Abstract:
The process described below is performed to output high-quality images without changing the resolution of the printer used. 
     A binarizing step using such a frequency modulation (FM) technique as error diffusion is performed. at a resolution higher than that which, can be output from the printer. A screen or grid corresponding to the resolution of the printer is superimposed on the binarized data. A size of a dot to be printed by the printer is obtained from the number of dots (or data indicative of a “1”) contained in one square of the screen. Then the diameter of the dot is modulated while an image data is output depending on the resolution of the printer.

Description:
This application is based on Application No. 10-005394 filed in Japan, the content of which is hereby incorporated by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to image processing apparatuses, and in particular to image processing apparatuses which employ frequency modulation (FM) techniques, such as error diffusion, to provide tone process. 
     2. Description of the Related Art 
     Conventionally, FM techniques, such as error diffusion, FM screening, are known in the field of image processing. They provide tone representation by modulating a frequency which drives a print head. According to the techniques, images have darker tones at higher-frequency portions and lighter tones at lower-frequency portions. 
     Also known as an image processing method is a dither method which uses a dither matrix to provide a pseudo tone representation. 
     In printing images after this image processes, smaller dots provided more densely can provide images of higher quality. In order to provide smaller dots, however, the resolution of the printer must be increased. When an ink-jet printer has its resolution increased, its output rate is decreased. When a laser beam printer has its resolution increased, the cost of its optical system and the like is increased. 
     When the dither method is employed, mechanical noises of printers disadvantageously affect image quality. Error diffusion generates a patterned noise (snake noise) in intermediate tones. 
     SUMMARY OF THE INVENTION 
     The present invention has been made to overcome the above disadvantages. The present invention contemplates an image processing apparatus capable of improving image quality without changing the resolution of the printer. 
     The present invention also contemplates an image processing apparatus providing for image quality hardly affected by mechanical noises from the printer. 
     The present invention also contemplates an image processing apparatus capable of reducing a patterned noise in intermediate tones. 
     To achieve the above objects, in an aspect of the present invention an image processing apparatus includes a first processing unit applying a tone process on input image data, and a second processing unit determining a size of a dot to be printed from the data of a plurality of pixels included in image data having been subjected to the tone process. 
     In another aspect of the present invention an image processing method includes the steps of: applying a tone process on input image data; and determining a size of a dot to be printed from data of a plurality of pixels included in image data having been subjected to the tone process. 
     The foregoing and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a perspective view of an ink jet printer according to a first embodiment of the present invention. 
     FIG. 2 is a perspective view of the FIG. 1 printer head  3 . 
     FIG. 3 is a plan view of printer head  3  as seen from the direction of its nozzle surface. 
     FIG. 4 is an exploded, perspective view of printer head  3 . 
     FIG. 5 is a plan view for illustrating an ink flow path in printer head  3 . 
     FIG. 6 is a cross section taken along line X—X of FIG.  5 . 
     FIG. 7 is a block diagram of a control circuit of an ink jet printer. 
     FIG. 8 is a block diagram showing a specific configuration of the FIG. 7 CPU  101 . 
     FIG. 9 is a block diagram showing a specific relation between the FIG. 8 image processing unit  114 , the FIG. 8 head jet drive unit  105  and each color head. 
     FIG. 10 describes a composition of yellow ink. 
     FIG. 11 describes a composition of magenta ink. 
     FIG. 12 describes a composition of cyan ink. 
     FIG. 13 describes a composition of black ink. 
     FIG. 14 represents a waveform of a voltage applied to a PZT  306 . 
     FIG. 15 is a graph of a voltage applied to PZT  306  versus a diameter of a dot on a recording sheet  2 . 
     FIG. 16 shows a relation between tones 0 to 8 and dot diameter. 
     FIG. 17 is a flow chart of a printing process performed by the ink jet printer according to the first embodiment. 
     FIG. 18 shows a relation between a screen depending on a resolution of a printer and a matrix for processing an image. 
