Patent Publication Number: US-2002008723-A1

Title: Printer and method of compensating for malperforming and inoperative ink nozzles in a print head

Description:
CROSS REFERENCE TO RELATED APPLICATIONS  
     [0001] This is a continuation-in-part of application Ser. No. 09/119,909, filed Jul. 21, 1998, entitled “Printer And Method Of Compensating For Inoperative Ink Nozzles In A Print Head” by Xin Wen, Lam J. Ewell, Douglas Couwenhoven and Edward Hauschild. 
    
    
     
       BACKGROUND OF THE INVENTION  
       [0002] This invention generally relates to ink jet printer apparatus and methods and more particularly relates to an ink jet printer and method of compensating for malperforming or inoperative ink nozzles in a print head, so that high quality images are printed although some ink nozzles are malperforming or inoperative.  
       [0003] An ink jet printer produces images on a receiver by ejecting ink droplets onto the receiver in an imagewise fashion. The advantages of non-impact, low-noise, low energy use, and low cost operation in addition to the capability of the printer to print on plain paper are largely responsible for the wide acceptance of ink jet printers in the marketplace.  
       [0004] It is known that quality printing by an ink jet printer requires repeated ejection of ink droplets from ink nozzles in the printer&#39;s print head. However, some of these ink nozzles may malperform. That is, some ink nozzles may indeed eject ink droplets; however, the ink droplets are ejected along a trajectory deviating from the droplets&#39; desired trajectory, thereby leading to artifacts in the printed image. Also, some ink nozzles may eject ink droplets having ink droplet volumes either less than or greater than the desired ink droplet volume. In addition, some ink nozzles may eject ink droplets at an undesired velocity. Moreover, some ink nozzles may completely fail to eject any ink droplets at all. When such malperforming nozzles are present, undesirable lines and artifacts will appear in the printed image, thereby degrading image quality. Also, when nozzle failures occur, unprinted lines will appear in the printed image along the direction of print head movement, thereby greatly degrading image quality.  
       [0005] Malperforming and inoperative nozzles may be caused, for example, by blockage of the ink nozzle due to coagulation of solid particles in the ink fluid in the nozzle. Malperforming and inoperative nozzles may also be due to inadvertent presence of foreign particles in the ink or faulty nozzle holes in a nozzle plate attached to the ink nozzles. Yet another reason for malperforming and inoperative nozzles may be inability to activate the ink droplets when required. That is, ink nozzles may fail to eject ink droplets as desired due to failures in an electric drive circuit which activates the nozzles in order to eject ink droplets. Moreover, ink nozzle malperformance due to failures in the electric drive circuit may give rise to ink droplets not having either a desired volume and/or a desired velocity, which in turn produce image artifacts. Also, such malperforming nozzles may only malperform intermittently. That is, such malperforming nozzles may operate as desired for a time and then malperform for a time only to return to the nozzle&#39;s desired operation. Moreover, in the case of thermal ink jet print heads, resistive heater elements that are in heat transfer communication with the ink in the nozzles for ejecting ink droplets may become degraded by repeated on-off heating duty cycles. Such heater element degradation compromises ability of the heater elements to supply the desired amount of heat when activated. For example, if a degraded heater element supplies less that the desired amount of heat to the ink, then an ink droplet may not be ejected from its associated ink nozzle. Therefore, it would be desirable to unclog such malperforming or inoperative ink nozzles or otherwise enable such malperforming inoperative ink nozzles to produce quality images.  
       [0006] Techniques for purging clogged ink nozzles are known. For example, U.S. Pat. No. 4,489,335 discloses a detector that detects nozzles which fail to eject ink droplets. A nozzle purging operation then occurs when the clogged ink nozzles are detected. As another example, U.S. Pat. No. 5,455,608 discloses a sequence of nozzle clearing procedures of increasing intensity until the nozzles no longer fail to eject ink droplets. Similar nozzle clearing techniques are disclosed in U.S. Pat. No. 4,165,363 and U.S. Pat. No. 5,659,342.  
