WO2023075777A1

WO2023075777A1 - Power levels in halftoned image data

Info

Publication number: WO2023075777A1
Application number: PCT/US2021/057069
Authority: WO
Inventors: Noam Shaham
Original assignee: Hewlett-Packard Development Company, L.P.
Priority date: 2021-10-28
Filing date: 2021-10-28
Publication date: 2023-05-04

Abstract

In an example, a method includes, with processing circuitry, receiving halftoned image data comprising a pixel array. Each pixel of the pixel array may be associated with a power level. The method may further include identifying a region of the pixel array which does not meet a predetermined power level criterion, and when a region is identified, increasing a power level associated with at least one pixel of the region.

Description

POWER LEVELS IN HALFTONED IMAGE DATA

BACKGROUND

[0001] In printing, print agents such as inks, toners, coatings and the like may be applied to a substrate. Substrates may in principle comprise any material, for example comprising paper, card, plastics, fabrics, metals or the like.

BRIEF DESCRIPTION OF DRAWINGS

[0002] Non-limiting examples will now be described with reference to the accompanying drawings, in which:

[0003] Figure 1 shows an example method of modifying power levels in halftoned image data;

[0004] Figure 2 shows an example of a method of modifying power levels in halftoned image data and providing a printed output;

[0005] Figure 3A and B illustrate, respectively, examples of original and modified halftoned image data;

[0006] Figure 4A, B and C illustrate, respectively, examples of a search mask, a portion of original halftoned data and a portion of modified halftoned image data;

[0007] Figure 5 is an example print apparatus; and

[0008] Figure 6 is an example of a machine-readable medium in association with a processor.

DETAILED DESCRIPTION

[0009] In some examples of printing techniques, charged print agents, such as charged toner particles or resins, may be applied to a selectively charged photoconductive surface. In some examples, such print agents are subsequently transferred (in some examples via at least one intermediate transfer member) to a substrate. In other examples, an image plate may be formed using photolithography techniques, in which a directed light source may be used to form an intended latent image in a photosensitive surface provided by an emulsion. An image formed using print agent on such a surface may be transferred directly or via an intermediate transfer member to a substrate. Techniques which utilise an intermediate transfer member may be referred to as offset printing techniques.

[0010] Images printed by print apparatus may be made up of dots or clusters of dots using a technique known as amplitude modulation, in which a size of a dot cluster provides an intended amount of a particular print agent, which may be expressed in terms of a surface coverage, a greyscale or grey level. The size of the dot cluster may be controlled by determining the number of dots therein and, in some examples, the size of the dots. While electrophotographic print apparatus is described herein, there are many other variations of print apparatus and methods which make use of amplitude modulation techniques, and the principles set out herein may be used with, or comprise, any such apparatus.

[0011] The terms ‘grey level’ and ‘greyscale’ arose in relation to monochrome images. The darker the image or image portion in a monochrome image, the higher its grey level, and the higher the density of black dots. The terms are now used more generally to refer to all print agents: for example, in an image composed of layers or separations of cyan, magenta, yellow and black colorants, each region of the image may be associated with a grey level, often between 0 and 255, for each colorant. Different image regions may have different grey levels associated therewith. ‘White’ may be provided by a substrate color in some examples, and may correspond to the absence of printed dots.

[0012] An image to be printed may be considered in terms of the color of individual pixels in a pixel grid. In general, grey levels may be achieved by at least conceptually selectively ‘turning on’ pixels in a cluster of pixels which form part of a pixel grid in which an image is to be built up. At low grey levels, some clusters may have a single pixel (usually a pixel which is at approximately the centre of the cluster) turned on, while other clusters may have all of the pixels turned off. In the context of a photoconductive surface, ‘turning on’ a pixel means that a spot within a region of the photosensitive surface corresponding to the cluster is exposed to light.

[0013] As the grey level increases above zero, a point may be reached where every cluster corresponding to an image region to which the grey level applies has at least one pixel turned on. As the grey level increases further, further pixel(s) may be turned on in an increasing number of clusters, and so on until a maximum grey level is reached in which all pixels are turned on in all clusters.

