Abstract:
One aspect of the invention checks mechanical misalignment of plural pens and shifts data to allow for at least part of misalignment—and automatically prints images with the shifted data. If pens are aligned within a dot row then preferably the image prints without data shift. In one other preference, the pens print respective ink types; in this regard a particular preference is that the inks include plural colors, or alternatively plural dilutions. The invention is particularly beneficial in printing on a particular printing medium that is insensitive to relative timing of deposition of ink types; in this case an ideal print medium is plain paper. In some such situations the data shift best compensates for only part of misalignment, and pen-nozzle selections for the rest. In other situations the shifting step best compensates for all the misalignment. In another aspect, the invention extends marking element life and thereby printhead life by distributing usage over a maximum number of elements. This is accomplished by a system that checks misalignment and shifts data, as above. Alternative preferences for finding alignment include data encoded on pens and a reader of the encoded data; or a system that uses the pens to print a test pattern and reads it to find alignment. A hardware aspect of the invention includes parts of a processor programmed to check alignment and print with essentially all nozzles, taking alignment into account.

Description:
RELATED PATENT DOCUMENTS  
       [0001]    A closely related document is another, coowned U. S. utility-patent application filed in the United States Patent and Trademark Office substantially contemporaneously with this document—and also hereby incorporated by reference in its entirety into this document. It is in the names of Askeland et al., identified as Hewlett Packard Company docket number PD-10982150-1, and entitled “ADAPTIVE INCREMENTAL-PRINTING MODE THAT MAXIMIZES THROUGHPUT WHILE MAINTAINING INTERPEN ALIGNMENT BY NOZZLE SELECTION”—subsequently assigned utility-patent application Ser. No. 09/______, and ______, issued as U.S. Pat. No. 5,______ on ______, 200______. 
     
    
     
       FIELD OF THE INVENTION  
         [0002]    This invention relates generally to machines and procedures for incremental printing of text or graphics on printing media such as paper, transparency stock, or other glossy media; and more particularly to alignment provisions in a scanning machine and method that construct text or images from individual ink spots created on a printing medium, in a two-dimensional pixel array. The invention employs threshold printmode techniques to optimize image quality vs. operating time.  
         BACKGROUND OF THE INVENTION  
         [0003]    The foregoing statement of the field of the invention speaks of “threshold” techniques in the printmode area. This term is used merely to emphasize that the printmode techniques required for practice of the present invention are extremely basic—almost primitive, in comparison with the very sophisticated present state of the printmode or printmasking art.  
           [0004]    For example, the related-art section of U.S. Pat. No. 5,677,716 of Cleveland, filed seven years ago, discusses space-rotation, sweep-rotation and autorotation of masks. More-recently filed patent documents introduce manual pseudorandomization, “neighborhood” conditions, balanced randomization and determinism, automatic generation, and real-time generation of printmasks—including columnwise mask generation within each swath, “precooked masks” and “pop-up” masks.  
           [0005]    Although entirely compatible with all of the advanced techniques just noted, the present invention by comparison actually needs only the most simple or pedestrian tools of the printmode art—little more, in fact, than the concept of a printmask height. Therefore, as will be seen, these techniques are indeed at the threshold of the print-mode arena; yet their use in the present invention confers potent advantages.  
           [0006]    (a) The need for registration—Many incremental printers use more than one array of multiple printing elements, such as for example multipen inkjet printers. Most often the different arrays (“pens” or “printheads”) print in different colors, including black; however, in certain cases some of the different arrays print in different dilutions or saturations of common colors. Other uses of plural arrays may occur.  
           [0007]    In such printers it is necessary that markings made by the different arrays be in register with one another. At least markings should be adequately registered to prevent a human viewer from seeing—with the unaided eye—the effects of misregistration.  
           [0008]    In general, however, such systems are subject to mutual misalignment of the arrays and therefore misregistration of the markings. Various different kinds of provisions are known for reducing such misalignment to the point at which registration at least satisfies visual requirements.  
           [0009]    (b) Tight tolerances—For convenience and definiteness such provisions can be described with reference to multipen inkjet printers, and more particularly thermal-inkjet units of the Hewlett Packard Company—such as the models known as PaintJet®, DeskJet® 1200C and HP® 2000C. In some of these printers, alignment is achieved mechanically: the pens are aligned to within a dot row, by virtue of individual machining to achieve fine mechanical tolerances.  
           [0010]    As a result these printers can print with all their nozzles, and with straightforward good alignment along the direction of printing-medium advance. This is true for the 7 dot/mm (180 dot/inch or “dpi”) PaintJet printer, and for the 12 dot/mm (300 dpi) model 1200C printer.  