     FIG. 19 illustrates a matrix included in one square of a screen. 
     FIG. 20 represents a relation between the number of dots included in one square and the sizes of dots. 
     FIG. 21 shows one example of an image output from a printer. 
     FIG. 22 shows one example of an output image, using a conventional dither method. 
     FIG. 23 shows one example of an image output from the printer according to the first embodiment. 
     FIG. 24 shows an image darker by one stage than the image shown in FIG.  23 . 
     FIG. 25 shows one example of an image output according to a conventional dither method. 
     FIG. 26 shows an image darker by one stage than the image shown in FIG.  25 . 
     FIG. 27 shows an image output from a printer according to the present embodiment. 
     FIG. 28 shows an image output according to error diffusion employing uniform dots conventionally used. 
     FIGS. 29-33 represent a relation between matrix and dot size according to a second embodiment. 
     FIG. 34 is a view for describing a variation of the printer according to the second embodiment. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Reference will now be made to the drawings to describe a printer provided with image processing apparatuses according to the preferred embodiments of the present invention. 
     First Embodiment 
     FIG. 1 is a perspective view showing a schematic structure of an ink jet printer  1  according to a first embodiment of the present invention. 
     Ink jet printer  1  includes a printer head  3  provided as an .ink-jetting printer head, a carriage  4  which holds printer head  3 , rods  5  and  6  along which carriage  4  reciprocates parallel to a recording surface of a recording sheet  2  corresponding to a recording medium, such as a paper sheet, a thin plastic plate, an OHP sheet and a roll of sheet, a drive motor  7  for driving carriage  4  so that carriage  4  reciprocates along rods  5  and  6 , a timing belt for transforming the rotation of drive motor  7  into the reciprocation of carriage  4 , and an idle pulley  8 . 
     Ink jet printer  1  also includes a platen  10  also serving as a guiding plate which guides recording sheet  2  along a transport path, a sheet pressing plate  11  which coordinates with platen  10  to press recording sheet  2  against platen  10  to prevent recording sheet  2  from lifting off platen  10 , a discharging roller  12  for discharging recording sheet  2 , a spurring roller  13 , a recovery system  14  washing the printer head  3  nozzle surface for jetting ink to recover a satisfactory amount of ink jetted, and a sheet feeding knob  15  for manually transporting recording sheet  2 . 
     Recording sheet  2  is fed manually or via a sheet feeding device such as a cut-sheet feeder to the recording position at which printer head  3  and platen  10  face each other, while the rotatability of a sheet feeding roller (not shown) is controlled to control the sheet transportation to the recording position. 
     Printer head  3  includes a piezoelectric element (PZT). Voltage is applied to the piezoelectric element to distort the piezoelectric element. 
     The distortion changes the volume of a channel filled with ink. The change of the volume allows the ink to be jetted from a nozzle provided at the channel so that an image is recorded on recording sheet  2 . 
     Carriage  4 , driven by drive motor  7 , idle pulley  8  and timing belt  9 , subjects recording sheet  2  to main scan in the lateral direction of recording sheet  2  (or the transverse direction of recording sheet  2 ) and printer head  3  mounted on carriage  4  records one line of image. Each time one line of image is completely recorded, recording sheet  2  is fed in the longitudinal direction thereof and subjected to subscan so that the subsequent line of image is recorded thereon. 
     Thus an image is recorded on recording sheet  2 . Recording sheet  2  which has passed across the recording position is discharged via discharging roller  12  and spurring roller  13  pressed against discharging roller  12 , both arranged downstream in the direction of the sheet transportation. 
     FIG. 2 is a perspective view for illustrating the configuration of a periphery of carriage  4 . 
