Patent Description:
When correcting cross-web spatial non-uniformity of a print engine with multiple fixed print heads, relatively large non-uniformities may occur at the junction between the print heads and within a printhead. However, correction of each print head nozzle using interpolated reflectance measurements does not fix this problem since the correction of each nozzle affects the reflectance of nearby locations, and because gaps or overlaps between print head segments are not taken account of when nozzles are independently compensated.

Moreover, fiducial marks at the print head segment boundaries can be used to accurately locate individual nozzles. This facilitates deriving a transfer function for each individual nozzle within a print head, so that the corrections based on measurements precisely centered on each nozzle can be applied to the proper nozzle. Nonetheless, the print head segment boundaries remain uncontrolled, and corrections to nearby nozzles will continue to interact.

Accordingly, a mechanism to perform nozzle uniformity compensation is desired.

<CIT> discloses a printing system which includes a printer to print image data to a medium. The printer includes a first pass channel including a first set of pel forming elements to print a first component of the image data and a second pass channel including a second set of pel forming elements to print a second component of the image data, wherein the first component of the image data and the second component of the image data occupy a same region on the print medium. The printing system also includes a print controller to perform uniformity compensation based on a combined compensation of the first pass channel and the second pass channel.

<CIT> discloses that CDE is measured for each nozzle array, to enable modification of a mapping between input image data and intended printing marks to compensate for the CDE. Printing proceeds using the modified mapping, which is either an optical-density transformation of data to printing marks or a spatial-resolution relation between image data and intended pixel grid. The density transformation preferably includes a dither mask (but can be error-diffusion thresholding instead); the resolution relation includes scaling of image data, to pixel grid. For some invention forms, CDE includes printing-density defects, measured and used to derive a correction pattern - in turn used to modify halftone thresholding. For other forms CDE includes swath-height error, but still this is measured and used to derive a correction pattern etc. For still other forms, however, CDE includes swath-height error and correction takes the form of scaling. When the halftoning forms are applied to plural-pass printing, a printmask is used to map the dither mask etc. to the nozzle array, enabling application of the correction to the mask. Halftone forms ideally uses a gamma function, though threshold or linear corrections are possible instead. Halftone correction is effective in single-pass printing. The swath-height correction can modify heights of all nozzle arrays. Computations are done at most only once for a full image.

A uniformity compensation mechanism is described. In the following description, for the purposes of explanation, numerous specific details are set forth to provide a thorough understanding of the present invention. It will be apparent, however, to one skilled in the art that the present invention may be practiced without some of these specific details. In other instances, well-known structures and devices are shown in block diagram form to avoid obscuring the underlying principles of the present invention.

Reference in the specification to "one embodiment" or "an embodiment" means that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one embodiment of the invention.

<FIG> is a block diagram illustrating one embodiment of a printing system <NUM>. A host system <NUM> is in communication with the printing system <NUM> to print a sheet image <NUM> onto a print medium <NUM> via a printer <NUM> (e.g., print engine). Print medium <NUM> may include paper, card stock, paper board, corrugated fiberboard, film, plastic, synthetic, textile, glass, composite or any other tangible medium suitable for printing. The format of print medium <NUM> may be continuous form or cut sheet or any other format suitable for printing. Printer <NUM> may be an ink jet, electrophotographic or another suitable printer type.

In one embodiment, printer <NUM> comprises one or more print heads <NUM>, each including one or more pel forming elements <NUM> that directly or indirectly (e.g., by transfer of marking material through an intermediary) forms the representation of picture elements (pels) on the print medium <NUM> with marking material applied to the print medium. In an ink jet printer, the pel forming element <NUM> is a tangible device that ejects the ink onto the print medium <NUM> (e.g., an ink jet nozzle) and, in an electro-photographic (EP) printer the pel forming element may be a tangible device that determines the location of toner particles printed on the print medium (e.g., an EP exposure LED or an EP exposure laser). The pel forming elements may be grouped onto one or more printheads. The pel forming elements <NUM> may be stationary (e.g., as part of a stationary printhead) or moving (e.g., as part of a printhead that moves across the print medium <NUM>) as a matter of design choice. The pel forming elements <NUM> may be assigned to one of one or more color planes that correspond to types of marking materials (e.g., Cyan, Magenta, Yellow, and blacK (CMYK)).