           [0011]    It will be understood, however, that this solution to good registration is achieved only at very great cost, since the machining required is time consuming and costly. Furthermore, this solution is commercially feasible only because spacing of the dot rows at {fraction (1/7)} or {fraction (1/12)} mm is relatively coarse in terms of machine-tooling standards.  
           [0012]    (c) Reserved elements—In the HP 2000C, resolution is 24 dot/mm (600 dpi) and at such fine spacings mechanical machining begins to be an uneconomic way to achieve registration. Instead, this printer uses a pen alignment scheme that reserves eight nozzles of each pen for pen-to-pen alignment.  
           [0013]    In that system, nominally four nozzles  13 K,  15 K (FIG. 1) are reserved at each end of the black-ink pen K, and only the nozzles  14 K between those two end zones are employed to print. Fixed upper and lower limits  11 ,  12  are established, reflecting these assumed reservations—and these fixed limits  11 ,  12  are applied to the color pens C, M, Y as well as the black pen K.  
           [0014]    With drift away from the nominal mechanical alignment, however, for example a particular pen, e. g. the cyan pen C as shown, might be two pixel rows lower than its nominal position, and this would require use of two nozzles from the top nominal end zone. This would leave only two nozzles  13 C actually reserved at the top, while in exchange the system would give up two nozzles just above the bottom nominal end zone, leaving unused six nozzles  15 C as the bottom end zone. The central group of used nozzles  14 C still has the same size in nozzles, but these used elements are shifted upward along the nozzle array by a distance equal to two nozzle spacings—which most commonly (though not necessarily) is two pixel rows.  
           [0015]    In practice any such possibility may occur for any of the pens, including the black pen K or the magenta or yellow pen M, Y rather than the cyan pen C. The particular pattern illustrated, with the magenta pen M shifted upward about six rows and the yellow pen Y at the nominal position, is purely exemplary.  
           [0016]    In any event, eight nozzles are always sacrificed to the needs of alignment, and in practice the limit lines  11 ,  12  are fixed in position relative to the world, or in other words to the pen carriage. The total number of nozzles in each pen is  304 ; therefore the maximum number of these printing elements that fall within the central regions  15  and can be used is 296.  
           [0017]    The remaining 8/304 or about 2.7 percent of the nozzle complement is abandoned at the outset. (Of course different reserved and total numbers of nozzles may be present in different models or from different manufacturers; these values are simply exemplary.) From these numbers it can be seen that the end zones  13 ,  15  of all the pens have been drawn greatly exaggerated relative to the corresponding central regions  14 , simply to facilitate clear discussion.  
           [0018]    To a person not skilled in this field, such a seemingly small fraction might not appear significant. In this extremely competitive commercial environment, however, these numbers represent a major handicap—for two reasons.  
           [0019]    (d) Direct cost—First, the provision of eight additional nozzles is far from a small matter. Each nozzle in the nozzle plate is not merely a hole in an amorphous structure, but to the contrary must be accompanied by ink-provision and firing components within the printhead—and these facilities are fashioned in what is the equivalent of a multilayer silicon circuit chip.  
           [0020]    The cost of printed-circuits and the like formed in silicon is notoriously expensive, to the extent that common slang in industry refers to silicon “real estate”. In addition, the cost of expanding the amount of space used in such a structure is not even linear: the bigger the chip, the more expensive per unit area because of the greater difficulty of working larger chips and the progressively escalating scrap factor.  
           [0021]    (e) Hidden costs—The first major handicap of cost does not end there, however, for the silicon real estate carries its operating environment along with it in an ever-widening series of ripple effects. The connecting circuitry must have more contacts, and the print zone must be therefore longer and more expensive.  
           [0022]    So must the printer chassis—and accordingly the shipping box. In turn the space and therefore the cost of shipment and inventory storage are implicated as well.  
           [0023]    (f) Performance: speed—The second major handicap, even more severe, is reduced throughput. Since the marking-element (e. g. nozzle) array height is in a sense artificially restricted, each swath of marks is likewise restricted in height. A greater number of such swaths is therefore needed to cover any given image height (for instance, a full page) on the printing medium.  
           [0024]    Each swath is marked in a single respective pass of the marking-element array (“pen”, or printhead) across the sheet of printing medium. The amount of time required to make each pass the marking-element array across the sheet is essentially independent of the array height.  
           [0025]    Therefore the restricted array height can be translated into a greater number of passes per image or page, and a longer time to make those passes. In short, the number of minutes per page increases and the manufacturer&#39;s advertised page-per-minute performance figure falls.  
           [0026]    The significance of this is well known to anyone who has ever glanced at a display of competing printers in a retail store. The number of pages printed per minute or hour is one of the two or three most heavily emphasized and most conspicuous characteristics of a desktop computer printer.  
