Seamless stitching of multiple image fields in a wide-format laser printer

Systems and methods are described for stitching multiple images together in a printer to form a single, seamless, composite image. The use of multiple laser sources and multiple scan lenses with one or more scanner assemblies and various image stitching methods achieves seamless image stitching in a manner that provides benefits over prior printers using single laser sources and single scan lenses. Such benefits include, for example, a wider image format, smaller optical spot size, reduced cost, and reduced overall size for the printer.

TECHNICAL FIELD

The present disclosure generally relates to imaging devices, and more particularly, to a wide-format printer in which an image is formed by stitching together partial images from multiple image fields produced by one or more laser scanner assemblies.

BACKGROUND

Conventional wide-format laser printers have generally been limited to printing on A3 (297×420 mm) size media in portrait mode with a scan line length not exceeding about 317 millimeters. Therefore, the size of images printed with conventional wide-format laser printers is typically about the size of a ledger-sized sheet of paper, or about 11×17 inches. Commercial printing businesses would benefit significantly from a wide-format laser printer that could print larger images onto larger media. For example, a printer with a scan length of 614 mm could print an image 594 mm wide on C2 or B2 size media. Such an image would be twice as wide as the usual 297 mm wide A3 image. (Note that, in this example, the scan length exceeds the nominal image width by 10 mm at each end of the scan to allow for image position correction, crop mark printing, etc.) One way to increase the image width in a wide-format laser printer is to design a “scaled-up” version of a current laser scanner system such that the scaled-up scanner is capable of scanning a wider image field (e.g., 614 millimeters) in one scan pass. However, preliminary analysis of such scaled-up laser scanner designs demonstrates various problems. One such problem with scaling up an existing scanner design to a larger format is that the residual aberrations in the optical design scale in proportion to the scan length, thereby increasing the geometric focused spot size, while the diffraction-limited spot size remains constant. Consequently, a scanner design which is initially diffraction limited (i.e., which has a focused spot size very close to the theoretical minimum), will not ordinarily remain diffraction limited when doubled in size. Instead, the spot size will increase because the optical performance is now dominated by aberrations rather than diffraction. The result of scaling a just-diffraction-limited scanner by 2×, for example, would be a doubling of both the scan line length and the geometric focused spot diameter. Such a scaling operation does not achieve any increase in the total number of resolvable focused spots in a scan line, and the resulting scanner could not print any more optically resolved pixels in a scan line than the initial system. Achieving a 2× increase in the number of resolvable pixels requires reducing the geometric spot size in the 2× system below the diffraction limit, which requires a new optical design if it can be accomplished at all.

Another problem with simply scaling up an existing scanner design to a larger format is an associated disproportionate increase in cost to produce the scaled-up design. This is true even if we ignore the need to make the scaled-up system optically superior to the original (i.e., parent) scanner in order to control optical aberrations to a level that will enable the scaled-up system to be substantially diffraction limited. As an example of the disproportionate increase in costs, scaling up the linear dimensions of an existing (1×) scanner design by a factor of two increases the production costs for the scaled-up (2×) version by roughly a factor of 8 over the production costs of the original 1× design. This large cost increase is explained by the fact that the volume of any object scales as the cube of its linear dimension. Thus, every part of the 2× scanner has eight times the volume of the corresponding part in the 1× scanner. Like many products, the manufacturing cost for a laser scanner assembly is very roughly proportional to its volume or, equivalently, its weight.

Accordingly, the need exists for a way to increase the printable image width in a wide-format laser printer that avoids the optical problems and added expense associated with scaling up existing optical designs to wider formats.

SUMMARY

Systems and methods are described for forming an image on a photosensitive element from separate image fields joined together at a seamless stitch.