       [0007] However, the art referred to hereinabove appear directed to recovery procedures when a nozzle completely fails to eject an ink droplet. Thus, this art appears to ignore the case in which, although the purged nozzle ejects an ink droplet, the droplet nonetheless does not possess desired characteristics (e.g., desired trajectory, desired volume, etc.). Moreover, the art referred to hereinabove appear to ignore the case in which not all failed nozzles can be recovered to be functional merely by performing nozzle clearing operations (e.g., wiping, purging, extensive firing and the like). For example, solid coagulates in the ink blocking the ink nozzles may strongly resist removal by nozzle clearing operations. That is, if only some of the solid coagulates are removed, then an ink droplet will eject; however, the ejected ink droplet may not have the desired trajectory, desired volume, e.t.c. Moreover, such nozzle clearing operations, even if successful in removing solid coagulates, cannot repair failed resistive heaters or failed electric driver circuits. Of course, presence of such permanently malperforming or inoperative nozzles compromises image quality.  
       [0008] Therefore, there has been a long-felt need to provide an ink jet printer and method capable of compensating for malperforming and inoperative ink nozzles in a print head, so that quality images are printed although some ink nozzles are malperforming or inoperative.  
       SUMMARY OF THE INVENTION  
       [0009] An object of the present invention is to provide an ink jet printer and method of compensating for malperforming and inoperative ink nozzles in a print head, so that quality images are printed although some ink nozzles are malperforming or inoperative.  
       [0010] With this object in view, the present invention resides in a printer comprising a plurality of drop-emitter nozzles arranged such that a first nozzle is adapted to print along a first path substantially the same as a second path previously printed by a second nozzle; and a control adapted to enable said first nozzle during a portion of the first path and to enable said second nozzle during a complementary portion of the first path, such that said first or said second nozzle is enabled during the entirety of the first path, said control being effective to disable said first or said second nozzle during the entirety of the first path to enable said first nozzle or said second nozzle during the entirety of the second path.  
       [0011] A feature of the present invention is the provision of an ink jet printer comprising a print head including operative ink nozzles that are capable of compensating for malperforming and inoperative ink nozzles.  
       [0012] An advantage of the present invention is that quality images are printed although some of the ink nozzles are malperforming or inoperative.  
       [0013] Another advantage of the present invention is that lifetime of the print head is increased and therefore printing costs are reduced.  
       [0014] These and other objects, features and advantages of the present invention will become apparent to those skilled in the art upon a reading of the following detailed description when taken in conjunction with the drawings wherein there is shown and described illustrative embodiments of the invention. 
     
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
     [0015] While the specification concludes with claims particularly pointing-out and distinctly claiming the subject matter of the present invention, it is believed the invention will be better understood from the following description when taken in conjunction with the accompanying drawings wherein:  
     [0016]FIG. 1 is a view in perspective of a printer with parts removed for clarity;  
     [0017]FIG. 2A illustrates a first mask pattern produced by an operative nozzle of the printer during a first printing pass;  
     [0018]FIG. 2B illustrates a second mask pattern produced by an operative nozzle of the printer during a second printing pass;  
     [0019]FIG. 3 illustrates a first algorithm for acquiring nozzle performance information (i.e., nozzles operative, malperforming or inoperative);  
     [0020]FIG. 4 is a plan view of the printer, with parts removed for clarity;  
     [0021]FIG. 5A illustrates a first mask pattern produced by an inoperative nozzle of the printer during a first printing pass;  
     [0022]FIG. 5B illustrates a second mask pattern produced by an operative nozzle of the printer during a second printing pass;  
     [0023]FIG. 5C illustrates a test image for detecting malperforming ink nozzles as well as fully operative ink nozzles; and  
     [0024]FIG. 6 illustrates a second algorithm providing image processing steps which result in compensating for malperforming or inoperative ink nozzles. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
     [0025] The present description will be directed in particular to elements forming part of, or cooperating more directly with, apparatus in accordance with the present invention. It is to be understood that elements not specifically shown or described may take various forms well known to those skilled in the art.  