In a further embodiment, printer <NUM> is a multi-pass printer (e.g., dual pass, <NUM> pass, <NUM> pass, etc.) wherein multiple sets of pel forming elements <NUM> print the same region of the print image on the print medium <NUM>. The set of pel forming elements <NUM> may be located on the same physical structure (e.g., an array of nozzles on an ink jet print head) or separate physical structures. The resulting print medium <NUM> may be printed in color and/or in any of a number of gray shades, including black and white (e.g., Cyan, Magenta, Yellow, and blacK, (CMYK)). The host system <NUM> may include any computing device, such as a personal computer, a server, or even a digital imaging device, such as a digital camera or a scanner.

The sheet image <NUM> may be any file or data that describes how an image on a sheet of print medium <NUM> should be printed. For example, the sheet image <NUM> may include PostScript data, Printer Command Language (PCL) data, and/or any other printer language data. The print controller <NUM> processes the sheet image to generate a bitmap <NUM> for transmission. Bitmap <NUM> may be a halftoned bitmap (e.g., a calibrated halftone bit map generated from calibrated halftones, or uncalibrated halftone bit map generated from uncalibrated halftones) for printing to the print medium <NUM>. The printing system <NUM> may be a high-speed printer operable to print relatively high volumes (e.g., greater than <NUM> pages per minute).

The print medium <NUM> may be continuous form paper, cut sheet paper, and/or any other tangible medium suitable for printing. The printing system <NUM>, in one generalized form, includes the printer <NUM> that presents the bitmap <NUM> onto the print medium <NUM> (e.g., via toner, ink, etc.) based on the sheet image <NUM>. Although shown as a component of printing system <NUM>, other embodiments may feature printer <NUM> as an independent device communicably coupled to print controller <NUM>.

The print controller <NUM> may be any system, device, software, circuitry and/or other suitable component operable to transform the sheet image <NUM> for generating the bitmap <NUM> in accordance with printing onto the print medium <NUM>. In this regard, the print controller <NUM> may include processing and data storage capabilities. In one embodiment, measurement module <NUM> is implemented as part of a halftone calibration system to obtain measurements of the printed medium <NUM>. The measured results are communicated to print controller <NUM> to be used in the halftone calibration process. The measurement system may be a stand-alone process or be integrated into the printing system <NUM>.

According to one embodiment, measurement module <NUM> may be a sensor to take measurements of printed images on print medium <NUM>. Measurement module <NUM> may generate and transmit measurement data <NUM>. Measurement data <NUM> may be OD (e.g., optical density) data corresponding to a printed image. In one embodiment, measurement module <NUM> may comprise one or more sensors that each or in total take measurements for printed markings produced for some or all pel forming elements <NUM>. In another embodiment, measurement module <NUM> may be a camera system, in-line scanner, densitometer or spectrophotometer. In a further embodiment, measurement data <NUM> may include a map information to correlate OD data to the corresponding pel forming elements <NUM>.

<FIG> is a block diagram illustrating one embodiment of a print controller <NUM>. The print controller <NUM>, in its generalized form, includes an interpreter module <NUM>, a halftoning module <NUM>, and a halftone calibration module <NUM>. These separate components may represent hardware used to implement the print controller <NUM>. Alternatively, or additionally, the separate components may represent logical blocks implemented by executing software instructions in a processor of the printer controller <NUM>.

The interpreter module <NUM> is operable to interpret, render, rasterize, or otherwise convert images (e.g., raw sheetside images such as sheet image <NUM>) of a print job into sheetside bitmaps. The sheetside bitmaps generated by the interpreter module <NUM> are each a <NUM>-dimensional array of pels representing an image of the print job (i.e., a Continuous Tone Image (CTI)), also referred to as full sheetside bitmaps. The <NUM>-dimensional pel arrays are considered "full" sheetside bitmaps because the bitmaps include the entire set of pels for the image. The interpreter module <NUM> is operable to interpret or render multiple raw sheetsides concurrently so that the rate of rendering substantially matches the rate of imaging of production print engines.