DETAILED DESCRIPTION

Overview

The following discussion is directed to systems and methods for stitching multiple images together in a wide-format printer to form a single, seamless, composite image. The human visual system is highly sensitive to linear features and can readily perceive errors as small as 10 μm in the placement of rows or columns of pixels on a page. Splicing pixelated image fields at their common boundaries without creating noticeable seams between the image fields or other visible artifacts is complicated by the difficulty of locating corresponding pixels in two separate image fields with sufficient accuracy to render a simple splice invisible. However, the use of multiple scanner assemblies and various image stitching methods as described herein reduces the visibility of image splices in the presence of such pixel placement errors and achieves seamless image stitching in a manner that provides benefits over prior wide-format printers using single laser scanner assemblies. Such benefits include, for example, a smaller optical spot size, reduced cost, reduced overall size for the printer, increased page rate, and in some cases, the ability to design and manufacture a laser scanner having a format width which exceeds the current state of the art for conventional laser scanner designs.

Exemplary System Environment

FIG. 1illustrates an exemplary environment for implementing one or more embodiments of an imaging device that provides seamless image stitching of multiple image fields. The exemplary environment100ofFIG. 1includes a wide-format imaging device102operatively coupled to a host computer104through a network106. The network106can be a direct or indirect link and may include, for example, a printer cable, a LAN (local area networks), a WAN (wide area networks), an intranet, the Internet, or any other suitable communication link. Network106can also include a wireless communications link such as an IR (infrared) or RF (radio frequency) link.

This disclosure is applicable to various types of imaging devices102that may be generally configurable to operate using, for example, a wide format, and that use a beam of light to record an image onto a photoconductive or otherwise photosensitive surface, such as devices capable of implementing an electrophotographic (EP) imaging/printing process or photographic imaging process for rendering PDL (page description language) data in printed form on a print medium. Therefore, imaging device102can include devices such as laser-based printers, photocopiers, scanners, fax machines, multifunction peripheral devices and other EP-capable devices, as well as photographic printers.

Host computer104can be implemented as a variety of general purpose computing devices including, for example, a personal computer (PC), a server, a Web server, and other devices configured to communicate with imaging device102. Host computer104typically provides a user with the ability to manipulate or otherwise prepare in electronic form, an image or document to be rendered as an image that is printed or otherwise formed onto a print medium by imaging device102after transmission over network106. In general, host computer104outputs host data to imaging device102in a driver format suitable for the device102, such as PCL or PostScript. Imaging device102converts the host data and outputs it in a wide format onto an appropriate recording medium, such as paper, transparency or photographic film.

Exemplary Embodiments

FIG. 2is a block diagram representation of an imaging device embodied as a wide-format laser printer102. The block diagram representation of wide-format laser printer102includes an embodiment of a controller200and an embodiment of an image forming system202. Computer104provides data, including print data, to controller200. Laser printer102is generally disposed to modulating the operating frequencies of a pulse width modulator (PWM) drive circuit204to place pixels onto the surface of a photosensitive element206for holding an image to be printed onto a print medium. AlthoughFIG. 2illustrates photosensitive element206in the form of a photoconductive drum206, it is understood that other forms of photosensitive elements206are possible. For example, photosensitive element206can optionally be configured as a continuous, photoconductive belt or other photoconductive image transfer medium, or as photographic film or paper in a direct exposure process.

Controller200is typically a printed circuit assembly that includes a memory208such as Random Access Memory (RAM) and/or non-volatile memory for holding an image to be printed, executable instructions, and other data for controller200. Controller200also includes a microprocessor210to process images, instructions and other data, in addition to other general data formatting circuitry such as that illustrated in data formatter212. Data formatter212is typically embodied as an ASIC (application specific integrated circuit) having various blocks of hardware implemented as logic gates. Thus, data formatter212includes a rasterizer block214, a phase lock circuit block216, a joint/splice locator block218, and PWM (pulse width modulation) drive circuit block204. Rasterizer block214and joint/splice locator block218might also be implemented as firmware instructions stored in a memory208and executable on processor210.

In general, rasterizer214converts print data from computer104into pixel/video data that PWM drive circuit204uses to form an image on photosensitive element206. More specifically, PWM drive circuit204uses pixel data from rasterizer214to control the flow of drive current to light sources, such as laser sources220, in image forming system202. In response to the drive current, laser sources220generate pulsating laser beams222to form images on photosensitive element206. The time period of the pulses of the laser beams222corresponds to the time period of the pulses of the pixel/video data. Laser sources220may be multi-element laser diode arrays, each capable of emitting multiple independently-modulated beams for simultaneously writing multiple scan lines, as is well known in the art.

