JP4521909B2 - Printing apparatus and printing method - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a technique for printing various images by forming dots on a recording medium, and more particularly to a technique for improving the image quality of a natural image printed by forming dots having different sizes.
[0002]
[Prior art]
As an output device for various images output from a computer or the like, a printing apparatus that prints an image by forming ink dots on a recording medium is widely used. These printing apparatuses can take only two states of whether or not to form dots, but by controlling the formation of dots, it is possible to print a natural image with a wide range of gradation expressions.
[0003]
Using a printer that can only take two states, whether or not to form dots, it is possible to print a natural image whose tone value changes continuously according to the tone value to be expressed. This is because the dot formation density is changed. For example, when dots are thinned out little by little from a state where black dots are formed on the entire surface of the recording paper, the color of the recording paper that was initially black gradually changes to gray as the dots are thinned out. In this way, if the dot formation density is controlled according to the gradation value to be expressed, a natural image expressed using a wide range of gradation values can be printed.
[0004]
In such a method, as the gradation value to be expressed decreases and the dot formation density decreases, the dots become more conspicuous and the image graininess deteriorates. As one means for solving this problem, a method is used in which large dots that are easily noticeable are replaced with small dots that are less noticeable. If the gradation value expressed by one large dot and the gradation value expressed by n small dots are the same, the gradation value can be changed by always replacing one large dot with n small dots. The graininess can be improved while keeping the same. In a low gradation region where gradation values are further lowered and even small dots are conspicuous, a method of replacing small dots with smaller dots can be employed in order to improve graininess.
[0005]
In order to improve the graininess, it is better to have as few large dots as possible in the low gradation value region, and it is desirable to replace the large dots with smaller dots as much as possible. That is, as long as large dots remain, or until small dots are formed in all the pixels and the number of small dots cannot be increased any more, the graininess can be improved by increasing the small dot formation rate. Even if a large dot is replaced with a small dot, the amount of ink applied to the recording paper is almost the same. The formation ratio of can be increased.
[0006]
[Problems to be solved by the invention]
However, if the proportion of small dots is increased too much in the intermediate gradation range from low to medium gradation values, the graininess may be improved but the image quality may be degraded. Was found. For this reason, the image quality in the intermediate gradation area may not be sufficiently high.
[0007]
The present invention has been made to solve the above-described problems in the prior art, and an object of the present invention is to provide a technique that enables high-quality printing in an intermediate gradation region.
[0008]
[Means for solving the problems and their functions and effects]
In order to solve at least a part of the above-described problems, the present invention employs the following configuration. That is, the printing apparatus of the present invention is
Three or more types of dots having different sizes can be formed, and each of the three or more types of dots is A single kind of dot All pixels Can be formed And Said single kind Dot every Is the formation rate of Single kind A printing apparatus capable of expressing a multi-gradation natural image by controlling the formation of each dot so that the dot recording rate becomes a predetermined value determined according to the gradation value of the image,
The gradation value of the image and each dot Single kind Regarding the correspondence with the dot recording rate, for each of the two or more types of dots excluding the largest dot, a gradation value that can be expressed by forming the dot in all pixels and a gradation value higher than that. Even if there is, an association means for associating the pixels in which the dots are not formed,
In accordance with the gradation value of the pixels constituting the image, each dot is based on the association means. Single kind Dot recording rate determining means for determining the dot recording rate;
Dot formation determination means for determining whether or not each dot is formed in the pixel so as to achieve the determined dot recording rate;
Dot forming means for forming each dot based on the determination result;
It is a summary to provide.
[0009]
Moreover, the printing method of the present invention corresponding to the printing apparatus is as follows.
Three or more types of dots having different sizes can be formed, and each of the three or more types of dots is A single kind of dot All pixels Formed into Is possible, Single kind Dot formation rate Single kind A printing method for expressing a multi-gradation natural image by controlling the formation of each dot so that the dot recording rate becomes a predetermined value determined according to the gradation value of the image,
The gradation value of the image and each dot Single kind Regarding the correspondence with the dot recording rate, for each of the two or more types of dots excluding the largest dot, a gradation value that can be expressed by forming the dot in all pixels and a gradation value higher than that. Even if there is, the association relationship in which the pixels in which the dots are not formed remains is stored,
According to the gradation value of the pixels constituting the image, the correspondence relationship is referred to and each dot is Single kind Determine the dot recording rate,
Judging whether or not each dot is formed in the pixel so as to achieve the determined dot recording rate,
The gist is to form each dot based on the determination result.
[0010]
Such a printing apparatus and printing method are inventions mainly related to a technique for preventing deterioration in image quality in the intermediate gradation region, and the inventors of the present application pay attention to the relationship between such deterioration in image quality and dot formation density. Thus, the present invention was completed. Therefore, before describing the operation and effect of the printing apparatus and printing method of the present application, the relationship between the deterioration of image quality and the dot formation density, which the present inventor has focused on, will be described.
[0011]
As described above, a printing apparatus that prints an image by forming dots on a recording medium expresses a wide range of gradation values by controlling the dot formation density. Further, in the intermediate gradation region from the low gradation value to the intermediate gradation value, in order to avoid the sparsely formed dots from being conspicuous and resulting in an image with poor graininess, The image is printed with medium dots or small dots. Therefore, when printing an image whose gradation value continuously changes in the intermediate gradation region, the density of small dots or medium dots formed on the recording medium also changes continuously.
[0012]
However, when visually recognizing the distribution of dots formed at a high density, human recognition exhibits the following characteristics. For example, even if the dot formation density increases or decreases from 90% to several percent, it is usually difficult for humans to clearly recognize this change in dot density. In other words, even if the dot formation density is increased or decreased from 90% to several percent, the ratio of pixels where dots are not formed is slightly increased or decreased, and it is difficult to visually recognize the boundary where such a change has occurred. .
[0013]
On the other hand, when the dot formation density is reduced from 100%, humans can clearly recognize even a slight decrease. For example, when the formation density is reduced from 100% to 97%, a state in which dots are not formed in some pixels (formation density of 97%) (a formation density of 97). %). Considering the image on the recording medium, an area where dots are formed in all pixels and an area where dots are not formed exist adjacent to each other. The boundary can be clearly recognized by human eyes.
[0014]
Although the case where the dot formation density decreases has been described above as an example, the same applies to the case where the dot formation density increases. That is, when the dot formation density is gradually increased, the dot density seems to increase naturally unless the dot is formed in all pixels (dot formation density 100%). . However, when a region where dots are formed appears in all pixels, it is recognized as if there is a clear boundary between that region and another region.
