Color registration strategy for preprinted form

A method for performing color registration on template media having template markings thereon comprises sensing the template media using a sensor to obtain first image data; printing a test pattern on the template media; sensing the template media along with the test pattern printed thereon using the sensor to obtain second image data; determining an output image data of the test pattern from the first image data, the second image data, and an estimated image data of the template media with the test pattern printed thereon; determining process direction and cross-process direction misregistrations from the output image data; and adjusting printheads based on the process direction and the cross-process direction misregistrations to provide adjusted color registration on subsequent template media. The estimated image data is representative of light scatter from the test pattern and light absorption by the test pattern.

BACKGROUND

The present disclosure relates to a method and a system for performing color registration on template media having template markings thereon by taking both light scatter from test pattern and light absorption by the test pattern into account.

2. Description of Related Art

In a continuous feed direct marking printer, e.g., based on solid inkjet technology, multiple printheads are distributed over a long print zone to obtain the desired color and image resolutions. Image Registration and Color Control (IRCC) technology is configured to achieve color to color registration using a closed feedback loop controller. At cycle up of the continuous feed direct marking printer, the closed feedback loop controller is configured to print a registration control target (i.e., test pattern), capture the registration control target using a linear array sensor, analyze the linear array sensor response profile, and determine the x-position and y-position of each printhead. The computed registration errors are corrected by y-registration actuators and x-registration actuators. This IRCC technology has been demonstrated in the continuous feed direct marking printer for a blank paper.

The transaction printing industry uses pre-printed forms. For example, these pre-printed forms are used as medical claim forms, shipping documents, purchase orders, insurance records, etc. These pre-printed forms are used, for example, to add color, logos, etc. to a large market mainly populated by monochrome (i.e., one color or shades of one color) web printers.

The pre-printed rolls are produced using offset technology. In offset technology, inked image is transferred or “offset” from a plate to an intermediate surface (e.g., rubber blanket), and then to the printing surface.

Full color digital web printers with the capability to produce excellent graphics are now being offered. The transition from preprinted forms to execute the entire print job in one machine may take some time, because the transition requires not only substituting monochrome printers but also, for example, changing the workflow, etc.

The use of preprinted forms (or offset “shells”) presents a problem for the registration strategy of the continuous feed direct marking printer. That is, setup test pattern (such as color registration test target) that is printed on top of the pre-printed form is confounded with the pre-printed form. This issue (i.e., the registration control target is confounded with the pre-printed form) precludes the actual analysis of the x- and y-positions of the printheads. Further, it is not practical to print the test patterns on blank paper since that would require swapping out the web roll.

The present disclosure provides improvements in registration strategy of preprinted forms.

SUMMARY

According to one aspect of the present disclosure, a computer-implemented method for performing color registration on template media having template markings thereon is provided. The method is implemented in a computer system comprising one or more processors configured to execute one or more computer program modules. The method includes sensing the template media using a sensor positioned along a process path of a web to obtain first image data; printing a test pattern on the template media; sensing the template media along with the test pattern printed thereon using the sensor to obtain second image data; determining an output image data of the test pattern from the first image data, the second image data, and an estimated image data of the template media with the test pattern printed thereon; determining at least one of a process direction misregistration and a cross-process direction misregistration from the output image data; and adjusting at least one of a cross-process position and a process position of printheads based on the process direction misregistration and cross-process direction misregistration to provide adjusted color registration on subsequent template media. The estimated image data is representative of light scatter from the test pattern and light absorption by the test pattern.

According to another aspect of the present disclosure, a system for performing color registration on template media having template markings thereon is provided. The system includes a print engine, a sensor, a processor, and a controller. The print engine is configured to print a test pattern on the template media. The sensor is positioned along a process path of a web. The sensor is configured to sense a) the template media to obtain first image data; and b) the template media along with the test pattern printed thereon to obtain second image data. The processor is configured to a) determine an output image data of the test pattern from the first image data, the second image data, and an estimated image data of the template media with the test pattern printed thereon; and b) determine at least one of a process direction misregistration and a cross-process direction misregistration from the output image data. The controller is configured to adjust at least one of a cross-process position and a process position of printheads based on the process direction misregistration and cross-process direction misregistration to provide adjusted color registration on subsequent template media. The estimated image data is representative of light scatter from the test pattern and light absorption by the test pattern.

According to another aspect of the present disclosure, a computer-implemented method for performing color registration on template media having template markings thereon is provided. The method is implemented in a computer system comprising one or more processors configured to execute one or more computer program modules. The method includes sensing the template media using a sensor positioned along a process path of a web to obtain first image data; printing a test pattern on the template media; sensing the template media along with the test pattern printed thereon using the sensor to obtain second image data; transforming the first image data and the second image data into an absorbance space to obtain a first absorbance and a second absorbance, respectively; determining a difference between the first absorbance and the second absorbance; applying a correction factor to the determined difference to obtain an output absorbance; transforming the output absorbance into a reflectivity space to obtain an output image data; determining at least one of a process direction misregistration and a cross-process direction misregistration from the output image data; and adjusting at least one of a cross-process position and a process position of printheads based on the process direction misregistration and cross-process direction misregistration to provide adjusted color registration on subsequent template media. The output absorbance is representative of absorbance corresponding to the test pattern, the correction factor is representative of light scatter from the test pattern, and the output image data is representative of image data of the test pattern.

Other objects, features, and advantages of one or more embodiments of the present disclosure will seem apparent from the following detailed description, and accompanying drawings, and the appended claims.

DETAILED DESCRIPTION

When using pre-printed forms (or “offset shells”), the prior art color registration strategy of the continuous feed direct marking printer may fail in some instances. The present disclosure addresses the drawbacks of the prior art color registration strategy.

For example, to correct for an error caused by mis-registration of the (e.g., non-ideal) preprinted form image, the present disclosure proposes additional processing steps. For example, these additional processing steps may include (1) an image registration step that performs form registration between the preprinted form image and the image of CRTT overprinted on the preprinted form, and (2) an edge/outline cleaning step which performs a morphological filtering on the resulting difference image (i.e., image of the CRTT) to clean residual outlines (i.e., while preserving the CRTT dash-lines). These residual outlines may be formed as a result of improper image registration. These additional processing steps are described in detail with respect to the method300(shown and explained with respect toFIG. 3). Specifically, the image registration step is described in detail below at procedure350of the method300and the form outline cleaning step is described in detail below at procedure370of the method300.