     [0026] Therefore, referring to FIGS. 1, 2A and  2 B, there is shown a printer, generally referred to as  10 , for printing an output image  20  on a receiver  30 , which may be a reflective-type receiver (e.g., paper) or a transmissive-type receiver (e.g., transparency). Printer  10  prints image  20  by means of a print head  40 , which is an ink jet print head having a plurality of ink ejection nozzles  50  formed therein. For reasons disclosed hereinbelow, each nozzle  50  is assigned a unique index number “N i ”, where i=0 . . . M. Here, the value “M” may be equal to the total number of nozzles  50  formed in print head  40 . By way of example only and not by way of limitation, there may be 200 index numbers N i  where i=0 to 199. That is, there may be 200 ink nozzles  50  in print head  40 .  
     [0027] Referring again to FIGS. 1, 2A and  2 B, it is seen that printer  10  generally comprises the following components: (a) a rotatable platen  60  and a receiver guide  70  for translating receiver  30  with respect to print head  40 ; (b) print head control electronics  80  connected to print head  40  for controlling activation of nozzles  50  in print head  40 ; (c) a computer  90  connected to print head control electronics  80  for providing image data to print head control electronics  80 ; (d) an image processor  100  coupled to computer  90  for processing the digital image data; (e) and motion control electronics  110  associated with print head  40  and platen  60  for controlling translation of print head  40  and rotation of platen  60 . Each of these components in addition to other components defining the invention are described more fully hereinbelow.  
     [0028] Still referring to FIGS. 1, 2A and  2 B, printer  10  further comprises a print head transport mechanism, generally referred to as  120 . Print head transport mechanism  120  is coupled to print head  40  for reciprocating print head  40  with respect to receiver  30  along a direction illustrated by a double-headed arrow  125 . In the preferred embodiment of the invention, print head transport mechanism  120  includes a first motor  130  engaging a gear  140 , which in turn engages a pulley-belt assembly  150 . Pulley-belt assembly  150  moves print head  40  with respect to receiver  30  along a fast scan direction as indicated by arrow  125  when first motor  130  operates. Although not shown, print head transport mechanism  120  may further include positional feedback, a linear encoder, and a direct current first motor  130 . Alternatively, print head transport mechanism  120  may be a screw-driven arrangement having an elongate lead-screw (not shown) extending parallel to platen  60  and threadably engaging print head  40  for reciprocating print head  40  along a longitudinal axis of the lead-screw. Moreover, printer  10  also comprises a receiver transport mechanism, generally referred to as  160 , for translating receiver  30  with respect to print head  40  along a direction illustrated by an arrow  165 . In the preferred embodiment of the invention, receiver transport mechanism  160  includes a second motor  170  connected to motion control electronics  110  and engaging a gear arrangement  180 . Second motor  170  operates platen  60  by means of gear arrangement  180 , such that receiver  30  moves in the direction of arrow  165  and slides along guide  70  when second motor  170  operates.  
     [0029] Referring yet again to FIGS. 1, 2A and  2 B, a digital image source  190  is connected to computer  90  for supplying an input digital image (not shown) to computer  90 , which input digital image comprises a plurality of pixel values characterizing the digital image by pixel color, pixel location, e.t.c. In this regard, digital image source  190  may be a digital camera, scanner, or the like (also not shown). Alternatively, this input digital image also may be created on computer  90  by means of a suitable user interface that may include a display, a keyboard, a stylus, and/or a “mouse” (also not shown). Computer  90  preferably includes at least one communication port (not shown) for transferring image files and other information to external devices, such as a computer network mass storage area. A nozzle performance information source  200  is stored in a memory (not shown), which is connected to computer  90 , for supplying information to computer  90  about performance of each nozzle  50 . In this regard, the nozzle performance information supplied to computer  90  specifies whether each nozzle  50  is “malperforming”, “inoperative” or “fully operative”, as described in detail hereinbelow.  
     [0030] In addition, in ink jet printing, an image row is often printed in more than one printing pass for at least two reasons. First, risk of ink coalescence on the ink receiver is minimized because only a subset of all image pixels is printed in each printing pass. This also reduces probability that ink spots at adjacent pixels will be in liquid contact. Secondly, visual artifacts caused by variabilities between ink nozzles are reduced. Such variabilities may be due to variabilities introduced in manufacturing the print head. In order to ameliorate such variabilities, each image row is printed by more than one ink nozzle in more than one printing pass. Therefore, variability, such as errors in ink drop placement or ink drop volume, between ink nozzles  50  can therefore cancel each other and make image artifacts less apparent to the naked eye when more than one printing pass is made.  
