GB2356601A - Faulty nozzle mapping in an ink jet printer - Google Patents

Description

2356601 FAuLTY NozzLE MAPPING IN AN INK JET PRINTER The present invention

is generally directed to compensating for malfunctioning ink jet nozzles in an ink jet print head. More particularly, the invention is directed to rerouting print data from a malfunctioning ink jet nozzle to a substitute nozzle.

Inkjet printers form images on paper by ejecting ink droplets from an array of nozzles on a print head. During the operational lifetime of an ink jet print head, the nozzles can become clogged, thus blocking the ejection of ink from the nozzles. Although most current ink jet printers include mechanisms for clearing clogged nozzles, these mechanisms are not always successful, and nozzles remained clogged.

Generally, when printer driver software generates print data to be sent to the print head, the software typically assumes that all of the nozzles of the print head are functioning properly. Thus, the print data may address nozzles that are malfunctioning. If this be the case, pixels that should be printed by the malfunctioning nozzles will remain blank on the paper. As more and more nozzles malfunction during a print head's lifetime, this situation becomes more and more noticeable in printed output.

Therefore, a system is needed that uses a fimctional ink jet nozzle on a print head to print dots that would otherwise be left blank due to a nonfunctioning nozzle.

The foregoing and other needs are met by a method for forming an image consisting of a plurality of dots. The image is formed by ejecting ink droplets onto a print medium from a plurality of functional ink jet nozzles in an ink jet print head as the ink jet print head makes multiple shingled passes across the print medium. The nozzles are disposed at nozzle positions in a substantially columnar array. The array is aligned substantially perpendicular to a direction of travel of the print head as the print head makes a pass across the print medium. During the pass, each of the ink jet nozzles prints dots within a corresponding raster line along the direction of travel of the print head, where the raster line printed by each nozzle corresponds to the position of the nozzle in the array. The print head also includes at least one nonfunctional ink jet nozzle.

The locations of the printed dots on the print medium correspond to pixel locations defffied by a plurality of shingle masks. Each of the shingle masks specify pixel locations that are printable during a corresponding one of the passes of the print head across the print medium. The shingle masks specify pixel locations corresponding to a plurality of interlaced spacing patterns. A combination of the interlaced spacing patterns defines all-printable pixel locations.

The method includes the step of assigning only one of the functional nozzles to be a substitute nozzle. The substitute nozzle prints dots during a particular pass of the print head at pixel locations that would normally be printed by the nonfunctional nozzle during an overlapping pass of the print head. The position of the substitute nozzle is separated from the position of the nonfunctional nozzle such that the raster line of dots that would normally be printed by the nonfunctional nozzle during a particular pass of the print head coincides with a raster line of dots that are printed by the substitute nozzle during an overlapping pass of the print head. The method also includes the step of determining, for each of the shingle masks, the pixel locations at which dots would normally be printed by the nonfunctional nozzle if the nonfunctional nozzle were functional. The method finther includes modifying each of the shingle masks to cause the substitute nozzle to print dots during each pass of the print head at pixel locations where the substitute nozzle would normally print, and to print dots at pixel locations where the nonfunctional nozzle would normally print during an overlapping pass.

Thus, by assigning a substitute nozzle to print in place of the nonfunctional nozzle during a previous or later overlapping pass, the present invention eliminates any blank pixels that would otherwise be left in the image due to the nonfurictioning nozzle. Further, since the substitute nozzle prints the dots for the nonfunctional nozzle during the same pass in which it prints its own dots, the invention does not introduce any extra print head passes to the printing process. Therefore, the invention corrects for the loss of the nonfunctional nozzle without significantly slowing the printing process.

Further advantages of the invention will become apparent by reference to the detailed description of preferred embodiments when considered in conjunction with

2 the drawings, which are not to scale, wherein like reference characters designate like or similar elements throughout the several drawings as follows:

Fig. I is a functional block diagram of an ink jet printer that compensates for nonfunctional inkjet nozzles; Fig. 2 depicts an ink jet print head and nozzle array according to a preferred embodiment of the present invention;_ Figs. 3a. and 3b depict graphic representations of first and second 50% shingling masks; Figs. 4a4d depict the printing of an image using first and second 50% shingling masks; Figs. 5a-5d depict the printing of an image using first and second 50% shingling masks when an inkjet nozzle has malfunctioned; Figs. 6a and 6b depict graphic representations of first and second 50% shingling masks that are modified according to a prefer-red embodiment of the present invention to compensate for a malfunctioning ink jet nozzle; Figs. 7a-7d depict the printing of an image using first and second 50% shingling masks that are modified according to a preferred embodiment of the present invention to compensate for a malfunctioning ink jet nozzle; Figs. 8a-8d depict graphic representations of first, second, third, and fourth 25% shingling masks; Figs. 9a-9h depict the printing of an image using first, second, third, and fourth 25% shingling masks; Figs. 10a-10h depict the printing of an image using first, second, third, and fourth 25% shingling masks when an inkjet nozzle has malfunctioned; Figs. lla-Ild depict graphic representations of first, second, third, and fourth 25% shingling masks that are modified according to a preferred embodiment of the present invention to compensate for a malfunctioning inkjet nozzle-, and Figs. 12a-12h depict the printing of an image using first, second, third, and fourth 25% shingling masks that are modified according to a preferred embodiment of the present invention to compensate for arnalfunctioning inkjetnozzle.