The present disclosure proposes replacing the density/absorbance subtraction model (i.e., used in the prior art color registration strategy), for example, by the Kubelka-Munk model that represents bulk scatter in addition to absorption, and optionally by a surface-scatter model. Alternatively, a correction factor may be used to empirically correct for light scatter in the inks or toners of the test pattern. The use of the Kubelka-Munk model and the correction factor are described in detail below at procedure360of the method300.

The present disclosure also proposes replacing global thresholding strategy (i.e., used in the prior art color registration strategy) used to analyze the CRTT image with a local thresholding strategy. As explained in detail below with respect to procedure390of the method300, the local thresholding used in the present disclosure is much more robust to the image content of the preprinted form.

It is contemplated that, for performing color registration on template media having template markings, the present disclosure proposes the usage of either all three of the above described improvements (i.e., (a) the image registration and the edge/outline cleaning steps, (b) the Kubelka-Munk model or the correction factor to correct for light scatter in the inks or toners of the test pattern, and (c) the local thresholding strategy), or one or more of the above described improvements.

FIG. 1illustrates a continuous web printing system100. The continuous web printing system100includes a print engine, a linear array sensor128, a processor220and a controller240.

The continuous web printing system100also includes a web supply and handling system that is configured to supply a very long (i.e., substantially continuous) web154of “substrate” or “media” (e.g., paper, plastic or other printable material) from a spool (not shown). In another embodiment, the web154is in the form of an extensible image receiving member, such as a belt, which defines an image receiving surface that is driven in a process direction between print modules of the print engine. The web154may be unwound as needed, and propelled by a variety of motors (not shown). The web supply and handling system is capable of transporting the web154at a plurality of different speeds. In one embodiment, the web154is capable of being moved at any speed between approximately zero inches per second (ips) and approximately 150 inches per second (ips). A set of rolls are configured to control the tension of the unwinding web as the web moves through the path114.

In the present disclosure, the process direction is the direction in which the web, onto which the image is transferred and developed, moves through the image transfer and developing apparatus. The cross-process direction, along the same plane as the web, is substantially perpendicular to the process direction. In the present disclosure, the x-direction is referred to as the cross-process direction and y-direction is referred to as the process direction.

The print engine of the continuous web printing system100includes a series of print (or color) modules102,104,106,108,110, and112, each print module102,104,106,108,110, and112effectively extending across the width of the web154in the cross-process direction. The print engine is configured to print a test pattern on a template media (having template markings thereon). As shown inFIG. 1, the print modules102,104,106,108,110, and112are positioned sequentially along the in-track axis of a process path114defined in part by rolls116. The process path114is further defined by upper rolls118, leveler roll120and pre-heater roll122. A brush cleaner124and a contact roll126are located at one end of the process path114. The linear array sensor128, a heater130and a spreader132are located at the opposite end of the process path114.

Each print module102,104,106,108,110, and112is configured to provide an ink of a different color. Six print modules are shown inFIG. 1although more or fewer print modules may be used. In all other respects, the print modules102,104,106,108,110, and112are substantially identical. Accordingly, while only print module102will be further described in detail, such description further applies to the print modules104,106,108,110, and112.

Print module102includes two print sub modules140and142. Print sub module140includes two print units144and146and print sub module142includes two print units148and150. The print units144and148each include four printheads152while the print units146and150each include three printheads152. Thus, each of the print sub modules140and142include seven offset printheads152. The printheads152are offset to provide space for positioning of control components. The use of multiple printheads152allows for an image to be printed on the web154, which is much wider than an individual printhead152. For example, seven printheads152, which are each three inches wide, may be used to produce a 20.5 inch image on the web154, which is 21 inches wide. The print width of the exemplary print module102may be increased or decreased by adding or eliminating printheads to each two print sub modules.

Each of the printheads152includes sixteen rows of nozzles156. Each of the nozzles156is individually controlled to jet a spot of ink on the web154. The matrix of nozzles156in one embodiment provides a density of 300 nozzles per inch in the cross-process direction of the process path114. Accordingly, each printhead152produces an image with a spot density of 300 spots of ink per inch (SPI).

The provision of two sub modules, such as sub modules140and142, for each of the print modules102,104,106,108,110, and112provides increased resolution. Specifically, the printheads152in the sub modules142are offset in the cross-process direction of the process path114with respect to the printheads152in the sub module140by a distance corresponding to the width of a spot or a pixel in a printhead configured to provide 600 SPI. The resultant interlacing of the jets produced by the nozzles152generates an image with a 600 SPI resolution. It is contemplated that increasing printing resolutions may be achieved by utilizing single printheads of higher nozzle density.

As shown inFIGS. 1 and 2, the multiple printheads are distributed in a print zone over a long span of the web154. The position of the printheads is determined using the Integrated Registration and Color Control (IRCC) technology. This IRCC technology includes the linear array sensor128, the processor220(i.e., signal processing and control algorithms, and actuator electronics to determine process (y) and cross-process (x) direction distances between printheads), and IRCC board or controller162to adjust process (y) and cross-process (x) direction distances between printheads.

Alignment of the print modules102,104,106,108,110, and112with the process path114is controlled by a control system160shown inFIG. 2(only print module102is shown inFIG. 2). The control system160may be used with the system ofFIG. 1to control generation and detection of test patterns (or registration patterns) and to control the process position and the cross-process position of printheads.

The control system160includes an image registration and color control (IRCC) board or controller162and a memory164. The IRCC board162is connected to the linear array sensor128, the processor220and a speed sensor166, which detects the speed at which the web154moves along the process path114. The IRCC board or controller162is further connected to each of the printheads152to control jetting of the nozzles156, and a head position board168.

The linear array sensor128is a full width image contact sensor, which monitors the ink on the web154as the web154passes under the linear array sensor128. In general, such a full width linear array sensor can capture the template media (or the pre-printed form) when the printheads are not printing, or can capture the overprinted image for image-quality check. When there is ink on the web154, the light reflection off the web154is low and when there is no ink on the web154, the amount of reflected light is high. When a pattern of ink is printed by one or more of the printheads152under the control of the IRCC board162, the linear array sensor128may be used to sense the printed mark and provide a sensor output to the processor220. Such a full width array sensor that is used in a printhead registration correction system to achieve the image registration in the direct marking continuous web printers is described in U.S. patent application Publication Ser. No. 12/274,566 (filing date: Nov. 20, 2008), hereby incorporated by reference in its entirety, and hence will not be explained in detail here.