[0015]
Considering the recognition characteristics of human vision as described above, the phenomenon in which the image quality deteriorates mainly in the intermediate gradation region is considered to have a close relationship with the dot formation density. That is, as described above, in the intermediate gradation region from the low gradation to the middle gradation, in order to avoid deterioration of the graininess, the small dots or the medium dots are usually formed with the highest possible density. When printing an image whose gradation changes continuously in the intermediate gradation area, the dot formation density of small dots and medium dots also changes continuously. As a result, when a state where the formation density of small dots or medium dots corresponds to 100%, that is, an area where dots are formed in all pixels, the boundary between this area and the other area is clearly recognized. In other words, a so-called pseudo contour is generated at the boundary portion to reduce the print image quality.
[0016]
The printing apparatus and printing method of the present invention have been completed by paying attention to the relationship between the deterioration of image quality and the dot formation density as described above. Printing is possible.
[0017]
In the printing apparatus and the printing method of the present application, regarding the correspondence between the gradation value of the image and the dot recording rate of various dots that can be formed, the dots other than the largest dot are subject to the conditions that can be formed on all pixels. Are associated so that a small number of pixels in which the dots are not formed remain. In the printing apparatus and printing method of the present application, whether or not dots of various dots are formed is determined based on the dot recording rates associated in this way, and dots are formed according to the determination result. For this reason, the dot formation density does not correspond to 100% in the intermediate gradation region, and it is possible to avoid the deterioration of the image quality caused by the generation of the pseudo contour and to perform high quality printing.
[0018]
In such a printing apparatus, for each dot that can be formed, a gradation value expressed by forming the dot in all pixels and a small number of pixels in which the dot is not formed in a gradation region higher than the gradation value. You may make it match so that it may always remain. For each dot that can be formed, the dot formation density can be equivalent to 100% in a gradation region that is higher than the gradation value expressed by forming the dot in all pixels. If a small number of pixels in which dots are not formed remain, it is possible to avoid a deterioration in image quality due to the occurrence of a pseudo contour.
[0020]
A correspondence between the gradation value of the image stored in the recording medium and the dot recording rate and a program for controlling dot formation with reference to the correspondence are read into a computer, and each dot is formed by the computer. By determining, it is possible to avoid the generation of pseudo contours mainly in the intermediate gradation region and to enable high-quality printing.
[0021]
DETAILED DESCRIPTION OF THE INVENTION
A. Device configuration
Embodiments of the present invention will be described based on examples. FIG. 1 is an explanatory diagram showing the configuration of a printing apparatus used in an embodiment of the present invention. As shown in the figure, the color scanner 21 and the color printer 20 are connected to a computer 80, and the printing apparatus functions as a printing apparatus as a whole by loading and executing a predetermined program on the computer 80. A color original to be printed is converted into color image data ORG recognizable by the computer 80 by the color scanner 21 and then input to the computer 80. The computer 80 performs predetermined image processing, converts the color image data ORG into image data that can be printed by a printer, and outputs the image data to the color printer 20. The image data handled by the computer 80 includes images created by the various application programs 91 on the computer 80 and images obtained by processing the images captured from the color scanner 21 in addition to the images captured by the color scanner 21. Used. The conversion results of these image data are output to the color printer 20 as image data FNL that can be printed by the printer, and the color printer 20 forms ink dots of each color on the printing paper according to the image data FNL. As a result, a color image corresponding to the color image data output from the computer 80 is obtained on the printing paper.
[0022]
The computer 80 includes a CPU 81, a ROM 82, a RAM 83, an input interface 84, an output interface 85, a CRT controller (CRTC) 86, a disk controller (DDC) 87, a serial input / output interface (SIO) 88, etc. These are connected by a bus 89 and can exchange data with each other. The CRTC 86 controls signal output to the color display CRT 23, and the DDC 87 controls data exchange with the flexible disk drive 25, the hard disk 26, or a CD-ROM drive (not shown). The ROM 82 and the hard disk 26 store various programs loaded into the RAM 83 and executed by the CPU 81, and various programs provided in the form of device drivers. If the SIO 88 is connected to the public telephone line PNT via the modem 24, necessary data and programs can be downloaded from the server SV on the external network to the hard disk 26.
[0023]
When the computer 80 is turned on, the operating system stored in the ROM 82 and the hard disk 26 is activated, and various application programs 91 are operated under the management of the operating system.
[0024]
The color printer 20 is a printer capable of printing a color image, and in this embodiment, an ink jet printer that prints a color image by ejecting a total of four colors of cyan, magenta, yellow, and black onto the printing paper. Is used. Of course, in addition to these four color inks, a color printer using a total of six inks including light cyan and light magenta inks may be used. However, the present invention is not limited to a color printer that ejects ink to form dots, and may be a color printer that forms dots by, for example, a sublimation type or a fusion type thermal transfer method. In addition, the ink discharge method of the ink jet printer used in this example employs a method using a piezo element PE as will be described later. However, it is assumed that a printer having a head for discharging ink by another method is used. Also good. For example, the present invention may be applied to a printer of a system that energizes a heater disposed in an ink passage and ejects ink by bubbles generated in the ink passage.
[0025]
The color printer 20 of this embodiment is a variable dot printer, that is, a printer that can form three types of dots of different sizes, large, medium, and small for each color. If the size of the dots to be formed is changed using a variable dot printer, it is possible to express multi-level gradation for each dot, so that an image with rich gradation expression can be printed. The color printer 20 of the present embodiment forms dots of three sizes using a single ink discharge nozzle by devising an ink discharge method. Such an ink ejection method will be described later. Further, as is apparent from the description of the ink ejection method, the size of the dots is not limited to three types, and even if two types of dots are formed, four or more types of dots are further formed. It doesn't matter.
[0026]
FIG. 2 is a block diagram conceptually showing the software configuration of the printing apparatus. In the computer 80, all application programs 91 operate under an operating system. A video driver 90 and a printer driver 92 are incorporated in the operating system, and image data output from each application program 91 is output to the color printer 20 via these drivers. An application program 91 such as retouch for processing an image can display an image captured from the color scanner 21 on the CRT 23 via the video driver 90 and perform predetermined processing while confirming the image.
[0027]
When the application program 91 issues a print command, the printer driver 92 of the computer 80 receives image data from the application program 91, performs predetermined image processing, and converts the image data into printable image data. As conceptually shown in FIG. 2, the image processing performed by the printer driver 92 is mainly composed of four modules: a resolution conversion module 93, a color conversion module 94, a multi-value conversion module 95, and an interlace module 96. . The contents of image processing performed by each module will be described later, but the image data received by the printer driver 92 is converted by these modules and then output to the color printer 20 as final image data FNL. The color printer 20 of this embodiment only serves to form dots according to the image data FNL and does not perform image processing. Of course, the color printer 20 performs part of image conversion. Also good.