As discussed in greater detail below, the described embodiment ofFIG. 2includes a joint/splice locator218that formats data from rasterizer214into two or more separate video data streams for the PWM drive circuit204. The PWM drive circuit204uses the video data streams to modulate two or more separate laser beams (represented inFIG. 2by beams222(A) and222(B) from laser sources220(A) and220(B), respectively) that, in turn, create two separate, adjacent, and overlapping image fields (i.e., Image Field1and Image Field2ofFIG. 2) on photosensitive element206that are joined together by a seamless joint or splice.

In the described embodiment ofFIG. 2, image forming system202focuses each beam emitted by laser sources220into a corresponding focused spot on the surface of photosensitive element206. Image forming system202further controls the movement of pulsating laser beams222from laser sources220across the surface of photosensitive element206by deflecting the beams222by reflection from rotating polygon scanners226after passing the beams222through respective collimating lenses224and cylinder lenses225that establish the desired wavefront curvature for the beams222, as is well known in the art. After reflection from polygon scanners226within respective scanner assemblies228, the laser beams222pass through respective scan lenses230and are incident upon photosensitive element206along one or more scan lines232. For purposes of illustration,FIG. 2shows laser beams222(A) and222(B) in three scan positions corresponding to three positions of polygon scanners226(A) and226(B).

As a pulsating beam222is scanned across photosensitive element206, it exposes regions or spots along a scan line232on the surface of photosensitive element206that have a dimension in the direction of the scan. The exposed regions or image spots represent pixel data (i.e., raster image data), and their dimensions along a scan line232are determined by the time periods of the pulses of the video data that drives PWM drive circuit204. In a typical electro-photographic process in which photosensitive element206is a photoconductor, exposed regions have a different electrostatic charge than unexposed regions. The electrostatic charge differential forms a latent image on photoconductor206that permits development of ink or toner to the photoconductor206in a pattern corresponding to the latent image. A transfer roller or belt (not shown) may be used to facilitate the transfer of ink or toner from the photoconductor onto a print medium in the form of a visible image.

Position sensors236in scanner assemblies228provide position feedback to a phase lock circuit216on controller200that controls the relative positions of scanner226(A) and226(B). The phase lock circuit216compares the relative positions of scanners226(A) and226(B) and attempts to drive any differences or errors in relative position to zero by accelerating or decelerating one of the scanner motors (not shown) in scanner assemblies228. Alternatively, start of scan detectors234(described below) can be used to provide position feedback to the phase lock circuit216. The use of such position sensors236or234and phase lock circuits216is generally well-known to those skilled in the art. Synchronizing the polygon scanners226(A) and226(B) in this manner ensures a more accurate and repeatable placement of image spots within each of the image fields (i.e., Image Field1and Image Field2ofFIG. 2). More specifically, synchronizing the scanners226promotes cross-scan (process direction) accuracy by limiting the magnitude of cross-scan direction placement errors between Image Field1and Image Field2.

Image forming system202also includes start of scan detectors234(A) and234(B) for determining the start of scan time for laser beams222(A) and222(B), respectively. A start of scan detector234detects the position of laser beam222in the scan direction as the beam approaches the portion of the photosensitive element206where the video data stream should be turned on. The start of scan detector234provides the laser beam222position information to the controller200in order to trigger the video data stream at the correct moment, thereby properly aligning each scan line in the scan direction at the edge of the scanned format As mentioned above, theFIG. 2embodiment includes a joint/splice locator218that formats data from rasterizer214into two separate video data streams. Each video data stream is formatted to generate one portion (e.g., one half) of an image (i.e., Image Field1and Image Field2ofFIG. 2) to be formed adjacent to one another on photosensitive element206by the two laser sources220and scanner assemblies228. Image Field1and Image Field2ofFIG. 2are separate but overlapping in an overlap region that is an area of photosensitive element206on which both laser/scanner assemblies can write image information. It is within the overlap region (FIG. 2) on photosensitive element206that image fields are joined together by a seamless joint or splice.