As shown inFIG. 1, the linear array sensor128is positioned along the process path114(as shown inFIG. 1) of the web154. When performing the registration strategy for pre-printed forms, a default sensor calibration that is stored in the sensor is used. In contrast, when performing the registration strategy for a blank paper, the sensor calibration is executed during every Cycle Up. In one embodiment, as shown inFIG. 1, the linear array sensor128is positioned upstream of the printheads to capture the pre-printed form or template media. The linear array sensor128is configured to sense a) the template media to obtain first image data; and b) the template media along with the test pattern printed thereon to obtain second image data.

In one embodiment, the template media is in the form of a continuous web having a plurality of template media. In one embodiment, the template media moves at 300 ft/min for high-quality applications and at 500 ft/min for low-quality applications. A first template media of the continuous web is sensed using the linear array sensor128positioned along the process path114of the web to obtain the first image data. A second or subsequent template media (with the test pattern printed thereon) of the continuous web is sensed using the linear array sensor128positioned along the process path114of the web to obtain the second image data.

In other words, the first template media of the continuous web is sensed using the linear array sensor128to obtain the linear array sensor response profile of the template media with template markings thereon (i.e., the first image data), then the test pattern is printed on the second or subsequent template media and the second or subsequent template media (i.e., along with the test pattern printed thereon) of the continuous web is sensed using the linear array sensor128to obtain the linear array sensor response profile of the template media along with the test pattern printed thereon (i.e., the second image data). The linear array sensor128is configured to provide the first image data and the second image data to the processor220.

In one embodiment, the processor220can comprise either one or a plurality of processors therein. Thus, the term “processor” as used herein broadly refers to a single processor or multiple processors. In one embodiment, the processor220can be a part of or forming a computer system. In one embodiment, the processor220can be a part of the image registration and color control (IRCC) board162(as shown inFIG. 2).

In one embodiment, the processor220is configured to a) determine an output image data of the test pattern from the first image data, the second image data, and an estimated image data of the template media with the test pattern printed thereon; and b) determine a process direction misregistration and a cross-process direction misregistration from the output image data. The estimated image data is representative of light scatter from the test pattern and light absorption by the test pattern.

In another embodiment, the processor220is configured to a) transform the first image data and the second image data into an absorbance space to obtain a first absorbance and a second absorbance, respectively; b) determine a difference between the first absorbance and the second absorbance; c) apply a correction factor to the determined difference to obtain an output absorbance; d) transform the output absorbance into a reflectivity space to obtain an output image data; and e) determine a process direction misregistration and a cross-process direction misregistration from the output image data. The output absorbance is representative of absorbance corresponding to the test pattern, the correction factor is representative of light scatter from the test pattern, and the output image data is representative of image data of the test pattern.

The above-mentioned embodiments where the processor220is configured to determine the output image data of the test pattern by taking into account light scatter from the test pattern, and to determine the process direction misregistration and the cross-process direction misregistration from the output image data are explained in detail below with respect to procedure360of the method300(as shown and explained with respect toFIG. 3).

In one embodiment, the processor220uses the output image data to determine the cross-process position of the nozzles156for the print units144,146,148, and150within the print module102(along with the nozzles156for the print units within the print modules104,106,108,110, and112). Based upon the relative positions, the processor220determines cross-process corrections for the print units144,146,148, and150. In other words, the processor220is configured to analyze the output image data to determine x-position and y-position of each printhead. In one embodiment, a registration algorithm (i.e., procedures380,390and395as shown and explained with respect toFIG. 3) of the processor220uses the amplitude of a repeating pattern at the expected spacing between dashes of the test pattern to compute the x- position and y-position of each printhead.

The system and method for determining registration errors in the cross-process direction is described in U.S. Patent Application Publication No. 2008/0062219, hereby incorporated by reference in its entirety, and hence will not be explained in detail here. U.S. patent application Publication Ser. No. 12/274,566 (filing date: No. 20, 2008), hereby incorporated by reference in its entirety, describes a printhead registration correction system and method for use with direct marking continuous web printers. This printhead registration correction system uses a full width array sensor to achieve the image registration in the direct marking continuous web printers. U.S. Patent Application Publication No. 2009/0265950, hereby incorporated by reference in its entirety, describes registration system for a continuous web printer.

In one embodiment, y-registration (i.e., process direction registration) of the image is achieved by a double reflex printing technology that determines jet timing of each printhead based on web motion measured by encoders230,240(as shown inFIG. 1) and tensiometers. The double reflex printing technology is described in U.S. Patent Application Publication No. 2008/0124158, hereby incorporated by reference in its entirety, and hence will not be explained in detail here. This patent application provides a more detailed description of a double reflex printing registration system and different methods of determining the double reflex printing offsets based on time varying changes in tension of the web. The double reflex printing registration system is configured to determine a double reflex printing offset for each printhead positioned along the web path which may be used to control system160to adjust the predetermined actuation time for each printhead so that each image applied by the various printheads is correctly registered on the web to form the desired composite color image.

In one embodiment, the printhead displacement offsets (i.e., process and cross-process direction misregistrations) may be used in conjunction with double reflex printing offsets to adjust actuation times for the printheads to compensate for registration errors that may be introduced due to time varying changes in tension of the web as well as registration errors that may be introduced due to printhead displacement that may occur over a period of time.

The controller or IRCC162is configured to adjust a cross-process position and a process position of printheads based on the process direction misregistration and cross-process direction misregistration to provide adjusted color registration on subsequent template media.

The IRCC board or controller162receives the process direction misregistration and the cross-process direction misregistration from the processor220and then passes the process direction misregistration and the cross-process direction misregistration to the head position board168, which in turn controls the cross-process position of the print units144,146,148, and150. In one embodiment, the computed process and cross-process misregistrations are corrected by y-registration actuators and x-registration actuators. The position of the print units144,146,148, and150may be individually controlled using stepper motors configured to change the location of the associated print units144,146,148, or150in one micron increments. Alternatively, piezoelectric motors may be used to reduce the potential for backlash when changing direction of the motors.

FIG. 3illustrates the method300for performing color registration on template media having template markings thereon by taking both light scatter from the test pattern and light absorption by the test pattern into account. The method300is a computer-implemented method that is implemented in a computer system comprising one or more processors220(as shown in and explained with respect toFIGS. 1 and 2) configured to execute one or more computer program modules.