[0014] In practice, amplitude modulation techniques may make use of halftoning using a threshold matrix, which may be made up of a plurality of cells (in some examples, repeated cells), each cell corresponding to a different dot cluster, and being made up of a two dimensional array of pixels. Applying a threshold matrix to image data means comparing, on a pixel by pixel basis, a greyscale value of a pixel of the image data with a pixel of the threshold matrix. For example, if a greyscale value of the image data exceeds a value in the threshold matrix of a corresponding pixel, then the pixel is ‘turned on’ in halftoned image data whereas otherwise the pixel is turned off. Once the pixel is turned on, a corresponding region of a photosensitive surface is exposed to light, and as a result, a dot of print agent may be formed on the surface in that region, for subsequent transfer to a substrate.

[0015] Moreover, in some examples, the size of a dot corresponding to a pixel may be controlled. Generally, in relation to electrophotographic printing, a lower laser power may discharge a region of a photoconductive surface in a manner which results in a small volume of print agent being transferred thereto, whereas a higher power level may result in a larger volume of print agent being transferred thereto. Therefore, by controlling the amount of power in the light source used to discharge the surface, the size of a print agent dot forming a cluster in a printed output may be controlled.

[0016] In amplitude modulation techniques, where just a single, or a few, dots of a cluster is/are to be printed, for example at lower greyscale levels as mentioned above, then it may be intended the printed dot or dots are to be of a relatively large size (bearing in mind that a ‘large’ dot may still be very small, for example on the order of 30 to 50 pm), wherein the large size may correspond to at least the minimum size print agent dot which is likely to survive transfer from the photoconductive surface to a substrate, in some examples via an intermediate transfer member as described above. Therefore, the first pixel to be ‘turned on’ in a cluster, which may be a central dot of the cluster (or an approximately central dot, noting that the cluster may not be completely symmetrical, and the position of the geometrical centre of the cluster may vary depending on the cluster size), may be associated with a power level which provides, at least, this threshold size and in some examples is intended to produce a dot of a calibrated size. This may mean, in some examples, that the power used to discharge the photoconductive surface in association with this ‘first’ dot, or the first few dots, of a cluster is relatively high, and in some examples higher than the power used when discharging the photoconductive surface to provide ‘subsequent’ dots of the cluster (i.e. dots associated with higher greyscale levels), even when those dots are to be at their maximum size. This higher power level may be referred to herein as a peak power level.

[0017] While the term ‘first pixel(s)’ and ‘first dot(s)’ are used herein, it may be noted that this is not intended to imply that the dot(s) is or are printed before other dots of the cluster, and/or that the corresponding region of the photosensitive member is treated with laser power before other regions of the cluster. Rather, the photosensitive member may for example be exposed to light using a raster scan method, and print agent transferred thereto according to a rotation of the photosensitive member relative to a print agent applicator, or the like. However, the ‘first pixel(s)’ and ‘first dot(s)’ are the first to be ‘turned on’ or ‘printed’ as the greyscale level increases above zero, or may be pixel(s)/dot(s) which are associated with a relatively high power level (e.g. the peak power level) compared to the power level which may be associated with other dots. Thus, in some examples a first dot is a dot which is to be printed when the greyscale level of a corresponding pixel in image data is any non-zero greyscale level, and the first pixel is a pixel associated with print instructions for printing that dot.

[0018] As mentioned above, an amplitude modulation scheme may be embodied as a grid of pixels associated with threshold values, arranged in a halftone screen. Generally, these comprise repeated cells, each cell comprising a two dimensional array of pixels wherein a number of pixels (usually a single pixel, or a small cluster of pixels at the centre of the cell) are associated with both lower and higher greyscale levels and a high power level, referred to herein as a ‘peak power level’, to ensure that the dot size is sufficient to survive the transfer operation when the greyscale level is low. These peak power pixel(s) are then surrounded by other pixels, which are associated with the higher greyscale levels. However, the power associated with these pixels may be variable. For example, there may be low, medium and large dot sizes, associated with low medium and high power levels. In some examples, all these power levels are lower than the peak power level associated with the first pixel(s).

[0019] For example, as the greyscale levels increase, clusters may grow in size gradually as different combinations of dot sizes are used to increase the amount of print agent transferred in respect of the clusters. In order to provide such different power levels, there may be a plurality of halftone screens, each corresponding to a different power level. Moreover, there may be a hierarchy associated with the screens such that, when a condition is fulfilled in relation to a corresponding pixel in more than one screen, one of the power levels takes precedent (for example this may be the highest of the power levels of the screens in which the condition is fulfilled).