It is noted, that while the presently described embodiment of the wide-format imaging device102illustrated inFIG. 2indicates an optical system having two laser sources, two collimators, two cylinder lenses and two scanner assemblies each having a two-element scan lens, this embodiment is not intended to limit the configuration of such an optical system. Thus, this disclosure contemplates an optical system having additional laser/scanner/lens assemblies, each driven by a separate video data stream to form a different image field on photosensitive element206as formatted by joint/splice locator218.

The joint/splice locator218is configured to format image data from rasterizer214using several different stitching methods in addition to generating two video data streams to drive PWM drive circuit204. Each image stitching method increases the accuracy of the intersection between Image Field1and Image Field2on photoconductor206and decreases the possibility of visible seams or other artifacts in the composite image when the image fields are written to the photosensitive element206.FIGS. 3-6illustrate examples of how Image Fields1and2may be written to photoconductor206based on the image stitching methods used by joint/splice locator218.

FIG. 3aillustrates an example of a printer output medium238identifying the printed areas corresponding to two image fields (e.g., Image Field1and Image Field2ofFIG. 2). Note that image field locations are generally reversed in printers which use a single-stage transfer process.

FIG. 3billustrates an example of two image fields (e.g., Image Field1and Image Field2ofFIG. 2) written to photosensitive element206that are joined through a random location of joints300along successive scan lines232within an image overlap region. InFIG. 3b, pixels exposed in Image Field1are illustrated at a highly enlarged scale by black or filled circles while pixels exposed in Image Field2are illustrated by white or non-filled circles. It is apparent from the illustration ofFIG. 3bthat for each scan line232, joint/splice locator218has randomly located the joint between Image Field1and Image Field2within the overlap region. The random location of the joints300breaks the linearity of the seam between Image Fields1and2, thereby making the seam less noticeable to the human visual system. In a color printer that superimposes multiple color layers (also called color planes or color separations) in a single output print, joint/splice locator218would, in general, independently randomize the joint locations for each color layer, further reducing the visibility of the seam between Image Fields1and2.

FIG. 3cillustrates an example of two image fields (e.g., Image Field1and Image Field2ofFIG. 2) written to photosensitive element206that are joined using a curvilinear joint randomization method that randomly locates joints300along successive scan lines232about a curvilinear seam midline302within an image overlap region. The curvilinear seam midline establishes a nominal joint location for each scan line232. The actual joint locations are determined by a randomization algorithm that displaces each joint a randomly variable distance to either side of the curvilinear seam midline.

FIG. 3dillustrates an example of two image fields (e.g., Image Field1and Image Field2ofFIG. 2) written to photosensitive element206that are joined using an interleaved segment joint randomization method. Segments304of a scan line232are exposed as a part of Image Field1while segments306of scan line232are exposed as a part of Image Field2. Segments304and306are interleaved such that segments304and306, taken from Image Fields1and2respectively, alternate with each other to form a splice or joint within an image overlap region. Alternating segments304and306may be of varying lengths and may be randomly positioned within a splice region308which is within the overlap region. The splice region308is shown by way of example only and is not intended to limit the number of pixels over which Image Field1and Image Field2may be joined. Thus, splice region308may occur over many more pixels within an overlap region than is indicated inFIG. 3d. Splice region308may be variably or randomly located within the overlap region and the position of segments304and306along a scan line232may be randomized independently for other scan lines in the same color layer and for the same scan line in other color layers.

FIG. 4illustrates an example of two image fields (e.g., Image Field1and Image Field2ofFIG. 2) written to photosensitive element206wherein successive scan lines232are joined within a splice region400which is within an image overlap region. InFIG. 4, Image Field1is illustrated by pixels having an “X” through their centers while Image Field2is illustrated by pixels having a cross “+” through their centers. The splice region400is shown by way of example only and is not intended to limit the number of pixels over which Image Field1and Image Field2may be joined. Thus, splice region400may occur over many more pixels within an overlap region than is indicated inFIG. 4. It is apparent from the illustration ofFIG. 4that for each scan line232, joint/splice locator218has randomly located a splice region400over which one end of Image Field1intersects one end of Image Field2. The splice regions400spatially distribute pixel placement errors between image fields over many pixels along each scan line232, tending to reduce the visibility of scan line joints, which in turn makes the intersection between the image fields less noticeable. In addition, the random location of the splice regions400between successive scan lines breaks up the linearity of the seam between Image Fields1and2, thereby making the seam less noticeable to the human visual system.