Shown in Fig. I is a functional block diagram of an ink jet printer 10 that compensates for nonfunctional ink jet nozzles. The printer 10 includes a printer 3 controller 12, such as a digital microprocessor, that receives image data from a host computer 14. The image data contains information describing an image 16 to be printed on a print medium 18, such as paper. Based on the image data, the printer controller 12 generates print signals, scan commands, and print medium advance commands, which collectively control the printing of the image as described in more detail below.

Generally, the image data generated by the host computer 14 describes the image 16 in a bit-map format. Such a format represents the image 16 as a collection of pixels, or picture elements, in a two-dimension rectangular coordinate system.

For each pixel, the image data indicates whether the pixel is on or off (printed or not printed), and the rectangular coordinates of the pixel on the print medium 18.

Typically, the host computer 14 "rasterizes" the image data by dividing the image 16 into horizontal rows of pixels, stepping from pixel-to-pixel across each row, and writing out the image data for each pixel according to each pixel's order in the row.

As shown in Figs. I and 2, the printer 10 includes a print head 20. On the print head 20 is a nozzle plate 24 that contains a columnar array of nozzles 22. For simplicity, the array of nozzles 22 is illustrated by a single linear colurm of nozzles. However, other array geometries are also suitable for use in the present invention, such as arrays wherein nozzles are not perfectly aligned, but are horizontally offset and staggered.

Based on the print signals from the printer controller 12, ink droplets are ejected from the array of nozzles 22 to form dots on the print medium 18 corresponding to pixels in the image 16. Ink is selectively ejected from the array of nozzles 22 when corresponding heating elements, disposed below the nozzle plate 24, are activated by the print signals from the controller 12. Each nozzle in the array 22 prints dots in a raster line as the print head 20 scans horizontally across the print medium 18. The raster lines of dots are aligned in a first direction, or horizontal direction, which corresponds with the direction of travel of the print head 20 as it scans across the print medium 18. Thus, each raster line is horizontally aligned with the nozzle position of the nozzle that printed the dots in the raster line. Vertically adjacent raster lines of dots printed by the array of nozzles 22 as the print 4 head 20 makes a complete pass across the print medium 18 are referred to herein as a swath of the image 16.

Throughout this invention description and the claims, the terms first pass, second pass, third pass, and so forth, are used to refer to particular passes of the print head across the print medium. Unless otherwise explicitly indicated herein, the terms first, second, third, and so forth, are not intended to describe any particular sequence of print head passes. Rather, these terms are used to differentiate one print head pass from another.

The printer 10 includes a print head scanning mechanism 26 for scanning the print head 20 across the print medium 18 in the first direction as indicated by the arrow 27. Preferably, the print head scanning mechanism 26 consists of a carriage which slides horizontally on one or more rails, a belt attached to the carriage, and a motor that engages the belt to cause the carriage to move along the rails. The motor is driven in response to scan commands generated by the printer controller 12.

As shown in Fig. 1, the printer 10 also includes a memory device 2 8, such as a RAM device, for storing image data and information that is indicative of operational characteristics of the printer 10. Preferably, the infon-nation. stored in the memory device 28 includes masks which indicate which of the nozzles in the array 22 is to print any particular pixel of the image 16 during a given pass across the print medium 18. Thus, these masks map nozzles to image pixels for any particular pass of the print head 20.

Typically, when horizontally or vertically adjacent dots are printed during the same pass of the print head 20, ink from the adjacent dots may bleed together before the ink has dried. This can cause undesirable aberrations in the printed image 16. To prevent the occurrence of adjacent wet ink dots, the printer 10 prints the image 16 using multiple interlaced and partially-overlapping spacing patterns of dots that are printed during multiple passes of the print head 20. This is a process known as "shingling". Preferably, none of the dots in a spacing pattern are immediately adjacent to any other dot in the same pattern. The variable p is used herein to represent the number of spacing patterns of dots that in combination form a complete image.

Figs. 3a and 3b graphically depict complementary 50% masks that are used in a 50% shingling technique. Using 50% shingling, one half of the pixels in each raster line of an image are printable during each pass of the print head 20. Thus, with 50% shingling, p is two.

In Figs. 3a and 3b, eight adjacent nozzles 178 of the columnar array of nozzles 22 are shown. In Fig. 3a, to the right of the array 22, is an arrangement of circles in a first 50% spacing pattern. Each of these circles represent a pixel location on the print medium 18 where a dot could be printed during a first pass of the print head 20 by one of the nozzles 1-8 that corresponds to the particular raster line. Whether or not any particular dot is actually printed during the first pass depends upon first print signals generated by the printer controller 12. Thus, the first 50% spacing pattern shown in Fig. 3a graphically represents a first 50% mask for a 50% checkerboard shingling technique. In Fig. 3b, each of the circles to the right of the array of nozzles 22 represents a location on the print medium 18 where a dot could be printed during a second pass of the print head 20. Thus, the second 50% spacing pattern shown in Fig. 3b graphically represents a second 50% mask for the 50% checkerboard shingling technique.

