of a print head. FIG. 21 shows a print position of each dot in a case of performing main scans with a print head including a small dot array and a middle dot array arranged at a distance of d=15 mm, and being inclined by an angle θ. The print head of Example 4 is designed so that print positions of small dots and print positions of middle dots are alternately arranged to be shifted in a sub scan direction by one pixel (approximately 21 μm) of 1200 dpi. However, in a case where the inclined print head includes the two arrays arranged at a long distance, the positional relationship between the print positions may be distorted on a printing pixel basis. FIG. 21 shows that the middle dots are shifted relative to the small dots by approximately 21 μm, and that the two kinds of dots are printed at substantially same positions in the sub scan direction.
Under this condition, gradation cannot be appropriately represented even with the INDEX patterns in Example 3 are employed, because the middle and small dots overlap with each other at the levels 2 and 3. At the level 3, especially, a dot alignment state becomes similar to the dot alignment pattern shown in FIGS. 3A and 3B, and thereby the variations in conveyance is more likely to have harmful effects on representation of gradation.
FIG. 22 is a schematic diagram for explaining INDEX patterns of Example 4 in comparison with the INDEX patterns of Example 3 shown in FIG. 18. FIG. 22 shows patterns each specifying whether or not to print large, middle and small dots in each printing pixel of 1200 dpi (vertical)×1200 dpi (horizontal), corresponding to density data having an image resolution of 600 dpi, and including 5-valued levels (levels 0 to 4). At the level 3 in Example 4, two middle dots included at the level 3 in Example 3, at which an influence of an inclination of the print head is more likely to appear, are replaced with a large dot. In this way, to print a dot with the diameter larger than one pixel area of the image resolution (600 dpi) from a relatively low tone level is a countermeasure against the displacement of print positions attributable to an inclination of the print head, and is a characteristic of Example 4. This is because the coverage does not change even with the print head inclined to a small extent, if at least one dot larger than one pixel area is printed in a pixel. Moreover, the level 3 of Example 4 is the same as the level 2 of Example 1 in that a dot of 2 pl and a dot of 5 pl are printed in one pixel of 600 dpi. Accordingly, banding attributable to variations in conveyance is reduced by the same effect as in Example 1.
It has been explained hereinabove that printing “a dot with a diameter larger than one pixel area of an image resolution” is effective in preventing damage attributable of the inclination of the print head. To be more precise, in the case of Example 4, it is effective to print a large dot with a diameter (60 μm) larger than one pixel width (42 μm) of 600 dpi. However, when a print head is inclined as is the case with Example 4, strictly speaking, the resolution of an ejection port array arranged on the print head is different from the resolution at which dots are actually arranged on a printing medium. In other words, the resolution of an image formed on a printing medium is changed according to the resolution of nozzles in a print head and an inclination of the print head. For this reason, it may not be said that printing “a dot with a diameter larger than one pixel area of an image resolution” is always effective even when the image resolution is 600 dpi.
However, if print positions are displaced to the approximately same extent as in Example 4 described by referring to FIG. 21, it can be said that the resolution of nozzles of a print head and the resolution of an image formed on a printing medium are substantially equal to each other in fact. The reason for this will be briefly described below. The ejection port array for middle dots in Example 4 is arranged at the position at a distance of approximately d=15 mm away from the ejection port array for small dots. A dot printed by this ejection port array for middle dots is printed to be shifted in the sub scan direction by approximately 21 μm. This means that an inclination amount θ is Sin θ=21 μm/15 μm, and is almost 0. On the other hand, the distance L between two middle dots actually arranged in the sub scan direction on a printing medium can be expressed as
L=D×cos θ=D×(1−Sin 2θ)1/2˜D
where D denotes the distance between nozzles arranged at 600 dpi. As is clear from this, the distance L between two middle dots is substantially equal to the distance D between nozzles. Accordingly, in other words, Example 4 shows that printing a dot with the diameter larger than the nozzle pitch of the print head is effective in preventing damage attributable to the inclination of the print head.
FIG. 23 is a diagram showing another example of INDEX patterns applicable to Example 4. In FIG. 23, there are prepared two kinds of patterns corresponding to the level 1, one of which allows one small dot to be printed in the upper-left printing pixel, and the other one of which allows one small dot to be printed in the upper-right printing pixel. In addition, there are prepared two kinds of patterns corresponding to the level 2, one of which allows one middle dot to be printed in the lower-left printing pixel in addition to one small dot, and the other one of which allows one middle dot to be printed in the lower-right printing pixel in addition to one small dot. Moreover, there are prepared two kinds of patterns corresponding to the level 3, one of which allows one large dot to be added in the upper-right printing pixel, and the other one of which allows one large dot to be added in the lower-right printing pixel. Preparing multiple patters corresponding to the same level, as described above, is effective in making less noticeable harmful effects on an image that are attributable to variations in carriage scans and various errors included in the apparatus main body. Various effects can be obtained by changing these multiple kinds of patterns column by column or raster by raster, by changing them every time a print data piece appears, or by changing them randomly. Although there are two kinds of patterns corresponding to each level in the case of FIG. 23, it is of course possible to prepare a larger number of patterns.
OTHER EXAMPLES
Note that it is not necessary to apply the INDEX patterns of each of the examples described above to all the ink colors used in the printing apparatus, uniformly. Precisely, the INDEX patterns may be uniformly used for all the ink colors, or may be used only for an ink color that causes banding attributable to variations in conveyance to be more noticeable.
Moreover, even when printing is made with the same ink color, banding attributable to variations in sub scans appears in various ways depending on a printing mode and a kind of printing medium. Accordingly, it is also possible to employ a configuration which using different types of image processing and different INDEX patterns depending on printing modes and kinds of printing media. For example, in a case of employing multi-pass printing in a serial-type ink jet printing apparatus, the greater the number of multi-passes, the less likely variations in sub scan is to appear in an image. For this reason, another possible configuration is that INDEX patterns according to the present invention are adapted in a high-speed printing mode with a small number of multi-passes, and conventional INDEX patterns are adapted in a printing mode with a large number of multi-passes. As a matter of course, such conventional INDEX patterns may include conventional ones as described by using FIG. 2 in the section of “DESCRIPTION OF THE RELATED ART.”
In addition, various modified examples of patterns having characteristics of the dot alignments presented in the above examples can be obtained in addition to the patterns presented in this description. The scope of the present invention also includes even a case of using any of these modified examples depending on an ink color, a printing mode, a kind of a printing medium or the like.
Note that, although the foregoing examples have been described as the system in which a series of image processing steps are shared by the host apparatus and the printing apparatus as shown in FIG. 9, the present invention is not limited to such a configuration. More steps may be performed by the host apparatus, or by the printing apparatus. For example, the image processing steps (F111, F112 and F113) employed in the host apparatus F101 in FIG. 9 may be employed in the printing apparatus F102.
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. 2006-227177, filed Aug. 23, 2006, which is hereby incorporated by reference herein in its entirety.
Patent Application
20080049060