Halftoning module <NUM> is operable to represent the sheetside bitmaps as halftone patterns of ink. For example, halftoning module <NUM> may convert the pels (also known as pixels) to halftone patterns of CMYK ink for application to the paper. A halftone design may comprise a pre-defined mapping of input pel gray levels to output drop sizes based on pel location.

In one embodiment, the halftone design may include a finite set of transition thresholds between a finite collection of successively larger drop sizes, beginning with zero and ending with a maximum drop size (e.g., threshold arrays such as single bit threshold arrays or multibit threshold arrays). In another embodiment, the halftone design may include a three-dimensional look-up table with all included gray level values.

In a further embodiment, halftoning module <NUM> performs the multi-bit halftoning using the halftone design consisting of a set of threshold values for each pel in the sheetside bitmap, where there is one threshold for each non-zero ink drop size. The pel is halftoned with the drop size corresponding to threshold values for that pel. These set of thresholds for a collection of pels is referred to as a multi-bit threshold array (MTA).

Multi-bit halftoning is a halftone screening operation in which the final result is a selection of a specific drop size available from an entire set of drop sizes that the print engine is capable of employing for printing. Drop size selection based on the contone value of a single pel is referred to as "Point Operation" halftoning. The drop size selection is based on the pel values in the sheetside bitmap.

This contrasts with "Neighborhood Operation" halftoning, where multiple pels in the vicinity of the pel being printed are used to determine the drop size. Examples of neighborhood operation halftoning include the well-known error diffusion method.

Multi-bit halftoning is an extension of binary halftoning, where binary halftoning may use a single threshold array combined with a logical operation to decide if a drop is printed based on the contone level for a pel. Binary halftoning uses one non-zero drop size plus a zero drop size (i.e., a drop size of none where no ink is ejected). Multi-bit halftoning extends the binary threshold array concept to more than one non-zero drop size.

Multi-bit halftoning may use multiple threshold arrays (i.e., multi-bit threshold arrays), one threshold array for each non-zero drop size. The point operation logic is also extended to a set of greater than and less than or equal to operations to determine the drop size by comparing the threshold and image contone data for each pel. Multi-bit defines a power of two set of drop sizes (e.g., two-bit halftone designs have four total drops, including a zero drop size). While power of two may be employed to define the number of drops, systems not following this such as a three total drop system may be used and are still considered multi-bit.

For multi-bit halftones, as shown in <FIG>, the MTA is a three-dimensional array including one two-dimensional array for each drop size transition. Thus an MTA includes a set of two-dimensional arrays of thresholds for transition between drop sizes: plane one provides the threshold for the Large output level, while plane <NUM> and plane <NUM> provide thresholds for the Medium and Small output levels respectively for a system having three drop sizes not including zero drop size (none or off). In other embodiments, different one-to-one relationship may be used since the correspondence between plane numbers and drop sizes is arbitrary.

To use these threshold arrays for halftoning, in the case where the threshold arrays are smaller than the sheetside map, each multibit threshold array is tiled across contone image data provided by the sheetside bitmap, which provides a set of threshold values for each pel in the sheetside bitmap. The contone image data (e.g., gray level data) is logically compared to the threshold data on a pel basis. In the case of Large drops, they are produced by the halftoning when the image contone data is greater than the respective large threshold values in plane <NUM>.

Medium drops are produced when the image data is greater than the medium drop plane <NUM> thresholds and also the image data is less than or equal to the large drop thresholds in plane <NUM>. Small drops are produced when the image data is greater than the small drop thresholds in plane <NUM> and also the image data is less than or equal to the medium drop thresholds in plane <NUM>.

Finally, the off/none drop size occurs for cases when the contone values is less than or equal to the small drop thresholds in plane <NUM>. In this embodiment of a two-bit multibit printing system, this set of four logical equations, used with thresholds from each plane of the multibit threshold array permit each printing drop size to be defined, based on the contone values.