Figs. 4a-4c show which dots are printed using the 50% shingling technique during each of three passes of the print head 20 to print the image 16 shown in Fig. 4d. As shown in Fig. 4a, on the first pass of the print head 20, the printer controller 12 generates print signals based on the image data that activate the print head 20 to cause ink droplets to be ejected from the nozzles 5-8. The dots that are printed during the first pass are represented in Fig. 4a as the cross-hatched circles. Note that each of the dots printed during the first pass coincide with the first 50% spacing pattern as determined by the first 50% mask. Dots that are disposed within a single horizontal raster line of the image 16 are printed by the nozzle that is aligned with that raster line.

Referring to Fig. 1, the printer 10 includes a print medium advance mechanism 30 which, based on print medium advance commands generated by the controller 12, causes the print medium 18 to advance in a second direction, as indicated by the arrow 32. In a preferred embodiment of the invention, the print 6 medium advance mechanism 30 is a stepper motor rotating a platen which is in contact with the print medium 18.

After the first pass of the print head 20, the printer controller 12 generates a print medium advance command which causes the print medium advance mechanism 30 to advance the print mediinn 18 by an advance distance. Preferably, the advance distance is equivalent to:

n x the separation distance between nozzles, P where n is the number of nozzles and p is the number of spacing patterns. Thus, with the 50% shingling of this example, the advance distance is four nozzle positions. After the print medium 18 has advanced by the advance distance, the array of nozzles 22 is positioned as shown in Fig. 4b.

Based on a scan command from the controller 20, the print head scan mechanism causes the print head 20 to make a second pass across the print medium 18. During the second pass of the print head 20, the printer controller 12 generates second print signals based on the image data. The second print signals activate the print head 20 to cause ink droplets to be ejected from all eight nozzles 1-8 in the second 50% spacing pattern as determined by the second 50% mask. The dots that are printed during the second pass are represented in Fig. 4b as the cross-hatched circles. Note that each of the dots printed during the second pass coincide with the second 50% spacing pattern. The black-filled circles of Fig. 4b represent the dots in the image 16 that were printed during the first pass.

After the second pass has been printed, the printer controller 12 again generates a print medium advance conunand which causes the print medium advance mechanism 30 to again advance the print medium 18 by the advance distance. After the print medium 18 has advanced, the array of nozzles 22 is positioned as shown in Fig. 4c.

Results of a third pass of the print head 20 across the print medium 18 is depicted in Fig. 4c. On the third pass, the printer controller 12 again generates first print signals based on the image data, thus causing ink droplets to be ejected from all eight nozzles 1-8 in the first 50% spacing pattem. The dots that are printed during the third pass are represented in Fig. 4c as the cross-hatched circles, each of 7 which coincides with the first 50% spacing pattern. The black-filled circles of Fig. 4c represent the dots in the image 16 that were printed during the first and second passes.

Figs. 5a-d illustrate how the process described above is affected when one of the nozzles, such as nozzle 6, malfunctions. The dots that would have been printed by nozzle 6 in the first and second passes are missing. Thus, blank pixels remain in the image 16, as shown in Fig. 5d.

If the faulty condition of a nozzle is detected and its position is recorded, the present invention provides a method of compensating for the missing nozzle and filling in the missing dots. The method and means of detecting the malfunctioning nozzle are not critical to the operation of the invention. The detection could be performed by electrical circuits within the print head 20 that monitor the condition of the nozzles, by printing a test pattern that is inspected by an optical sensor or inspected by a hurnan operator, or both.

Preferably, the print head 20 includes a memory device 29 for storing inforrnation that is indicative of operational characteristics of the print head 20. At least two memory locations of at least two bytes each are reserved in the memory device 29 for storing the nozzle locations of missing or malfimctioning nozzles.

This missing nozzle information may be accessed by the printer controller 12 and used in modification of the shingle masks, as desciibed below.

After detection of a faulty nozzle, the masks stored in the memory device 28 are modified according to the present invention so that the dots that would be printed by the malfunctioning nozzle are instead printed by one other nozzle in the array, referred to herein as a substitute nozzle. The substitute nozzle is selected to be in a nozzle position that, in a later pass of print head 20, passes over the same raster line as did the malfunctioning nozzle in a previous pass. Alternatively, the substitute nozzle is selected to be in a nozzle position that, in a earlier pass of print head 20, passes over the same raster line as will the malfimctioning nozzle in a later pass.

To prevent overly complicating the examples of the operation of the invention as described below, it is assumed that a single nozzle has malfunctioned. However, it should be appreciated that the invention as described herein is also 8 applicable when more than one nozzle on a print head has malfunctioned. The total number of malfunctioning nozzles for which the invention can compensate depends on the locations of the malfunctioning nozzles and the type of shingling used.