The method300includes, during Cycle Up, sensing a blank preprinted form using the linear array sensor128, printing the control registration test target (or test pattern) on a subsequent and identical blank preprinted form, sensing the preprinted form with the control registration test target printed thereon using the linear array sensor128, using optical models incorporating bulk light scatter (e.g., Kubelka-Munk model) and/or surface light scatter (e.g., the optical model disclosed in “The Effect of Gloss on Color” by E. N. Dalal and K. Natale-Hoffman,Color Res. &App.,24, 369-376, 1999, incorporated herein by reference) to obtain the reflectance of the control registration test target and executing the IRCC (i.e., Image Registration and Color Control) analysis on the obtained control registration test target image. Alternatively, instead of using optical models incorporating bulk light scatter and/or surface light scatter, the method300is configured to use a correction factor to empirically correct for light scatter in the inks or toners of the test pattern.

The method300begins at procedure310, where cycle up of the continuous web printing system100is started. The method300then proceeds to procedure320. At procedure320, the template media having template markings thereon is sensed using the linear array sensor128positioned along the process path114of a web to obtain first image data. In one embodiment, such linear array sensor may be positioned upstream of the printheads to capture the template media (or the pre-printed form).FIG. 5illustrates a simulated image capture of the template media (i.e., with template markings thereon) by the linear array sensor128. The first image data is a linear array sensor response profile of the template media with template markings thereon.

An exemplary template media400having template markings thereon is illustrated inFIG. 4. The exemplary template media400as shown inFIG. 4is a pre-printed form of a sales receipt. In one embodiment, as shown inFIG. 4, the template markings include form images401, marks, report formats, banners, logos401, letterhead, data heading for spaces for data, pre-printed text402, pre-printed boxes403, pre-printed lines, and/or questions with corresponding spaces for answers.FIG. 6illustrates a simulated image capture of a test pattern printed on a blank paper by the linear array sensor128.

At procedure330, during cycle up, a test pattern is printed on the template media500. In one embodiment, the test pattern may include a plurality of dashes, the dashes being process direction dashes. The test pattern may include repeated single pixel dashes or dash lines (e.g., 20 to 25 pixels long), addressing all the printheads in the system. In one embodiment, the test pattern may include 1-on and 4-off dash lines.

The method300then proceeds to procedure340, where the template media along with the test pattern printed thereon is sensed or captured using the linear array sensor128to obtain second image data. The second image data is a linear array sensor response profile of the template media along with the test pattern printed thereon.FIG. 7illustrates a simulated captured template media along with the test pattern printed thereon. For illustrative purposes, a monochromatic linear array sensor was used to capture the exemplary images shown inFIGS. 5-7.

In one embodiment, the template media is in the form of a continuous web having a plurality of template media. A first template media of the continuous web is sensed using the linear array sensor128positioned along the process path114of the web to obtain the first image data. A second or subsequent template media (with the test pattern printed thereon) of the continuous web is sensed using the linear array sensor128positioned along the process path114of the web to obtain the second image data.

In other words, the first template media of the continuous web is sensed using the linear array sensor128to obtain the linear array sensor response profile of the template media with template markings thereon, then the test pattern is printed on the second or subsequent template media and the second or subsequent template media (i.e., along with the test pattern printed thereon) of the continuous web is sensed using the linear array sensor128to obtain the linear array sensor response profile of the template media along with the test pattern printed thereon.

Optionally, at procedure350, the method300is configured to perform image registration between the first input image data and the second input image data by aligning reference points on the first input image data with reference points on the second input image data. This image registration procedure is configured to remove the noises caused by preprinted form differences in the web roll as well as the paper to sensor mis-registration due to Motion Quality (MQ). These preprinted form differences in the roll are caused by, for example, imperfect preprinted form, Image-On-Paper (IOP), MQ, etc.

In one embodiment, the image registration procedure is performed by aligning the reference points (i.e., anchor points/objects) between the first input image data and the second input image data. For example, such reference points may include corners, boxes, lines, and edges that are likely to be present in the template media or the preprinted form.

In one embodiment, the reference points may be pre-selected offline (since the preprinted form is the same for the whole roll of paper). The pattern matching techniques may be used in real-time to locate these pre-selected reference points on captured images for image registration.

In another embodiment, a pre-defined list of characteristics (e.g., a cross-mark, a circular dot, a L-shaped mark, etc) that are likely to occur in the preprinted forms are stored in a database. Reference points may be identified in real-time with these pre-defined list of characteristics. These pre-defined list of characteristics are chosen such that they occur in the preprinted forms but are not exhibited on the CRTT (single-pixel lines with known length and known on-off frequency). This database may be constantly updated with new characteristics.

In yet another embodiment, reference points may be identified in a real-time with the pre-defined list of characteristics. If the reference points are not identified in the pre-defined list of characteristics, then the reference points may be selected offline (since the preprinted form is the same for the whole roll of paper).

At procedure360, the method300is configured to derive output image data of the test pattern from the first image data and the second image data by taking light scatter from the test pattern and light absorption of the test pattern into account.

The output image data of the test pattern may be obtained using two approaches. As will be clear from the discussions below, in the first approach, a correction factor is used to empirically correct for light scatter in the inks or toners of the test pattern. In the second approach, the Kubelka-Munk model is used to represent light scatter in the inks or toners of the test pattern.

In the first approach, the method300is configured to transform the first image data and the second image data first into an absorbance space to obtain a first absorbance and a second absorbance, respectively. In the first approach, an RGB sensor or a monochromatic sensor may be used.

The first absorbance is obtained by taking a decimal logarithm for the first image data (i.e., in reflectivity space), according to the Equation (1):
AF(x, y)=−log10[RF(x, y)]  Equation (1)where AF(x, y) is the first absorbance; andRF(x, y) is the first image data.

In other words, RF(x, y) is the reflectance of the template media, without the test pattern printed thereon, as sensed by the sensor at location (x, y), and AF(x, y) is the corresponding absorbance at that location.

The second absorbance is obtained by taking a decimal logarithm for the second image data (i.e., in reflectivity space), according to the Equation (2):
A(x, y)=−log10[R(x, y)]  Equation (2)where A(x, y) is the second absorbance; andR(x, y) is the second image data.