[0020] Such methods can reproduce even fine text with high clarity, but when very fine lines and features are to be produced, there is a possibility that the fine feature may not coincide with the ‘peak power’ pixels of the cluster, instead coinciding with the lower power pixels. This may result in print quality issues. Such print quality issues may affect both the printed output comprising the fine line/features and subsequent printed outputs. This is because print agent which is not successfully transferred to a substrate may accumulate throughout the apparatus and may transfer to a subsequent substrate in an unpredictable manner.

[0021] Figure 1 is a flowchart of an example method which may be a computer implemented method executed by processing circuitry, for example processing circuitry of a print apparatus. The method may be used in an Amplitude Modulation (AM) printing operation, in which dot clusters of varying size are printed to provide an intended coverage of a print agent.

[0022] The method comprises, in block 102, receiving halftoned image data comprising a pixel array, wherein each pixel of the pixel array is associated with a power level. In some examples, the power level is binary - for example, each pixel may be ‘off’ (a zero-power level) or ‘on’ (a non-zero power level, which may be the ‘peak power level’). In another example, each pixel may be associated with one of a zero power level, at least one intermediate power level, or a peak power level. In some examples, each pixel may be associated with one of three power levels (for example, peak, medium and zero), one of four power levels (for example, peak, high, low and zero) or one of five power levels (for example, peak, high, medium, low and zero). In examples, one of the power levels is the peak power level and any other power level(s) is/are lower than the peak power level. [0023] In some examples, at least some of the pixels may have additional data associated therewith. In particular, the zero-power level pixels may be associated with data indicative of whether the greyscale level associated therewith was zero, or a non-zero value (which may result in a zero-power level for that pixel through a halftoning process). In some examples, at least some of the pixels of the halftoned data may have an indication of the greyscale level which was associated with that pixel in the image data which was halftoned to provide the halftoned image data. In other examples, pixels may be associated with one of a zero greyscale level, an intermediate greyscale level (which may be any greyscale level between zero and the maximum) or a maximum greyscale level.

[0024] Block 104 comprises identifying a region (e.g. at least one region, or any region) in the pixel array which comprises a pixel region which does not meet a predetermined power level criterion.

[0025] In some examples, this may comprise identifying a region which is associated with a low, or marginal, likelihood of print agent transfer. This may for example be associated with a size of a continuous print agent spot, wherein the size may comprise a volume of print agent, noting that the thickness of a print agent spot may not be uniform, for example having thicker portions associated with higher power levels. In other examples, it may be associated with a dimension, for example a height and/or width, or a thickness, of the print agent spot. In other examples, it may be associated with the cumulative power level of the region, taken as a whole, for example a total power for all pixels within a pixel window having a predetermined size.

[0026] In some examples, block 104 may comprise identifying a region or regions of the pixel array which is associated with a single pixel, a small continuous cluster or group of pixels and/or a narrow continuous cluster/group of pixels, wherein the single pixel/group of pixels are associated with a non-zero power level (i.e. a pixel which is associated with a print agent dot to be printed). In some examples, block 104 may comprise inspecting a neighbourhood associated with each of a plurality of pixels of the halftoned image data. In some examples, this may comprise matching patterns which may be embodied in a search mask, as is set out in greater detail below.

[0027] In some examples, once such regions, groups, clusters or pixels have been identified, the power level associated with the pixels thereof may be considered and may be compared to at least one predetermined power level criterion. For example, if there are less than a threshold number of pixels of a predetermined energy level or energy levels (for example, a predetermined number of peak power level pixels, or a predetermined number of non-zero power level pixels), and/or the cumulative power level of the pixels is less than a threshold, then a region may be identified as not meeting a power level criterion.

[0028] In some examples, a region may be a region associated with, or intended to print, a continuous print agent spot. A ‘continuous’ print agent spot may be made up of print agent dots associated with a continuous pixel region, or a continuous cluster associated with at least one non-zero power level. A continuous pixel region or cluster associated with at least one non-zero power level may be made up of non-zero power level pixels which share at least one edge with another non-zero power level pixel, and/or a non-zero power level pixels which share at least one corner with another non-zero power level pixel. In some examples, a continuous pixel region or cluster may be made up of a column and row which intersect with a particular pixel, wherein if either the column or row are less than a threshold size, the power level associated with the pixels thereof may be considered, for example being compared to a predetermined criterion.

