Patent Application
20090315939
This disclosure relates generally to imaging devices that eject ink from inkjets onto an image substrate and, more particularly, to imaging devices that detect inkjets in a printhead that are unable to eject ink to form a pixel on an image receiving member.
Drop on demand inkjet technology for producing printed media has been employed in commercial products such as printers, plotters, and facsimile machines. Generally, an inkjet image is formed by selectively ejecting ink drops from a plurality of drop generators or inkjets, which are arranged in a printhead or a printhead assembly, onto an image substrate. For example, the printhead assembly and the image substrate are moved relative to one other and the inkjets are controlled to emit ink drops at appropriate times. The timing of the inkjet activation is performed by a printhead controller, which generates firing signals that activate the inkjets to eject ink. The image substrate may be an intermediate image member, such as a print drum or belt, from which the ink image is later transferred to a print medium, such as paper. The image substrate may also be a moving web of print medium or sheets of a print medium onto which the ink drops are directly ejected. The ink ejected from the inkjets may be liquid ink, such as aqueous, solvent, oil based, UV curable ink or the like, which is stored in containers installed in the printer. Alternatively, the ink may be loaded in a solid form that is delivered to a melting device, which heats the solid ink to its melting temperature to generate liquid ink that is supplied to a print head.
During the operational life of these imaging devices, inkjets in one or more printheads may become unable to eject ink in response to a firing signal. The defective condition of the inkjet may be temporary and the inkjet may return to operational status after one or more image printing cycles. In other cases, the inkjet may not be able to eject ink until a purge cycle is performed. A purge cycle may successfully unclog inkjets so they are able to eject ink once again. Execution of a purge cycle, however, requires the imaging device to be taken out of its image generating mode. Thus, purge cycles affect the throughput rate of an imaging device and are preferably performed during periods in which the imaging device is not generating images.
Methods have been developed that enable an imaging device to generate images even though one or more inkjets in the imaging device are unable to eject ink. These methods cooperate with image rendering methods to control the generation of firing signals for inkjets in a printhead. Rendering refers to the processes that receive input image data values and then generate output image values. The output image values are used to generate firing signals for a printhead to cause the inkjets to eject ink onto the recording media. Once the output image values are generated, a method may use information regarding defective inkjets detected in a printhead to identify the output image values that correspond to a defective inkjet in a printhead. The method then searches to find a neighboring or nearby output image value that can be adjusted to compensate for the defective inkjet. Preferably, an increase in the amount of ink ejected near the defective inkjet may be achieved by replacing a zero or nearly zero output image value with the output image value that corresponds to the defective inkjet. Another method increases neighboring or nearby output image values to boost the amount of ink to be ejected by a plurality of inkjets in the vicinity of the defective inkjet. Another method is able to compensate for the defective inkjet because a normalization process may be used to establish a maximum output image value for inkjets that is less than the output value that causes an inkjet to eject the maximum amount of ink that can be ejected by an inkjet. Thus, an output image value can be increased beyond the normalized maximum output image value to enable an inkjet to eject an amount of ink corresponding to the maximum output value plus some incremental amount. By firing several nearby inkjets in this manner, the ejected ink density can approximate the ink mass that would have been ejected had the defective inkjet been able to eject the ink for a missing pixel.
The previously known methods for re-distributing the ink to be ejected by a defective inkjet to other neighboring or nearby inkjets are useful as long as the nearby inkjets and the defective inkjet are printing a generally uniform area. When a defective inkjet is located in or near an edge of an object, indiscriminate re-distribution may result in increased ink density in areas that are noticeably outside an object's boundaries. Particularly when the object is a textual character, such redistribution of the missing pixel may cause the textual character to have ragged edges. To attenuate this effect, the output image value adjustments may be constrained to those values that are adjacent to the value for the defective inkjet. A previously known method evaluated halftone areas and determined if a region in the vicinity of a defective inkjet was in a uniformly halftone area. If it was not in a uniformly halftone area, the compensating values were constrained accordingly. In this method, some areas that were not located at an object boundary were identified as being non-uniform halftone areas. In an error diffused or hybrid rendered image, this method identifies all regions as being non-uniform halftone regions. Consequently, defective inkjet compensation methods that enable more selective placement of the ink used to compensate for a defective inkjet may be useful.