In other words, R(x, y) is the reflectance of the template media along with the test pattern printed thereon, as sensed by the sensor at location (x, y), and A(x, y) is the corresponding absorbance at that location.

It should be appreciated that the foregoing equations (i.e., Equation (1) and Equation (2)) denote the conversion of the linear array sensor response profiles from a pure reflectivity space (e.g., a color space such as RGB) to a density space.

It should be also appreciated that the image data is transformed into the absorbance space so as to subtract the absorbances of the two images (i.e., the template media, and the template media with the test pattern printed thereon) and obtain the output absorbance (i.e., absorbance of the test pattern). In other words, the reflectivity is not an additive quantity and is generally in a percentage form, thus, the data in the reflectivity space is not subtracted. Therefore, the image data is converted into absorbance space to calculate the difference between the captured images (i.e., captured template media (or the first image data, as shown inFIG. 5) and the captured template media along with the test pattern printed thereon (or the second image data, as shown inFIG. 7)).

After transforming the image data into the absorbance space, the method300is configured to a) determine a difference between the first absorbance AF(x, y) and the second absorbance A(x, y), and b) apply a correction factor f[A(x, y),AF(x, y)] to the determined difference to obtain an output absorbance. The output absorbance is representative of absorbance corresponding to the test pattern. The output absorbance is determined according to the Equation (3):
AT(x, y)=[A(x, y)−AF(x, y)]−f[A(x, y),Af(x, y)]  Equation (3)where AT(x, y) is the output absorbance;AF(x, y) is the first absorbance;A(x, y) is the second absorbance; andf[A(x, y),AF(x, y)] is the correction factor.

In one embodiment, the correction factor is a function of the first absorbance and the second absorbance and is representative of light scatter from the test pattern.

In one embodiment, an off-line calibration procedure is first performed (i.e., before the real-time measurements) to determine the correction factor. During this off-line calibration procedure, solid (100%) patches of, for example, C, M, Y, and K inks (i.e., in case of CMYK color model) are printed over various colors of the template media and over the bare substrate. This off-line calibration procedure yields a measured absorbance of the test pattern printed over the bare substrate, ATjas a function a) a measured absorbance of the test pattern printed over the template media, Aijand b) a measured absorbance of the template media without a test pattern printed thereon, AFi. The correction factor, f└Aij, AFi┘ is obtained using the Equation (4):
TTj=Aij−AFi−f└Aij, AFi┘  Equation (4)where f└Aij, AFi┘ is the correction factor relative to a simple non-scattering model;i represents various preprinted colors (e.g., i=1˜n) of the template media;j represents various inks or toners (e.g., C, M, Y or K) of the test pattern;AFiis the measured absorbance of the various preprinted colors of the template media (i.e., without a test pattern printed thereon);ATjis the measured absorbance of the various inks of the test pattern printed over the bare substrate; andAijbe the measured absorbance of the various inks or toners of the test pattern printed over the template media with the various preprinted colors (e.g., i=1˜n).

In the Equation (4), a color system having a set of inks or toners j, such as cyan, yellow, magenta, black (CMYK) is used by the image printing device for printing a test pattern. However, it is contemplated that any other color system having set of N inks or toners (e.g., N>3), for example CMYKOV, CMYKO, CMYKOG, CcMmYK, or CMYKOB may be used by the image printing device for printing the test pattern.

After obtaining the output absorbance AT(x,y) from Equation (3), the method300is then configured to transform the output absorbance AT(x, y) into a reflectivity space to obtain an output image data. The output image data is representative of image data of the test pattern. The output image data is obtained by taking an exponential function of the output absorbance, according to the Equation (5):
RT(x,y)=10[−AT(x,y)]Equation (5)where RT(x, y) is the output image data; andAT(x, y) is the output absorbance.

It should be appreciated that the foregoing equation (i.e., Equation (5)) converts absorbance (i.e., corresponding to the test pattern) in the density space to the image data of the test pattern in the reflectivity space (i.e., its original color space).

In the second approach, the method300, at procedure360, is configured to determine an output image data of the test pattern from the first image data, the second image data, and an estimated image data of the template media with the test pattern printed thereon. The estimated image data is representative of light scatter from the test pattern and light absorption by the test pattern.

In this second approach, a spectral sensor is used. In one embodiment, the first image data and the second image data may be obtained at each wavelength in the spectral range of the sensor. In another embodiment, the first image data and the second image data may be obtained at some pre-selected wavelengths (i.e., subset of the spectral range of the sensor). In this approach, the first image data and the second image data may be obtained at each location (x, y) of the sensor, where x is the cross-process direction and y is the process direction.

In one embodiment, the estimated image data of the template media with the test pattern printed thereon for each ink or color of the test pattern is determined using equations (6) and (7).

Specifically, the estimated image data of the template media without any ink or color of the test pattern printed thereon is obtained using Equation (6). In other words, in Equation (6), j is equal to 0 corresponding to no ink or toner in the test pattern.
i Rj(x, y)=RF(x, y)   Equation (6)where Rj(x, y) is the estimated image data of the template media without the test pattern printed thereon (i.e., for no ink (or toner) of the test pattern, or j=0);j corresponds to inks or toners of the test pattern, and j=0; andRF(x, y) is the first image data;

In other words, RF(x, y) is the reflectance of the template media, without the test pattern printed thereon, as sensed by the sensor at location (x, y) .

The estimated image data of the template media with various toners or inks of the test pattern printed thereon is obtained using Equation (7). In other words, in Equation (7), j is greater than 0 corresponding to various inks or toners in the test pattern.

Rj⁡(x,y)=1-RF⁡(x,y)*⌊aj-bj*coth⁡(bj⁢Sj⁢X)⌋aj-RF⁡(x,y)+bj*coth⁡(bj⁢Sj⁢X)Equation⁢⁢(7)where Rj(x, y)is the estimated image data of the template media with the test pattern printed thereon for each ink or toner, j in the test pattern;j corresponds to each toner or ink, j in the test pattern, and j>0;RF(x, y) is the first image data;ajis a predetermined value;bjis a predetermined value;Sjis a predetermined value and is representative of light scatter from the test pattern;coth is the hyperbolic cotangent function; andX is thickness of the color and is set to be 1.