Consider a situation in which nozzle 6 malfimctions. As illustrated in Figs. 5a-5d, nozzle 2 may be assigned to be the substitute nozzle for nozzle 6. In this situation, the first 50% mask for the 50% checkerboard shingling technique is modified as shown in Fig. 6a. In the modified first 50% mask, nozzle 6 is completely masked off, such that no dots would be printed on raster lines corresponding to the position of nozzle 6. The first 50% mask is further modified so that pixels in the second 50% spacing pattern that would normally be mapped to nozzle 6 are instead mapped to nozzle 2. As shown in Fig. 6b, the second 50% mask for the 50% checkerboard shingling technique is also modified to completely mask off nozzle 6. The second 50% mask is further modified so that pixels in the first 50% spacing pattern that would normally be mapped to nozzle 6 are instead mapped to nozzle 2. Thus, the dots that would have been printed by nozzle 6 in the first pass are instead printed by nozzle 2 during the second pass.

Figs. 7a-d illustrate how the modified 50% masks compensate for faulty nozzle 6. After the first pass, as depicted in Fig. 7a, no dots are printed in the raster line corresponding to nozzle 6. However, as shown in Fig. 7b, during the second pass, dots in the first 50% spacing pattern that would have been printed by nozzle 6 in the first pass are mapped to nozzle 2. Thus, nozzle 2 prints the dots missed by nozzle 6 on the first pass, as well as dots that would normally be printed by nozzle 2 during the second pass. During the third pass, as shown in Fig. 7c, dots in the second 50% spacing pattern that would have been printed by nozzle 6 in the second pass are mapped to nozzle 2. Thus, the dots that would have been printed by nozzle 6 in the second pass are instead printed by nozzle 2 during the third pass.

As the above example indicates, the substitute nozzle (nozzle 2 in the above example) prints dots in both the first and second 50% spacing patterns during each pass across the print medium 18. Thus, generally, the substitute nozzle prints twice the number of dots as it otherwise would.

Figs. 8a-8d graphically depict complementary 25% masks that are used in a 25% shingling technique. Using 25% shingling, approximately one quarter of the 9 pixels in each raster line of an image are printable during each pass of the print head 20. Thus, with 25% shingling, p is four. In Fig. 8a, to the right of the array 22, is an arrangement of circles in a first 25% spacing pattern. Each of these circles represent a pixel location on the print medium 18 where a dot could be printed during a first pass of the print head 20 by one of the nozzles 1-8 that corresponds to the particular raster line. As is the case with the 50% masks described above, whether or not any particular dot is actually printed during the first pass depends upon print signals generated by the printer controller 12. Thus, the first 25% spacing pattern shown in Fig. 8a graphically represents a first 25% mask for the 25% shingling technique. In Figs. 8b-8d, each of the circles to the right of the array of nozzles 22 represents a location on the print medium 18 where a dot could be printed during second, third, and fourth passes of the print head 20, respectively.

Thus, the second, third, and fourth 25% spacing patterns shown in Fig. 8b-8d graphically represent second, third, and fourth 25% masks for the 25% shingling technique.

Figs. 9a-9h illustrate the operation of the 25% shingling technique. Fig. 9h depicts the printed image 16 that is formed by the 25% shingling technique, and Figs. 9a-9g show which dots are printed during each of seven passes of the print head 20 over the image 16. As shown in Fig. 9a, during the first pass of the print head 20, the printer controller 12 generates print signals based on the image data that activate the print head 20 to cause ink droplets to be ejected from nozzles 7 and 8. The dot that is printed during the first pass is represented in Fig. 9a as the crosshatched circle.

After the first pass has been printed, the printer controller 12 generates a print medium advance command which causes the print medium advance mechanism 30 to advance the print medium 18 by an advance distance. In the embodiment illustrated by this example, the advance distance is:

n x separation distance between nozzles = 8 x separation distance = 2 x separation distance.

P 4 Thus, in this example, the advance distance is two nozzle positions. After the print medium 18 has advanced by the advance distance, the array of nozzles 22 is positioned as shown in Fig. 9b.

The print head 20 then makes a second pass across the print medium 18. On the second pass of the print head 20, the printer controller 12 generates print signals based on the image data to activate the print head 20 to cause ink droplets to be ejected from the nozzles 1-8 in the second 25% spacing pattern as determined by the second 25% mask. The dots that are printed during the second pass are represented in Fig. 9b as the cross-hatched circles. Note that each of the dots printed during the second pass coincide with the second 25% spacing pattern. The black-filled circle of Fig. 9b represents the dot in the image 16 that was printed during the first pass.

After the second pass has been printed, the printer controller 12 generates a print medium advance command which causes the print medium advance mechanism 30 to again advance the print medium 18 by the advance distance. After the print medium 18 has advanced by the advance distance, the array of nozzles 22 is positioned as shown in Fig. 9c.