     In the periphery of carriage  4  are included a casing  401  which accommodates an ink cartridge  403  for storing ink, a lid  405  of casing  401 , a pin  402  which allows ink cartridge  403  to be mounted removably and also receives and feeds ink to printer head  3 , a biased clasp  406  for fastening lid  405  to casing  401  when lid  405  is closed, a piece  407  which engages with biased clasp  406 , and a spring plate  408  which coordinates with lid  405  to hold ink cartridge  403  while pressing ink cartridge  403  in the direction opposite to the direction in which ink cartridge  403  is stored (i.e. the direction indicated by an arrow D 3 ). When carriage  4  moves in the direction indicated by an arrow D 1  shown in FIG. 2, main scan is performed and ink drops are jetted in the direction of arrow D 2 . 
     FIG. 3 shows the FIG. 1 printer head  3  as seen from a side thereof having a nozzle provided thereon. 
     Referring to FIG. 3, printer head  3  includes a head  3 Y for yellow ink, a head  3 M for magenta ink and a head  3 C for cyan ink which respectively jet yellow ink, magenta ink and cyan ink, and a head  3 K for black ink which jets black ink. 
     It should be noted that although the present invention does not employ photo ink, the present invention is applicable to printers which employ photo ink. 
     FIG. 4 is an exploded, perspective view of a portion of the FIG. 3 printer head  3 . FIG. 5 is a plan view of printer head  3  as seen from the nozzle plate  301  side, for illustrating the flow of ink in printer head  3 . FIG. 6 is a cross section taken along line X—X of FIG.  5 . 
     Referring to the figures, the printer head is configured by a head holder  307 , a piezoelectric element (PZT)  306 , a diaphragm  305 , a channel plate  304 , an inlet plate  303 , a shared ink-chamber plate  302  and a nozzle plate  301  that are deposited successively from the bottom. 
     PZT  306  is connected to lead frames  314   a ,  314   b.    
     As shown in FIG. 6, the deposition of all of the parts allows an ink introducing path  313 , a shared ink chamber  311 , an ink chamber  312  and a nozzle  315  to form a series of spaces through which ink flows and ink  320  is jetted via nozzle  315  onto recording sheet  2  to form an image. 
     Reference will now be made to FIGS. 5 and 6 to describe the flow of ink in printer head  3 . 
     Ink is supplied from ink cartridge  403  (FIG. 2) via pin  402  (FIG. 2) to printer head  3 . The ink is introduced to shared ink chamber  311  via ink introducing path  313  in the printer head. The ink in the shared ink chamber is fed to ink chamber  312 . 
     When voltage is applied across lead frames  314   a  and  314   b , PZT  306  deforms in the vertical direction of FIG.  6 . Thus the volume of ink chamber  312  is reduced and ink  320  is thus jetted toward recording sheet  2  (FIG. 1) via nozzle  315 . 
     The degree of deformation of PZT  306  varies in proportion to the voltage applied to PZT  306 . By controlling the voltage applied to PZT  306 , the amount of ink jetted can be controlled each time the PZT is deformed, to change the size of dots to be printed on a recording sheet. 
     FIG. 7 is a block diagram showing a schematic configuration of a control unit of ink jet printer  1 . 
     The control unit of ink jet printer  1  includes a CPU  101 , a RAM  102 , a ROM  103 , a data receiving unit  104 , a head jet drive unit  105 , a head-movement drive unit  106 , a sheet-feeding drive unit  107 , a recovery-system motor drive unit  108 , and a unit of various sensors  109 . 
     CPU  101 , which provides general control, uses RAM  102  as required to run a program stored in ROM  103 . The program includes a portion which uses image data read in from data receiving unit  104  to control head jet drive unit  105 , head-movement drive unit  106 , sheet-feeding drive unit  107  and the unit of various sensors  109  to record an image on recording sheet  2 , and a portion which controls recovery-system motor drive unit  108  and the unit of various sensors  109  as required to recover a satisfactory condition of the nozzle surface of printer head  3 . 
     Data receiving unit  104  is connected to a host computer and the like to receive image data to be recorded. 