     [0031] Therefore, FIGS. 2A and 2B illustrate printing of a single image row  210  in two passes when all nozzles  50  are fully operative. The terminology “fully operative” with respect to nozzles  50  is defined herein to mean nozzles  50  that eject ink drops having desired characteristics, such as desired ink drop trajectory, desired ink drop volume, and desired ink drop velocity. Entry values of mask patterns in image row  210  comprises a plurality of pixel locations  220  having pixel location index numbers P ij , where i=0 . . M and j=1 . . . C. In this exemplary embodiment of the invention, “M” is the total number of pixel rows that extend horizontally on receiver  30  and “C” is the total number of pixel columns that extend vertically on receiver  30 . Thus, the subscript “i” for pixel location P ij  denotes a row location and the subscript “j” for pixel location P ij  denotes a column location. Therefore, location of each pixel in image  20  can be described by its two-dimensional pixel location number P ij . However, it should be noted that values of P ij  are values for mask patterns in image rows  210  rather than pixel values obtained from the digital input image, as disclosed more fully hereinbelow. In order to determine whether a pixel is printed, the mask pattern value and the pixel value from the digital input image are logically multiplied (i.e., logically an “AND” arithmetic operation).  
     [0032] Referring to FIGS. 2A and 2B, it may be appreciated that a perfectly operating printer  10  has all nozzles  50  operative. The printing process begins when receiver transport mechanism  160  positions receiver  30  so that image row  210  comes into registration with nozzles N 0 . Next, print head transport mechanism  120  translates print head  40  along the fast scan direction (i.e., direction of arrow  125 ) to print a swath plane comprising M image rows. More specifically, image row  210  is printed using a first mask pattern  250  corresponding to the first printing pass. For purposes of illustration, first mask pattern  250  for nozzle  50  is illustrated as containing entry values of “0&#39;s” and “1”, where the entry value of “1” is used herein to indicate that nozzle N 0  has been enabled to print a pixel at a predetermined pixel value at pixel location P ij  and the entry value of “0” is used herein to indicate that nozzle N 0  has been disabled to not print a pixel location P ij .  
     [0033] Referring to FIG. 2B, receiver  30  is advanced by receiver transport mechanism  160  so that image row  210  comes into registration with nozzle N 100 . At this point, the next swath plane of image  20  is printed. More specifically, image row  210  is printed using a second mask pattern  260 . As described hereinabove, the values of “0&#39;s” and “1&#39;s” at pixel locations P ij  in second mask pattern  260  represent enabling and disabling, respectfully, of printing at each pixel for that particular pass.  
     [0034] Referring again to FIGS. 2A and 2B, entry values in second mask pattern  260  are complementary to values in first mask pattern  250 . That is, where an entry value of “0” appears in a column “j” of first mask pattern  250 , such as at pixel location P 0,1 , a complementary entry value of “1” appears in the same column “j” of second mask pattern  260 , such as at pixel location P 100, 1 . Conversely, where entry value of “1” appears in a column “j” of first mask pattern  250 , such as pixel location P 0, 2 , an entry value of “0” appears in the same column “j” of second mask pattern  260 , such as at pixel location P 100, 2 .  
     [0035] Referring yet again to FIGS. 2A and 2B, image row  210  is printed by nozzle  50  having index number N 0  in the first printing pass and then overprinted by nozzle  50  having index number N 100  in the second printing pass. In this manner, the combined effect of mask patterns  240  and  250  produced by the first and second printing passes, respectively, allows all pixels in image row  210  to be printed. Thus, during the first printing pass, nozzle N 0  is activated to print a predetermined portion of image row  210  using mask pattern  250 . Similarly, during the second printing pass, nozzle N 100  is activated to print the remaining portion of image row  210  using mask pattern  260 . In the present invention, nozzles that print over the same image rows, such as nozzles N 0  and N 100 , are assigned to a nozzle group. Another nozzle group may include nozzles N 2  and N 102 . Yet another nozzle group may include nozzles N 99  and N 199 . It may be appreciated from the teachings herein that the present invention is compatible with other ways of organizing nozzle groups which may vary depending on the specific printing mode selected and may be different from the example disclosed immediately hereinabove. Such specific printing modes may, for example, be number of printing passes, paper transport amount after each pass, e.t.c.  