[0028]
FIG. 3 shows a schematic configuration of the color printer 20 of the present embodiment. As shown in the figure, the color printer 20 includes a mechanism for driving a print head 41 mounted on a carriage 40 to eject ink and forming dots, and the carriage 40 is reciprocated in the axial direction of a platen 36 by a carriage motor 30. The moving mechanism, the mechanism for transporting the printing paper P by the paper feed motor 35, and the control circuit 60 are included. The mechanism for reciprocating the carriage 40 in the axial direction of the platen 36 is an endless drive between the carriage motor 30 and the slide shaft 33 slidably holding the carriage 40 laid in parallel to the axis of the platen 36. A pulley 32 that stretches the belt 31 and a position detection sensor 34 that detects the origin position of the carriage 40 are configured. The mechanism for transporting the printing paper P includes a platen 36, a paper feed motor 35 that rotates the platen 36, a paper feed auxiliary roller (not shown), and a gear train that transmits the rotation of the paper feed motor 35 to the platen 36 and the paper feed auxiliary roller. (Not shown). The control circuit 60 appropriately controls the movement of the paper feed motor 35, the carriage motor 30, and the print head 41 while exchanging signals with the operation panel 59 of the printer. The printing paper P supplied to the color printer 20 is set so as to be sandwiched between the platen 36 and the paper feed auxiliary roller, and is fed by a predetermined amount according to the rotation angle of the platen 36.
[0029]
An ink cartridge 42 for storing black (K) ink and an ink cartridge 43 for storing cyan (C) / magenta (M) yellow (Y) ink are mounted on the carriage 40. Of course, K ink and Y ink may be stored in the same ink cartridge. The carriage 40 can be made compact if a plurality of inks can be stored in one cartridge in any combination. Ink heads 44, 45, 46, and 47 are formed on the print head 41 below the carriage 40 for K, C, M, and Y inks, respectively. An introduction tube (not shown) is erected for each ink at the bottom of the carriage 40. When an ink cartridge is mounted on the carriage 40, each ink in the cartridge passes through the introduction tube to each of the ink ejection heads 44 to 47. Supplied. The ink supplied to each head is ejected from the print head 41 by the method described below to form dots on the printing paper.
[0030]
FIG. 4A is an explanatory diagram showing the internal structure of each color head. The ink discharge heads 44 to 47 for each color are provided with 48 nozzles Nz for each color, and each nozzle is provided with an ink passage 50 and a piezoelectric element PE on the passage. As is well known, the piezo element PE is an element that transforms electro-mechanical energy at an extremely high speed because the crystal structure is distorted by application of a voltage. In the present embodiment, by applying a voltage having a predetermined time width between the electrodes provided at both ends of the piezo element PE, the piezo element PE expands for the voltage application time as shown in FIG. One side wall of the passage 50 is deformed. As a result, the volume of the ink passage 50 expands and contracts according to the expansion of the piezo element PE, and the ink corresponding to the contraction becomes particles Ip and is ejected from the nozzle Nz at high speed. The ink Ip soaks into the printing paper P mounted on the platen 36, thereby forming dots on the printing paper P.
[0031]
FIG. 5 is an explanatory diagram showing the arrangement of the ink jet nozzles Nz in the ink ejection heads 44 to 47. As shown in the figure, on the bottom surface of the ink ejection head, six sets of nozzle arrays for ejecting ink of each color are formed, and 48 nozzles Nz per group of nozzle arrays have a constant nozzle pitch k. Arranged in a staggered pattern. The 48 nozzles Nz included in each nozzle array need not be arranged in a staggered manner, and may be arranged in a straight line. However, when arranged in a zigzag pattern as shown in FIG. 5A, there is an advantage that the nozzle pitch k can be easily set small.
[0032]
As shown in FIG. 5, the positions of the ink ejection heads 44 to 47 for the respective colors are shifted in the conveyance direction of the carriage 40. Further, the nozzles for each color head are also displaced in the transport direction of the carriage 40 because the nozzles are arranged in a staggered manner. When the nozzles are driven while transporting the carriage 40, the control circuit 60 of the color printer 20 drives each head at an appropriate timing while considering the difference in head driving timing due to the difference in nozzle position. .
[0033]
The color printer 20 of the present embodiment includes a nozzle Nz having a constant diameter as shown in FIG. 5, but three types of dots having different sizes can be formed using the nozzle Nz. This principle will be described below. FIG. 6 is an explanatory diagram showing the relationship between the drive waveform of the nozzle Nz and the ejected ink Ip when the ink is ejected. The drive waveform indicated by the broken line in FIG. 6 is a waveform when a normal dot is ejected. In the section d2, once a voltage lower than the reference voltage is applied to the piezo element PE, the piezo element PE is deformed in the direction in which the cross-sectional area of the ink passage 50 is increased, contrary to the case described above with reference to FIG. Since there is a limit to the ink supply speed to the nozzle, the ink supply amount is insufficient with respect to the expansion of the ink passage 50, and the ink interface Me is dented inside the nozzle Nz as shown in the state A in FIG. It becomes a state. Further, if the voltage is drastically lowered as shown in the section d1 using the drive waveform shown by the solid line in FIG. 6, the ink supply amount is further insufficient, and the inside of the state A is greatly increased as shown in the state a. It becomes indented.
[0034]
Next, when a high voltage is applied to the piezo element PE (section d3), the ink in the passage is compressed due to the reduction in the cross-sectional area of the ink passage 50, and ink droplets are ejected from the ink nozzles. At this time, if the ink supply amount is insufficient, the ejected ink droplets are also reduced. Accordingly, from the state where the ink interface is not very recessed (state A), a large ink droplet is ejected as shown in state B and state C, and from the state where the ink interface is greatly recessed (state a), state b and Small ink droplets are ejected as shown in state c. Thus, if the rate of change when the drive voltage is lowered (sections d1 and d2) is changed, the size of the dots to be formed can be changed.
[0035]
The color printer 20 continuously outputs two types of drive waveforms. This situation is shown in FIG. Comparing the rate of change when the voltage is lowered, it can be seen that the drive waveforms W1 and W2 correspond to a small ink droplet Ips and a large ink droplet Ipm, respectively. Consider a case where the drive waveform W1 is output while the carriage 40 moves in the main scanning direction, and then the drive waveform W2 is output. The small ink droplet Ips ejected by the driving waveform W1 has a relatively low flying speed, and the large ink droplet Ipm ejected by the driving waveform W2 has a high flying speed. Therefore, the time required from the ejection to the printing paper is reached. Is longer for small ink droplets Ips. Naturally, the movement distance in the main scanning direction from the ink ejection position to the printing paper is longer for the small ink droplet Ips than for the large ink droplet Ipm. Therefore, by adjusting the timing of the drive waveform W1 and the drive waveform W2, as shown in FIG. 7, it is possible to eject small ink droplets Ips and large ink droplets Ipm to the same pixel.