FIG. 5illustrates an extension of the randomly placed image splice regions400discussed above with respect toFIG. 4.FIG. 5illustrates a single scan line232wherein Image Field1and Image Field2intersect in a spliced region400and wherein the exposure level from the respective laser sources220(FIG. 2) generating the image fields is adjusted within the spliced region400by the joint/splice locator218. As laser beam222(A) which is writing image spots in Image Field1enters the spliced region400, the exposure level from respective laser source220(A) is at 100%. As laser beam222(A) progresses through the spliced region400, the exposure level from laser source220(A) is reduced (i.e., ramped down) to zero %. Conversely, as the corresponding scan line for Image Field2enters the spliced region400, the exposure level from respective laser source220(B) is at zero %. As laser beam222(B) progresses through the spliced region400, the exposure level from the respective laser source220(B) is increased (i.e., ramped up) to 100%. Ramping the exposure level from laser sources220adjusts the exposure of the image fields along the scan lines232, thereby decreasing the exposure of Image Field1as it comes to an end within the overlap region while increasing the exposure of Image Field2as it begins within the overlap region. Ramping the exposure of the image regions can be achieved by either changing the intensity (i.e., changing the power) of a laser element in a laser source220or by changing the pulse duration of the laser element220.

FIG. 6illustrates an example of the exposure levels of two image fields (e.g., Image Field1and Image Field2ofFIG. 2) written to photosensitive element206in a color printer that are joined within splice regions (e.g., such as regions400) randomly located by the joint/splice locator218within the overlap region such that splice locations are independently randomized for each color layer. For each color layer the exposure levels within the splice regions are additionally ramped as discussed above with respect toFIG. 5. For each scan line in each color layer in an Image Field1, the joint/splice locator218determines the location of a splice region within the overlap region where the same scan line in the same color layer in an Image Field2will be joined. For that scan line in that color layer, the exposure level from laser source222(A) in Image Field1is ramped down within the splice region while exposure from laser source222(B) in Image Field2is ramped up. The splice region for each of the color layers (e.g., Black, Cyan, Magenta, Yellow) is randomly located within the overlap region for each scan line. Therefore, splice locations for the same color layers are randomized among neighboring scan lines, while splice locations for corresponding scan lines in different color layers are also randomized with respect to one another.

Numerous combinations of the image stitching methods described with reference toFIGS. 3a,3b,3c,3d,4,5and6are possible, but will not be described or illustrated here. An example of such a combination is interleaved segment image stitching, illustrated inFIG. 3d, which is randomly located about a curvilinear seam midline such that the shape and position of the midline varies from one color layer to the next. In the context of this disclosure, the terms “randomly located”, “randomly positioned”, “randomized”, and the like, refer to algorithms used by joint/splice locator218to generally randomize the visually perceived location of scan line joints or splices within an overlap area between image fields. Numerous such randomization algorithms are possible which have a variety of statistical properties. It may be desirable, for example, to use a randomization algorithm that produces a quasi-normal distribution of scan line joints about a predetermined central location, rather than a uniform distribution within the limits of the overlap region, It may also, for example, be desirable to avoid certain combinations of scan line splice locations that would occur in a truly random distribution but which are known to cause visible artifacts. Consequently, referring to splice/joint positions as “randomly located”, or the like, is not intended to limit the choice of algorithms used by joint/splice locator218.