The print head 20 then makes a third pass across the print medium 18. On the third pass of the print head 20, the printer controller 12 generates print signals based on the image data to activate the print head 20 to cause ink droplets to be ejected from the nozzles 1-8 in the third 25% spacing pattern as determined by the third 25% mask. The dots that are printed during the third pass are represented in Fig. 9c as the cross-hatched circles. Note that each of the dots printed during the third pass coincide with the third 25% spacing pattern. The black-filled circles of Fig. 9c represent the dots in the image 16 that were printed during the first and second passes.

After the third pass has been printed, the printer controller 12 generates a print medium advance command which causes the print medium advance mechanism 30 to again advance the print medium 18 by the advance distance. After the print medium 18 has advanced by the advance distance, the array of nozzles 22 is positioned as shown in Fig. 9d.

The print head 20 next makes a fourth pass across the print medium 18. On the fourth pass of the print head 20, the printer controller 12 generates print signals 11 based on the image data to activate the print head 20 to cause ink droplets to be ejected from nozzles 1-8 in the fourth 25% spacing pattern as determined by the fourth 25% mask. The dots that are printed during the fourth pass are represented in Fig. 9d as the cross-hatched circles. Note that each of the dots printed during the fourth pass coincide with the fourth 25% spacing pattern. The black-filled circles of Fig. 9d represent the dots in the irnagt 16 that were printed during the first through third passes.

After the fourth pass has been printed, the printer controller 12 generates a print medium advance command which causes the print medium advance mechanism 30 to again advance the print medium 18 by the advance distance. After the print medium 18 has advanced by the advance distance, the array of nozzles 22 is positioned as shown in Fig. 9e.

The print head 20 then makes a fifth pass across the print medium 18. On the fifth pass of the print head 20, the printer controller 12 again generates the print signals based on the image data to activate the print head 20 to cause ink droplets to be ejected from the nozzles 1-8 in the first 25% spacing pattern as determined by the first 25% mask. The dots that are printed during the fifth pass are represented in Fig. 9e as the cross-hatched circles, each of which coincide with the first 25% spacing pattern. The black-filled circles of Fig. 9e represent the dots in the image 16 that were printed during the first through fourth passes.

After the fifth pass has been printed, the printer controller 12 generates a print medium advance command which causes the print medium advance mechanism 30 to again advance the print medium 18 by the advance distance. After the print medium 18 has advanced by the advance distance, the array of nozzles 22 is positioned as shown in Fig. 9f The print head 20 then makes a sixth pass across the print medium 18. On the sixth pass, the printer controller 12 again generates the print signals based on the image data to activate the print head 20 to cause ink droplets to be ejected from the nozzles 1-8 in the second 25% spacing pattern as determined by the second 25% mask. The dots that are printed during the sixth pass are represented in Fig. 9f as the cross-hatched circles, each of which coincide with the second 25% spacing pattern.

12 The black-filled circles of Fig. 9f represent the dots in the image 16 that were printed during the first through fifth passes After the sixth pass has been printed, the printer controller 12 generates a print medium advance command which causes the print medium advance mechanism 30 to again advance the print medium 18 by the advance distance. After the printmedium 18 has advanced by the advance distance, the array of nozzles 22 is positioned as shown in Fig. 9g.

Finally, the print head 20 makes a seventh pass across the print medium 18.

On the seventh pass, the printer controller 12 again generates the print signals based on the image data to activate the print head 20 to cause ink droplets to be ejected from the nozzles 1-8 in the third 25% spacing pattern as detern-iined by the third 25% mask. The dots that are printed during the seventh pass are represented in Fig. 9g as the cross-hatched circles, each of which coincide with the third 25% spacing pattern. The black-filled circles of Fig. 9g represent the dots in the image 16 that were printed during the first through sixth passes.

Figs. 1 Oa-h illustrate how the process described above with reference to Figs. 9a-h is affected when one of the nozzles, such as nozzle 6, is faulty. The dots that would have been printed by nozzle 6 in the second through fifth passes are missing. Thus, blank pixels remain in the image 16 as indicated in Fig. I Oh.

Depicted in Figs. I la-1 Id are the four 25% masks of Figs. ga-d as modified according to the preferred embodiment of the invention to compensate for a malfunctioning nozzle 6 using substitute nozzle 2. As Figs. Ila-lld indicate, nozzle 6 is completely masked off in each of the modified 25% masks, such that no dots will be printed on the raster lines corresponding to the position of nozzle 6.

The first 25% mask is further modified so that the dots in the third 25% spacing pattern that would normally be mapped to nozzle 6 are instead mapped to nozzle 2. The second 25% mask is further modified so that the dots in the fourth 25% spacing pattern that would normally be mapped to nozzle 6 are instead mapped to nozzle 2. The third 25% mask is further modified so that the dots in the first 25% spacing pattern. that would normally be mapped to nozzle 6 are instead mapped to nozzle 2. The fourth 25% mask is further modified so that the dots in the second 25% spacing pattern that would normally be mapped to nozzle 6 are instead mapped to nozzle 2.

