Title of Invention	INK JET FAULT TOLERANCE USING OVERSIZE DROPS
Abstract	A printing method identifies where parts of an image will not be printed correctly due to partial or total device failure and if possible adjusts the size of ink dots in adjacent rows or columns so as to lessen the visual effect of failure to print at the original location. FIG 3

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INK JET FAULT TOLERANCE USING OVERSIZE DROPS Field of the Invention This invention relates to digital printing and more particularly to printing sing devices which eject ink onto the printed substrate. However, the invention is ot limited to ink ejection devices and is also applicable to laser, light emitting diode printers and to digital photocopiers. background of the Invention In ink ejection devices a printhead has an array of nozzles through which ink 5 selectively ejected onto the substrate as the substrate moves relative to the printhead. The printhead may print by scanning across the substrate to print horizontal bands or. if it is a full page width printhead, it may pass along the length if the page. A blocked nozzle will result in multiple horizontal blank lines, in the ase of a scanning type printhead. or a blank vertical line in the case of a page width printhead. Such blank lines are undesirable since they detract from the printed result. The present invention provides a method of modifying the printing of an mage so as to reduce or effectively eliminate the visual effect of one or more such blocked nozzles apparent to the eye of an observer in normal use. However, the nvention is applicable to other forms of printing where a device, whether passive or ictive, is repeatedly used to produce dots of ink or the like on a substrate. The nvention has potential application to laser and LED type printers and photocopiers vhere a fault in the imaging drum or light source can result in repeated faults in the mage produced. As used above and throughout the description and claims the term mage is to be understood to have a broad meaning and includes anything printed. ;uch as text and line drawings. Disclosure of the Invention In one broad form the invention provides a method of modifying an image to be digitally printed by a printing device to compensate for failure to correctly print dots of ink at specific locations, the method including the steps of: a) identifying said specific location or locations, and b) adjusting the dot size of at least one a dot at a location adjacent or near to the respective specific location from that required by the image data. In another broad form the invention provides a printer having a row of activatable devices which, when activated, cause rows of dots to be deposited onto a substrate and means to move the substrate relative to the row of devices in a direction generally perpendicular to the row of dots, said printer including: a) means to determine if one or more of said devices is not operating correctly; and b) control means for analysing images or image data and for identifying a specific location or locations where a dot of ink should be printed by activation of a incorrectly operating device and for adjusting the size of dot produced by one or both of the devices on either side of the failed device. The incorrectly operating device will result in a defect line or lines in the image printed. Usually the incorrectly operating device will produce no ink or not enough ink and so a blank or faint line will be produced. To compensate adjacent ink dots will be caused to be larger than required by the raw image data. Conversely if the incorrectly operating device is producing oversized ink dots, the dot size of adjacent dots will be reduced. Where a part of an image requires the incorrectly operating device to deposit a continuous or substantially continuous column of dots, the dots in adjacent columns are preferably all adjusted in size. If there are a small minority of locations in the column of the incorrectly operating device which do not require ink, dots in adjacent columns may or may not be adjusted in size. Dots in more than the two adjacent columns may be adjusted in size. Dots m adjacent columns may be adjusted in size only if they are within predetermined vertical or horizontal distances or both of one or more specific location. For example only dots in the columns either side of the failed column may be adjusted in size but dots in those columns two or three rows above and/or below the respective location may be adjusted in size. Brief Description of the Drawings The invention shall be better understood from the following non-limiting description of preferred embodiments and the drawings, in which Figure 1 shows a schematic illustration of a set of nozzles of an ink jet printing head. Figure 2 shows a schematic illustration of an array of ink dots formed by the printhead of Figure 1 without fault correction operational. Figure 3 shows a schematic illustration of the same array of ink dots as in Figure 2 formed by the printhead of Figure 1, but with fault correction operational. Figure 4 shows a second schematic illustration of an array of ink dots formed by the printhead of Figure 1 without fault correction operational. Figure 5 shows a schematic illustration of the same array of ink dots as in Figure 4 formed by the printhead of Figure 1 but with fault correction operational. Description of Preferred and Other Embodiments Referring to Figure 1, a printhead 10 has an array of ink jet nozzles 12 arranged in a singe line. For the purpose of explanation only 14 nozzles are shown but in practice there will be from tens to thousands of nozzles arranged in a line. Paper is passed underneath the printhead in a direction generally perpendicular to the line of ink jet nozzles, as indicated by arrow 14. The printhead may be a stationary or a movable printhead. As the paper passes under the printhead the Inkjet nozzles A to N are selectively operated to cause an array of