           [0027]    While a 2½% shift alone may not be visible in the published information, naturally several such factors from different causes do become visible and significant. Hence the importance of the eight unused nozzles in terms of performance.  
           [0028]    The actual impact of a 2.7% loss of nozzle complement is likely to be extremely nonlinear. For example, in a two-pass printmode if a system comes up just four nozzles short, two additional entire passes must be made to complete the pattern.  
           [0029]    At the other extreme, if a system in a two-pass mode happens to reach the end of a pattern and only needs, say, half the nozzles in the printhead for the final swath, then loss or gain of four nozzles might be entirely inconsequential. Since neither of these extreme kinds of circumstances can be predicted generally in advance, it can only be said that the number of available nozzles is very important.  
           [0030]    In many practical cases, just a few nozzles more or less can assume a throughput importance out of all proportion to their number. This is surely true in human terms, taking into account the engineering effort regularly devoted even to merely trying to evaluate that importance.  
           [0031]    When an overall printhead is just one-half inch tall (including any nozzles that are candidates for reservation) and the image is an integral multiple of a half inch, loss of only two or three nozzles can regularly trigger a requirement for additional passes. This is an adverse case which occurs with particular frequency—because half-inch nozzle arrays are currently in favor, and many desired images are in integral-inch (e. g., eight by ten) sizes.  
           [0032]    (g) Performance: life—A related phenomenon is the effect on useful life of each printhead—now the number of pages that can be printed before the head wears out. This is particularly important for printheads that are refillable after the ink supply is exhausted.  
           [0033]    When the number of swaths per page rises, the number of rows on each page that must be printed by each nozzle also rises. This means that a greater part of each nozzle&#39;s useful life must be expended on each page, and so lowers that life in terms of number of pages.  
           [0034]    When several nozzles are worn out, the entire head must be discarded. Hence a 2½% reduction in nozzle complement translates directly into a 2½% reduction in pen life.  
           [0035]    (h) Statistical waste—The reserved-nozzle approach to alignment is ingenious and extremely useful in comparison with machining to better than {fraction (1/24)} mm. Nevertheless, as has now been shown, that approach is squeezed by two pressures: performance penalties suffered by failing to use all the silicon real estate (and its supporting installed environment) that is actually in place and paid for, and high mechanical cost of adding more capacity to compensate.  
           [0036]    If either of these is allowed to control, however, then alignment suffers. This is a particularly acute aggravation because statistically, given the nature of tolerances and departure from nominal conditions, usually the most likely condition is the nominal one—i. e., the used nozzles being the 296 in the middle.  
           [0037]    The least likely condition is the extreme case of the used nozzles extending all the way to one or the other end of the array. The intermediate two cases are in general of intermediate likelihood.  
           [0038]    Hence the major fraction of the eight-nozzle sacrifice outlined above is made on behalf of a relatively minor fraction of the actually occurring cases of misalignment. To put it another way, the minimum nozzle loss is 8/304 or 2.7 percent—regardless of the actual amount of mechanical misalignment.  
           [0039]    Every user must pay the full penalties outlined above, even though the printer owned by a representative user has an actual need for perhaps less than half of those penalties.  
           [0040]    (i) Conclusion—These registration problems have continued to impede achievement of uniformly excellent incremental printing—at high throughput and very low cost—on all industrially important printing media. Thus important aspects of the technology used in the field of the invention remain amenable to useful refinement.  
         SUMMARY OF THE DISCLOSURE  
         [0041]    In preferred embodiments of its first major independent facet or aspect, the invention is a method of printing an image from image data, using plural pens that in general are not perfectly aligned. The method includes the step of determining pen-to-pen mechanical misalignment.  
           [0042]    It also includes the step of, based upon the determined misalignment, automatically shifting the data to compensate for at least a portion of the determined misalignment. In addition the method includes the step of automatically printing with the shifted data.  
           [0043]    The foregoing may represent a description or definition of the first aspect or facet of the invention in its broadest or most general form. Even as couched in these broad terms, however, it can be seen that this facet of the invention importantly advances the art.  
           [0044]    In particular, this facet of the invention obviates the need to sacrifice usable nozzles in return for alignment between pens.  
           [0045]    Although the first major aspect of the invention thus significantly advances the art, nevertheless to optimize enjoyment of its benefits preferably the invention is practiced in conjunction with certain additional features or characteristics. In particular, preferably if the determining step establishes that the pens are aligned within a dot row, then the printing step prints without shifting the data.  
           [0046]    Another preference is that the plural pens print respective plural kinds of ink. In this case the plural kinds of ink respectively include plural ink colors—and, still more preferably, plural ink dilutions. It is also preferable in this case that the invention be for use with a particular printing medium that is substantially insensitive to relative timing of application of the plural kinds of ink respectively.  