     Controlled by CPU  101 , head jet drive unit  105  drives the printer head  3  PZT  306 , head-movement drive unit  106  drives driving motor  7  which moves carriage  4  carrying printer head  3  in the lateral direction, and sheet-feeding drive unit  107  chives a sheet feeding roller. Also controlled by CPU  101 , recovery-system motor drive unit  108  drives a motor required for recovering the satisfactory condition of the printer head  3  nozzle surface. 
     FIG. 8 is a block diagram showing a configuration of the FIG. 7 CPU  101 . 
     Referring to the figure, CPU  101  includes a tone correction unit  111  receiving signals r, g and b corresponding to the colors of red, green and blue from data receiving unit (or image-source input unit)  104  and applying tone correction thereon, a color conversion unit  112  converting the data of r, g and b subjected to tone correction into the data of c, m and y corresponding to the colors of cyan, magenta and yellow, a black generation+under-color removal (UCR) unit  113  which separates a gray component from the converted signals of the three colors (i.e. under-color removal) and replaces the gray component with a black signal and thus outputs data k corresponding to the color of black, and an image output unit  114  which corrects the resolution of each of the data, binarizes them according to the present embodiment and then outputs data for each color depending on the screen corresponding to the resolution of the printer. 
     Head jet drive unit  105  receives data from image processing unit  114 . Head jet drive unit  105  drives each color-head. 
     FIG. 9 is a block diagram showing a relation between the FIG. 8 image processing unit  114 , the FIG. 8 had jet drive unit  105 , and each color-head. 
     Referring to the figure, head jet drive unit  105  includes a cyan-head drive circuit  120   c  driving a head  3 C for cyan ink, a magenta-head drive circuit  120   m  driving a head  3 M for magenta ink, a yellow-head drive circuit  120   y  driving a head  3 Y for yellow ink, and a black-head drive circuit  120   k  driving a head  3 K for black ink. 
     The drive circuits receive from image processing unit  114  their respective data c 1 , m 1 , y 1  and k 1  for driving their respective heads. 
     FIG. 10 describes a composition of yellow ink used in an ink jet printer of the present embodiment. 
     The yellow ink contains water of 74.5%, polyhydric alcohol/diethylene glycol (DEG) of 11%, polyhydric alcohol ether/triethylene glycol monobutyl ether (TGB) of 6.5%, and a thickerner/polyethylene glycol (PEG) #400 of 4.5% as the solvent. It also contains a dye/Bayer Y-CA 51092 of 2.5% as a coloring material. It also contains a surfactant/Olfine E1010 of 0.8% and a pH adjusting agent/NaHCO 3  of 0.2% as additives. 
     FIG. 11 describes a composition of magenta ink used in an ink jet printer of the present embodiment. 
     The magenta ink contains water of 74.5%, polyhydric alcohol/DEG of 11%, polyhydric alcohol ether/TGB of 6.5%, and a thickener/PEG #400 of 4.5% as the solvent. It also contains a dye/BASF RED FF-3282 of 2.5% as a coloring material. It also contains a surfactant/Olfine E1010 of 0.8% and a pH adjusting agent/NaHCO 3  of 0.2% as additives. 
     FIG. 12 describes a composition of cyan ink used in an ink jet printer of the present embodiment. 
     The cyan ink contains water of 74%, polyhydric alcohol/DEG of 11%, polyhydric alcohol ether TGB of 6.5%, a thickener/PEG #400 of 4.5% as the solvent. It also contains a dye/Bayer CY-BG of 3.0% as a coloring material. It also contains a surfactant/Olfine E1010 of 0.8% and a pH adjusting agent/NaHCO 3  of 0.2% as additives; 
     FIG. 13 describes a composition of black ink used in an ink jet printer of the present embodiment. 
     The black ink contains water of 77.9%, polyhydric alcohol/DEG of 6.0%, polyhydric alcohol ether/TGB of 6.0%, and a thickener/PEG #400 of 4.5% as the solvent. It also contains a dye/Bayer BK-SP of 4.6% as a coloring material. It also contains a surfactant/Olfine E1010 of 0.8% and a pH adjusting agent/NaHCO 3  of 0.2% as additives. 