FIG. 7is a block diagram illustrating another embodiment of an imaging device implemented as a wide-format laser printer102. TheFIG. 7embodiment contains many of the same elements as theFIG. 2embodiment discussed above, however theFIG. 7embodiment uses only one polygon scanner700and enables improved control over scan line placement. In addition, the current embodiment ofFIG. 7has two plane folding mirrors702(A) and702(B) to direct the two scanning laser beams222(A) and222(B) to separate image fields (i.e., Image Field1and Image Field2) on photosensitive element206. Alternatively, folding prisms could be used in place of folding mirrors702(A) and702(B). The joint/splice locator218functions in the same general manner as discussed above to format image data from rasterizer214using several different stitching methods while generating two or more video data streams that are processed by PWM drive circuit204and used to modulate two or more laser beams (represented inFIG. 7by beams222(A) and222(B) from laser sources220(A) and220(B), respectively) that, in turn, create two separate, adjacent, and overlapping image fields (i.e., Image Field1and Image Field2ofFIG. 7) on photosensitive element206that are joined together by a seamless joint or splice. Therefore, the image stitching solutions discussed above with reference toFIGS. 3-6and the embodiment ofFIG. 2are equally applicable to the embodiment ofFIG. 7.

The single polygon scanner700in theFIG. 7embodiment provides the two overlapping image fields (i.e., Image Field1and Image Field2) without the need to phase lock two separate polygons as in the prior embodiment ofFIG. 2. Thus, there is no need for a phase lock circuit216(FIG. 2), and costs associated with a second polygon assembly and a phase lock circuit are eliminated. Likewise, uncorrected residual position errors that inevitably result from phase locking two separate polygon scanners are eliminated.

The single polygon scanner700enables an image stitching solution wherein any errors in facet tilt on the polygon scanner700introduced during fabrication affect both the Image Field1portion and the Image Field2portion of a stitched scan line similarly. This is accomplished by writing the Image Field1and Image Field2portions of each scan line with the same polygon facet. Using the example ofFIG. 7, a selected polygon facet located at position710(A) causes laser beam222(A) to scan Image Field1thereby writing a first portion of a scan line232. As polygon scanner700rotates in a counter-clockwise direction, the selected polygon facet moves two facet increments to position710(B) where it causes laser beam222(B) to scan Image Field2thereby writing a second portion of scan line232. Unless otherwise corrected, the process motion of photosensitive element206will cause the first and second portions of scan line232to be displaced in the process-direction by a distance equal to the surface speed of photosensitive element206multiplied by the time interval between the end of scan for the first portion of scan line232and the start of scan for the second portion of scan line232. In the current embodiment, this process-direction position error is eliminated by introducing a compensating process-direction alignment offset between Image Field1and Image Field2. This is accomplished, for example, in an alignment step during the manufacture of printer102wherein the path of laser beam222(B) along a second portion of scan line232in Image Field2is advanced in position relative to the path of laser beam222(A) that exposes a first portion of the same scan line232in Image Field1, the amount of position advance at the surface of photosensitive element206being equal to the required process-direction alignment offset. This process-direction alignment offset is readily introduced, for example, by shifting one or both of laser sources220(A) and220(B) in the process direction. In one solution, laser sources220(A) and220(B) are shifted equal and opposite amounts to introduce the desired process-direction alignment offset at the photosensitive element while minimizing the distance either laser source is moved relative to the axis of its respective optical system.

Where laser sources220(A) and220(B) are multi-element laser sources, the process-direction alignment offset required to compensate the process-direction position error will increase in proportion to the number of laser elements in each laser source. Thus, the process-direction alignment offset at the surface of photosensitive element206, measured in increments of the scan line spacing, is equal to M×N, where “M” is the number of scanning beams emitted by each laser source and “N” is the number of polygon facet intervals separating input beams222(A) and222(B). This process-direction alignment offset enables a given polygon facet to first write a first portion of a scan line, (the portion in Image Field1, for example), after which it writes a second portion of the same scan line (the portion in Image Field2, for example) in process-direction registration but N facet intervals later in time. Thus, Image Field1and Image Field2for every scan line are written using the same polygon facet, and any cross-scan tilt error of that facet will have a substantially identical affect on the position of both image fields of the scan line, further reducing the visibility of the splice between the two image fields.