In this second approach, an off-line characterization of the absorption and scattering of the various inks or toners (e.g., C, M, Y, and K inks) in the test pattern is performed, for example, using the Kubelka-Munk model to determine ak, bj, and Sj. aj, bj, and Sjare determined using the Equation (8). As can be clearly seen from Equation (8) below, the predetermined values aj, and bjare functions of Kj(i.e., light absorbance) and Sj(i.e., light scatter).

In general, at each wavelength of light, Kubelka-Munk model relates the light absorption and the light scattering behavior of a material to its reflectivity. The Kubelka-Munk equation may be written in several forms. The commonly used “coth” form is given by Equation (8).

To determine aj, bj, and Sjin Equation (7), the off-line characterization of the absorption and scattering of the various inks or toners (e.g., C, M, Y, and K inks) in the test pattern is performed. This is done, for example, by printing solid (100%) patches of the various inks or toners (e.g., C, M, Y, and K inks) in the test pattern over at least a white substrate and a black substrate. The spectral measurements are made over a range of wavelengths, for example, from 400 nm to 700 nm in 10 nm increments.

All the terms in Equation (8), except for X, are spectra, and are treated independently at each wavelength λ.

If the Rgand Rjspectra are measured, then at each wavelength λ for each ink j there are only two unknowns, Kjand Sj, in Equation (8). Therefore, performing the measurements on two different substrates, such as a white substrate and a black substrate, yields two equations which may be solved for the two unknowns Kjand Sj. If more than two substrates are used, the two unknowns Kjand Sjmay be determined more robustly, for example, by a regression procedure.

Once the predetermined values (i.e., aj, bj, and Sj) are determined from Equation (8), these values are used in Equation (7) to determine the estimated image data (i.e., Rj(x,y)) of the template media with the test pattern printed thereon for each ink or toner, j in the test pattern.

In the Equations (7) and (8), a color system having a set of inks or toners j, such as cyan, yellow, magenta, black (CMYK) is used by the image printing device for printing a test pattern. However, it is contemplated that any other color system having set of N inks or toners (e.g., N>3), for example CMYKOV, CMYKO, CMYKOG, CcMmYK, or CMYKOB may be used by the image printing device for printing the test pattern.

After determining the estimated image data (i.e., Rj(x,y)) of the template media with the test pattern printed thereon for each ink or toner, j in the test pattern, the method300is configured to compare the second image data with the estimated image data obtained for each ink or toner in the test pattern to identify the estimated image data for one ink or toner that closely matches the second image data. In other words, the spectrum of the reflectance of the template media (i.e., R(x,y)), with the test pattern printed thereon, as sensed by the sensor at location (x,y) is compared with the five spectra (i.e., j=0 for no ink, and j=1-4 corresponding to C, M, Y, or K inks) for the estimated image data (i.e., Rj(x,y)) to identify the closest match between them (i.e., R(x, y)and Rj(x,y)). In one embodiment, identifying the closest match is done, for example, by minimizing the least-squares error.

If the closest match corresponds to j=J, then the spectrum of the reflectance of the test pattern, as sensed by the sensor at location (x, y) (i.e., RT(x, y)) given by Equation (9).

RT⁡(x,y)=1-RP*[aJ-bJ*coth⁡(bJ⁢SJ⁢X)]aJ-RP+bj*coth⁡(bJ⁢SJ⁢X)Equation⁢⁢(9)where RT(x, y) is output image data of the test pattern;RPis an image data of a bare substrate;J is an ink or toner in the test pattern where the estimated image data closely matches the second image data.aJis a predetermined value and is equal to

1+(KJSJ)bJis a predetermined value and is equal to √{square root over (aJ2−1)}KJis a predetermined value and is light absorbance of the test pattern;SJis a predetermined value and is the light scatter from the test pattern;coth is the hyperbolic cotangent function; andX is thickness of the color and is set to 1.

In Equation (9), aJ, bJ, KJ, and SJare predetermined values that are determined using Equation (8) at j=J.

In one embodiment, some pre-processing of the measured reflectance spectra (i.e., R(x, y)) may be performed. For example, such pre-processing may include Saunderson correction for reflection at interfaces.

FIGS. 8A and 8Billustrate exemplary images captured by the linear array sensor after the procedure360of the method300.

Specifically,FIG. 8Aillustrates an exemplary output image data806obtained after a perfect image registration (i.e., at procedure350) between the first image data (i.e.,FIG. 5) and the second image data (i.e.,FIG. 7). As can be seen inFIG. 8A, when a perfect image registration between the first image data and the second image data is achieved, there are no residual errors in the output image data806.

In contrast,FIG. 8Billustrates an exemplary output image data808obtained after an imperfect image registration (i.e., at procedure350) between the first image data (i.e.,FIG. 5) and the second image data (i.e.,FIG. 7). As can be seen in FIG.8B, the output image data808, for example, is off by one pixel in cross-process direction and two pixels in process direction. This imperfect image registration between the first image data and the second image data resulted in residual errors802and804in the output image data808as shown inFIG. 8B.

To remove any residual errors (i.e., resulted from an imperfect image registration), the method300, at procedure370, is configured to optionally perform a form outline cleaning procedure. These residual errors in the output image data are removed while preserving the CRTT image.

In one embodiment, morphological filters with a defined structure (e.g., that excludes the elements that resemble CRTT) may be used to perform the form outline cleaning procedure. In other words, the morphological filters with the defined structure are configured to clean out everything except elements that looks like single pixel lines of known length.

Morphological filtering techniques generally include erosion and/or dilation steps. Various morphological filtering techniques, including erosion and dilation of an image, are disclosed, for example, inComputer and Robot VisionVol. I by Haralick, R. M and L. G. Shapiro, Addison-Wesley Publishing, 1992, pp. 158-205; and in “Methods for Fast Morphological Image Transforms Using Bitmapped Images,”Computer Vision, Graphics, and Image Processing: Graphical Models and Image Processing, van den Boomgard, R, and R. van Balen, Vol. 54, Number 3, pp. 254-258, May 1992, herein incorporated by reference in their entirety.

In another embodiment, an edge detection is performed on the output image data and any pixels around these edge pixels are discounted. This edge detection technique (i.e., for removing residual errors) is a passive approach but works well when the error in image registration is within a pixel or two pixels.

FIGS. 9A and 9Billustrate exemplary images captured by the linear array sensor after the form outline cleaning procedure (i.e., procedure370) of the method300.