13 According to a preferred embodiment of the invention, the 25% masks are modified to map the faulty nozzle to a substitute nozzle that is separated 6om. the faulty nozzle by an integer multiple of n1, times the separation distance between nozzles, where n is the number of nozzles in the array and the separation distance is the center-to-center spacing between adjacent nozzles. Thus, in the example shown in Figs. I I a- I I d, since the array has eight nozzles, the substitute nozzle is separated from the faulty nozzle by an integer multiple of twice the nozzle-to-nozzle separation distance. The integer multiple chosen for the example is two. Therefore, the substitute nozzle of Figs. I I a- I I d is separated from the faulty nozzle by four nozzle positions. Although nozzle 2 was selected in the example to be the substitute nozzle to compensate for faulty nozzle 6, it will be appreciated that nozzles 4 or 8 could also have been selected as the substitute nozzle.

Figs. 12a- I 2h illustrate how the modified 25% masks of Figs. I I a- I I d are used to form the image 16. As Figs. 12a- I 2h indicate, since nozzle 6 is masked off, it does not produce any of the dots in the image 16. As shown in Fig. 12d, dots that would have been printed by nozzle 6 in the second pass are instead printed by nozzle 2 during the fourth pass. As shown in Fig. l2e, the modified first 25% mask is again used during the fifth pass. On the fifth pass, dots in the third 25% spacing pattern that would normally be mapped to nozzle 6 in the third pass are mapped to nozzle 2. Thus, the dots that would have been printed by nozzle 6 in the third pass are instead printed by nozzle 2 during the fifth pass. As shown in Fig. 12f, the modified second 25% mask is again used during the sixth pass. On the sixth pass, dots in the fourth 25% spacing pattern that would normally be mapped to nozzle 6 in the fourth pass are mapped to nozzle 2. Thus, the dots that would have been printed by nozzle 6 in the fourth pass are instead printed by nozzle 2 during the sixth pass. As shown in Fig. 12g, the modified third 25% mask is again used during the seventh pass. On the seventh pass, dots in the first 25% spacing pattern that would normally be mapped to nozzle 6 in the fifth pass are mapped to nozzle 2. Thus, the dots that would have been printed by nozzle 6 in the fifth pass are instead printed by nozzle 2 during the seventh pass.

The invention has been described herein with reference to an array consisting of eight nozzles. It should be appreciated that the use of an eight-nozzle 14 array is exemplary only, and that the invention is not limited to any particular number of nozzles. As the number of nozzles increases, it should be appreciated that the spacing patterns illustrated in the figures herein repeat in the vertical dimension.

Further, it should be appreciated that the invention is not limited to any particular number of pixels within- a raster line. As the raster lines extend horizontally to the limits of the width of the print medium, the spacing patterns illustrated in the figures repeat in the horizontal dimension.

It is contemplated, and will be apparent to those skilled in the art from the preceding description and the accompanying drawings that modifications and/or changes may be made in the embodiments of the invention. Accordingly, it is expressly intended that the foregoing description and the accompanying drawings are illustrative of preferred embodiments only, not limiting thereto, and that the true spirit and scope of the present invention be determined by reference to the appended claims.

CLAMS 1. A method for forming an image consisting of a plurality of dots by ejecting ink droplets onto a print medium from a plurality of functional ink jet nozzles in an ink jet print head as the ink jet print head makes multiple shingled passes across the print medium, the plurality of nozzles being disposed at nozzle positions in a substantially columnar array aligned substantially perpendicular to a direction of travel of the print head as the print head makes a pass across the print medium, wherein each of the ink jet nozzles prints dots within a corresponding raster line along the direction of travel of the print head, the raster line printed by each nozzle corresponding to the position of the nozzle in the array, where locations of the dots on the print medium correspond to pixel locations defined by a plurality of shingle masks, each of the shingle masks specifying pixel locations that are printable during a corresponding one of the passes of the print head across the print medium, the plurality of shingle masks speciying pixel locations corresponding to a plurality of interlaced spacing patterns, where a combination of the interlaced spacing patterns defines all printable pixel locations, and where the ink jet print head has a nonfunctional ink jet nozzle, the method comprising the steps of.

(a) assigning only one of the functional nozzles to be a substitute nozzle to print dots during a particular pass of the print head at pixel locations that would normally be printed by the nonfunctional nozzle during an overlapping pass of the print head, the position of the substitute nozzle being separated from the position of the nonfunctional nozzle such that the raster line of dots that would normally be printed by the nonfunctional nozzle during a particular pass of the print head coincides with a raster line of dots that are printed by the substitute nozzle during an overlapping pass of the print head; (b) for each of the shingle masks, determining the pixel locations at which dots would normally be printed by the nonfunctional nozzle if the nonfunctional nozzle were functional; and (c) modifying each of the shingle masks to cause the substitute nozzle to print dots during each pass of the print head at pixel locations where the substitute nozzle would non-nally print, and to print dots at pixel locations where the 16 nonfunctional nozzle would normally print during an overlapping pass of the print head.