ink dots to be placed on the paper. This array is a series of columns and rows, the spacing of which is dependent on the spacing of the inkjet nozzles and the minimum paper feed step respectively. Whilst it is preferred that the horizontal and vertical spacing of the dots is the same, this is not necessarily achievable due to the different sources of the spacing. The printhead may be a page width printhead or a smaller printhead which scans across the page to lay down a series of transverse bands of printing. For the purposes of explanation it is assumed that inkjets a-g and i-n inclusive are operating correctly but, for whatever reason, inkjet h is not operating correctly or at all. It is also assumed that the diagnostic systems of the printer, which will be well understood by those skilled in the art, have detected that nozzle h is not functioning correctly. In most cases, a malfunctioning device will be partially or totally blocked resulting in insufficient or no ink being deposited on the paper. Referring to Figure 2, which schematically shows a portion of printing performed by the printhead 10 without fault correction, there is a blank column, labelled "h' corresponding to inkjet h, whilst columns a-g and i-n have been correctly selectively printed. This leads to one or more blank lines appearing in the printing depending on whether the printhead 10 is a full page width printhead or a scanning type printhead. The unshaded circles numbered 16, 18, 20 and 22 represent drops of ink which should have been printed in column h but were not. Figure 3 shows the same image printed by the printhead 10 but with fault correction according to an embodiment of the invention operational. Referring to Figure 3 the ink drops in columns g and i are caused to be larger than normal, as will be explained below. This reduces the amount of white space between the dots and between the columns g and i. The effect is that the un-printed column h is not apparent to the eye of the user. When printingon A4 or letter size paper for reading at normal distances, such as at 20 to 30cm, the effect occurs at about 1600dpi and upwards. In the Figure 3 print, only dots intended to be printed anyway in columns g and i have been increased in size but it is within the scope of the invention that extra dots of ink, whether of normal size or of adjusted size, may be printed in the columns either side of the failed column in locations when the image data does not require a dot. As seen in figure 3 there are dots in the image at only about 50 /o of possible locations and so, even with oversize dots, there is still significant white space. This white space may be reduced by printing dots in vacant areas to reduce and/or break up the visual effect of the un-printed column mechanical type ink ejection devices the pulse width and/or driving voltage or current may be increased. The invention is also applicable to situations where individual devices are producing too much ink, in which case the adjacent devices may be adjusted lo reduce the dot size of ink dots produced. It will also be appreciated that this technique may be used with laser and LED printers and photocopiers and other types of digital printers where the placement of an ink dot is dependent on individual activation of a device or component. For example, an LED in a LED printer may fail or there may be a detect in the photoconductive imaging drum of a laser printer. In both cases, adjusting the size of adjacent dots can hide or reduce the visual effect of the defect in the device or component. In the case of a laser or light emitting device type printer dot size may be modified by modulating the intensity and or total amount of the light falling on the corresponding portion of the photoelectric imaging drum. I Claim: 1. A method of modifying an image to be digitally printed by a printing device to compensate for failure to correctly print dots of ink at specific locations, the method including the steps of: a) identifying said specific location or locations, and b) adjusting the dot size of at least one a dot at a location adjacent or near to the respective specific location from that required by the image data. 2. The method of claim 1 wherein the dot size of said adjusted dot or dots is increased if no dot or an undersize dot is printed at the respective specific location. 3. The method of claim 1 wherein the dot size of said adjusted dots is decreased if an oversize drop is printed at the respective specific location. 4. The method of claim 1 wherein dots located both transversely and longitudinally spaced from the respective location are adjusted in size. 5. The method of claim 1 wherein selected oversize adjusted dots contact or overlap adjacent dots. 6. The method of claim 1 wherein selected adjusted size dots'do not contact or overlap adjacent dots. 7. A printer having a row of activatable devices which, when activated, cause rows of dots to be deposited onto a substrate and means to move the substrate relative to the row of devices in a direction generally perpendicular to the row of dots, said printer including: means to determine if one or more of said devices is not operating correctly: and control means for analysing images or image data and for identifying a specific location or locations where a dot of ink should be printed by activation of a incorrectly operating device and for adjusting the size of dot produced by one or both of the devices on either side of the failed device. 8. The printer of claim 7 wherein the control means adjusts the size of dots deposited in the same row as the respective specific location by one or both of the devices on either side of the failed device. 16. A method of modifying an image to be digitally PRINTED BY PRINHEAD device substantially as herein described with reference to the accompanying drawings. 17. A printer substantially as herein described with reference to the accompanying drawings.

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