A system compensates for a defective inkjet in a printhead by distributing compensation image values to other image data positions with reference to edges detected in the image data values around the image data position corresponding to the defective inkjet. The system includes an image data memory having a plurality of locations in which image data values are stored, and a processor configured to search the image data memory locations to detect one or more edges in the image data values and to modify a first ordered sequence search to identify compensation candidate locations proximate a defective inkjet image data array location in response to an edge being detected proximate to the defective inkjet image data array location.
A method is also disclosed that compensates for a defective inkjet in a printhead by distributing compensation image values to other image data memory locations in an image data memory. The method includes searching image data stored in an image data array to detect one or more edges in the image data, modifying an ordered sequence search to search for compensation candidate positions in image data array positions proximate a defective inkjet image data array position in response to an edge being detected proximate to the defective inkjet image data array position, searching the image data array positions proximate the defective inkjet image data array position in accordance with the modified ordered sequence, identifying a compensation candidate image data array position to which a compensation image value can be moved, and moving the compensation image value to the identified compensation candidate image data array position.
A computer readable medium is described below that enables a computer to perform a method for distributing compensation values in an image data memory. The computer readable medium contains instructions that, when executed by the computer, cause the computer to perform a method for detecting one or more edges in image data values stored in image data memory locations, and for modifying an ordered sequence search used to identify at least one image data memory location for storage of an image data compensation value in response to an edge being detected proximate to a defective inkjet image data memory location.
The foregoing aspects and other features of a printer that enables compensation for defective inkjet with reference to edges in images are explained in the following description, taken in connection with the accompanying drawings, wherein:
For a general understanding of the environment for the system and method disclosed herein as well as the details for the system and method, reference is made to the drawings. In the drawings, like reference numerals have been used throughout to designate like elements. As used herein, the word “printer” encompasses any apparatus that performs a print outputting function for any purpose, such as a digital copier, bookmaking machine, facsimile machine, a multi-function machine, etc.
The imaging device 10 in
As shown in
The printhead assembly 14 is appropriately supported to eject drops of ink directly onto the media web 20 as the web moves through the print zone 18. In other imaging systems in which the defective inkjet compensation system and method may be used, the printhead assembly 14 may be configured to eject drops onto an intermediate transfer member (not shown), such as a drum or belt, for subsequent transfer to a media web or media sheets. The printhead assembly 14 may have two or more printheads. Within each printhead, a plurality of inkjets is arranged in a row and column fashion. Each of the inkjets is coupled to a source of liquid ink and each one ejects ink through an inkjet nozzle in response to a firing signal being received by an inkjet actuator, such as a piezoelectric actuator, in the inkjet.
In the illustrated system of
In the printing system shown in
In order to form an image with the ink ejected by the printhead assembly 14, input image data are rendered into output image values that are used to generate firing signals that selectively actuate the inkjets in the printheads to eject ink onto the web as it moves past the printhead assembly. Typically, digital image data are received by the device 10. These digital image data may include an image for each color to be printed in the image. The input image data for a single color is called a color separation for the overall image. Each datum in a color separation corresponds to an input image value for a particular location in the color separation. The processing of the input image values is typically performed by a marking engine, which is controlled by a processor executing instructions stored in a memory operatively coupled to the processor.
The processor for the marking engine may be implemented with one or more processors, one of which may be configured to perform the defective inkjet compensation method described below. The processor may be a general purpose processor having an associated memory in which programmed instructions are stored. Execution of the programmed instructions enables the processor to process the input image values to detect edges and adjust output image values about the detected edges. The processor may, alternatively, be an application specific integrated circuit or a group of electronic components configured on a printed circuit. Thus, the processor may be implemented in hardware alone, software alone, or a combination of hardware and software. In one embodiment, the processor for the marking engine comprises a self-contained, microcomputer having a central processor unit (not shown) and electronic storage (not shown). The electronic storage may be a non-volatile memory, such as a read only memory (ROM) or a programmable non-volatile memory, such as an EEPROM or flash memory. The image data source may be any one of a number of different sources, such as a scanner, a digital copier, a facsimile device, etc.