In other embodiments, the number of planes of threshold data can be extended to handle any number of drop sizes. The data from these two-dimensional arrays may be segmented into separate memory regions and stored in any convenient order. For example, the thresholds for each drop size transition may be stored contiguously in memory, and it is often advantageous to do so.

Halftone calibration module <NUM> performs a calibration process on an un-calibrated halftone <NUM>, or previously generated uniformity compensated halftone, received at print controller <NUM> to generate one or more calibrated halftones <NUM>. Calibrated halftones <NUM> are then received at halftoning module <NUM> along with the sheetside bitmap. In one embodiment, an un-calibrated halftone <NUM> represents a reference halftone design that is modified to create the calibrated halftones. In such an embodiment, measurements of the system response (e.g., measurement data <NUM>) are received via measurement module <NUM> using the un-calibrated halftone <NUM> for printing.

According to one embodiment, halftone calibration module <NUM> performs an iterative process to achieve uniformity compensation of a calibrated halftone. As used herein, uniformity compensation is defined as a calibration to compensate for measured response differences at a single pel, by a pel forming element <NUM> (e.g., print head nozzle).

In one embodiment, an un-calibrated halftone <NUM> is initially received at halftone calibration module <NUM> and is used to generate a uniformity calibrated halftone. Subsequently, an iterative process is performed that results in a generation of refined uniformity calibrated halftones after each iteration in order to improve uniformity, where each iteration compensates for marking differences of pel forming elements <NUM>. In each case the halftone from the previous iteration is used to print the markings that are measured and used to generate the current uniformity compensation data. In other words, the calibrated halftone created for iteration number two is based on measurements of marks printed using the calibrated halftone from iteration number one.

In a further embodiment, the iterative process is completed once a determination is made that optical density (OD) variations for the pel forming elements <NUM> for a calibrated halftone are less than a predetermined uniformity threshold associated with a target uniformity compensation specification. The uniformity threshold may be received. In yet a further embodiment, a calibrated halftone is saved as a uniformity compensated halftone upon a determination during an iteration that that the OD variations are less than the predetermined threshold.

<FIG> illustrates one embodiment of halftone calibration module <NUM>. As shown in <FIG>, halftone calibration module <NUM> includes a step chart generator <NUM> implemented to print a calibration step chart. In one embodiment, step chart generator <NUM> generates an initial step chart based on un-calibrated halftone <NUM>. In such an embodiment, the initial step chart uses a threshold array associated with the uncalibrated halftone design to generate the calibration step chart. In subsequent iterations, step chart generator <NUM> generates a step chart based on a threshold array associated with a current uniformity compensated halftone.

In one embodiment, the calibration step chart is printed by the pel forming elements <NUM> of printer <NUM>, and an image of the calibration step chart is measured by measurement module <NUM> (e.g., via a scanner). In such an embodiment, measurement module <NUM> generates print image measurement data, which includes OD data measured for a uniform grid across the web of the print medium for each pel forming element <NUM>.

The calibration step chart typically includes a number of steps (e.g. bars or stripes) of uniform density in which there may be at least one halftone pattern for each color of ink used by the printer. The stripe densities range from paper white (no ink) to maximum density for each ink color. Enough intermediate densities are included to allow interpolation to accurately recover the average measured density corresponding to each color value by interpolation. The steps or bars are arranged so that every segment or portion of the print head prints every color and shade of every color of ink at some point in the chart pattern. Enough pixels are included in the height of the bar so that the random variations in the halftone design are averaged away by averaging measured densities over the bar height. In one embodiment, there is one row in each bar for each row in the halftone threshold array, so that each bar constitutes a complete sample of the halftone design's threshold distribution, and there is no sampling noise included in the measurement due to halftoning of the step chart before printing.