Specifically,FIG. 9Aillustrates an exemplary image906that is obtained after performing the form outline cleaning procedure on the image806. As can be seen inFIGS. 8A and 9A, when a perfect image registration between the first image data and the second image data is achieved, there are no residual errors in the output image data806, and hence the form outline cleaning procedure need not be performed. In other words, the form outline cleaning procedure (i.e., the procedure370) is a null-process, when a perfect image registration (at the procedure350) is achieved.

In contrast,FIG. 9Billustrates an exemplary image908that is obtained after performing the form outline cleaning procedure on the image808. During the form outline cleaning procedure on the image808, a series of “open” filtering were used to remove structure elements that are larger than 5×5, 30×1, or 1×10 array of pixels. By comparingFIG. 8BwithFIG. 9B, it is clear that the form outline procedure (i.e., procedure370) of the method300improved the resulting image908of the test pattern. It is contemplated that the structure elements may be further optimized for individual preprinted forms. It can be seen fromFIGS. 8A-9Bthat the form outline cleaning procedure improves the resulting image in all cases except in the case where a perfect image registration is achieved.

At procedure380, the method300is configured to provide digital image enhancement to the output image data (i.e., image data of the test pattern). This image enhancement may include improving image contrast by reducing additional noise. Image processing algorithms for improving the image contrast are for example described in detail in “Contrast Limited Adaptive Histogram Equalization,” Graphic Gems IV, San Diego: Academic Press Professional, 1994. 474-485, by Karel Zuiderveld, hereby incorporated by reference in its entirety, and hence will not be explained in detail here.

At procedure390, the method300is configured to determine a process direction misregistration and a cross-process direction misregistration from the output image data (i.e., image data of the test pattern).

The system and method for determining registration errors in the cross-process direction is described in U.S. Patent Application Publication No. 2008/0062219, hereby incorporated by reference in its entirety, and hence will not be explained in detail here. U.S. patent application Publication Ser. No. 12/274566 (filing date: Nov. 20, 2008), hereby incorporated by reference in its entirety, describes a printhead registration correction system and method for use with direct marking continuous web printers. This printhead registration correction system uses a full width array sensor to achieve the image registration in the direct marking continuous web printers. U.S. Patent Application Publication No. 2009/0265950, hereby incorporated by reference in its entirety, describes registration system for a continuous web printer.

In one embodiment, y-registration (i.e., process direction registration) of the image is achieved by a double reflex printing technology that determines jet timing of each printhead based on web motion measured by encoders230,240(as shown inFIG. 1) and tensiometers. The double reflex printing technology is described in U.S. Patent Application Publication No. 2008/0124158, hereby incorporated by reference in its entirety, and hence will not be explained in detail here. This patent application provides a more detailed description of a double reflex printing registration system and different methods of determining the double reflex printing offsets based on time varying changes in tension of the web. The double reflex printing registration system is configured to determine a double reflex printing offset for each printhead positioned along the web path which may be used to control system160(as shown inFIG. 2) to adjust the predetermined actuation time for each printhead so that each image applied by the various printheads is correctly registered on the web to form the desired composite color image.

In one embodiment, the printhead displacement offsets (i.e., process and cross-process direction misregistrations) may be used in conjunction with double reflex printing offsets to adjust actuation times for the printheads to compensate for registration errors that may be introduced due to time varying changes in tension of the web as well as registration errors that may be introduced due to printhead displacement that may occur over a period of time.

At the procedure390, the method is configured to obtain a profile in the cross-process direction and a profile in the process direction. Centriods are identified on each of these profiles to obtain the x offset and the y-offset of the printhead, respectively.

For example, during the extraction of the x-offset of the printhead, the method300is configured to average the reflectances RT(x, y) of the derived CRTT image (i.e., output image data) along y (or process) direction to obtain the profile p(x) in the cross-process direction. The profile in the cross-process direction is given by the Equation (11).

where RT(x, y) be the reflectance of the output image data at location (x, y), where x is the cross-process direction and y is the process-direction;

p(x) is the profile in the cross-process direction;

Σ is the summation function; and

count ( ) is the count function.

Once the profile in the cross-process direction is obtained, centroids of the CRTT's are identified to obtain the x offset of the printhead. An equation similar to Equation (11) may be used to derive a profile in the process direction, and hence the y-offset of the printhead.

In the prior art, CRTT lines were analyzed as group with a global thresholding strategy to identify the centroids of the CRTT's. Though a filtering step was applied prior to the global thresholding strategy to remove background non-uniformity due to printer and/or sensor, this filtering step is not effective enough to deal with the “background non-uniformity” caused by the image content of the template media or the preprinted forms. In the prior art, centroids of the CRTT's are identified by calculating a threshold η. For example, the threshold η is set globally for entire profile p(x) regardless of position x (i.e., in the cross-process direction). The threshold η may be obtained using Equation (12).
η=(max(p(x))+min(p(x))/2   Equation (12)where η is the threshold;p(x) is the profile in the cross-process direction; andmax( ) and min( ) represent maximum and minimum functions of the profile in the cross-process direction, respectively.

The max and min values of the profile p(x) in Equation (12) may also be replaced with 95-percentile and 5-percentile of the profile p(x), respectively.

FIG. 10Aillustrates a zoom-in view of an area (i.e., upper left corner) of the exemplary image illustrated inFIG. 7showing the lines of the test pattern printed on the template media andFIG. 10Billustrates a zoom-in view of an area (i.e., upper left corner) of the exemplary image illustrated inFIG. 6showing the lines of the test pattern printed on the blank paper. Though the image of the captured template media with CRTT are shown inFIG. 10Ait is contemplated that an image of the derived CRTT image obtained after procedure360in the method300may be used.

FIGS. 11A and 11Billustrate exemplary profiles (i.e., p(x) in the cross-process direction) of the two-sub regions of interest of the areas shown inFIGS. 10A and 10B, respectively after a global thresholding strategy is performed. The graphs inFIGS. 11A and 11Billustrates the position (i.e., in the cross-process direction) on a horizontal x-axis. On a vertical y-axis, the graph illustrates the profile in the cross-process direction. As shown inFIGS. 11A and 11B, the global threshold η inFIG. 11Ais between 100-150 and the global threshold η inFIG. 11Bis close to200. Although the global threshold strategy works well forFIG. 11B, the global threshold strategy fails forFIG. 11Ain identifying all dash lines (too low for first few lines and too high for the rest) and thus cannot properly extract printhead x-offsets in the case where the image content of the preprinted form was not cleaned up successfully.