2. A method for forirting an image by ejecting ink droplets onto a print medium from a substantially columnar array of ink jet nozzles in an ink jet print head as the ink jet print head makes multiple shingled passes across the print medium, where the array comprises n number of nozzles disposed at nozzle positions aligned substantially perpendicular to a direction of travel of the print head as the print head makes the passes across the print medium, where -each of the nozzles is separated from an adjacent nozzle by a separation distance, where each of the nozzles that is functional prints dots in corresponding raster lines of the image aligned along the direction of travel of the print head, where each raster line of dots consists of dots disposed in multiple interlaced and overlapping spacing patterns, and where the print head has a nonfunctional ink jet nozzle that is incapable of printing in any raster line of dots corresponding to its nozzle position in the array, the method comprising the steps of (a) determining only one of the functional nozzles to be a substitute nozzle to print the raster line of dots corresponding to the nozzle position of the substitute nozzle and to print the dots in a raster line that would be printed by the nonfunctional nozzle if the nonfunctional nozzle were functional; (b) during a first pass of the print head across the print medium, printing with the functional nozzles dots in a fust of the multiple spacing patterns within the raster lines that correspond to the nozzle positions of the functional nozzles, and printing with the substitute nozzle dots in another of the multiple spacing patterns in the raster line of the pattern that would be printed by the nonfunctional nozzle during another pass if the nonfunctional nozzle were functional; (c) moving the print medium relative to the print head by an advance distance in a direction perpendicular to the direction of travel of the print head, such that, after the moving, the nozzle position of the substitute nozzle is aligned with a 17 raster line that would be printed by the nonfunctional nozzle during another pass if the nonfunctional nozzle were functional; (d) during a second pass of tile print head across the print medium, printing with the functional nozzles dots in a second of the multiple spacing patterns within the raster lines that correspond to the nozzle positions of the functional nozzles, and printing with the substitute nozzle dots in another of the multiple spacing patterns in the raster line of the pattern that would be printed by the nonfunctional nozzle during another pass if the nonfunctional nozzle were functional; and (e) moving the print medium relative to the print head by an advance distance in a direction perpendicular to the direction of travel of the print head, such that, after the moving, the nozzle position of the substitute nozzle is aligned with a raster line that would be printed by the nonfunctional nozzle during another pass if the nonfunctional nozzle were functional.

3. The method of claim 2 further comprising:

the determining step (a) further comprising determining the substitute nozzle such that the position of the substitute nozzle is separated from the position of the nonfunctional nozzle by an integer multiple of n times the separation P distance, where p is a total number of the multiple interlaced and overlapping spacing patterns of dots that together form the image; and the moving steps (c) and (e) further comprising moving the print head by an n advance distance which is substantially equal to times the separation distance.

P 4. The method of claim 3 wherein p is two, and:

the step of printing with the functional nozzles during the first pass further comprises selectively ejecting ink droplets from the functional nozzles to forni dots in the image coinciding with a first checkerboard pattern, where rows in the first checkerboard pattern correspond to the raster lines of dots; and 18 the step of printing with the functional nozzles during the second pass further comprises selectively ejecting ink droplets from the functional nozzles to form dots in the image coinciding with a second checkerboard pattern, where rows in the second checkerboard pattern correspond to the raster lines of dots, the second checkerboard pattern being an inverse of and being interlaced with the first checkerboard pattern, the substitute -nozzle ejecting droplets during the first and second passes to form dots in the image coinciding with both the first and second checkerboard patterns.

5. A method for forming an image by ejecting ink droplets onto a print medium from a substantially columnar array of ink jet nozzles in an ink jet print head as the ink jet print head makes multiple shingled passes across the print medium, where the array comprises n number of nozzles disposed at nozzle positions aligned substantially perpendicular to a direction of travel of the print head as the print head makes the passes across the print medium, where each of the nozzles is separated from an adjacent nozzle by a separation distance, where each of the nozzles that is functional prints dots in corresponding raster lines of the image aligned along the direction of travel of the print head, where each raster line of dots consists of dots disposed in first, second, third, and fourth interlaced spacing patterns, where dots disposed in the first spacing pattern are printed during a first pass of the print head, dots disposed in the second spacing pattern are printed during a second pass of the print head, dots disposed in the third spacing pattern are printed during a third pass of the print head, and dots disposed in the fourth spacing pattern are printed during a fourth pass of the print head, and where the print head has a nonfunctional ink jet nozzle that is incapable of printing in any raster line of dots corresponding to its nozzle position in the array, the method comprising the steps of.