     [0036] Referring to FIGS. 1, 2A and  2 B, the input digital image is transmitted from digital image source  190  to computer  90  wherein the input digital image is processed by image processor  100 . In this regard, image processor  100  is capable of resizing, cropping, tone scale transformation, color transformation, and/or halftoning the input digital image. Moreover, image processor  100  places the input digital image in a format useful for input to ink jet print head  40 , which image format may be in the form of separate color planes comprising the input digital image (e.g., yellow, magenta, cyan and black color planes); or a plurality of swath planes that are each printed during different printing passes, as described hereinabove. As described more fully hereinbelow, image processor  100  also includes a first algorithm  270  (see FIG. 3) that acquires nozzle performance information such as whether nozzles  50  are either operative malperforming or inoperative. Also as described more fully hereinbelow, image processor  100  further includes a second algorithm  370  (see FIG. 6) for compensating for any inoperative nozzles  50 . These algorithms  270  and  370  are used to acquire nozzle performance information and to compensate for presence of inoperative nozzles  50  by using only operative nozzles  50 .  
     [0037] Referring yet again to FIGS. 1, 2A and  2 B, the processed digital image data provided by image processor  100  is transmitted from image processor  100  to the previously mentioned print head control electronics  80 . The print head control electronics  80  receives this processed digital image data and transforms this data into electrical signals that selectively drive (i.e., selectively activate) nozzles  50 . These selectively driven nozzles  50  produce output image  20  on receiver  30  by printing a plurality of image rows  210  onto receiver  30 . In addition, motion control electronics  110  controls first motor  130 , so that print head  40  is controllably translated with respect to receiver  30  in order to print each image row  210  in first mask pattern  250 . In addition, after each swath plane is printed, motion control electronics  110  controls second motor  170 , such that platen  60  rotates to advance receiver  30  in a direction illustrated by arrow  165 . Receiver  30  is advanced in this manner in order to prepare the ink nozzles in the same nozzle group for printing a different image mask pattern  260  on image row  210  of image  20 . It may be appreciated from the description hereinabove that a single image row  210  belonging to image  20  may be completely printed in 3, 4, 6 or any number of such printing passes, if desired.  
     [0038] The description hereinabove was directed to the nominal case where all nozzles  50  are operative and no nozzles  50  are malperforming or inoperative. However, some of these nozzles  50  in fact may be malperforming or inoperative. It is desirable to detect and compensate for malperforming or inoperative nozzles  50  by activating fully operative nozzles  50 , so as to provide high quality output image  20 .  
     [0039] Referring to FIGS. 1, 2A,  2 B and  3 , first algorithm  270  for providing nozzle performance information begins with detecting inoperative nozzles  50 , as at step  310  of first algorithm  270 . The inoperative nozzles  50  are detected in a manner disclosed presently. Next, nozzles  50  are organized into nozzle groups, as at step  320 , and as described hereinabove. Some of the index numbers N i  are associated with malperforming and inoperative nozzles  50 , while other ones of the index numbers N i  are associated with fully operative nozzles  50 . In step  347 , these nozzle index numbers N i  representing either malperforming, inoperative or operative nozzles  50  are stored as nozzle performance information in nozzle performance information source  200 . This nozzle performance information is then transmitted from performance information source  200  to computer  90  where it is processed for use by image processor  100 . It may be appreciated that nozzle performance information source  200  may be stored in an electronic memory connected to computer  90  for storing nozzle indices N i .  