[0029] Block 106 comprises increasing a power level associated with at least one pixel of the region. For example, a pixel associated with a lower power level may have the association changed, such that it is associated with a higher power level. In some examples more than one pixel of the region may be ‘promoted’, i.e. associated with a higher power level than in the original halftoned data, and/or a cumulative power level of a region may be increased until the condition discussed in block 104 is met.

[0030] In some examples, increasing a power level associated with at least one pixel of the region comprises increasing the power level of at least one pixel of the region associated with a non-zero power level. This ensures that no new pixels are turned from ‘off’ to ‘on’, and thus no additional print agent dots will be printed, rather that print agent dots will be increased in size when compared to the intended dot size prior to the application of the method. This may assist in preventing undue thickening of fine features or loss of fine features in ‘knock out’ features (i.e. features where print agent is applied around text or the like, wherein the absence of print agent may provide an intended feature or content). In some examples, this condition may apply to those pixels which are associated with a zero power level in the halftoned data and a zero greyscale level in image data, and not to those pixels which are associated with a zero power level in the halftoned data and a non-zero greyscale level in image data, as this may more effectively represent underlying image data in some examples. [0031] In some examples, block 106 may be carried out conditionally on there being at least one pixel of the region which meets predetermined criterion for being associated with an increased power level. For example, if there are isolated single pixels or a small number of pixels, it may be that a threshold cumulative power condition cannot be achieved unless additional pixels are ‘switched on’. In some such examples, some such pixels may be switched on (i.e. pixels which were associated with a zero power level may instead be associated with a non-zero power level). However, depending on the intended output of the print operation, it may be the case that the condition for increasing a power level is that the halftoned data indicates that there are at least n other pixels to be printed in the neighbourhood of a pixel, wherein n is any integer (for example, at least two neighbour pixels, or at least three). In some such examples, where the criterion is not met, such isolated single pixels/small pixel groups may instead be switched off, i.e. associated with a zero power level instead of a non-zero power level, thus avoiding thickening of features of the image while also preventing potential agent transfer issues as mentioned above.

[0032] Figure 2 is another example of a method which may again be at least partially a computer implemented method executed by processing circuitry, for example processing circuitry of a print apparatus. Again, the method may be used in an Amplitude Modulation (AM) printing operation.

[0033] Block 202 comprises obtaining image data. For example, the image data may relate to text, patterns, images and the like. In some examples, the image data comprises fine features. For example, this may comprise text which is to be printed in a point size of less than point 3. Point sizes are used to indicate the height of text characters. Each point corresponds to 1/72 of an inch, or around 0.353mm. Thus, text printed in point 3 size may be less than 1mm in height. Such text may be used for example to provide security features or micro text. In other examples, fine features may replicate intricate designs, which may be security features (e.g. features on a banknote or certificate of authenticity or the like), or may be intended to provide intricate designs which are pleasing to a human observer.

[0034] The image data may specify a target greyscale level for each of a plurality of pixels of an image. In one example, the image data relates to one separation of an image and the method may be carried out on each separation. In other examples, for example where the image is intended to be a monochromatic image, there may be a single separation. For example, the image data may be acquired by processing circuitry of a printer or print apparatus, or processing circuitry which is intended to determine print instructions for a printer or print apparatus. The image data may for example be obtained over a network, from a memory or the like.

[0035] Block 204 comprises, for example by the processing circuitry, applying at least one threshold matrix to the image data. A threshold matrix comprises an array of values, which usually have the same range as the number of greyscale values of the image. However, in some examples, different threshold matrices may include values associated with the whole or just part of the number of greyscale values. In some examples, a threshold matrix may be made up of a plurality of repeated cells, each cell corresponding to a different dot cluster. Applying a threshold matrix to image data means comparing, on a pixel by pixel basis, a greyscale value of a pixel of the image data with a pixel of the threshold matrix.

[0036] For example, there may be different threshold matrices associated with different power levels. If a greyscale value of the image data exceeds a value in the threshold matrix of a corresponding pixel, then the power level of that threshold matrix is associated (at least initially) with that pixel. Subsequent matrices may for example alter the power level associated with each pixel. For example, if a pixel value exceeds a threshold value of a corresponding pixel in a subsequently applied threshold matrix, then any previously associated power level is overwritten. In one example of a halftone matrix, the peak power level may for example be associated with a group of four pixels at the centre of a cell of the matrix, wherein the four pixels are associated with low greyscale levels (for example, greyscale levels of 1). However, in other examples, there may be different arrangements. For example, there may be a single pixel associated with the peak power level.