Exemplary Methods

Example methods for seamlessly stitching images in a wide-format imaging system such as those discussed above will now be described with primary reference to the flow diagrams ofFIGS. 8-11. The methods apply generally to the exemplary embodiments discussed above with respect toFIGS. 2-7. The elements of the described methods may be performed by any appropriate means including, for example, by hardware logic blocks on an ASIC or by the execution of processor-readable instructions defined on a processor-readable media, such as a disk, a ROM or other such memory device.

FIG. 8shows an exemplary method800for stitching images in a wide-format imaging system. At block802, image data is received at a wide-format imaging system such as laser printer102. The data is typically received from a host computer. At block804, the image data is formatted into a plurality of separate image fields that will form a composite image with a seamless stitch when written to a photosensitive element. As discussed below, several image stitching methods continue from block804.

Continuing method800at block805, formatted image data is converted into a plurality of video data streams according to the number of image fields and the number of laser elements in each laser source. For example, a printer102having two image fields each exposed by a four-element laser source will have a total of eight laser elements and will require eight video data streams, one for each laser element. At block806, each laser element is modulated by the corresponding video data stream causing it to emit a pulsating laser beam. Laser sources that comprise multiple laser elements emit multiple independent pulsating beams. At block808, one or more pulsating beams from each laser source are scanned onto a photosensitive element by a distinct polygon scanner to write each separate image field. Thus, each laser element emits a pulsating beam that writes a scan line across an image field on a photosensitive element in response to a video data stream during the passage of a polygon facet across the beam as the polygon rotates. Typically, two separate image fields are generated, one by each separate laser source in a system having two laser sources. The laser elements within the laser sources are driven by a PWM circuit operating on the video data streams formatted in a printer controller.

At block810, the separate polygon scanners are synchronized to rotate in unison. Synchronization typically involves rotational position feedback to a phase lock circuit that functions to zero out differences in the relative positions of the polygon scanners. At block812, a decision is made whether to generate additional image fields for the current printed page. Additional image fields would be required, for example, to print additional color layers in a multi-color print. If additional color layers are required in the printed image, the method continues at block814which instructs block804to independently randomize joint or splice locations for the next color layer with respect to previous color layers while formatting image data. The method then continues at block804as previously described. If no additional color layers are required, image exposure is complete and the method ends at block816. Thus, joint or splice locations for each color layer in a multi-color image are separately and independently randomized and will not generally coincide with joint or splice locations in other color layers, further reducing the visibility of seams between image fields in multi-color images.

After image field data is converted into a plurality of video data streams according to the number of laser elements in each laser source, as shown at block805, an alternative method continues at block818. In the alternative method, each laser element in a first laser source is modulated with a corresponding video data stream causing it to emit a pulsating laser beam as indicated in block818. At block820, one or more pulsating laser beams emitted by the first laser source are scanned onto the photosensitive element by means of a particular facet of a single polygon scanner to write one or more scan lines in a first image field. At block822, an N-facet delay time is introduced into each video data stream used to modulate a laser element in a second laser source. As previously defined with reference toFIG. 7, “N” is the number of polygon facet intervals separating a facet that reflects beams from the first laser source and a facet that reflects beams from the second laser source at any given time. In the embodiment illustrated inFIG. 2, for example, N is equal to 2 and the N-facet time delay would equal 2/6 times the rotational period of the scanner. Note that this scanner has 6 facets, making each facet interval 1/6 of the rotational period. At block824, each laser element in the second laser source is modulated by a corresponding time-delayed video data stream, causing it to emit a pulsating laser beam. At block826, one or more pulsating laser beams emitted by the second laser source are scanned onto the photosensitive element my means of the same facet of the same single polygon scanner to write the same one or more scan lines in a second image field. Use of the same facet to scan both image fields reduces the visibility of the splice between the two image fields because of the substantially identical effect that a tilt in the facet will have on both image fields. The alternative method continues at block812as previously described.