(a) determining only one of the functional nozzles to be a substitute nozzle to print the raster line of dots corresponding to the nozzle position of the substitute nozzle and to print the dots in a raster line that would be printed by the nonfunctional nozzle if the nonfunctional nozzle were functional; (b) during the first pass of the print head across the print medium, 19 printing with the functional nozzles dots in the first spacing pattern within the raster lines that correspond to the nozzle positions of the functional nozzles, and printing with the substitute nozzle dots in the second, third, or fourth spacing pattern in the raster line of the pattern that would be printed by the nonfunctional nozzle during the corresponding second, third, or fourth pass if the nonfunctional nozzle were functional; (c) moving the print medium relative to the print head by an advance distance in a direction perpendicular to the direction of travel of the print head, such that, after the moving, the nozzle position of the substitute nozzle is aligned with the raster line that would be printed by the nonfunctional nozzle during the corresponding first, third, or fourth pass if the nonfaxictional nozzle were functional; (d) during the second pass of the print head across the print medium, printing with the functional nozzles dots in the second spacing pattern within the raster lines that correspond to the nozzle positions of the functional nozzles, and printing with the substitute nozzle dots in the first, third, or fourth spacing pattern in the raster line of the pattern that would be printed by the nonfunctional nozzle during the corresponding first, third, or fourth pass if the nonfunctional nozzle were functional; (e) moving the print medium relative to the print head by an advance distance in a direction perpendicular to the direction of travel of the print head, such that, after the moving, the nozzle position of the substitute nozzle is aligned with the raster line that would be printed by the nonfunctional nozzle during the first, second, or fourth pass if the nonfunctional nozzle were functional; during the third pass of the print head across the print medium, printing with the functional nozzles dots in the third spacing pattern within the raster lines that correspond to the nozzle positions of the functional nozzles, and printing with the substitute nozzle dots in the first, second, or fourth spacing pattern in the raster line of the pattern that would be printed by the nonfunctional nozzle during the corresponding first, second, or fourth pass if the nonfunctional nozzle were functional; (g) moving the print medium relative to the print head by an advance distance in a direction perpendicular to the direction of travel of the print head, such that, after the moving, the nozzle position of the substitute nozzle is aligned with the raster line that would be printed by the nonfunctional nozzle during the first, second, or third pass if the nonfunctional nozzle were functional; (h) during the fourth pass of the print head across the print medium, printing with the functional nozzles dots in the fourth spacing pattern within the raster lines that correspond to the nozzle positions of the flinctional nozzles, and printing with the substitute nozzle dots in the first, second, or third spacing pattern in the raster line of the pattern that would have been printed by the nonfunctional nozzle during the corresponding first, second, or third pass if the nonfunctional nozzle were functional; and (i) moving the print medium relative to the print head by an advance distance in a direction perpendicular to the direction of travel of the print head, such that, after the moving, the nozzle position of the substitute nozzle is aligned with the raster line that would be printed by the nonfunctional nozzle during the second, third, or fourth pass if the nonfunctional nozzle were flinctional.

6. The method of claim 5 further comprising repeating steps (b) - (i) until the image is completely formed on the print medium.

7. The method of claim 5 further comprising:

the determining step (a) further comprising determining the substitute nozzle such that the position of the substitute nozzle is separated from the position of the nonfunctional nozzle by an integer multiple of n times the separation 4 distance; and 21 the moving steps (c), (e), (g), and (i) further comprising moving the print head by an advance distance which is substantially equal to n times the separation 4 distance.

8. An ink jet printer that compensates for nonfiinctional inkjet nozzles, comprising: an ink jet print head having a plurality of ink jet nozzles for ejecting droplets of ink to forrn dots on a print medium, the nozzles being disposed at nozzle positions lying in a substantially columnar array, each of the nozzles for printing a corresponding raster line of the dots aligned a first direction corresponding to the direction of travel of the print head as the print head makes a pass across the print medium, wherein the raster line printed by each nozzle corresponds to the nozzle's position in the array, one of the functional nozzles being a substitute nozzle for printing a raster line of dots corresponding to the nozzle position of the substitute nozzle and for printing all of the dots in a raster line that would be printed by a nonfunctional nozzle if the nonfunctional nozzle were functional, where the nozzle position of the substitute nozzle is separated from the nozzle position of the nonfunctional nozzle by a separation distance; a print head scan mechanism for scanning the print head across the print medium in the first direction in response to a scan command; a memory device for storing nozzle information indicative of which of the nozzles are functional and which of the nozzles are nonfunctional; a printer controller for generating the scan command to cause the print head scan mechanism to scan the print head in a first pass across the print medium, for retrieving from the memory device the nozzle infort-nation, for receiving image data from a host computer, for generating print signals based on the image data and the nozzle information to activate the print head to eject ink droplets from the functional nozzles in a first spacing pattern within the raster lines corresponding to the nozzle positions of the functional nozzles as the print head makes the first pass across the print medium, for generating print signals based on the image data and the nozzle information to activate the print head to eject ink droplets from the substitute nozzle 22 during the first pass within a raster line that would be printed by the nonfunctional nozzle during a different pass if the nonfunctional nozzle were functional, where the ink droplets ejected from the substitute nozzle during the first pass are in a spacing pattern that overlaps and is interlaced with the first spacing pattern.

9. The ink jet printer of claim 8 wherein the printer controller is further operable to generate print signals to activate the print head to eject ink droplets from the substitute nozzle during the first pass within a raster line that would have been printed by the nonfunctional nozzle during a prior pass if the nonfunctional nozzle were functional.

10. The ink jet printer of claim 8 wherein the printer controller is ftirther operable to generate print signals to activate the piint head to eject ink droplets from the substitute nozzle during the first pass within a raster line that would be printed by the nonfunctional nozzle during a subsequent pass if the nonfunctional nozzle were functional.

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