[0037] In summary therefore, the method comprises using threshold matrices wherein if the greyscale level, or ‘tone’, of a pixel in image data is higher than the value of the relevant pixel in a particular matrix, the relevant power is assigned to that particular pixel. However, the threshold matrices have a hierarchy, or priority order, such that if the condition is also met for that pixel on a subsequent matrix, the power assigned to that pixel will be of that subsequent matrix. Thus, subsequent matrices may be considered to have a higher priority than earlier matrices. In other examples, the power levels may be assigned in some other way.

[0038] Once all the matrices have been applied in order, this provides a power level associated with each pixel of the image data in halftoned image data. In relation to some pixels, the power level may be zero. In some examples, each zero power level pixel of the halftoned image data is also associated with an indication of the greyscale level in the pre-halftoned image data. In particular, in some examples, each zero power level pixel may be associated with an indication of whether the greyscale level was non-zero or zero.

[0039] The method proceeds by scanning the halftoned image data on a pixel by pixel basis. In summary, in this example, for each pixel associated with a non-zero power level, the neighbourhood around that pixel is considered. An example of such a method is now described, although it will be appreciated that there may be many variations of such a method, some of which are discussed in greater detail below.

[0040] Block 208 comprises setting a pixel index i to 0, and block 210 comprises determining if the pixel p(i) is associated with a nonzero power level. If the pixel p(i) is associated with a zero power level, then, assuming that the pixel is not the final pixel of the halftoned image data, i is incremented (block 212) and the method loops back to block 210. If however the pixel p(i) is associated with a nonzero power level then the method proceeds to block 214. In other examples, the method may operate on pixels which have a zero power level in halftoned data, but a non-zero greyscale level (or a greyscale level which exceeds a threshold value) in image data. These may be pixels which could have been associated with a non-zero power level if the alignment with the halftone matrix was different. Such pixels may be eligible for an increased power level whereas pixels which are associated with a zero greyscale level may not be eligible for an increased power level. This may reduce undue thickening of features, or loss of fine features in ‘knock out’ features or text. In such examples, block 210 may prevent consideration of pixels which are associated with zero power level and a zero greyscale level in image data, but not pixels which are associated with zero power level and a nonzero greyscale level in image data.

[0041] In this example block 214 comprises determining a continuous cluster of pixels associated with a non- zero power level. In this example, determining a continuous cluster comprises determining a continuous row length and a continuous column length of pixels associated with a non-zero power level (wherein each of the continuous row and the continuous column may be a continuous cluster) and which include the pixel p(i), and determining if both of these meet a threshold. If so, it is determined that no action is required in relation to this local spot and, assuming that the pixel is not the final pixel of the halftoned image data, the method proceeds with incrementing i (block 212), then considering the next pixel in block 210. However, if the row or column length is shorter than the threshold, this may be indicative of a narrow feature, and it may be intended to identify short columns and rows in block 214 as such features may be associated with a low likelihood of print agent transfer.

[0042] If the pixel is in a row/column of nonzero power level pixels which is less than the threshold, then in block 216, it is determined whether the row and/or column which is less than the threshold size comprises at least two peak power pixels. In other words, in this example, the power level criterion is that the row/column comprises at least two peak power level pixels. In other examples, the criterion may be different. For example, the criterion may comprise determining a total power associated with a region around pixel p(i), for example in a n by m pixel window centred on p(i), where n and m may be any integer (for example n and/or m may be equal to 2, 3, 4, 5 or the like), and comparing this to a threshold (regardless of whether any of the pixels are associated with the peak power level).

[0043] If it is determined, in block 216 that the row and column do comprise at least two peak power pixels, then, assuming that the pixel p(i) is not the final pixel of the halftoned image data, the index i is incremented (block 212) and the method loops back to block 210.

[0044] If however the power level criterion is not met, it is considered if there is a pixel which is eligible for promotion (block 218). For example, a pixel may be eligible for promotion if there is at least one other pixel associated with a non-zero power in contact there with (or at least n, where n is any integer. In this example, the pixel p(i) is to be considered for the increased power level (and not any other pixels of the local region). In this example, p(i) will only be considered if it is non-zero due to the operation of block 210. Thus, in this example, only non-zero power level pixels are eligible for promotion. Moreover, in this example, p(i) may be eligible for promotion if it meets criteria such as having at least a threshold number of non-zero neighbouring pixels.