           [0047]    For such sequence-sensitive inks, the ideal medium in this regard is plain paper. This preference is valid for sequence-sensitive inks generally, but in particular for inks such as typically used in inkjet photographic-quality incremental printers—and most particularly those manufactured by the Hewlett Packard Company.  
           [0048]    Also with sequence-sensitive inks one preference is to design the shifting step to compensate for only part of the determined misalignment. In this situation it is also preferable to employ pen-nozzle selections to compensate for at least part of the rest of the misalignment. One common ramification of resorting to such nozzle selections is a small but significant loss of throughput.  
           [0049]    On the other hand, it is also possible to configure the shifting step to compensate for all of the determined misalignment. Depending on the degree of sensitivity of the inking system to deposition sequence, this way of operating may be adequate with particular printing media—and if so then this operating mode is preferable, as it does not entail any of the throughput loss just mentioned.  
           [0050]    In preferred embodiments of its second major independent facet or aspect, the invention is a printer for printing an image from image data. The printer includes plural printheads, each having a multiplicity of marking elements.  
           [0051]    Each element is subject to deterioration and shortening of operable life through use, and the plural heads are subject to mechanical misalignment.  
           [0052]    The printer also includes some means for extending the life of the marking elements—and thereby the life of the printheads. For purposes of generality and breadth in discussing the invention, these means will be called simply the “life-extending means”.  
           [0053]    The life-extending means accomplish their life-extending effect by distributing use of the marking elements over a maximum number of marking elements. The life-extending means include some means for automatically shifting the data to compensate for at least a portion of the misalignment.  
           [0054]    The foregoing may represent a description or definition of the second aspect or facet of the invention in its broadest or most general form. Even as couched in these broad terms, however, it can be seen that this facet of the invention importantly advances the art. In particular, this second aspect of the invention helps counters the problem of limited printhead life, which of course is a concern to all users.  
           [0055]    Although the second major aspect of the invention thus significantly advances the art, nevertheless to optimize enjoyment of its benefits preferably the invention is practiced in conjunction with certain additional features or characteristics. In particular, preferably the life-extending means further include some means for automatically determining the mechanical misalignment.  
           [0056]    In this latter case, several valuable alternative preferences obtain. In one of these, the alignment-determining means include alignment data encoded on the pens, and some means for reading the encoded data. In another, the alignment-determining means instead include some means for using the pens to print a test pattern, and some complementary means for reading the test pattern to determine the pen alignment therefrom.  
           [0057]    In preferred embodiments of its third major independent facet or aspect, the invention is a printer for printing an image from image data. The printer includes plural pens, each pen having multiple nozzles.  
           [0058]    The pens in general are not perfectly aligned. The printer also includes some means for determining pen-to-pen physical misalignment.  
           [0059]    Also included are portions of a processor programmed to automatically print the image using substantially all of the multiple nozzles of all of the pens. The processor is programmed to take into account the determined alignment.  
           [0060]    The foregoing may represent a description or definition of the third aspect or facet of the invention in its broadest or most general form. Even as couched in these broad terms, however, it can be seen that this facet of the invention importantly advances the art.  
           [0061]    In particular this third aspect of the invention, like the first, in suitable circumstances eliminates the alignment-driven need for discarding a significant part of the nozzle complement. This is a significant advance, because nozzle “real estate” —and corresponding capacity on the underlying silicon chip used for firing control are limiting factors in the cost of inkjet pens.  
           [0062]    Although the third major aspect of the invention thus significantly advances the art, nevertheless to optimize enjoyment of its benefits preferably the invention is practiced in conjunction with certain additional features or characteristics. In particular, preferably the processor portions are programmed to provide a relative shift of data for the plural pens, to compensate for imperfect alignment of the pens.  
           [0063]    In this case ordinarily the physical misalignment includes relative displacement of the pens toward lower or higher positions. A corresponding preference is that the data shift include raising data for a lowest pen or lowering data for a highest pen, or both.  
           [0064]    Another preference is that the determining means include alignment data encoded on the pens, and some means for reading the encoded data. In this case the determining means preferably further include carriage datum-point alignment calibration data ascertained at manufacture of the apparatus, and portions of a processor programmed to automatically take into account the carriage datum-point alignment calibration data.  
           [0065]    Yet another preference is that the determining means include some means for using the pens to print a test pattern. This preference also includes provision of corresponding means for reading the test pattern, to determine the pen alignment from the read pattern.  
           [0066]    Alternative preferences in this regard include using the pens to eject drops, under automatic processor control, and a drop detector to determine their location. For example a shutter and optical sensor may preferably serve as such a detector.  