[0036]
In the color printer 20 of this embodiment, small dots are supplied by supplying only the drive waveform W1 to the piezo element PE, medium dots are supplied by supplying only the drive waveform W2 to the piezo element PE, and both the drive waveforms W1 and W2 are supplied. Large dots are formed by supplying and ejecting two ink droplets to the same pixel. Of course, by increasing the number of types of drive waveforms, it is possible to form dots of various sizes.
[0037]
FIG. 8 is an explanatory diagram showing the internal configuration of the control circuit 60 of the color printer 20. As shown in the figure, the control circuit 60 includes a PC interface 64 that exchanges data with the CPU 61, PROM 62, RAM 63, and computer 80, a peripheral device that exchanges data with the paper feed motor 35, the carriage motor 30, and the like. An output unit (PIO) 65, a timer 66, a drive buffer 67, and the like are provided. The drive buffer 67 is used as a buffer for supplying dot on / off signals to the ink ejection heads 44 to 47. These are connected to each other by a bus 68 and can exchange data with each other. The control circuit 60 is also provided with an oscillator 70 that outputs a drive waveform at a predetermined frequency, and a distribution output device 69 that distributes the output from the oscillator 70 to the ink ejection heads 44 to 47 at a predetermined timing.
[0038]
When the control circuit 60 having the configuration shown in FIG. 8 receives the image data FNL from the computer 80, the control circuit 60 stores the dot ON / OFF signal in the temporary RAM 63. The CPU 61 outputs dot data to the drive buffer 67 at a predetermined timing while synchronizing with the movements of the paper feed motor 35 and the carriage motor 30.
[0039]
Next, the mechanism by which dots are ejected when the CPU 61 outputs a dot on / off signal to the drive buffer 67 will be described. FIG. 9 is an explanatory diagram showing the connection of one nozzle row of the ink ejection heads 44 to 47 as an example. The nozzle rows of the ink ejection heads 44 to 47 are interposed in a circuit having the drive buffer 67 as a source side and the distribution output device 69 as a sink side, and each piezo element PE constituting the nozzle row has its electrode. One of these is connected to each output terminal of the drive buffer 67, and the other is connected to the output terminal of the distribution output unit 69 all together. The distribution output device 69 outputs a drive waveform of the oscillator 70 as shown in FIG. When the CPU 41 outputs a dot ON / OFF signal for each nozzle to the drive buffer 67, only the piezo element PE that has received the ON signal is driven by the drive waveform. As a result, the ink particles Ip are simultaneously ejected from the nozzles of the piezo element PE that has received the ON signal from the drive buffer 67.
[0040]
The color printer 20 having the hardware configuration as described above drives the carriage motor 30 to move the ink ejection heads 44 to 47 of the respective colors in the main scanning direction with respect to the printing paper P, and the paper feed motor. By driving 35, the printing paper P is moved in the sub-scanning direction. Under the control of the control circuit 60, the color printer 20 prints a color image on printing paper by driving the print head 41 at an appropriate timing while repeating main scanning and sub-scanning of the carriage 40.
[0041]
B. Overview of image processing
As described above, the color printer 20 has a function of printing a color image upon receipt of the image data FNL. The image data FNL is generated by the computer 80 performing predetermined image processing on the color image. FIG. 10 is a flowchart showing an outline of image processing performed by the CPU 81 in the printer driver 92 of the computer 80. The outline of image processing will be described below with reference to FIG.
[0042]
When image processing is started, the CPU 81 inputs image data (step S100). This image data is data supplied from the application program 91 as described with reference to FIG. 2, and 256 gradations having values of 0 to 255 are provided for each color of R, G, and B for each pixel constituting the image. Data. The resolution of the image data changes according to the resolution of the original image data ORG.
[0043]
The CPU 81 converts the resolution of the input image data into a resolution for printing by the color printer 20 (step S102). When the resolution of the image data is lower than the printing resolution, resolution conversion is performed by generating new data between adjacent original image data by linear interpolation. Conversely, when the resolution of the image data is higher than the printing resolution, the resolution conversion is performed by thinning out the data at a certain rate.
[0044]
Next, the CPU 81 performs color conversion processing (step S104). The color conversion process is a process for converting image data composed of R, G, and B gradation values into gradation value data of each color such as C, M, and Y used in the color printer 20. This processing is performed using a color conversion table LUT (see FIG. 2). In the LUT, C, M, and Y for expressing the color composed of the combination of R, G, and B by the color printer 20 are used. -The combination of K is stored. Various known techniques can be applied to the process of performing color conversion using the color conversion table, and for example, processing by interpolation calculation can be applied.
[0045]
When the color conversion process ends, the gradation number conversion process starts (step S106). In the present embodiment, the image data after color conversion is a 256-gradation image of four colors of C, M, Y, and K. On the other hand, in the color printer 20 of the present embodiment, only a total of four states can be taken: “do not form dots”, “form small dots”, “form medium dots”, and “form large dots”. . Therefore, it is necessary to convert an image having 256 gradations into an image expressed in 4 gradations that can be expressed by the color printer 20. A process for performing such conversion is a gradation number conversion process. That is, the color printer 20 can represent 256 gradations of the original image by changing the ease of forming large, medium, and small dots on the recording medium in accordance with the gradation value of the original image. It is expressed by gradation values.
[0046]
When the CPU 81 finishes the gradation number conversion process, the CPU 81 starts an interlace process (step S108). This process is a process of rearranging the image data converted into a format representing the presence or absence of dot formation by the gradation number conversion process in the order to be transferred to the color printer 20. That is, as described above, the color printer 20 drives the print head 41 while repeating the main scanning and sub-scanning of the carriage 40 to form a dot row (raster) on the printing paper P. As described with reference to FIG. 4, since the plurality of nozzles Nz are provided in the ink ejection heads 44 to 47 for each color, a plurality of rasters can be formed in one main scan. . The rasters are separated from each other by a nozzle pitch k. Although it is desirable that the nozzle pitch k be as small as possible, it is difficult to reduce the nozzle pitch k to a pixel interval (corresponding to the case where the nozzle pitch k is 1) for the convenience of head manufacture. As a result, in order to form rasters arranged at pixel intervals, first, a plurality of rasters separated by the nozzle pitch k is formed, and then the head position is slightly shifted to form new rasters between the rasters. Such control is required.
[0047]
In addition, in order to improve the print image quality, one raster is formed by dividing it into a plurality of main scans, and further, in order to shorten the printing time, at each of the forward and backward movements of the main scan. Control such as formation of dots is also performed. When these controls are performed, the order in which the color printer 20 actually forms the dots is different from the order of the pixels on the image data. Therefore, the image data is rearranged in the interlace processing.
[0048]
When the interlace processing is completed, the image data is output to the color printer 20 as image data FNL that can be printed by the printer (step S110).