[0045] If p(i) is not eligible for promotion, then it may be determined that it is an isolated pixel, or a pixel of a small group, and, in block 220 it is reassigned a zero power level such that no print agent dot will be printed in respect thereof. Assuming that the pixel is not the final pixel of the array, the pixel index is then incremented in block 212, and the method loops back to block 210.

[0046] If however the pixel is eligible for an increased power level, the pixel p(i) is reassigned with the peak power level (block 222). [0047] In some examples, pixels other than pixel p(i) may be considered for an increased power level. In some examples, pixels neighbouring an existing peak power pixel in a local neighbourhood, if any, may be reassigned to the peak power level before non-neighbouring pixels, as this creates a group of peak energy pixels, which in turn may increase the likelihood of print agent transfer. If there are no pixels meeting that specification, a pixel associated with the highest non-zero power level may be reallocated to the peak power level first, and so on, through reducing power levels.

[0048] As noted above, in this example, where the pixel p(i) is a single isolated pixel and/or has a number of neighbours which is below a threshold, this pixel is reassigned a zero power level in block 220 such that no print agent dot will be printed in respect thereof. In other examples however, pixels around such a pixel may be associated with a peak power level, as may the pixel itself, for example to meet the specification set out in block 216.

[0049] The method continues to loop until all the pixels of the array have been inspected.

[0050] In this example, therefore, each continuous row and/or column of non-zero pixels of less than a threshold length will, by operation of the method, contain a minimum of two peak power pixels, which may tend to be clustered.

[0051] As noted above, in other examples, other criteria may be applied to identify a region of the halftoned data which may be at risk of failing to print successfully and/or whether a region fulfils the power level criterion. For example, a local spot size may be determined by identifying neighbours which share an edge with the pixel under consideration. Once a neighbour is identified, it is added to the local spot, and its neighbours, if any, are also identified and added to the local spot. Once no more neighbours can be identified, it is determined if the local spot meet some criteria, for example, includes at least n ‘peak power’ pixels, where n may be any integer. If there are less than n peak power pixels, then at least one pixel of the spot may be reassigned the peak power level. Other criteria may be based on a cumulative total power level, or a threshold number of non-zero power level pixels, or the like.

[0052] In another variation on the above method, pixels which are associated with one of the intermediate power levels may be inspected (i.e., both pixels with a power level of zero and a peak power level may be ignored) and, in the event that a small or narrow feature is identified in the vicinity thereof, and a power level criterion is not met, that pixel may itself be associated with a higher power level.

[0053] In this example, the method further comprises, in block 224, controlling a laser of a print apparatus using the associated power level for each pixel (some of which may have been reassigned in block 222), wherein the laser is controlled to discharge regions corresponding to the pixels on a photoconductive surface using the power level associated with that pixel. For example, the laser may be controlled by the processing circuitry of print apparatus, or by a controller of print apparatus according to instructions provided by processing circuitry which performed the method of blocks 202 to 222. The method further comprises, in block 226, applying print agent to the photoconductive surface to form an image, for example using the print apparatus and, in block 228, transferring the print agent image to a substrate such as paper, card, plastic or the like. In some examples, the print agent may be transferred to the substrate via an intermediate transfer member of a print apparatus as described above.

[0054] For example, the print apparatus may comprise an electrophotographic print apparatus such as a Liquid Electro Photographic (LEP) print apparatus which may be used to print a print agent such as an electrostatic printing fluid or composition (which may be more generally referred to as “an electronic ink” in some examples). Such a printing fluid may comprise electrostatically charged or chargeable particles (for example, resin or toner particles which may be colored particles) dispersed in a carrier fluid. In other examples, the print agent may comprise a dry toner. A photo charging unit may deposit a substantially uniform static charge on a photosensitive surface, which in this example is a photoconductive surface (which may be termed a photo imaging plate, or ‘PIP’). In some examples, such a charge is transferred to the photoconductive surface via a charge transfer roller which is in contact with the photoconductive surface, although non-contact methods of charge transfer may be used. A write head comprising a light source (for example at least one laser), may be used to dissipate the static charge in selected locations of the image area on the photoconductive surface to leave a latent electrostatic image. Both the locations and the power of the write head may be controlled based on the halftoned image data, which in this example may have been modified relative to original halftoned image data in block 222. In other examples, the printing apparatus may be some other form of printing apparatus, which may for example comprise a photolithographic printing device and/or an offset printing apparatus. [0055] The electrostatic print agent is transferred to the photoconductive surface from a print agent source using a print agent supply unit (which may be termed a Binary Ink Developer (BID) unit in some examples), which may present a substantially uniform film or layer of the print agent to the photoconductive surface for example via a print agent application roller.