           [0067]    All of the foregoing operational principles and advantages of the present invention will be more fully appreciated upon consideration of the following detailed description, with reference to the appended drawings, of which: 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0068]    [0068]FIG. 1 is a diagram of a representative group of pens with representative mechanical misalignments (greatly exaggerated for clarity), and masked off to obtain effective alignment by reservation of nozzles according to the prior-art scheme discussed above;  
         [0069]    [0069]FIG. 2 is a like diagram of a novel system and method that maximize throughput while maintaining effective interpen alignment by shifting image data, to print with physically unaligned nozzles;  
         [0070]    [0070]FIG. 3 is a like diagram of another novel system and method that employ an adaptive printing mode to maximize throughput while maintaining interpen alignment by nozzle selection, or in other words partial reservation of nozzles, but without shifting the data;  
         [0071]    [0071]FIG. 4 is a like diagram of the overall data sets for two colors of an entire image, and also showing the data subsets automatically chosen for a particular swath in both the FIG. 1 prior-art system and FIG. 3 novel system;  
         [0072]    [0072]FIG. 5 is a like diagram of overall data sets and swath subsets according to the FIG. 2 novel system;  
         [0073]    [0073]FIG. 6 is a block-diagrammatic representation of a hardware system according to the FIG. 2 or  3  operation;  
         [0074]    [0074]FIG. 7 is a like representation of an alternative form of a portion of the FIG. 6 system, namely a drop-detector form of the alignment-determining means;  
         [0075]    [0075]FIG. 8 is a further elaboration of another portion of the FIG. 6 system, namely provision for incorporating carriage-bay misalignment information into the encoded-pen form of the alignment-determining means; and  
         [0076]    [0076]FIG. 9 is a flow chart showing method aspects of the FIG. 2 or  3  operation. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0077]    1. The Data-Shift Approach  
         [0078]    In one novel system and method, no nozzles at all are reserved. The top ends  21  (FIG. 2) of the used-nozzle sections  24 , for the same exemplary pen misalignment pattern as illustrated and discussed earlier for the prior art, are no longer at a common level (as at  11 , FIG. 1).  
         [0079]    Rather, the top ends  21 K,  21 C,  21 M,  21 Y are independent of one another and coincide with the extreme top ends of the overall nozzle arrays K, C, M, Y. In general the top ends  21 K,  21 C,  21 M,  21 Y of the used-nozzle sections can be at four different positions, but naturally they can also be aligned—as is the case for the black and yellow pens K, Y in the illustration—if the pens themselves happen to be aligned.  
         [0080]    Analogous relationships hold for the bottom ends  22 K,  22 C,  22 M,  22 Y of the used-nozzle sections. Those sections themselves  24 K,  24 C,  24 M,  24 Y thus occupy the entire arrays, enabling complete elimination of the 2.7 percent, eight-nozzle waste for the exemplary system.  
         [0081]    To make these improvements possible, for each color the image data are shifted with respect to the corresponding pen, independently—to compensate for the misalignment. For each color the selection of data for use in each swath is also similarly shifted.  
         [0082]    Preliminarily, consider the relationships between the black and cyan image data  36 K,  36 C (FIG. 4) for the prior-art reserved-nozzle system. In that prior-art system and method, all four image-data sets are considered to be aligned. This is shown exemplarily for the black and cyan planes  36 K,  36 C only, the other two data sets (not shown) for magenta and yellow being identical in both size and position.  
         [0083]    The data subsets  37 K,  37 C for any given swath, too, are likewise aligned, as is the case for the other two corresponding data subsets (not shown) for magenta and yellow. Thus the top edges of the two representative data subsets  37 K,  37 C have a common level  11 , and the bottom edges too have a common level  12 —these being the same artificially imposed top and bottom common cutoff points  11 ,  12  identified in FIG. 1 and the earlier discussion.  
         [0084]    In the novel system of FIG. 2, by comparison, the cyan data for transmission to the pen in each swath must be higher, by the same distance that the cyan pen itself is lower. It is necessary that every original pixel row of cyan data must align, on the printing medium, with the original corresponding pixel row of black data. Since the pens are misaligned, all the data must be oppositely misaligned to compensate.  
         [0085]    This is accomplished by shifting the cyan data  38 C as they are to be fed (FIG. 5) to the cyan pen C, in the opposite direction from the unintended shifting of that pen itself. Hence, when the cyan pen C is lower than the black pen K by some amount illustrated in FIGS. 1 and 2, the going-to-the-pen cyan data  38 C must be higher than the black data  38 K by an equal but opposite offset  39 .  
         [0086]    This shift completely corrects for data misalignment so that when the deposited cyan data  38 C′ are considered, i. e. the cyan markings as they are printed on the printing medium, the downward pen shift and upward data shift cancel out. The cyan data  38 C′ therefore appear on the printing medium in register with the corresponding black data  38 C.  