[0049]
C. Overview of gradation conversion processing
In this embodiment, generation of pseudo contour is avoided by appropriately determining the formation of medium and small dots in the intermediate gradation region from the low gradation to the intermediate gradation, and the dot formation determination is described above. This is performed during the gradation number conversion process. As a preparation for explaining the details of the tone number conversion process of the present embodiment, a general tone number conversion process will be briefly described first.
[0050]
As described above, the gradation number conversion process is a process for expressing an image having 256 gradations by the presence or absence of dots, and in this embodiment, the image is expressed by the presence or absence of three types of large, medium, and small dots. become. Since it has been explained in the beginning that an image having 256 gradations can be expressed by the presence or absence of dots if the dot formation density is appropriately controlled, the following two points will be described here. First, as a first point, the following describes how the color printer 20 sets three types of dot densities of large, medium, and small in order to express continuous tone. Next, as a second point, a method used to match the density of dots actually formed and the set dot density will be described.
[0051]
(1) Setting the dot recording rate
FIG. 11 is an explanatory diagram showing how the three types of dot recording rates Dp of large, medium, and small are set to express an image having continuous gradation. The dot recording rate Dp refers to the rate at which dots are formed with respect to the pixels in the area when printing a solid area having a certain gradation value. Note that since the correction is actually performed due to a difference in dot gain or the like, the relationship between the gradation value and the dot recording rate is not linear, but in this specification, for easy understanding, FIG. As shown in the figure, it is assumed that the tone value and the dot recording rate have a linear relationship. FIG. 11A shows the dot recording rate Dp when a continuous tone is expressed using only large dots. As shown in the figure, the gradation value to be expressed and the dot recording rate Dp are in a directly proportional relationship, and when the gradation value is 255, the dot recording rate Dp is 100%, that is, a dot is formed in all the pixels. It becomes. Such a relationship can also be considered to indicate that the amount of ink applied to the recording paper unit area corresponds to the gradation value expressed on the recording paper.
[0052]
FIG. 11B is an explanatory diagram showing a dot recording rate set for each dot when a continuous tone is expressed using a large dot and a small dot. The dot recording rate of large and small dots shown in FIG. 11B is based on the fact that the large dots are replaced with a plurality of small dots based on the dot recording rate of the large dots shown in FIG. Can think. The number of small dots to be replaced with one large dot may be set so that the gradation values to be expressed are equal, in other words, the amount of ink to be ejected onto the recording paper is the same. In the example shown in FIG. 11B, one large dot corresponds to four small dots.
[0053]
A method for replacing dots will be specifically described with reference to FIG. For example, at a gradation value of 32, as shown in the figure, the dot recording rate DpL for large dots is 12.5%, that is, 1/8 pixel dots are formed. Since one large dot is replaced with four small dots, if all large dots are replaced with small dots, a state where small dots are formed in half of all pixels, that is, the dot recording rate Dps for small dots is 50%. become. Similarly, since the dot recording rate DpL for large dots is 25% at a gradation value of 64, when all large dots are replaced, the dot recording rate Dps for small dots is 100%, that is, small dots are formed in all pixels. It becomes a state. Since the dot recording rate Dps for small dots is 100% at a gradation value of 64, a large dot must be formed in order to express a gradation value higher than the gradation value of 64. However, if a large dot is formed in a state where a small dot is formed in all pixels (dot recording rate Dps is 100%), the small dot and the large dot are formed over the same pixel. In order to avoid this, it is necessary to reduce the number of small dots in accordance with the formation of large dots. As a result, in the gradation region higher than the gradation value 64, the dot recording rate Dps for small dots decreases as the dot recording rate DpL for large dots increases.
[0054]
As described above, the formation of large dots in the low gradation area deteriorates the granularity of the printed image. To avoid this, use small dots instead of large dots as much as possible in the low gradation areas. It is preferable to express an image. This will be specifically described with reference to FIG.
[0055]
FIG. 12A is an explanatory diagram showing a state in which an image in a low gradation region is expressed using only large dots, and is enlarged and displayed for easy viewing of the drawing. As shown in the figure, since dots are formed sparsely in the low gradation region, each large dot is noticeable, and the graininess is greatly deteriorated. FIG. 12B shows the result of replacing the three large dots A1 to A3 with the small dots among the five large dots shown in FIG. The replacement ratio of large dots to small dots is assumed to be four small dots for one large dot. The following can be understood from FIG. Although the graininess is improved by replacing the large dots A1 to A3, the large dots A4 and A5 still remain, and these two large dots deteriorate the graininess. FIG. 12 (c) shows the result of replacing all large dots A1 to A5 with small dots. In this case, the large dots that deteriorate the graininess no longer remain, and the graininess is sufficiently improved. As can be seen from the above description, the graininess deteriorates even if a large dot remains even in the low gradation area. Therefore, in order to obtain a high-quality image, it is necessary to replace all large dots with small dots. Is preferred.
[0056]
As a result of replacing the large dots with the small dots as much as possible, the recording ratio of the large and small dots inevitably has a distribution as shown in FIG. That is, the dot recording rate Dps for small dots increases almost linearly from 0%, and when the recording rate reaches 100%, this time decreases almost linearly. The dot recording rate DpL for large dots is 0% until the dot recording rate Dps for small dots reaches 100%. Thereafter, the dot recording rate DpL increases gradually as the small dots decrease, and the small dot recording rate increases. When it reaches 0%, the dot recording rate when only large dots are formed is restored. Comparing the case of expressing using only large dots (FIG. 11A) and the case of expressing using large and small dots (FIG. 11B), large dots when using large and small dots are used. The dot recording rate DpL is obtained by subtracting the dot recording rate by the amount corresponding to the formation of small dots, as indicated by hatching in FIG.
[0057]
The setting of the dot recording rate Dp when using three types of large, medium, and small dots can be considered in the same manner as when using two types of large, small, and small dots. FIG. 11C is an explanatory diagram showing the dot recording rate Dp of each dot when a continuous tone is expressed using large, medium, and small dots. In the illustrated example, one large dot corresponds to two medium dots. In the state shown in FIG. 11B, a large dot is formed from the gradation value 64, but in order to avoid deterioration of the graininess in such a low gradation region, the large dot is made as large as possible. As a result of the replacement, the dot recording rate Dpm for medium dots also becomes a substantially triangular dot recording rate, similar to the small dots. The dot recording rate DpL for large dots is obtained by reducing the dot recording rate by the amount corresponding to the formation of medium dots, as indicated by the oblique lines in FIG.
[0058]
The above is the basic idea of setting the dot recording rate Dp for various dots. The same concept can be applied to the case of forming more dots in addition to the three types of dots, large, medium, and small. For example, when forming an extremely small dot in which one dot corresponds to half the small dot, as shown in FIG. 13A, the dot recording rate of the small dot is set to be reduced in accordance with the formation of the minimal dot. Good.