     [0040] As best seen in FIGS. 3 and 4, any inoperative nozzles  50  are detected by an optical detection system, generally referred to as  325 , comprising a light source  330  laterally disposed to one side of print head  40  and a light sensor  340  laterally disposed to an opposite side of print head  40 . Light sensor  340  is coupled to nozzle performance information source  200  for transmitting an electrical signal to nozzle performance information source  200 , as described in more detail presently. Light source  330 , which may be a laser light source, is colinearly aligned with light sensor  340  and emits a light beam along a light beam path  342  passing adjacent to nozzles  50 . Of course, light sensor  340 , which may be a photodiode, receives light emitted by light source  330 . Thus, in order to detect operative nozzles  50 , motion control electronics  110  translates print head  40  to a position between light source  330  and light sensor  340 , so that when an ink droplet  290  is ejected from operative nozzle  50 , the light beam is interrupted. When the light beam is interrupted in this manner, an electrical signal produced by light sensor  340  causes this nozzle  50  to be recorded in nozzle performance information source  200  as an operative nozzle  50 . On the other hand, if ink droplet  290  fails to eject from nozzle  50  when nozzle  50  is activated, then the light beam is uninterrupted and no electrical signal is produced by light sensor  340 . In this latter case, nozzle  50  is recorded in nozzle performance information source  200  as an inoperative nozzle  50 . Using this information, mask patterns  250  and  260  are applied to nozzle groups having all operative nozzles. Mask patterns  345  and  348  are subsequently applied to nozzle groups that include inoperative nozzles.  
     [0041] However, some nozzles  50  may be malperforming in the sense that ink droplets  290  are ejected but not as intended. Such nozzles are not completely “inoperative” and not “fully operative”. For example, some ink nozzles  50  may indeed eject ink droplets  290 ; however, the ink droplets  290  are ejected along a trajectory deviating from the droplets&#39; desired trajectory; that is, the trajectory normal to a nozzle plate (not shown) belonging to printhead  40 . Other ink nozzles may eject ink droplets  290  having ink droplet volumes either less than or greater than the desired ink droplet volume. Such ink nozzle behavior may lead to artifacts appearing in output image  20 . That is, when such malperforming nozzles  50  are present, image artifacts, such as banding, will appear in the printed image, thereby degrading image quality. As described presently, the invention compensates for such malperforming nozzles  50 , as well as for completely failed nozzles, in order to obtain a high quality output image  20 .  
     [0042] Therefore, as best seen in FIG. 5C, a test image  361  is first printed by a specific print head  40  for acquiring nozzle performance information. The purpose of printed test image  361  is to detect nozzles that are malperforming as well as nozzles that have completely failed. In this regard, printed test image  361  includes a plurality of ink marks, such as lines  362 , with each line  362  being printed by a different nozzle N i , where i=0 to 199. For purposes of clarity, test printing results for only a subset of all two-hundred nozzles are shown in FIG. 5C. That is, test printing results only for nozzles N i , where i=0 to 19 are shown.  
     [0043] Still referring to FIG. 5C, a desired (i.e., perfectly formed) line  363  printed by a fully operative nozzle N 12  comprises a plurality of generally aligned ink dots  364   a  of substantially equal size, each ink dot  364   a  being formed by individual ink droplet  290 . However, if any one of nozzles  50 , such as nozzle N 2 , completely fails to eject ink droplet  290 , then a space  365  is observed where line desired  363  should be. In addition, if any one of nozzles  50 , such as nozzle N 7 , ejects ink droplet  290  along an undesired trajectory, then a line  366  is displaced from its intended location in printed test image  361 . Moreover, if any one of nozzles  50 , such as nozzle N 17 , ejects an insufficient volume of ink for ink droplet  290 , then a lighter and thinner than desired line  367  is produced. In this case, lighter than desired line  367  comprises ink dots  364   b  that are smaller than ink dots  364   a . In addition, if any one of nozzles  50 , such as nozzle N 19 , ejects more than desired volume of ink for ink droplet  290 , then a darker and thicker than desired line  368  is produced. In this case, darker than desired line  375  comprises ink dots  366   c  that are larger than ink dots  364   a . The nozzle indices N i  for fully operative, as well as malperforming nozzles, are stored in nozzle performance information source  200 .  