         [0087]    As FIGS. 4 and 5 confirm, each swath being printed is significantly taller than the corresponding height in the reserved-nozzle approach. (Again, the improvement is exaggerated for clarity of illustration.)  
         [0088]    The data-shift approach requires reevaluation of the necessary synchronizing data-shift distances whenever any of the pens is replaced, but only for that particular pen. Hence the necessary measurements or determinations, and corresponding calculations, need not be performed for every image or page, or even each time the printer is turned on, but only on occasions that—for most users—occur very infrequently.  
         [0089]    2. Limitations of the Data-Shift Approach  
         [0090]    What can cause problems, however, is that deposition order of different colors, within the few pixel rows of data shift along the tops or bottoms of swaths, is now different from the normal deposition order. The result can be noticeable hue or saturation shifts, or other inking peculiarities of sheen etc., in these shallow bordering regions.  
         [0091]    For example, in the example of FIG. 5 the solid-outline portion of cyan data  38 C that is above (i. e. outside) the hatched portion of cyan swath  38 C′ is correctly positioned relative to (namely, above) the top of black swath  38 K. It is not, however, printed in the same pass as that black swath  38 K—as it would be in the conventional approach.  
         [0092]    Instead it is printed in the preceding pass. Reversal of color deposition order will occur in such regions—or in analogous regions below the bottoms of the original swaths, depending on the order of the pens on the carriage and the order of carriage motion.  
         [0093]    Where hue or analogous shifts happen to be conspicuous, within the context of subject matter in a particular color image, distinct banding artifacts can appear in the image. Accordingly it is important to determine the conditions under which such shifts are noticeable.  
         [0094]    For the product example discussed above, it has been observed that hue shifts and related phenomena are insignificant when printing on plain paper. For other types of printing medium such as special glossy media, transparencies, or other plastic media, however, in general these color effects are objectionable and this novel system and method are usually best avoided.  
         [0095]    Again, these statements are true for the particular printing medium under discussion. Results should be tested for each new product environment, printing medium, and other major variation of operating conditions which is contemplated.  
         [0096]    3. The Adaptive Approach  
         [0097]    For such circumstances in which the data-shift method is not acceptable, or more generally whenever preferred, another novel method and system can be used instead. This alternative is not sensitive to deposition sequence, but compared with data shift does not produce as great a benefit—or in fact as consistent a benefit—in terms of saved pixel rows per pass.  
         [0098]    This second novel approach proceeds according to a principle of using all the commonly aligned nozzles that are available for each actual multipen set in each actual printer. This principle replaces the earlier philosophy of establishing a simple, common and consistent nozzle complement for all pens and all printers.  
         [0099]    As will be recalled, in the data-shift approach the top end  21  of the usable portion  24  of every pen K, C, M, Y is actually at the top end of that respective pen itself. In the second novel approach, that condition cannot be met for every pen, but in general can be provided for at least one pen—namely, the pen (C in the FIG. 3 example) which is lowest.  
         [0100]    Thus the top end  31  (FIG. 3) of the usable portion  34 C of the lowest pen, here the cyan pen C, is at the overall top end of that cyan pen. In other words, for that pen no nozzle is sacrificed to alignment. The top ends  31  of the usable portions of the other three pens K, M, Y are defined by the top end of the cyan pen C, so that the top ends  31  of the usable portions of all four pens are coincident.  
         [0101]    If more than one pen is aligned at the lowest possible position, then the top conditions just described can be satisfied for all those pens which have that alignment. This is so even if all four pens are aligned together.  
         [0102]    Analogously the bottom ends  32  of the usable portions of all four pens C, M, Y, K are defined in common by the bottom end of the pen which is highest—here the magenta pen M. No nozzle is sacrificed for alignment at the bottom of the magenta pen, and the usable portion  34 M of that pen coincides with the overall bottom end of that magenta pen M.  
         [0103]    Also analogously with the top-end conditions, the bottom-end conditions just described can be met by whatever number of the four pens have the same highest alignment in common. This is true even if the four pens are all aligned in common at the highest position.  
         [0104]    In the latter case there is no misalignment at all, and the entire height of every pen becomes the common usable portion  34 K,  34 C,  34 M,  34 Y. This condition is illustrated in FIG. 3 in the sense that the general showing of endzones  32 K,  32 M,  32 Y,  34 K,  34 C,  34 Y encompasses every possible endzone height from eight nozzles down through zero inclusive.  
         [0105]    Now it can be appreciated that the usable portions  34  of the four pens K, C, M, Y extend from the top  31  of the lowest pen C down to the bottom of the highest pen M. To the extent that the lowest and highest pens e. g. C, M are not perfectly aligned, bottom and top endzone portions  32 C of the lowest pen and  31 M of the highest pen, respectively, are sacrificed to obtain useful effective alignment.  