[0059]
Further, in the color printer 20 of the present embodiment, the size of the small dot is substantially equal to the size of the pixel. As is well known, when dots having the same size as a pixel are formed at a high density, a so-called banding phenomenon occurs in which a region where dots are not formed streaks at the pixel boundary and deteriorates image quality. . For this reason, as shown in FIG. 13B, the maximum dot recording rate of small dots is suppressed to a low value that avoids the occurrence of banding, and the amount of suppression of the formation of small dots is compensated by the formation of medium dots. It is also preferable to set. Of course, when a very small dot smaller than the small dot can be formed, the small dot is replaced with the minimum dot, and the dot recording rate as shown in FIG.
[0060]
(2) Method for realizing the set dot recording rate Dp
If each dot is formed on the recording medium so that the dot recording rate Dp set by the above method is obtained, that is, if the set dot recording rate can be realized, a natural image having 256 gradations is printed. Can do. Hereinafter, a method for realizing the set dot recording rate Dp in the color printer 20 of the present embodiment will be described.
[0061]
In the color printer 20 of the present embodiment, the tone number conversion process (see FIG. 10) is performed using a technique called a so-called systematic dither method, but the set dot recording rate Dp is realized in the systematic dither method. As described above, there is an effect of determining whether or not dots are formed for each pixel. Hereinafter, this operation will be briefly described.
[0062]
In order to realize the set dot recording rate, the dot recording rate Dp is once read as level data Ld in the systematic dither method. The level data Ld is data obtained by rereading the scale so that the dot recording rate 100% becomes the level data 255. In the systematic dither method, the level data Ld and a predetermined threshold are compared for each pixel, and if the level data Ld is larger, it is determined that a dot is formed in that pixel, and if the level data Ld is smaller, a dot is formed. Is determined not to form. The predetermined threshold is set for each pixel using a dither matrix. FIG. 14 is an explanatory diagram conceptually showing how the level data Ld and a threshold value determined by a dither matrix are compared to determine the presence or absence of dot formation for each pixel. In the figure, the hatched pixels are pixels that are determined to form dots. The reason why dots are formed at a preset recording rate in the systematic dither method is that the dither matrix is set by a special method as described below.
[0063]
As an example, if a dither matrix having a size of 16 × 16 in length and width and composed of a total of 256 pixels is taken as an example, each pixel of the dither matrix has a value of 0 to 255 once. It is set to appear randomly. For example, a pixel for which the dot recording rate Dp is set to 100% is converted into a value 255 of the level data Ld. Since the value set in the dither matrix as the threshold value can only take a value from 0 to 255, the level data Ld is always greater than the threshold value for this pixel, and a dot is always formed. That is, for this pixel, the dot recording rate is 100%. If the pixel has a dot recording rate of 50%, the level data Ld is 128. Since the threshold value determined by the dither matrix can take a value from 0 to 255 at random, the probability that a dot is formed in the pixel is 50%. That is, for example, if there is an area where the dot recording rate is set to 50% and the number of pixels constituting the area is 20 pixels, dots are formed in 10 pixels. Since the size of the image to be printed is usually larger than the set dither matrix, the position of one dither matrix is moved in order to set a threshold value for all pixels constituting the print image. To use.
[0064]
As described above, in the systematic dither method, the dot recording rate Dp is read as the level data Ld, and the threshold value randomly set in the range of 0 to 255 by the dither matrix is compared with the level data Ld. Thus, the density of dots to be formed and the dot recording rate Dp are matched.
[0065]
In the color printer 20 of the present embodiment, the tone number conversion process is performed using the systematic dither method. However, other widely known methods such as a method called an error diffusion method may be used. Of course.
[0066]
D. Setting the dot recording rate in this embodiment
As described above, in the intermediate gradation region from the low gradation to the middle gradation, the dot recording rate of each dot is 100%, that is, if a single type of dot is formed on all the pixels, a pseudo contour is generated and the image quality is lowered. A problem was found. In order to avoid such a problem, the color printer 20 of the present embodiment is set so that the dot recording rate of each dot does not become 100%. FIG. 15A shows the dot recording rates set for the large, medium, and small dots in the color printer 20 of this embodiment. As already described with reference to FIGS. 11 and 12, since it is preferable to make the formation ratio of small dots as high as possible in order to improve the graininess, the dot recording rate of medium dots at a gradation value of 128 is set to 100. % (See FIG. 13B), the color printer 20 of the present embodiment dares to form large dots, and the dot recording rate of medium dots is set not to be 100%. This is because it is better to avoid the generation of pseudo contours by setting so that large dots are formed slightly rather than generating pseudo contours with the dot recording rate of medium dots being 100%, which may deteriorate the graininess. This is because a preferable result can be obtained. As described with reference to FIG. 12, when the gradation value to be expressed is low, even if a slightly large dot is formed, the graininess is greatly deteriorated. If small dots or medium dots are formed at a certain ratio, even if a slightly large dot is formed, the graininess is not so much deteriorated.
[0067]
The reason why the dot recording rate for small dots is set to around 50% in FIG. 15A is to avoid the occurrence of banding described above. In the color printer 20 of this embodiment, in order to avoid the occurrence of banding, it is necessary to reduce the dot recording rate of small dots to near 50%. However, in order to avoid the occurrence of pseudo contour, dot recording of medium dots is performed. It was sufficient to lower the rate slightly, and it was sufficient to set it to about 95% as shown. Of course, the dot recording rate suppression value required for improving the image quality may be higher depending on the type of color printer.
[0068]
The color printer 20 of this embodiment forms three types of dots, large, medium, and small. Of course, the number of dots that can be formed is not limited to three. For example, when forming an extremely small dot smaller than a small dot, the dot recording rate may be set as shown in FIG. In the example of FIG. 15A or FIG. 15B, a pseudo contour is generated. No For this reason, the dot recording rate of both medium dots and extremely small dots is limited to a value where the maximum recording rate is around 95%, but of course it can be set to a lower value as long as the graininess does not deteriorate. is there. For example, as shown in FIG. 15C, the maximum dot recording rate of medium dots may be set to a lower value.
[0069]
In the color printer 20 according to the present embodiment, since dot formation is determined based on the dot recording rate as shown in each drawing of FIG. 15, when printing an image in the intermediate gradation area, Thus, it is possible to avoid the generation of an area where all pixels are filled with a single type of dot. When such a region occurs on the recording medium, a pseudo contour is generated at the boundary with another region and the image quality is greatly deteriorated. However, in this embodiment, such a problem does not occur, and The print quality can be improved.
[0070]
E. Tone number conversion processing in this embodiment
Finally, the tone number conversion process performed by the color printer 20 of this embodiment will be briefly described. Needless to say, the present invention is not limited to the method described below, and various methods known for gradation number conversion processing can be applied.