Several image stitching methods continue from block804of method800. Method900ofFIG. 9is a continuation of the method800from block804. At block902of method900, a seamless stitch in the form of a joint between two image fields is randomly located along a scan line within an overlap region of the two image fields. The joint is formed where the last pixel from a first image field abuts the first pixel from a second image field. Randomly locating such joints within the overlap region of the two image fields for successive scan lines, breaks up the linearity of the seam between the two image fields and makes the seam less noticeable to the human visual system.

Method1000ofFIG. 10is another image stitching method that may continue from method800at block804. At block1002of method1000two image fields written to photosensitive element206are joined along a curvilinear seam midline that crosses successive scan lines within an overlap region. A joint along a scan line between a last pixel in a first image field and a first pixel in a second image field is randomly located in proximity to the curvilinear seam midline. There is a random distribution of distances between the curvilinear seam midline and individual joint locations, causing most joints to lie near the seam midline rather than on it. The curvilinear shape of the midline of the seam between the two image fields breaks up the linearity of the seam and reduces its visibility. Randomly distributing the individual scan line joint locations with respect to the curvilinear seam midline further reduces the visibility of the seam.

Method1100ofFIG. 11is another image stitching method that may continue from method800at block804. At block1102of method1100a scan line in a first image field is joined with a corresponding scan line in a second image field by randomly interleaving scan line segments from the first image field with segments from the second image field in a splice region within the overlap region. Thus, a splice is made between corresponding scan lines in two image fields by alternating scan line segments taken from one image field with segments taken from the other. The alternating scan line segments may be of variable length and position within the splice region and the splice region may be randomly located within the overlap region. Thus, the alternating or “interleaved” segments may be randomly located with respect to other scan lines in the same color layer and with respect to the corresponding scan line in the other color layers.

Method1200ofFIG. 12is another image stitching method that may continue from method800at block804. At block1202of method1200, a spliced region that includes pixels from one end of a first image field intersecting pixels from one end of a second image field is randomly located within the overlap region of the two image fields. At block1204, the exposure of one end of a scan line in the first image field is ramped down within the spliced region from the point the scan line enters the splice region to the opposite end of the splice region. In addition, as shown at block1206, the exposure of one end of the corresponding scan line in the second image field is ramped up within the spliced region from the point the scan line enters the splice region to the opposite end where the scan line leaves the splice region. The ramping of the exposures can be done linearly, providing the same total exposure to each pixel that it would have received from a single un-ramped scan, or the ramping can be nonlinear to compensate for inherent nonlinearities in the response of the photosensitive element and the remainder of the printing process. Nonlinear ramping may give better results for certain printing processes in the presence of pixel placement errors. Ramping exposures within the splice regions spatially distributes pixel placement errors thereby reducing their visibility between image fields and making the intersection between the images fields less noticeable. Randomizing the location of the splice region within the overlap region between successive scan lines breaks up the linearity of the seam between image fields, thereby making the seam less noticeable to the human visual system.

Methods900,1000,1100and1200have the common property that splice locations within the same color layer are randomized from one scan line to another scan line in the cross-scan direction. In addition, splice locations for the same scan line in different color layers of a multi-color image are independently randomized with respect to each other.

As previously discussed, many randomization algorithms are possible for randomizing the location of splices or spliced regions within the overlap region between image fields. Different randomization algorithms will generally have different statistical properties and will cause different visual perceptions of a splice between image fields. References in this disclosure to randomly locating splices or spliced regions are not intended to specify a particular randomization algorithm.

Methods900,1000,1100, and1200may be used in a variety of combinations to achieve the desired suppression of artifacts associated with the stitching of image fields. A printer may be capable of using multiple stitching methods alone or in combination, and the stitching methods used may be dynamically determined during printing according to the nature of the printing task.

Although the description above uses language that is specific to structural features and/or methodological acts, it is to be understood that the invention defined in the appended claims is not limited to the specific features or acts described. Rather, the specific features and acts are disclosed as exemplary forms of implementing the invention.

Additionally, while one or more methods have been disclosed by means of flow diagrams and text associated with the blocks of the flow diagrams, it is to be understood that the blocks do not necessarily have to be performed in the order in which they were presented, and that an alternative order may result in similar advantages. Furthermore, the methods are not exclusive and can be performed alone or in combination with one another.