Halftone calibration module <NUM> also includes OD comparison logic <NUM> to receive the OD data and determine whether the OD variations for the pel forming elements <NUM> are less than a predetermined threshold associated with the target uniformity compensation specification. OD variations are differences in the average measured OD values of the printing resulting from one print head nozzle (e.g., pixel) versus measured OD values from one or more other nozzles in response to the same color value input being printed. The uniformity compensation specification comprises a maximum allowable value for these differences or of limits on statistics related to the distribution of differences, such as limits on the mean and standard deviation of the differences. The statistics may include a weighting factor for the OD values wherein the weighting factor is higher for other nozzles that are closer to the one nozzle than other nozzles farther away. In one embodiment, the predetermined threshold is received at halftone calibration module <NUM> from a system user via a graphical user interface (GUI) <NUM>.

Upon a determination that the variations are less than the pre-determined threshold, OD comparison logic <NUM> transmits a message indicating that a uniformity compensated halftone has been achieved (e.g., the OD variations are less than the pre-determined threshold). In one embodiment, the message may comprise the uniformity compensated halftone, including threshold values associated with each of the pel forming elements <NUM>. Upon a determination that the variations are greater than the pre-determined threshold, nozzle measurement generation logic <NUM> generates measurement data associated with each of the pel forming elements <NUM> (e.g., nozzle measurement data). In this embodiment, the nozzle measurement data comprises an interpolation of the measured OD data. In another embodiment, upon a determination that the OD variations are greater than the pre-determined threshold, OD comparison logic <NUM> transmits a message indicating that a uniformity compensated halftone has not been achieved (e.g., the OD variations are greater than the pre-determined threshold). Printing System <NUM> or Print Controller <NUM> may initiate the generation and printing of a revised calibration step chart upon receiving the message.

Transfer function generation engine <NUM> generates an inverse transfer function (or ITF) for each of the pel forming elements <NUM> based on the OD data and a received uniformity compensation target function. In one embodiment, a transfer function comprises a mapping of an input digital count to an output digital count for a system, where digital count is the gray level or color value representing the pels in a bitmap <NUM> (<FIG>). Transfer functions may be received or generated (e.g., generated based on target OD versus input digital count data and measured OD versus output digital count data).

An inverse transfer function is the reversed (e.g., inverted) application of the digital count data, where the output digital count values of the transfer function form the input digital count values of the inverse transfer function and the input digital count values of the transfer function form the output digital count values of the inverse transfer function.

<FIG> illustrates one embodiment of an inverse transfer function of a nozzle. As shown in <FIG>, the inverse transfer function of a pel forming element (e.g., nozzle) is represented as Uk(g) = c = TFk-<NUM>(g). Target OD response T(c) is used as a uniformity objective for all nozzles for a current halftone. Where {hkl}i is the three-dimensional Threshold Array for iteration i, column location k and drop size level <NUM>. The Threshold Array includes threshold for all drop sizes. Thresholds for column k are used in the halftoning process to print using nozzle k. Therefore, there is a one to one correspondence between nozzles from the ink jet array to columns of the threshold array. The rows of the threshold array are not included for simplicity. The curly brackets are used to indicate sets of threshold values. It should be understood that the entire threshold array is three dimensional, with rows in the web movement direction, columns corresponding to each nozzle and planes corresponding to each nonzero drop size. In one embodiment, the same target is used for all nozzles to achieve uniform printing. Uxrepresents the inverse transfer function for nozzle k, which maps color value g to color value c in <FIG>. Uk may be determined as shown in <FIG> by following the dotted line path in the opposite direction of the indicated arrows. In one embodiment, the color values are the Digital Count (DC) levels, which are <NUM> to <NUM>, for a typical <NUM>-bit printing system.

In such an embodiment, the inverse transfer function of a color value g is the color value c, whose target value T(c) is the measured value Mk(g) that results when the color value g is printed. T(c) may be set the same for all nozzles to provide uniformity between all nozzles. The value Mk(g) is the measured OD at the cross print medium (e.g. web) location corresponding to nozzle k and the current halftone (e.g. threshold array). Uk may be computed directly, rather than by inverting a TF, starting from Mk(g), and then finding T-<NUM>(Mk(g)). The inverse of the target function T-<NUM> may be found by tabulating the target function in two columns having all c and g values, and subsequently swapping the columns. The measured response Mk(g) at nozzle location k may be measured with the entire Threshold Array employed for printing, so as to include contributions from adjacent compensated nozzles. Employing scanned image data, the data is interpolated to obtain the measured response for the nozzle at location k.