     FIG. 14 shows a waveform of a pulse of a voltage applied to PZT  306 . 
     Referring to the figure, PZT  306  receives a voltage V 0  which varies depending on the diameter (or size) of the dot to be printed. It requires 4 μsec from initiation of the voltage application until the voltage attains the value of V 0 . Then voltage V 0  is applied for 6 μsec. Then the voltage takes 40 μsec to attain zero. In other words, the application time of one pulse is 50 μsec in total. 
     FIG. 15 is a graph of the diameter (μm) of a dot on recording sheet  2  when voltage V 0  is applied to PZT  306  versus the application of voltage V o  to PZT  306 . 
     As shown in the figure, the diameter of the dot on recording sheet  2  increases as the voltage applied is increased. 
     In the present embodiment, voltage V 0  applied to PZT  306  is varied between 0V, 5V, 7.5V, 10V, 12.5V, 15V, 20V, 25V and 30V to change dot size between tones 0 to 8 as shown in FIG. 16 to provide an 8-tone representation. 
     FIG. 17 is a flow chart of a printing process performed by an ink jet printer according to the present embodiment. 
     Referring to the figure, at step S 101 , image data is received via data receiving unit  104 . At step S 102 , the received image data is corrected by tone correction unit  111  to have a tone level suitable for an image process. Other processes, such as color conversion and black generation, are also performed at step S 102 . 
     At step S  103 , the image data is converted to have a resolution appropriate for a subsequent image process to allow for a tone process using an FM method at a resolution larger than that of the printer used. For example, when the printer has a resolution of 360 dpi and the tone process is performed at a resolution four times larger than the resolution of the printer, the input image data is converted to have a resolution of 1440 dpi. 
     At step S  104 , an FM method such as error diffusion is used to apply the tone process on image data. The image data thus becomes binary data. 
     At step S 105 , the binary data is developed into a matrix. At step S 106 , a screen depending the resolution of the printer used is superimposed on the image data developed into the matrix at step S 106 . 
     Referring now to FIG. 18, when the matrix into which the image data is developed is represented with broken line the grid represented with solid line corresponds to a screen depending on the resolution of the printer used, since the resolution of the printer is lower than that of the matrix. 
     At step S 107 , the size of a dot to be printed by the printer is obtained depending on the number of dots contained in one square of the screen. At step S 108 , image data depending on the resolution of the printer is printed, depending on the dot size obtained. 
     The dot-size determination actually made at the FIG. 17 step S 107  will now be described. 
     FIG. 19 is an enlarged view of one square included in a screen depending on the resolution of the printer represented in solid line as shown in FIG.  18 . 
     Referring to FIG. 19, one square contains 16 pixels a 1  to a 16  provided within a matrix into which image data is developed. The size of a dot to be printed is obtained depending on the number of dots (the number of data “1”) in pixels a 1  to a 16 . 
     FIG. 20 represents a relation between the number of dots contained in the square and the size of a dot to be printed. 
     In the present embodiment, any of characteristics γ 1  to γ 4  can be selected as a correspondence between the number of dots contained in one square and dot size. This selection may depend on the user&#39;s favor or may be changed via a printer driver. 
     More specifically, any of dot sizes 0 to 8 is determined depending on any number of dots, i.e. 0 to 16, contained in one square as shown in FIG.  19 . 
     It should be noted that as shown in FIG. 20, characteristic γ 1  represents a linear relationship, γ 2  a downwardly curved relationship, γ 3  an upwardly curved relationship and γ 4  an S-shaped relationship between the number of dots and dot size. 
     An effect of an ink jet printer according to the present embodiment will now be described. 
     FIG. 21 is an enlarged view of an image output from the ink jet printer according to the present embodiment. The black dots in the figure represent those printed in ink of a color and the white dots in the figure represent those printed in ink of a different color. 
