Patent Abstract:
Hard imaging methods, hard imaging device fabrication methods, hard imaging devices, hard imaging device optical scanning systems, and articles of manufacture are described. A hard imaging method includes providing image data corresponding to a hard image to be formed and generating light responsive to the image data. The method further includes scanning the light to form a latent image corresponding to the hard image to be formed and accessing correction data corresponding to scanning errors of a scan lens intermediate a rotating reflection device and a photoconductor. The method also includes modifying the image data using the correction data before the generating and the modifying including modifying to reduce the introduction of image errors resulting from the scanning using the scan lens.

Full Description:
FIELD OF THE INVENTION 
     At least some embodiments of the invention relate to hard imaging methods, hard imaging device fabrication methods, hard imaging devices, hard imaging device optical scanning systems, and articles of manufacture. 
     BACKGROUND OF THE INVENTION 
     Computer systems including personal computers, workstations, hand held devices, etc. have been utilized in an increasing number of applications at home, the workplace, educational environments, entertainment environments, etc. Peripheral devices of increased capabilities and performance have been developed and continually improved upon to extend the functionality and applications of computer systems. For example, imaging devices, such as digital presses or printers, have experienced significant advancements including refined imaging, faster processing, and color reproduction. 
     Some imaging devices form latent images upon a photoconductor during imaging operations. A scan lens may be used to focus light (from a laser) along a scan line of the photoconductor to write data for a plurality of pixels on the scan line. The focused light may be used to selectively discharge pixels of the scan line to form latent images which are subsequently developed using a marking agent, such as toner. 
     Some printers have utilized scan lens having little or minimal scan geometry error which may be represented as a linear displacement along a scan line with respect to a scan angle of a rotating polygon mirror used to reflect the light from the light source towards the scan lens. These scan lens configurations may utilize a significant number of degrees of freedom (e.g., numerous lenses and/or lens surfaces) resulting in complex and perhaps costly designs. 
     Aspects described herein provide improved apparatus and methods for optical scanning in hard imaging implementations. 
     SUMMARY OF THE INVENTION 
     At least some embodiments of the invention relate to hard imaging methods, hard imaging device fabrication methods, hard imaging devices, hard imaging device optical scanning systems, and articles of manufacture. 
     According to one embodiment, a hard imaging method comprises providing image data corresponding to a hard image to be formed and generating light responsive to the image data. The method also includes scanning the light to form a latent image corresponding to the hard image to be formed and accessing correction data corresponding to scanning errors of a scan lens intermediate a rotating reflection device and a photoconductor. The method also provides modifying the image data using the correction data before the generating to reduce the introduction of image errors resulting from the scanning using the scan lens. 
     According to an additional embodiment, a hard imaging device comprises an interface configured to access image data corresponding to images to be formed using a hard imaging device and processing circuitry coupled with the interface and configured to access the image data. The processing circuitry is additionally configured to access correction data corresponding to scanning error of an optical scanning system of the hard imaging device, and to modify the image data according to the correction data to reduce image errors introduced during optical scanning of the image data using the optical scanning system. 
     Other aspects of the invention are disclosed herein as is apparent from the following description and figures. 
    
    
     
       DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a functional block diagram of a hard imaging device according to one embodiment. 
         FIG. 2  is an illustrative representation of a scanning system according to one embodiment. 
         FIG. 3  is a graphical representation of exemplary scan geometry error of a scanning system according to one embodiment. 
         FIG. 4  is an exemplary hard image generated by a scanning system having scan geometry error according to one embodiment. 
         FIG. 5  is a flow chart of a methodology of modifying image data to accommodate scan geometry error of a scanning system according to one embodiment. 
         FIG. 6  is a flow chart of a methodology of determining correction data corresponding to scan geometry error of a scanning system according to one embodiment. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     According to at least some embodiments or aspects, apparatus and methods for enabling generation of quality hard images using optical lens having: scan geometry error are described. Exemplary aspects enable the utilization of less complex and/or expensive scan optics within a hard imaging device to, provide scanning operations to produce hard images of acceptable quality. In one embodiment, image data to be imaged is modified prior to scanning operations using the image data to accommodate the scan geometry error of the scan optics and to provide acceptable output. In another embodiment, a timing of outputting information for pixels of a scan line is varied to accommodate scan geometry error. Other aspects are disclosed below. 