     [0044] Referring again to FIG. 5, any malperforming nozzles  50  including any completely failed nozzles  50  can be detected visually or by means of automatically operated apparatus (not shown). With regard to visual detection, an operator of printer  10  examines nozzles  50  and determines the malperforming nozzles including the completely failed nozzles. Next, the operator nozzle index numbers N i  corresponding to those nozzles ejecting ink droplets  290  in an undesirable manner as well as those nozzles that completely fail to eject ink droplets. The operator then inputs this information into computer  90 , which stores the information in nozzle performance information source  200 . On the other hand, with regard to detection by means of automatically operated apparatus, printed test image  361  is imaged by an image sensor (not shown), preferably integrally connected to printer  10 . The image is then analyzed by at least one of a plurality of image pattern recognition programs well known in the art, to detect malperforming nozzles including completely failed nozzles. This information is then stored in nozzle performance information source  200 . Such an automatic detection technique is disclosed in commonly assigned U.S. patent application Ser. No. 09/135,308 titled “Ink Jet Printing With Enhanced Image Stability” filed Aug. 17, 1998, the disclosure of which is hereby incorporated by reference.  
     [0045] Referring to FIGS. 4 and 6, it may be appreciated from the discussion hereinabove that previously mentioned light source  330  and light sensor  340  are used to detect completely failed nozzle  50 . Also, it may be appreciated from the discussion hereinabove that test image  361  is also used to detect a completely failed nozzle  50 , as well as detecting other malperforming nozzles  50 . Therefore, if it is desired merely to detect completely failed nozzles  50 , light source  330  and light sensor  340  may be used. Alternatively, test image  362  may be used to detect completely failed nozzles. An advantage of using light source  330  and light sensor  340  to detect a completely failed nozzle  50  is that test image  362  need not be printed. This results in a concomitant time savings because time spent printing and analyzing test image  362  is avoided.  
     [0046]FIGS. 5A and 5B provide an exemplary illustration of how such malperforming and inoperative nozzles  50  are compensated for by operative nozzles  50 . In the example described presently, nozzle N 0  is assumed to be an inoperative (i.e., failed) nozzle. This nozzle N 0  will define a third mask pattern  345  in the first printing pass. In this regard, third mask pattern  345  defined by nozzle N 0  is illustrated as containing entry values of all “0&#39;s” (i.e., nozzle N 0  inoperative). On the other hand, nozzle N 100  is assumed to be an operative nozzle. This nozzle N 100  defines a fourth mask pattern  348  in the second printing pass. In this regard, fourth mask pattern  348  defined by nozzle N 100  is illustrated as containing entry values of all “l&#39;s” (i.e., nozzle N 100  operative). Thus, it may be understood that entry values appearing in fourth mask pattern  348  are complementary to entry values appearing in third mask pattern  345 . That is, where entry value of “0” appears in column “j” for third mask pattern  345 , a complementary entry value of “1” appears in the same column “j” for fourth mask pattern  348 .  
     [0047] Referring again to FIGS. 5A and 5B, and as described hereinabove, third mask pattern  345  is illustrated as containing entry values of all “0&#39;s” (i.e., nozzle N 0  inoperative) and fourth mask pattern  348  is illustrated as containing entry values of all “1&#39;s” (i.e., nozzle N 100  operative). It may be appreciated from the description hereinabove that when the entry values in third mask pattern  345  are “0” for a specific inoperative nozzle  50 , then no pixel locations P 0j  (where j=1 . . . C) will be printed in the first printing pass regardless of the image value at those pixel locations. Similarly, it may be further appreciated from the description hereinabove, that if the entry values in fourth mask pattern  348  are “1” for a specific operative nozzle  50 , then pixel locations P 100, j  will be printed in the second printing pass consistent with the image values for those pixel locations. In this manner, all pixels for image row  210  are printed even though some nozzles  50  are inoperative. Also, the combined effect of fourth mask pattern  348  when overlaid onto third mask pattern  345 , after completion of the first printing pass and second printing pass, allows all pixels in image row  210  to be printed using operative nozzles  50  in place of inoperative nozzles  50 .  