     As is apparent from the figure, the present embodiment uses error diffusion and dots are thus printed randomly. This prevents generation of a pattern in output dots, as with a dither method, and thus prevents significant mechanical noises from the printer. 
     Since dots are printed randomly, any secondary noise of frequency can be prevented if colors are superimposed on one another. Furthermore, the smearing and bleeding of ink can be prevented, since dots of various colors are hardly superimposed on one another. 
     Furthermore, a printer having a single resolution can provide a smaller dot to form an image at a less dense portion to form as fine an image as that provided by a printer higher in resolution. Furthermore, larger dots can be used at denser portions to process images rapidly. 
     Furthermore, in accordance with the present embodiment a tone process according to an FM method can be initially performed and a dot size can then be obtained to reduce a patterned noise in intermediate tones that is characteristic to error diffusion. 
     FIG. 22 is a view for illustrating an image output when only the dither method is used. In the figure, the area surrounded by the wide line corresponds to one dither matrix. 
     As is apparent from the figure, when the dither method is employed dots are printed periodically. Thus any mechanical noise generated will be noticeable. 
     FIG. 23 represents an image output from an ink jet printer of the present embodiment as a matrix of eight by eight, and FIG. 24 shows the FIG. 23 pattern with the density increased by one stage. 
     For the dither method, increasing the number of tones to output a low-density intermediate tone requires a larger dither matrix and this disadvantageously results in a lower resolution. By contrast, the present embodiment, basically employing error diffusion, can be free from such a problem and also achieve fine changes in density. That is, when a dither matrix of four by four in size is used according to the dither method, an image having its density increased from that shown in FIG. 25 by one stage in density is as shown in FIG.  26 . This shows that when the dither method is employed, increasing a density by one stage doubles the density. The present embodiment is free from such a disadvantage. 
     FIG. 27 shows an exemplary image output from an ink jet printer according to the present embodiment, and FIG. 28 shows the FIG. 27 image that is output only according to a conventional error diffusion. 
     As shown in FIG. 27, in accordance with the present embodiment, error diffusion is initially performed and a dot diameter is then modified to perform a tone process (an areal tone method). Thus, smooth representation can be achieved when the density of an image shifts. By contrast, it is difficult for the conventional error diffusion using uniform dots to represent smooth shift in image data. 
     Second Embodiment 
     An ink jet printer according to a second embodiment of the present invention is characterized in that the size of dots to be printed is divided into four stages and in that four pixels correspond to one square contained in the screen depending on the resolution of the printer. 
     More specifically, any dot is not printed or tone 0 is provided when the four pixels contained in one square of the screen are all “0”, as shown in FIG.  29 . 
     When one of the four pixels is “1” or in the state at which a dot is printed, the smallest dot is printed or tone 1 is provided, as shown in FIG.  30 . 
     When there are two pixels indicative of a “1”, the second smallest dot is printed or tone 2 is provided, as shown in FIG.  31 . 
     When there are three pixels indicative of a “1”, the third smallest dot is printed or tone 3 is provided, as shown in FIG.  32 . 
     When all of the pixels indicate a “1”, the largest dot is printed or tone 4 is provided, as shown in FIG.  33 . 
     Thus in contrast with the first embodiment the present embodiment does not require the correspondency table shown in FIG.  20 . Thus the procedure of processing images can be simplified. It should be noted that the above tones 1 to 4 that are adapted to correspond to the tones 1 to 4 of a laser beam printer, as shown in FIG. 34, to print images allows the present invention applicable to the laser beam printer. 
     Furthermore, the present invention is also applicable to heat-transfer printers if heat application time or the number of pulses for heat application is changed depending on the tone. 
     Although the present invention has been described and illustrated in detail, it is clearly understood that the same is by way of illustration and example only and is not to be taken by way of limitation, the spirit and scope of the present invention being limited only by the terms of the appended claims.