       FIG. 1  shows an exemplary configuration of a hard imaging device  10 . Hard imaging device  10  is configured to form hard images. Hard images comprise images physically rendered upon output media  22 , such as sheet paper, roll paper, envelopes, transparencies, labels, etc. Hard imaging device  10  may be implemented as an electrophotographic device such as a digital press (e.g., an HP1000 or HP3000 Indigo press available from Hewlett-Packard Company) in one embodiment. Other possible embodiments of hard imaging device  10  include laser printers, copiers, facsimile devices, multiple function peripheral (MFP) devices, or any other configuration arranged to form hard images upon media  22 . 
     The illustrated exemplary hard imaging device  10  includes a communications interface  12 , processing circuitry  14 , a storage device  16 , and an image engine  18 . The depicted example of hard imaging device  10  comprises a digital press for discussion purposes. Other implementations are possible as mentioned previously. 
     Communications interface  12  is configured to communicate electronic data externally of hard imaging device  10 . In one embodiment, interface  12  is arranged to provide input/output communications with respect to external devices, via for example, a communications medium (not shown) implemented in a networked arrangement of private and/or public devices in one example. Image data may be provided from an external device (e.g., host) to hard imaging device  10  using communications interface  12 . Alternately, image data may be generated internally of device  10  or otherwise obtained. Exemplary image data includes page description language (PDL) data (e.g., computer readable list of objects to be hard imaged on a page and may include text and/or line art, for example; along with location, size, color and other attributes of the individual objects), or any other data comprising content to be hard imagined. A page description for a page to be hard imaged may be accessed and processed using processing circuitry  14 . Image data before modification to correct for scan geometry errors described below may be referred to as original or initial image data of an original or initial image or initial format. 
     Processing circuitry  14  is configured to access and process image data (e.g., rasterize PDL image data) and control operations of hard imaging device  10  (e.g., communications, imaging, etc.). Processing circuitry  14  may comprise circuitry configured to implement desired programming (e.g., a microprocessor or other structure configured to execute software and/or firmware instructions). Other exemplary embodiments of processing circuitry  14  include hardware logic, PGA, FPGA, ASIC, and/or other processing structures. These examples of processing circuitry  14  are for illustration and other configurations are possible for processing image data and controlling operations of hard imaging device  10 . 
     Storage device  16  is configured to store electronic data (e.g., initial image data, raster image data, etc.), programming such as executable instructions (e.g., software and/or firmware), and/or other digital information and may include processor-usable media. Processor-usable media includes any article of manufacture which can contain, store, or maintain programming, data and/or digital information for use by or in connection with an, instruction execution system including processing circuitry in the exemplary embodiment. For example, exemplary processor-usable media may include any one of physical media such as electronic, magnetic, optical, electromagnetic, infrared or semiconductor media. Some more specific examples of processor-usable media include, but are not limited to, a portable magnetic computer diskette, such as a floppy diskette, zip disk, hard drive, random access memory, read only memory, flash memory, cache memory, and/or other configurations capable of storing programming, data, or other digital information. 
     Image engine  18  is configured to form hard images upon output media  22 . In one embodiment, image engine  18  comprises development and fusing assemblies configured to form the hard images using a marking agent, such as liquid ink or toner. An exemplary development assembly comprises an optical scanning system  20  configured to form latent images upon a photoconductor (not shown in  FIG. 1 ). Image engine  18  may be configured to generate monochrome and/or color hard images. 
     Referring to  FIG. 2 , details of one configuration of optical scanning system  20  of image engine  18  of an exemplary electrophotographic color digital press is shown. The illustrated scanning system  20  includes a light source  22 , a collimating lens  24 , a reflection device  26 , and a scan lens  28  comprising a plurality of lens elements  30 . Other arrangements of scanning system  20  are possible. The illustrated exemplary optical scanning system  20  is embodied as a polygonal scanner operable to scan a laser beam over a photoconductor  32  in a raster pattern in the described arrangement. 