In contrast, the method300of the present disclosure, at procedure390, is configured to optionally perform a local thresholding strategy (i.e., as opposed to the global thresholding strategy of the prior art), which first localizes individual CRTT lines or a small set of CRTT lines and then estimates local background as the threshold. In one embodiment, pattern-matching methods are used to isolate each CRTT line (i.e., dash line with known size). The IRCC algorithms are configured for localized processing of these isolated CRTT line(s).

The method300, at procedure390, is configured to calculate a local threshold η′for each of the multiple regions in the images shown inFIGS. 10A and 10B, where each region includes a small group/set of lines (i.e., color registration test targets). The local threshold η′may be calculated using Equation13.
η′=(max(p(x))+min(p(x))/2   Equation (13)where η′is the local threshold for a region having a small group of lines;p(x) is the profile in the cross-process direction for that region of interest; andmax( ) and min( ) represent maximum and minimum functions of the profile in the cross-process direction for that region of interest, respectively.

For example, for the region between x=1 and x=15, the local threshold η1′is calculated based on profile in that section only. In other words, the local threshold η1′=(max(p(x))+min(p(x))/2, where x=1˜15. Similarly, for the region between x=91 and x=105, the local threshold η7′is calculated based on profile at that section only. In other words, the local threshold η7′=(max(p(x))+min(p(x))/2, where x=91˜105.

FIGS. 12A and 12Billustrate exemplary profiles (i.e., p(x) in the cross-process direction) of the two-sub regions of interest of the areas shown inFIGS. 10A and 10B, respectively after a local thresholding strategy is performed. The graphs inFIGS. 12A and 12Billustrates the position (i.e., in the cross-process direction) on a horizontal x-axis. On a vertical y-axis, the graph illustrates the profile in the cross-process direction.

As shown inFIG. 12A, the local threshold η1′shown in the first graph ofFIG. 12Ais close to200and the local threshold η7′shown in the second graph ofFIG. 12Ais close to50. As shown inFIG. 12A, η7′is much smaller than η1′due to the impact of residual image contents of the template media behind the CRTT's in that line groups. This is, however, appropriate threshold for this line groups in order to correctly identify the centroids of these lines. As can be seen fromFIG. 12B, in the case of CRTT's printed on the blank paper, the resulting local thresholds η′'s will be close to each other and close the global threshold η derived from prior art.

As is clear from a comparison between the graphs inFIGS. 11A and 11Bwith the graphs inFIGS. 12A and 12B, the global thresholding strategy used in the prior art works well for the image inFIG. 10B(i.e., when the test pattern is printed on blank paper) but does not work for the image inFIG. 10A(i.e., where the image contents of the template media are not fully removed after procedure370of the method300). Therefore, impact of the “background non-uniformity” caused by the image content of the template media or the preprinted forms may be reduced dramatically by a) identifying individual CRTT lines (or group of CRTT lines) and b) processing these identifying individual CRTT lines (or group of CRTT lines) in isolation, as proposed by the local thresholding strategy of the present disclosure.

At procedure395, the method300is configured to determine whether the determined process direction misregistration and cross-process direction misregistration are less than a threshold. In one embodiment, the threshold may be a predetermined value or range. If it is determined that the determined process direction misregistration and cross-process direction misregistration are less than the threshold, then the method300proceeds to procedure398. If not (i.e., the determined process direction misregistration and cross-process direction misregistration are not less than the threshold), the method300returns to procedure330where the test pattern is printed on the template media (i.e., during cycle up), then to procedure340and so on. In one embodiment, if the determined process direction misregistration and cross-process direction misregistration are not less than the threshold, then the method300may be configured to adjust the cross-process position and process position of printheads before returning to procedure330.

In one embodiment, if it is determined that the determined process direction misregistration and cross-process direction misregistration are less than the threshold, then the method300(i.e., before proceeding to procedure398) is configured to adjust cross-process position and process position of printheads to provide adjusted color registration on subsequent template media.

In one embodiment, the registration algorithm (i.e., procedures380,390and395as shown and explained with respect toFIG. 3) uses the amplitude of a repeating pattern at the expected spacing between dashes to compute the x- and the y-positions.

The method300ends at procedure398, where cycle up of the continuous web printing system100ends and printing (i.e., runtime print job) starts.

In one embodiment, the procedures310-398can be performed by one or more computer program modules that can be executed by one or more processors220(as shown in and explained with respect toFIGS. 1 and 2).

As used herein, “template markings” are any type of marks, visible to the human eye or otherwise detectable by some kind of sensor, that are positioned on the web so that marks or images subsequently made on the web in a printing process in some way fit with or correspond to the template markings, either whereby the template markings and the printed images form a single coherent visible image, or for some other purpose, such as fiducial or encoding marks. A template marking may also be in the form a physical feature of the web, such as perforations, notches, or stickers disposed on a backing web, in cases where a printer is used to make labels.

Embodiments of the present disclosure, the processor, for example, may be made in hardware, firmware, software, or various combinations thereof The present disclosure may also be implemented as instructions stored on a machine-readable medium, which may be read and executed using one or more processors. In one embodiment, the machine-readable medium may include various mechanisms for storing and/or transmitting information in a form that may be read by a machine (e.g., a computing device). For example, a machine-readable storage medium may include read only memory, random access memory, magnetic disk storage media, optical storage media, flash memory devices, and other media for storing information, and a machine-readable transmission media may include forms of propagated signals, including carrier waves, infrared signals, digital signals, and other media for transmitting information. While firmware, software, routines, or instructions may be described in the above disclosure in terms of specific exemplary aspects and embodiments performing certain actions, it will be apparent that such descriptions are merely for the sake of convenience and that such actions in fact result from computing devices, processing devices, processors, controllers, or other devices or machines executing the firmware, software, routines, or instructions.

While the present disclosure has been described in connection with what is presently considered to be the most practical and preferred embodiment, it is to be understood that it is capable of further modifications and is not to be limited to the disclosed embodiment, and this application is intended to cover any variations, uses, equivalent arrangements or adaptations of the present disclosure following, in general, the principles of the present disclosure and including such departures from the present disclosure as come within known or customary practice in the art to which the present disclosure pertains, and as may be applied to the essential features hereinbefore set forth and followed in the spirit and scope of the appended claims.