     [0048] Referring to FIGS. 3, 5A and  5 B, if nozzle N 0  is detected as inoperative in the manner disclosed hereinabove, then third mask pattern  345  for nozzle N 0  is stored in nozzle information source  200 , as at step  347  of the previously mentioned first algorithm  270 . Next, the inoperative nozzle  50  having index number N 0  is disabled, as at step  350  of first algorithm  270 . This disabled nozzle  50  having index number N 0  is illustrated in FIG. 5A, wherein each entry value for each pixel location is “0”. These entry values of “0” indicate that no pixels in image row  210  are printed in the first printing pass. Put another way, the printing function of disabled nozzle  50  having index number N 0  (i.e., disabled nozzle  50  having entry values of “0”) are reassigned, as at step  360  of first algorithm  270 , to operative nozzle  50  having index number N 100  (i.e., enabled nozzle  50  having entry values of “1”). That is, printing function of disabled nozzle N 0  is reassigned to operative nozzle N 100 . Thus, entry values in image row  210  have a value of “1” during the second printing pass, so that all unprinted pixels associated with inoperative nozzle N 0  in the first printing pass are printed by operative nozzle N 100  in the second printing pass.  
     [0049] Turning now to FIG. 6, there is shown a second algorithm, generally referred to as  370 . Second algorithm  370  illustrates imaging processing steps performed by image processor  100 . In this regard, at step  380  the input image is operated upon in order to resize, crop, tone scale, halftone, transform color, and separate image row planes for each printing pass and each color. It may be appreciated that image processor  100  may perform other desired image preprocessing operations, as needed. As illustrated at step  390 , a swath plane including a plurality of image rows  210 , is extracted; that is, all pixel values of the swath plane are read by image processor  100 . Next, an image column is extracted from the swath plane, as at step  400 . An image pixel is then extracted from the image column “j” (where j=1 . . . C), as at step  410 . In addition, step  420  determines whether nozzle  50  falls into a nozzle group containing inoperative nozzles. If all nozzles in a nozzle group are operative, nominal (i.e., regular) mask patterns are applied as shown in FIGS. 2A and 2B and at step  430 . On the other hand, if nozzles  50  include inoperative nozzles, new mask patterns  345  and  348  are applied, as at step  440 . At this point, steps  390  through  410  are repeated for all pixels P ij  in steps  450  through  470 . It should be observed that first algorithm  270  and second algorithm  370  preferably reside in computer  90  in machine language.  
     [0050] It is appreciated from the description hereinabove that an advantage of the present invention is that high quality images are printed although some ink nozzles are malperforming or inoperative. This is so because pixels that would otherwise be printed by inoperative ink nozzles  50  in a first printing pass are instead printed by operative ink nozzles  50  in a second printing pass.  
     [0051] Another advantage of the present invention is that printing costs are reduced. This is so because purchase of a new print head merely to replace malperforming and inoperative nozzles is virtually avoided.  
     [0052] While the invention has been described with particular reference to its preferred embodiments, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements of the preferred embodiments without departing from the invention. For example, printer  10  may include a nozzle purging apparatus in communication with each nozzle  50 . Such nozzle purging may be performed by an ink pump and a vacuum suction device. Thus, any malperforming or inoperative nozzles may be purged before using the invention to compensate for the inoperative nozzles. This technique has the advantage of restoring function of malperforming and inoperative nozzles, if possible, so that a minimum number of malperforming and inoperative nozzles need be compensated for by operative nozzles. In this manner, printing speed is not significantly reduced. Nonetheless, some of these malperforming and inoperative nozzles nonetheless may resist purging operations. According to this technique, compensating for such permanently malperforming and inoperative nozzles by using operative nozzles would only occur after any unsuccessful purging operations.  
     [0053] As is evident from the foregoing description, certain other aspects of the invention are not limited to the particular details of the examples illustrated, and it is therefore contemplated that other modifications and applications will occur to those skilled in the art. It is accordingly intended that the claims shall cover all such modifications and applications as do not depart from the true spirit and scope of the invention.  
     [0054] Therefore, what is provided is an ink jet printer and method of compensating for malperforming and inoperative ink nozzles in a print head, so that high quality images are printed although some ink nozzles are malperforming or inoperative.