     Light source  22  is configured to emit light to be used to form latent images responsive to raster image data from processing circuitry  14  (e.g., the data may be buffered or internally stored in storage device  16  following processing by circuitry  14  and before application to light source  22 ). Light source  22  is arranged as a laser diode (e.g., VCSEL) in one possible configuration. 
     Collimating lens  24  is arranged to receive the generated light and to apply the light to reflection device  26 . Collimating lens  24  outputs a parallel bundle of light towards reflection device  26  responsive to the received light from light source  22 . 
     Reflection device  26  is configured to receive the parallel bundle of light from collimating lens  24  and to direct the light towards a photoconductor  32 . The illustrated exemplary reflection device  26  comprises a rotating polygon mirror comprising a plurality of facets. Individual facets of the device  26  are configured to form respective individual scan lines upon a surface  34  of photoconductor  32 . For example, in the illustrated embodiment, device  26  rotates in a counter-clockwise direction. The rotation causes the light to move in an x direction from right to left (also referred to as a scan direction) to form individual scan lines of a resultant latent image formed upon photoconductor surface  34 . 
     Scan lens  28  operates to alter the light reflected from device  26 . For example, the scan lens  28  may focus the received light to a single pixel location of the scan line being imaged at a given moment of time. Accordingly, a plurality of pixels of a scan line are written from right to left in the depicted exemplary configuration. 
     Photoconductor  32  may be cylindrical and configured to rotate along an axis in the scan direction and orthogonal to the axis of rotation of reflection device  26 . Photoconductor  32  receives the scanned laser beam light in a raster pattern (i.e., two-dimensional array of pixels). Light source  22  is turned on and off responsive to raster image data to selectively discharge some pixels and to form a latent image. In one embodiment, the discharged pixels attract toner during subsequent development operations, and the developed image may be subsequently transferred onto media  22  to produce a hard image. 
     Scan lens  28  determines the scan geometry along a scan line. More specifically, as emitted light reflects of individual facets of refection device  26 , it passes through scan lens  28  and onto photoconductor  32  forming a single scan line. In the described embodiment, photoconductor  32  rotates and successive scan lines are laid down sequentially on surface  34  forming a desired two-dimensional raster. The scan geometry is the relationship between the angle of rotation of the reflection device  26  (θ) and the displacement (in the x direction) of the spot (e.g., pixel) where the focused light lands on surface  34  of photoconductor  32 . In some embodiments, the spot of the light moves along a scan line at a constant velocity so processing circuitry  14  may clock, out pixels to light source  22  at a substantially constant rate. A configuration of scan lens  28  meeting this design criteria may be referred to as a f-θ lens (wherein the spot displacement in an x direction substantially equals the lens focal length f times the respective polygon angle of device  26  in the illustrated embodiment). 
     Some arrangements of scan lens  28  comprise a plurality of degrees of freedom to meet design constraints. Degrees of freedom may be increased by adding lens elements  30  or lens surfaces to the scan lens  28  resulting in increased complexity and cost. Scan lens  28  having less degrees of freedom may introduce errors in scan geometry, causing variations in pixel spacing and pixel size as data is read out from circuitry  14  at a constant rate along individual scan lines, and thus a geometric warping of what is hard imaged. 
     Referring to  FIG. 3 , an exemplary graphical representation of scan geometry error of different scan lens is shown. Line  36  corresponds to a scan lens configuration having sufficient degrees of freedom to function as an ideal scan lens. Line  38  corresponds to a scan lens configuration having insufficient degrees of freedom and the resultant scan geometry error (also referred to as scanning error) associated with the particular scan lens (e.g., less complex than scan lens  36 ). As described below, disclosed aspects enable the use of a scan lens  28  having associated scan geometry or scanning error within scanning system  20  and device  10  configured to accommodate the error and provide acceptable hard imaging operations. 