As shown in <FIG>, the Transfer Function TF(c) = g. In one embodiment, the transfer function of color value c is a color value g that, when color value g is printed by the print system, causes the target optical density at color value c to be obtained as the measured value. TF(c) may be determined as shown in <FIG> by following the dotted line path in the direction of the indicated arrows. This is because c and g both have the same Optical Density OD<NUM> associated with them. Color c occurring in the target domain and color g in the measured domain systems. The TF(c) may be derived from T(c) and Mk(g) by tabulating the matching color pairs for optical densities as explained here for colors c and g. Uk may then be determined by inverting TFk.

Once the inverse transfer functions have been generated, halftone update engine <NUM> generates an updated uniformity compensated halftone by modifying all of the thresholds in each column of the threshold array corresponding with the current halftone using the generated inverse transfer function for each respective nozzle. In one embodiment, the inverse transfer function for each column (or nozzle) (k) is used at each iteration to transform the current threshold values {hkl}i-<NUM> of the compensated halftone from the previous iteration. This creates threshold values for a compensated halftone, {hkl}i. This process generates thresholds in the compensated threshold array, based on the corresponding thresholds from the current threshold array.

According to one embodiment, each threshold of the compensated threshold array is determined by using the transformation defined by the corresponding ITF applied to each threshold for all corresponding pels and drop sizes. If the ITF is assumed to be a continuous function, threshold values after transformation may be rounded to integer threshold values. The range of threshold values in the Threshold Arrays created may match the bit depth of image data in the imaging path. Halftones having bit depths higher than that of the imaging path may be employed, by generating down sampled halftones or bit shifting the image data to match the halftone thresholds.

For example, each level (<NUM>) of a multi-level halftone represents a drop or dot size, where level l = <NUM> represents no drop or dot. In a web wide halftone, each nozzle/column k of a print head has a set of halftone thresholds for each level l of the multilevel halftone, {hk,l}. After iteration (i) the uniformity compensated halftone threshold array is: {hk,l}i = Uk,i(Uk,i-<NUM>(. Uk,<NUM>({hkl}<NUM>). )), where Uk,i is the inverse transfer function for nozzle k at iteration i. The uncalibrated initial halftone threshold array is given by {hkl}<NUM>. This results in convergence towards uniform response in the halftoned image.

In an alternative embodiment not encompassed by the wording of the claims, transfer functions may be implemented to apply directly to image data. In this case the uncalibrated halftone is not modified. Instead, the image data is transformed prior to halftoning. In this embodiment, transfer function generation engine <NUM> generates a transfer function for each nozzle, rather than inverse transfer functions, while halftone update engine <NUM> modifies the transfer function for each nozzle, by applying the current transfer function to it, to generate the current cumulative transfer function. It is important to distinguish between the cumulative transfer function determined in this way: <MAT> from the current transfer function at iteration i, TFi(c). The cumulative transfer function is, in other words, the composition of transfer functions determined by measurement at each iteration, i. Additionally, this embodiment includes step chart generator <NUM> generating a calibration step chart using the halftone (e.g., threshold array) and transfer function applied to the image data.

In one embodiment, the same uncalibrated halftone is employed for printing in this embodiment and only the cumulative transfer function is recomputed to obtain the current cumulative transfer function after iteration i. Assuming the cumulative transfer function is TFCk,i for nozzle k after iteration i, the current cumulative transfer function for all nozzles is <MAT> where TFk,i is the transfer function for nozzle k, as measured after iteration i. An initial transfer function TFk,<NUM> is used, which typically is the identity transfer function. In another embodiment an initial transfer function derived based on engine calibration might be used. This results in convergence towards uniform response in the uniformity compensated halftoned image, employing cascaded transfer functions instead of uniformity calibrated halftones.