[0071]
FIG. 16 is a flowchart in which the printer driver 92 performs the tone number conversion process using a method called a systematic dither method. In this embodiment, tone number conversion processing is performed in parallel for each color of C, M, Y, and K. However, in order to avoid complicated explanation, in the following explanation, the color is not specified. explain.
[0072]
When the gradation number conversion process is started, the CPU 81 reads the image data Cd of the pixel (target pixel) from which the dot formation is to be determined (step S200). This image data Cd is image data for each color having 256 gradations after color conversion. Next, the dot recording rates Dpl, Dpm, and Dps set for each of large, medium, and small dots are acquired, and the respective level data Ldl, Ldm, and Lds are calculated (step S202). Since the dot recording rate of each dot is stored as a table for the gradation value, the dot recording rate of each dot for the image data Cd can be obtained by referring to this table.
[0073]
Next, the large dot level data Ldl and the first threshold thl are compared (step S204). If the large dot level data Ldl is larger, a large dot is formed in the multi-value quantization result Cdr. Is substituted with a value “3” (step S206).
[0074]
When the large dot level data Ldl is smaller than the first threshold thl, it is determined whether or not dots are formed for the medium dot. That is, the medium dot level data Ldm is compared with the second threshold thm (step S208). If the medium dot level data Ldm is greater than the second threshold thm, the multi-value quantization result Cdr is included. A value “2” representing the formation of a dot is substituted (step S210). In this way, since it is determined whether or not a medium dot is formed for a pixel that has not formed a large dot, the same pixel is not formed for the large dot and the medium dot. The value of the second threshold thm for determining whether or not medium dots are formed is set in the dither matrix set for medium dots. When the dither matrix for medium dots is shared with the one for large dots, for example, it is difficult to form both large dots and medium dots for pixels in which a large value near 255 is set for the threshold value, which in turn reduces image quality. There is a risk of it coming. To prevent this, the printer driver 92 of this embodiment prepares a dither matrix for each large, medium, and small dot. Of course, when it is highly necessary to save the storage capacity of the computer 80, the dither matrix of each dot may be shared.
[0075]
If the medium dot level data Ldm is smaller than the second threshold thm, it is determined whether or not a small dot is formed. That is, the small dot level data Lds is compared with the third threshold ths (step S212). If the small dot level data Lds is larger than the third threshold ths, the small dot of the multi-value quantization result Cdr. Is substituted with a value “1” that means forming a dot (step S214), and if the small dot level data Lds is smaller, a value “0” that means that no dot is formed is set in the multi-value quantization result Cdr. Substitution is performed (step S216). Thus, when the dot determination is completed for all the pixels (step S218), the gradation number conversion process is ended, and the process returns to the image processing routine shown in FIG.
[0076]
As described above, the gradation number conversion process using the systematic dither method has been described. However, the gradation number conversion process can be performed by combining the error diffusion method. Since the error diffusion method is a well-known technique, a detailed description of the method itself will be omitted, and only an outline will be described here using a dot recording rate setting example shown in FIG. For example, in FIG. 15A, as described above, the dot recording rate of large dots is set slightly higher near the gradation value 128, and a gradation error is generated accordingly. For this reason, if the gradation conversion process for medium dots is performed using the error diffusion method, the formation of medium dots is reduced in order to eliminate errors due to the formation of a large number of large dots.
[0077]
Further, the applicant of the present application controls the dot recording rate of each dot to be a preset value by actively controlling the threshold value used for determining whether or not dots are formed in the error diffusion method. A technique is disclosed (Japanese Patent Application No. 10-196793), and it is also preferable to apply such a technique. FIG. 17 shows an example of a flowchart to which this technique is applied. Hereinafter, a brief description will be given according to the flowchart of FIG.
[0078]
When the gradation number conversion process is started, the CPU 81 reads the image data Cd of the pixel (target pixel) from which the dot formation is to be determined (step S300), adds a diffusion error to the image data Cd, and corrects the correction data Cx. Is generated (step S302). In the error diffusion method, an error that has occurred for a processed pixel is distributed in advance with weighting to surrounding pixels. In step S302, the corresponding error is read out and reflected in the image data Cd of the target pixel. It is. The details of the error will be described later.
[0079]
Next, the CPU 81 sets threshold values ths, thm, and thl (step S304). These threshold values are set by reading values corresponding to the image data Cd from a table stored in advance in the ROM. A method for setting each threshold will be described later.
[0080]
The threshold value ths set in this way is compared with the correction data Cx for the target pixel (step S306). When the correction data Cx is smaller than the threshold ths, it is determined that no dot is formed in the target pixel, and a value “0” indicating that no dot is formed is substituted for the value Cdr indicating the multi-value quantization result, and the result value RV A value “0” is substituted into (step S308). The result value RV is a gradation value expressed in the target pixel by determining whether or not dots are formed. If it is determined that no dot is to be formed, no dot is formed on the target pixel, and thus the value of the result value RV is “0”.
[0081]
If the correction data Cx is greater than the threshold ths, it is determined whether the correction data Cx is greater than the threshold thm (step S310). If the correction data Cx is smaller than the threshold value thm, it is determined that a small dot is to be formed in the target pixel, and a value “1” that means forming a small dot is substituted for the value Cdr indicating the multi-value quantization result. The value “Vs” is substituted for the value RV (step S312). The value “Vs” is a gradation value expressed in a pixel in which one small dot is formed. When the correction data Cx is larger than the threshold thm, the magnitude relation with the threshold thl is determined (step S314). If the correction data Cx is smaller than the threshold value thl, it is determined that a medium dot is to be formed, a value “2” indicating the formation of a medium dot is substituted for the multi-value quantization result Cdr, and a value “Vm” is substituted for the result value RV ( Step S316). The value “Vm” is a gradation value expressed in a pixel in which one medium dot is formed. If the correction data Cx is larger than the threshold value thm, it is determined that a large dot is to be formed, a value “3” meaning the formation of a large dot is substituted for the multi-value quantization result Cdr, and a value “Vl” is substituted for the result value RV ( Step S318).
[0082]
As is clear from the above description, the threshold values ths, thm, and tkl are threshold values used as criteria for determining whether small, medium, and large dots are to be formed on the target pixel. It is possible to control the ease with which the corresponding dots are formed by changing. For example, the smaller the threshold ths that is the criterion for determining the formation of small dots, the easier it is to form small dots, and the larger ths is set, the more difficult it is to form small dots. In order to set the dot recording rate of each dot to a desired value, how to set the respective threshold values ths, thm, and thl can be determined by performing a simulation calculation under each condition. it can. That is, when one combination of threshold values ths, thm, and thl is determined in a certain image data, the dot recording rate of each dot is obtained by simulating on the computer the gradation number conversion process as shown in FIG. Can do. Such calculation is executed for various image data with various combinations of threshold values, and the calculation results are accumulated. Based on the accumulated data, a combination of threshold values is selected so that the dot recording rate of each dot is realized for the image data.