     Referring to  FIG. 4 , an example of geometric warping is illustrated  FIG. 4  depicts a grid (e.g., graph paper)  40  having a plurality of vertical lines  42 . Vertical lines  42  depict the lines and spacing in the scan (x) direction of the original image to be formed. Vertical lines  42   a  illustrate resultant lines which may be actually formed due to geometric warping. In one example, vertical lines  42   a  may be spaced farther apart than desired adjacent the left and right borders of an imaged sheet (e.g., in the scan direction) and spaced closer together than desired in a middle portion of the imaged sheet in one example. The character “e” indicates image error introduced into formed latent images responsive to the scanning error of the scan lens  28  and represents erroneous displacement of the image data either to the left or right in the scan direction. 
     Aspects described herein permit hard imaging operations with satisfactory results while utilizing of scan lens  28  having scan geometry distortion (e.g., insufficient degrees of freedom). In one embodiment, processing circuitry  14  may be configured to provide image processing operations with an assumption that pixels in a raster to be imaged are not evenly spaced. Processing circuitry  14  may pre-warp image data to cancel geometric warping introduced by scan lens  28  to provide hard images representing the original image data of acceptable accuracy. In one embodiment, the scan tens  28  to be utilized in device  10  is analyzed to determine the scan geometry error (e.g.,  FIG. 3 ). For example, a scan lens design package, such as Code V available from Optical Research Associates of Pasadena, Calif. (http://www.opticalres.com), may be utilized to design a desired scan lens  28 . Constraints of a desired scan lens  28  may be provided. One or more appropriate scan lens  28  may be designed using different lens geometries and/or materials. Once a particular design which meets desired constraints is selected (or is otherwise acceptable), the design package may also provide the geometric distortion of the scan lens  28  (e.g., curve  38  of  FIG. 3 ). The optical design package can output a graph, table, or equation that characterizes the geometric distortion of the optics. The inverse graph, table, or equation characterizes the inverse geometric distortion which may be applied in the raster image processor described below or other structure. 
     Using the determined scanning error of scan lens  28  (e.g., geometric distortion), correction data to be used by processing circuitry  14  to “pre-warp” image data to be hard imaged may be calculated. In one embodiment, correction data comprising geometric or perspective transforms may be calculated from the geometric distortion. In one illustrative example, the correction data may be determined from the geometric distortion using techniques described in “Computer Graphics: Principles and Practice (Second Edition in C),” Foley, James D., Andres van Dam, Steven K. Feiner, and John F. Hughes authors, 1996, Addison-Wesley Publishing Company, ISBN 0-201-84840-6, the teachings of which are incorporated herein by reference. The correction data may be an inverse perspective transform of the geometric distortion curve determined from the design of scan lens  28 . In one embodiment, the correction data is stored within storage device  16  and is accessible by processing circuitry  14 . 
     Once the correction data is determined, processing circuitry  14  can modify the image data to account for the geometric distortion and reduce, cancel or minimize image errors resulting from the geometric distortion. In one embodiment, processing circuitry  14  operates as a raster image processor (RIP). A raster image processor may accept a high-level description of a page to be printed and produce binary raster image data. The raster image data is given to image engine  18  which generates images upon media  22 . In one example, the high-level page description is cast into the Adobe PostScript language. This description may include characters in various fonts and colors, line art (lines, polygons, arcs, circles, strokes, outlines, etc.), and continuous tone images, in the form of a PostScript program. 
     Initially, the raster image processor processes the high-level pages description to parse the page description and extract one or more graphical objects it defines. According to the described example, the execution of a PostScript program, by the raster image processor produces,a display list which comprises a list of primitive graphical objects and their attributes (size, location, color, etc.). Objects to be hard imaged (or “displayed”) on the page are contained in the display list. This extends even to individual halftone dots produced by the application of a halftone screen to any continuous tone images or colored objects in the page. Complex objects may be broken down into simple graphics primitives for inclusion in the display list. 
     Thereafter, the raster image processor may rasterize the display list. The raster image processor may calculate an intersection of individual scan lines with primitive graphical objects in the display list and determine which pixels to turn on and turn off to draw individual scan lines. The raster image processor produces binary raster image data for output to an image engine  18 . 