<FIG> is a flow diagram illustrating one embodiment of a process <NUM> for performing uniformity compensation. Process <NUM> may be performed by processing logic that may include hardware (e.g., circuitry, dedicated logic, programmable logic, microcode, etc.), software such as instructions run on a processing device, or a combination thereof. In one embodiment, process <NUM> is performed by halftone calibration module <NUM>.

At processing block <NUM>, a halftone is received. In the first iteration of the process, the halftone is an uncalibrated halftone. At processing block <NUM>, a uniformity calibration step chart is generated. This chart may have cross web stripes having the same tint level, ranging from zero to <NUM>% tint levels in multiple steps. At processing block <NUM>, the uniformity calibration chart is printed and print image measurement data from measuring the chart, including the OD measurement data, is received. At decision block <NUM>, a determination is made as to whether OD variations are less than or equal to the predetermined threshold. If so, a message is transmitted indicating that a uniformity compensated halftone has been achieved, processing block <NUM>. As discussed above, the message may include uniformity compensated halftone values. In some embodiments, a message may also be transmitted indicating that the variations are greater than the predetermined threshold (e.g., indicating that another iteration is to be performed).

At processing block <NUM>, nozzle measurement data is generated upon a determination that the OD variations are greater than the predetermined threshold. As discussed above, the nozzle measurement data may be generated by interpolating the OD measurement data. At processing block <NUM>, inverse transfer functions are generated for each nozzle to achieve the target OD response for each nozzle.

At processing block <NUM>, a uniformity compensated halftone is generated by modifying the thresholds in each column of the threshold array using the inverse transfer function for each nozzle. By performing these steps on a per nozzle basis, improved uniformity compensation (e.g., more accurate, faster, less operator burden, etc.) over conventional methods may be achieved. In one embodiment, a uniformity compensated halftone is generated for each drop size of the halftone design, which produces the current threshold array which is employed for printing or printing the uniformity calibration step chart. In another embodiment, at processing block <NUM>, the uniformity compensated halftone for each of the pel forming elements is transmitted. In yet another embodiment, at processing block <NUM> a message is transmitted indicating that the uniformity compensated halftone has not been achieved (e.g., request revised print image measurement data corresponding to printing a revised uniformity calibration step chart with the revised uniformity compensated halftones).

Subsequently, control is returned to processing block <NUM>, where the uniformity compensated halftone is received as an updated halftone. This loop is repeated for as many iterations that are needed to satisfy the condition of decision block <NUM>. When a threshold array is generated which satisfies the OD variation criteria it may be used for all subsequent printing. The uniformity compensated threshold arrays may be saved with the current conditions, such as the paper type. Subsequent printing, using the same paper type, may employ the saved uniformity compensated threshold array.

<FIG> is a flow diagram illustrating another process <NUM> for performing uniformity compensation. In this process transfer functions for each nozzle based on a cascaded process are employed. Process <NUM> may be performed by processing logic that may include hardware (e.g., circuitry, dedicated logic, programmable logic, microcode, etc.), software such as instructions run on a processing device, or a combination thereof. In one embodiment, process <NUM> is performed by halftone calibration module <NUM>.

At processing block <NUM>, a halftone is received. At processing block <NUM>, a uniformity calibration step chart is generated. At processing block <NUM>, the uniformity calibration chart is printed and print image measurement data from measuring the chart, including the OD measurement data, is received. At decision block <NUM>, a determination is made as to whether OD variations are less than or equal to the predetermined threshold. If so, a message is transmitted indicating that a uniformity compensated set of transfer functions has been obtained, processing block <NUM>.

However, nozzle measurement data is generated upon a determination that the OD variations are greater than the predetermined threshold, processing block <NUM>. At processing block <NUM>, transfer functions are generated for each nozzle. At processing block <NUM>, a refined set of cumulative transfer function (cascaded single iteration transfer functions) are generated by modifying the previous transfer function sets for the nozzles. In another embodiment, the refined set of cumulative transfer functions are transmitted. Subsequently, control is returned to processing block <NUM>, where the process is repeated. Once a refined set of cumulative transfer functions has been generated that satisfies the OD variation criteria, the cumulative transfer functions may be used for all subsequent printing. The uniformity compensated set of cumulative transfer functions may be saved with the current conditions, such as the paper type. Subsequent printing, using the same paper type, will may utilize the saved sets of refined cumulative transfer functions.