         [0106]    To the extent that only one pen C is lowest and only one pen M is highest, both the top and the bottom endzone portions  31 K,  31 Y,  32 K,  32 Y of the other two pens too are sacrificed. In short, this approach adapts the effective height of the nozzle array to the common available nozzles in the four pens, which is controlled by interpen alignment—hence the phrase “adaptive printing mode”.  
         [0107]    4. Benefits and Limitations of the Adaptive Approach  
         [0108]    A merit of this approach is that every printer, taking into account the particular printheads that are installed, and their relative alignments, receives the maximum possible usable nozzle-array height  34 . As FIG. 3 shows, the usable portion cannot be extended further upward—because in that direction there is no additional usable nozzle of the lowest pen C—and conversely for extension downward, with respect to the bottom-most usable nozzle of the highest pen M.  
         [0109]    As this approach requires no data shift, FIG. 5 is not applicable. The illustration of FIG. 4, however, includes an indication of the data effects of this second novel approach: each adaptive-mode swath  37 K′,  37 C′ etc. (drawn hatched in FIG. 4) is in general taller than the corresponding prior-art swath  37 K,  37 C etc.  
         [0110]    These swath heights, however, are not consistent among printers, or even among different pen combinations in any given printer, for they depend upon the uncontrolled variations of pen alignment as explained above. These swath heights vary within a range between the same, restricted height of the prior-art swaths  37 K,  37 C—as a minimum—and the unrestricted height  38 K,  38 C of the data-shift method as a maximum.  
         [0111]    A drawback of this adaptive approach is that print-masking must vary with the number of nozzles actually available in common on the pens in use. Since pens are replaced from time to time in nearly every printer, the programmed processor that operates the printer must be capable of selecting or generating suitable printmask sets to match the pen set currently installed.  
         [0112]    This requirement is not unduly complex. As mentioned at the outset, it is merely at the threshold of printmode techniques.  
         [0113]    Still, this requirement in general may require more printmask storage capacity, if the printer relies upon masks provided at the factory—or more computing power, in the case of masking generated by the printer itself. This topic of required storage or power is potentially very wide-ranging because in some printers the printmask used is very small and is tiled over the image, and even some relatively complicated masks though relatively wide are not very tall, etc. Furthermore some infrastructure of hardware and software, or printer architecture, must be present anyway.  
         [0114]    Nevertheless it remains generally true that storing or being able to generate several different masks, for use whenever the invention calls for different swath heights, can sometimes add to the overall required silicon-chip size in a printer. Such requirements should be taken into consideration.  
         [0115]    Although the art of printmask generation as such continues to be subject to ongoing refinement and important advances, variation of masks with swath height is well understood and is generally independent of the method used for generating the masks. Therefore implementation of this second novel nozzle-saving approach is well within the state of the art as to needed accommodations in print-masking.  
         [0116]    5. Hardware  
         [0117]    The present invention is embodied in, for example, incremental printers of the thermal-inkjet type. Such printers have been disclosed and illustrated in many patent documents coowned with the present document, and in many others; accordingly, general information about such devices need not be repeated here.  
         [0118]    Hardware for implementing the novel approaches outlined above include, first, the generally misaligned pens or printheads K, C, M, Y (FIG. 6), each with their respective arrays of nozzles or printing elements  41 . Also required is provision for determining relative alignment or misalignment of the pens and their element arrays, and this provision can take any of several different forms.  
         [0119]    One example of such provision is encoded information  42  associated with each pen, conveying the alignment information—in combination with one or more readers  43  for reading the information  42  from the pens. Thus for instance the information may be simply printed in ordinary numerical form  42  on the pen, and the readers may be optical numeral readers  43  disposed or moved to read those numerals.  
         [0120]    Alternatively the indicia may be in bar-code form  42  and the readers optical bar-code readers  43 , or the indicia may be magnetically impressed on a magnetic element  42  affixed to the pens and the reader may be a magnetic-strip reader  43 . Any of such systems in the aggregate make up a first type of alignment-determining means  44 A; these means cooperate with reader-control portions  42  of a programmed processor  51 , in collecting the alignment information to a data path  61  where it can be received in a final printing control block  55  of the processor  51 .  
         [0121]    When this type of determining means  44 A is used, it may also be desirable or necessary to include auxiliary provision for taking into account possible misalignment contributions  81  from the alignment datum points  82 - 85  that locate the printheads K, C, M, Y in the carriage bays  72 - 75 , and move those heads across the printing medium. (For definiteness the datum points  82 - 85  are illustrated as discrete pads, although in practice they may be merely surfaces of the respective bays  72 - 75 .)  