     If the optics of image engine  18  have no substantial geometric distortion, a conventional raster image processor may be employed to convert a page description into a display list and then into binary raster image data for the image engine  18 . If the optics of image engine  18  have geometric distortion, use of a conventional raster image processor will produce output with geometric distortion in the scan direction with no geometric distortion in the process direction. 
     In accordance with one embodiment, the correction data may be utilized to modify the raster image processor to generate binary raster image data that is geometrically distorted in the scan direction with the inverse geometric distortion (i.e., correction data), also referred to as a correction warp. When the raster data from this configured raster image processor is printed with image engine  18  having geometric distortion, the two distortions cancel each other, and the desired output is produced. Accordingly, in one embodiment, processing circuitry  14  may comprise a raster image processor configured to modify initial image data using the correction data, for example, during rasterization. 
     The modified raster image processor may be implemented in a plurality of exemplary embodiments. In one arrangement, the raster image processor may apply the correction warp to the primitive graphical objects in the display list to produce a new display list. The new display list contains pre-wrapped primitive graphical objects which may be rasterized in a conventional manner to produce the desired binary raster image data. However, if the correction warp is sufficiently complicated, simple primitive graphical objects (e.g., a circle) may become too, complex after the correction warp is applied to be described as “primitive.” In one example, a small circle (e.g., 10 or 100 pixels in diameter) may become a small ellipse. The respective axis in the process direction would still be 10 or 100 pixels, but the axis in the scan direction may increase or decrease to cancel the optical distortion. The axis in the scan direction might be 8 or 12 pixels, or 91 or 107 pixels, for example. The geometric description of an ellipse is typically simple enough for a list of primitive graphical objects. 
     In another example, if a circle were the size of the page, it may traverse all or many regions of the geometric warp of the optics, and of the correction warp. Parts of the circle may be stretched in the scan direction and other parts may be compressed. According to the currently described embodiment, there would be no simple primitive description of the circle after the application of the correction warp unless the circle were first broken down into a number of small arcs, and the correction warp applied to each arc. A modified raster image processor implemented in this manner may decompose individual large primitive graphical objects in the display list into a number of small primitive graphical objects in the new display list and apply the correction warp to each one. 
     Another exemplary implementation of a modified raster image processor may begin with a geometric intersection of individual scan lines with the list of primitive graphical objects in the display list. Since a scan line is one dimensional, the intersection can merely be a list of line segments, where each segment is the intersection of the scan line with one of the primitive graphical objects. The correction warp may be readily applied to the locations of the end points of individual line segments in the intersection, producing a new intersection. The new intersection may be rasterized by the raster image processor in the conventional manner to produce the desired binary raster image data. 
     In a more specific description, a scan line is not truly one dimensional but may be one pixel high and as many pixels wide as a scan line (e.g., thousands) comprising a rectangle. The modified raster image processor may calculate the intersection of this rectangle with the primitive graphical objects in the display list, producing (to an excellent approximation) a list of one-pixel-high trapezoids and less-than-one-pixel-high triangles as the intersection. The correction warp may be applied to the vertices of these trapezoids and triangles to produce a new intersection, still comprising a list of trapezoids and triangles. The new intersection may be converted to binary raster image data in the conventional manner. This technique avoids aliasing errors and Moiré patterns, which otherwise would, be produced by processing the pixel raster with collections of primitive graphical objects, such as the dots, in a halftone screen pattern. 
     Following rasterization of the image data using the correction data, processing circuitry  14  may output the modified image data comprising raster data to light source  22  to control the emission of light to form the latent image. The modified image data may be outputted to light source  22  at a constant rate in one embodiment. Additional details regarding exemplary processing of image data to offset lens distortion are described in a U.S. Pat. No. 5,751,863, the teachings of which are incorporated by reference herein. 
     Accordingly, scan lens  28  in one arrangement, produces images upon photoconductor  32  which differ from images of the generated light from light source  22  due to scanning errors or geometrical distortion of the scan lens  28 . Processing circuitry  14  may utilize correction data corresponding to the geometrical distortion to modify the raster image data such that latent images subsequently produced using scan lens  28  are correct or satisfactory representations of the initial or original image data before the described modification using processing circuitry  14 . 