Note that the cascaded refined cumulative transfer function approach described may be employed in printing systems that do not have point operation halftoning, using threshold arrays. The described uniformity system, employing a refined set of cascaded transfer functions, can be employed in printers having neighborhood halftoning implemented, such as error diffusion.

Although shown as a component of print controller <NUM>, other embodiments may feature halftone calibration module <NUM> included within an independent device, or combination of devices, communicably coupled to print controller <NUM>. For instance, <FIG> illustrates one embodiment of a halftone calibration module <NUM> implemented in a network <NUM>.

As shown in <FIG>, halftone calibration module <NUM> is included within a computing system <NUM> and transmits calibrated halftones <NUM> to printing system <NUM> via a cloud network <NUM>. Printing system <NUM> receives calibrated halftones <NUM>. In this embodiment, processing blocks <NUM> - <NUM> (and <NUM> - <NUM>) may be performed at computing system <NUM>, while processing blocks <NUM> and <NUM> (and <NUM> and <NUM>) may be performed at printing system <NUM>.

<FIG> illustrates a computer system <NUM> on which printing system <NUM> and/or halftone calibration module <NUM> may be implemented. Computer system <NUM> includes a system bus <NUM> for communicating information, and a processor <NUM> coupled to bus <NUM> for processing information.

Computer system <NUM> further comprises a random access memory (RAM) or other dynamic storage device <NUM> (referred to herein as main memory), coupled to bus <NUM> for storing information and instructions to be executed by processor <NUM>. Main memory <NUM> also may be used for storing temporary variables or other intermediate information during execution of instructions by processor <NUM>. Computer system <NUM> also may include a read only memory (ROM) and or other static storage device <NUM> coupled to bus <NUM> for storing static information and instructions used by processor <NUM>.

A data storage device <NUM> such as a magnetic disk or optical disc and its corresponding drive may also be coupled to computer system <NUM> for storing information and instructions. Computer system <NUM> can also be coupled to a second I/O bus <NUM> via an I/O interface <NUM>. A plurality of I/O devices may be coupled to I/O bus <NUM>, including a display device <NUM>, an input device (e.g., an alphanumeric input device <NUM> and or a cursor control device <NUM>). The communication device <NUM> is for accessing other computers (servers or clients). The communication device <NUM> may comprise a modem, a network interface card, or other well-known interface device, such as those used for coupling to Ethernet, token ring, or other types of networks.

Embodiments of the invention may include various steps as set forth above. The steps may be embodied in machine-executable instructions. The instructions can be used to cause a general-purpose or special-purpose processor to perform certain steps. Alternatively, these steps may be performed by specific hardware components that contain hardwired logic for performing the steps, or by any combination of programmed computer components and custom hardware components.

Claim 1:
A system (<NUM>) comprising:
at least one physical memory device to store halftone calibration logic; and
one or more processors coupled with the at least one physical memory device to execute the halftone calibration logic to:
receive print image measurement data corresponding to a first halftone design comprising a set of threshold values for each of a plurality of pel forming elements in a sheetside bitmap;
generate nozzle measurement data by interpolating the print image measurement data for each color value for each of the pel forming elements based on the print image measurement data;
generate a uniformity compensated halftone for each of the pel forming elements based on inverse transfer functions corresponding to each of the pel forming elements and the first halftone design, wherein each inverse transfer function comprises a mapping from an output gray level or color value to an input gray level or color value representing the pels in a bitmap (<NUM>);
generate, from the uniformity compensated halftone, a refined uniformity calibrated halftone which compensates for marking differences of the pel forming elements, wherein the refined uniformity calibrated halftone is generated by performing an iterative process, where each iteration uses the halftone from the previous iteration to print markings that are measured and used to generate a current uniformity compensated halftone; and
transmit the refined uniformity compensated halftone for each of the pel forming elements.