         [0122]    Such auxiliary provision may take the form of misalignment measurement data  81  recorded at the factory and deposited in electronic storage  76  that is part of or otherwise associated with the programmed processor  51  mentioned earlier. At printing time the processor interrelates these locating-datum alignment data  81  with the pen alignment data  42  encoded on the pens K, C, M, Y and recovered through the readers  43  as mentioned above—all information from the print engine  70 —to generate overall comparative alignment information for use in one or the other of the novel approaches introduced above.  
         [0123]    Using this overall information, the processor  51  develops or implements printmode details which eventuate as control signals  57 ,  58  (FIGS. 6 and 8). These signals pass back into the print engine  70  for firing the pens and operating the printing-medium advance mechanism  59 .  
         [0124]    A different strategy for determining alignment generates information in real time, rather than through factory measurements and encoding. This strategy may be effected by portions  53  of the processor  51 , programmed to control firing  45  of the pens K, C, M, Y to mark test patterns  46  onto a printing medium  48 . The hardware implementing this strategy makes up alternative alignment-determining means  44 B (FIG. 6).  
         [0125]    Other portions  54  of the processor  51  operate one or more line readers  49 , which optically sense details of the test patterns  46 —to generate corresponding detector signals  62  that in turn pass to the print-control block  55 . The test patterns  46  can be designed and the signals  62  interpreted in the ways taught by U.S. Pat. No. 5,600,350 to Cobbs et al., U.S. Pat. No. 5,796,414 to Sievert et al., and U.S. Pat. No. 5,980,016 to Nelson et al.—all three coowned with the present document.  
         [0126]    An alternative to the test patterns  46  and line sensors  49  as such is a combination of shutter  47 ′ (FIG. 7) and drop detector  49 ′. These receive the inkdrop stream  45  and in response generate corresponding alignment data for passage to the print-control block  55  within the processor  51 .  
         [0127]    The alignment information obtained in any of these real-time, inking-measurement strategies is used substantially as in the encoded-data strategy. Here, however, it is not necessary to correct for misalignment of the carriage-bay datum points  72 - 75 .  
         [0128]    Whatever alignment-determining strategy is employed, the resulting computations within the final printing-control block  55  are used in a printmasking subblock to generate the final firing-control signals  57 ,  58  (FIG. 6 and  8 ). These signals operate the pens and printing-medium advance mechanism in the print engine  70 , to print the desired image.  
         [0129]    6. Printhead Life Extension  
         [0130]    Both the data-shift and the adaptive approaches have benefits other than greater throughput. In particular they have the effect of allocating or distributing the printing work over a larger number of marking elements, e. g. nozzles—and thereby extending the life of those elements.  
         [0131]    Accordingly the several portions of the apparatus (FIGS. 6 through 8) illustrated and discussed above function as means for extending the life of the marking elements and thereby the life of the printheads, by distributing use of the marking elements over a maximum number of marking elements.  
         [0132]    Thus the life-extending means include means  44 A,  44 B,  47 ′,  49 ′,  52 - 54 , for printing with the maximum number of nozzles that can be used while printing with the used nozzles of all pens substantially aligned—whether by data shift or by adaptation. They print in this way based on known pen-to-pen mechanical misalignment.  
         [0133]    In the case of the adaptive approach, this is accomplished substantially without relative shift of respective data for the plural pens. The life-extending means further include the means for automatically establishing the mechanical misalignment.  
         [0134]    7. Method  
         [0135]    From the foregoing, those skilled in the art will understand the method as well as the apparatus aspects of the invention. For completeness FIG. 8 shows representative program flow for practice of the method aspects of the invention.  
         [0136]    This flow chart will be essentially self explanatory in its presentation of the determining or establishing step  101 , a query step  102  that leads to branching  103 / 108  and eventually printing  107  with the nozzles available under the particular novel approach selected. The automatic ascertaining step  104  displays the disjunctive condition “with/out data shift”—meaning that the indicated ascertaining is performed under the assumption that:  
         [0137]    (1) there is data shift, for the data-shift approach; or  
         [0138]    (2) there is not data shift, for the adaptive approach.  
         [0139]    The ascertaining step includes a nozzle-elimination substep  105  as shown. The printmode-using step  106 , as mentioned earlier, although novel in the sense that it is first used in the novel approaches described above, is entirely straightforward and well within the state of the printmode or printmasking art. It entails simply preparing one or more printmasks suited to the known, to-be-used height of the marking-element array, and in the case of the data-shift approach also incorporating the simple data shifts previously described.  
         [0140]    The above disclosure is intended as merely exemplary, and not to limit the scope of the invention—which is to be determined by reference to the appended claims.