     Referring to  FIG. 5 , an exemplary methodology of processing circuitry  14  is shown. Other methods are possible including more, less, or alternative steps. 
     At a step S 10 , the processing circuitry accesses original or initial image data (e.g., PDL data). 
     At a step S 12 , the processing circuitry operates as a raster image processor (RIP) to rasterize the image data. In one embodiment, the processing circuitry modifies the initial image data using the correction data corresponding to scanning error of the scan lens. 
     At a step S 14 , the processing circuitry outputs the modified image data to a light source of the image engine. 
     Referring to  FIG. 6 , an exemplary methodology is depicted for configuring a hard imaging device. Other methods are possible including more, less or alternative steps. 
     At a step S 20 , an optical lens designer provides lens design criteria data for a scan lens to be utilized within the image engine of the hard imaging device. 
     At a step S 22 , scanning error information of the scan lens is determined. The scanning error information corresponds to geometric distortion of the scan lens in one embodiment and may be represented as pixel displacement in the scan direction relative to scan angle (e.g., angle of the facet reflecting light at a given moment in time). 
     At a step S 24 , correction data for the respective scan lens may be determined. The correction data may be provided in a form of a perspective transform corresponding to an inverse of the geometric distortion of the scan lens. 
     At a step S 26 , the correction data may be stored within the storage device for access by the processing circuitry during modification operations of the raster image data. 
     In accordance with another arrangement, the timing of the outputting of raster image data (information for individual, pixels) from processing circuitry  14  to the light source  22  may be varied to compensate for geometric distortion (e.g., scanning error) of scan lens  28  and resulting in variation in pixel spacing (e.g., image error) along a scan line. In one exemplary embodiment, processing circuitry  14  may be configured to output raster image data according to a variable output of a variable frequency clock circuit (not, shown). Exemplary details regarding a variable frequency clock circuit which may be utilized to control the timing of outputting of raster image data for pixels from processing circuitry  14  are described in U.S. Pat. No. 4,717,925, the teachings of which are incorporated by reference herein. 
     In one exemplary arrangement, the output of the frequency clock circuit could be controlled using the correction data to control the timing of the outputting of the raster image data from processing circuitry  14  (or other internal buffering or storage circuitry) to light source  22  to account for the scanning errors of scan lens  28  and reduce the presence of image errors. In the described example of  FIG. 4 , processing circuitry  14  could be controlled to output the information of the most left line  42  at a later moment in time corresponding to line  42   a  plus the respective scanning error “e” to provide the information at the correct location of line  42 . The modifying in the described example includes modifying the outputting of the timing of the raster data using the correction data and corresponding to the initial image data without modification of the image data using the correction data described above. 
     The exemplary embodiments described herein enable the utilization of a relatively simple scan lens  28  (e.g., less degrees of freedom) within an optical scanning system. The described embodiments correct or accommodate scan geometry error without utilization of complex scan lens having minimal or otherwise reduced scan geometry error (e.g., scan lens having an increased number of degrees of freedom compared with scan lens usable in the described embodiments). Exemplary embodiments enable production of hard images with unevenly spaced pixels if actual pixel spacing is known prior to outputting raster data to a light source  22  of the optical scanning system  20 . At least one embodiment implements correction operations without the complexity and cost of a variable frequency clock circuit, and the correction to the scan geometry error may additionally be implemented in programming. Further, modulation of light source  22  according to at least some of the described embodiments may be readily accomplished, at relatively high frequencies as a scan line is being written. In one example, an 800 dots-per-inch (DPI) hard imaging device  10  may modulate the light source  22  at three times the pixel rate for an effective resolution of 2400 DPI along scan lines providing relatively fine resolution Accordingly, it is possible to achieve a relatively high degree of geometric correction producing high-quality hard images. 
     The protection sought is not to be limited to the disclosed embodiments, which are given by way of example only, but instead is to be limited only by the scope of the appended claims.

Technology Classification (CPC): 7