Once the input image values have been used by the printhead controller to generate firing signals for the inkjets in the printheads of the printhead assembly, drops of ink are ejected onto the moving web to form an image. The web continues to move so the image passes through a fixing assembly 50, which fixes the ink drops to the web. In the embodiment of
A block diagram of a system that processes output image values to compensate for defective inkjets is shown in
In the discussion presented below, edge searching is performed in the image data array positions that are proximate to an image data array position that corresponds to a defective inkjet. Edge searching, however, may also be performed by searching through all of the image data array positions to detect one or more edges in the array before defective inkjet image data array positions are identified. This type of searching may be preferred for detecting edges in the image as a whole rather than edges associated with particular features near a defective inkjet. Numerous distinguishing features may be used to detect edges within or for an image. For example, some edges transition from dark to light and/or vice versa. These transitions may occur in any of a number of directions, e.g., vertical, horizontal, or both directions. Edges may also occur in one particular color plane of a color separated image. Thus, different types of edges may be detected and classified accordingly. For example, edges may be classified according to a spatial direction (vertical, horizontal, etc.), a color plane, an intensity direction (light to dark), or a spatial direction in a color plane, to name a few possible classifications. After detecting and classifying one or more edges, the search for compensation candidate positions may be performed with reference to the type of edge detected. For one example, only image data array positions in the vicinity of edges in the same color plane as the defective jet may be searched. For another example, any image data array position on a darker edge side may be searched, but the search on the lighter side of the edge may be limited to those positions within a single pixel of the edge. In another example, the image data array positions in vertical single pixel line may be considered to be at both a light-to-dark edge and a dark-to-light edge, though this type of edge may be classified as a special type of edge with a corresponding search pattern. Generally, the search pattern for this type of edge is limited to only one side or another of the edge, rather than alternating the search between the right and left sides of the edge.
An ordered sequence search may correspond to the search pattern 300 shown in
The processor configuration is capable of myriad variations to enhance the detection and compensation performed by the processor. In one embodiment, the processor modifies the ordered sequence for the next ordered sequence search in an effort to distribute the compensation values more evenly. In this embodiment, the processor commences the search on the side of the column that is opposite the detected image data position. For example, if the detected image position was position 7 (
In another embodiment, the processor is configured to detect edges in the output image data of the image data memory. Edge detection is well-known in the art and the known methods may be used to detect edges in the image data stored in the memory 204. Once an edge is detected, the ordered sequence is altered to use the search pattern 400 shown in
In another embodiment using the pattern discussed above, pixels are not distributed beyond an object edge or possibly distributed to a single pixel position beyond the edge that is detected within an extended region, such as the one depicted in
As noted above, numerous techniques for edge detection are known and may be used. In one embodiment, the processor is configured to filter the output image values and to compare the filtered values to a threshold to detect an edge in the memory. The threshold comparison helps prevent small transitions and noise from being identified as edges. In another embodiment, the processing may be performed on rendered image data and the edges are derived from the rendered data. Edge detection techniques are well-known in the art and they may be used to detect edges in the rendered image data or in binary image data. Binary image data may be derived from image data by comparing the image data to a threshold value in a threshold array and assigning a bit value of either 1 or 0 to an image data position in response to a comparison of the image data at a storage location to the threshold value. Additionally, a detected edge may be dilated to enhance treatment of the areas about the detected edges. Enhanced treatment includes modification of the ordered sequence search at image data positions in the vicinity of the dilated edge.
In another embodiment, the rendered image data includes multiple levels corresponding to the number of ink drops per pixel. Consequently, more than one ink drop to be ejected by a defective inkjet at a pixel location may need to be moved. Drops may be moved to neighbors that already contain drops as long as the number of drops at the candidate location is less than the maximum number of drops for a pixel. In one embodiment, the search patterns may be different for different drops at a pixel location. For example, the search for candidate locations may start on one side for the first drops at a location and on the other side for the next drop at the location. In response to a detected edge, the ordered sequence search may be modified to start only one side of the detected edge.
A method that may be used to compensate for output image values corresponding to defective inkjets is shown in
The method shown in
The methods disclosed herein may be implemented by a processor being configured with instructions and related circuitry to perform the methods. Additionally, processor instructions may be stored on computer readable medium so they may accessed and executed by a computer processor to perform the methods for distributing compensation image values to image data positions located around an image data position corresponding to a defective inkjet.
It will be appreciated that various of the above-disclosed and other features, and functions, or alternatives thereof, may be desirably combined into many other different systems or applications. Various presently unforeseen or unanticipated alternatives, modifications, variations, or improvements therein may be subsequently made by those skilled in the art, which are also intended to be encompassed by the following claims.