[0083]
In step S320 of FIG. 17, an error is calculated and error diffusion processing is performed. The error here refers to a difference between the correction data Cx of the target pixel calculated in step S302 and the gradation value RV determined by the dot formation determination. For example, assuming that the gradation value 255 is expressed when a large dot is formed, the correction data Cx of the pixel of interest has a gradation value 199. By forming a large dot in a pixel that should express the gradation value 199, an error of 199−255 = −56 is generated in the target pixel.
[0084]
The error diffusion process is a process for diffusing the error thus obtained to pixels around the target pixel with a predetermined weight. FIG. 18 shows an example of a weighting coefficient when an error is diffused to surrounding pixels. The error generated in the target pixel PP1 is diffused to the pixels PP2 to PP8 at the rate shown in the figure.
[0085]
When the above process is completed for the target pixel, it is determined whether or not all the pixels have been processed (step S322). If there are any unprocessed pixels, the process returns to step S300 to continue the tone number conversion process. To do.
[0086]
As described above, the dot recording rate of each dot can be set to a desired value also by a method in which each threshold is positively controlled according to the gradation value of the image data. Therefore, as described with reference to FIG. 15, the dot recording rate of each dot excluding large dots is set not to be 100%, and the above-described gradation number conversion process is performed based on this dot recording rate. For example, it is possible to avoid the occurrence of an area where all pixels are filled with a single type of dot in the intermediate gradation area.
[0087]
Although various embodiments have been described above, the present invention is not limited to all the embodiments described above, and can be implemented in various modes without departing from the scope of the invention. For example, a software program (application program) that realizes the above functions may be supplied to a main memory or an external storage device of a computer system via a communication line and executed.
[Brief description of the drawings]
FIG. 1 is a schematic configuration diagram of a printing apparatus according to an embodiment.
FIG. 2 is an explanatory diagram showing a configuration of software.
FIG. 3 is a schematic configuration diagram of a printer according to the present exemplary embodiment.
FIG. 4 is an explanatory diagram illustrating the principle of dot formation in the printer of this embodiment.
FIG. 5 is an explanatory diagram showing a nozzle arrangement in the printer of this embodiment.
FIG. 6 is an explanatory diagram for explaining the principle of forming dots of different sizes by the printer of this embodiment.
FIG. 7 is an explanatory diagram illustrating a nozzle driving waveform and a state of dots formed by the driving waveform in the printer of the present embodiment.
FIG. 8 is an explanatory diagram illustrating an internal configuration of a printer control apparatus according to the present exemplary embodiment.
FIG. 9 is an explanatory diagram illustrating a state in which the printer head according to the present embodiment receives data from the drive buffer and forms dots.
FIG. 10 is a flowchart showing a flow of an image processing routine in the present embodiment.
FIG. 11 is an explanatory diagram showing a method of setting the dot recording rate of various dots that can be formed.
FIG. 12 is an explanatory diagram for explaining the reason why the graininess deteriorates due to the formation of large dots.
FIG. 13 is an explanatory diagram showing a general dot recording rate setting for various dots that can be formed.
FIG. 14 is an explanatory diagram conceptually illustrating a tone number conversion process by a systematic dither method.
FIG. 15 is an explanatory diagram showing an example of a dot recording rate set in the present embodiment.
FIG. 16 is a flowchart illustrating a flow of tone number conversion processing by a systematic dither method in the present embodiment.
FIG. 17 is a flowchart illustrating an example of performing tone number conversion processing using an error diffusion method in the present embodiment.
FIG. 18 is an explanatory diagram showing an example of a weighting factor for diffusing an error in error diffusion processing.
[Explanation of symbols]
20 Color printer
21 ... Color scanner
23 ... CRT
24 ... modem
25 ... Flexible disk drive
26: Hard disk
30 ... Carriage motor
31 ... Driving belt
32 ... Pulley
33 ... Sliding shaft
34 ... Position detection sensor
35 ... Paper feed motor
36 ... Platen
40 ... carriage
41 ... CPU
41 ... Print head
42, 43 ... Ink cartridge
44-47 ... Ink discharge head
50 ... Ink passage
59 ... Control panel
60 ... Control circuit
61 ... CPU
62 ... PROM
63 ... RAM
64 ... PC interface
66 ... Timer
67 ... Drive buffer
68 ... Bus
69 ... Distribution output device
70: Oscillator
80 ... Computer
81 ... CPU
82 ... ROM
83 ... RAM
84 ... Input interface
85 ... Output interface
86 ... CRTC
87 ... DDC
88 ... SIO
89 ... Bus
90 ... Video driver
91 ... Application program
92 ... Printer driver
93 ... Resolution conversion module
94 ... Color conversion module
95 ... Halftone module
96 ... Interlace module

Claims (2)

Three or more dots of different sizes are possible forms, and each of said three or more types of dots are formed available-for all pixels a single type of dots, for each of the single type of dots A multi-tone natural image is expressed by controlling the formation of each dot so that the single-type dot recording rate, which is the formation rate, becomes a predetermined value determined according to the tone value of the image. A possible printing device,
Regarding the correspondence between the gradation value of the image and the dot recording rate of a single type of each dot, for each of the two or more types of dots excluding the largest dot, the dots are formed in all the pixels. An associating means for associating and leaving a pixel in which the dot is not formed even if the gradation value can be expressed and a gradation value higher than the gradation value;
Dot recording rate determining means for determining a single type of dot recording rate for each dot based on the association means according to the gradation value of the pixels constituting the image;
Dot formation determination means for determining whether or not each dot is formed in the pixel so as to achieve the determined dot recording rate;
A printing apparatus comprising: dot forming means for forming the dots based on the determination result. Three or more dots of different sizes are possible forms, and each of said three or more types of dots, a single type of dots can be formed on all the pixels, the formation ratio of a single type of dots Printing method for expressing a multi-gradation natural image by controlling the formation of each dot so that a single kind of dot recording rate becomes a predetermined value determined according to the gradation value of the image Because
Regarding the correspondence between the gradation value of the image and the dot recording rate of a single type of each dot, for each of the two or more types of dots excluding the largest dot, the dots are formed in all the pixels. Even if the gradation value can be expressed and a gradation value higher than that, a correspondence relationship in which a pixel in which the dot is not formed remains is stored,
In accordance with the gradation value of the pixels constituting the image, the single type dot recording rate of each dot is determined with reference to the correspondence relationship,
Judging whether or not each dot is formed in the pixel so as to achieve the determined dot recording rate,
A printing method for forming the dots based on the determination result.

Images