[0056] In an example, particles and/or a resin component of the print agent may be electrically charged by virtue of an appropriate potential applied to the print agent in the print agent source. The charged particles and/or resin component, by virtue of an appropriate potential on the electrostatic image areas of the photoconductive surface, are attracted to the latent electrostatic image on the photoconductive surface. In this example, the print agent does not adhere to the charged areas and forms an image in print agent on the photoconductive surface in the uncharged locations (although the reverse could be true if the polarity of the print agent was reversed). The photoconductive surface will thereby acquire a print agent pattern on its surface.

[0057] In some examples, the pattern may then be transferred to an Intermediate Transfer Member (ITM), by virtue of pressure and/or an appropriate potential applied between the photoconductive surface and the ITM such that the charged print agent is attracted to the ITM. The ITM may for example comprise an endless loop and/or a rubber ‘blanket’, for example comprising a belt arranged about rollers or material arranged about the surface of a drum. The ITM may be urged towards the photoconductive surface to be in close proximity thereto. In some examples, the ITM is biased towards the photoconductive surface such that, but for the presence of any print agent on the photoconductive surface, it would be in contact with the photoconductive surface.

[0058] In some examples, the print agent pattern may be dried and/or at least partially fused on the ITM before being transferred to a substrate (for example, adhering to the colder surface thereof). In other examples, print agent may be transferred from a photoconductive surface directly to a substrate.

[0059] In some examples, an image on a substrate may be built up in layers (i.e. ‘separations’, as mentioned above) produced using different print agents.

[0060] Figure 3A and 3B show, schematically, an example of operation of the method. A snapshot of part of an original halftoned image is shown in Figure 3A, each pixel represented by a square. In this example, there are four power levels: a zero power level, indicated by an empty square, a low power level indicated by a small circle, a medium power level associated with a medium-sized circle and a peak power level associated with a large circle. In Figure 3A, three pixels are highlighted, pixels 302, 304 and 306.

[0061] Figure 3B shows the same data after operation of an example of the method of Figure 1 and 2. In this example, pixels 302, 304 and 306 were identified as being in rows and/or columns which are shorter than four pixels in length, and which comprise less than two peak power pixels. Therefore, a number of pixels have been associated with higher power levels in Figure 3B, such that no individual pixel is associated with a row or column of less than four pixels, unless that row/column comprises at least two peak energy pixels (noting that, in some cases, the condition may be fulfilled by pixels which are outside the field of view of Figure 3A and 3B). In the case of the pixel 306, this pixel was isolated from any surrounding non-zero power level pixels. Rather than instructing application of a print agent dot in relation to a pixel which was not previously associated with printing print agent, the power level for this pixel has been reduced to zero.

[0062] Figure 4A, 4B and 4C schematically illustrate another example of how a power level criterion may be applied. Figure 4A illustrates a search mask, which may be applied to halftoned image data to identify a region of interest having predetermined features. The pixels marked with a 0 are to be matched with pixels associated with a zeropower level in the halftoned image data, whereas the pixels marked with x may be matched with pixels associated with any power level. The central pixel may be matched with any pixel which is not either a zero power level of pixel or a peak power level pixel. Thus, the search mask may be scanned across the halftoned image data and where there are two columns of three white pixels around a central non-zero, non-peak power pixel, having the layout or pattern set out in the mask this may be identified as a region of interest.

[0063] It may be appreciated that in this example, the pattern defined by the example search mask serves to identify potential ‘thin’ vertical features which are to be printed between ‘white’, or non-printed, bounding columns. However, in other examples, the content of the region of interest may be matched according to some other predetermined pattern which may be embodied in a search mask. Moreover, there may be more than one search mask, for example to identify features of potential interest in another dimension (horizontal rather than vertical, for example), and the halftoned data may therefore be scanned multiple times with different search masks. Such search masks may be applied to identify regions of interest, in some examples based on logical combinations of matched regions of the halftoned data. For example, regions of interests could be identified using an AND condition (i.e. having been matched to more than one pattern defined in a search mask) and/or an OR condition (i.e. matching any one of at least two search mask patterns).

[0064] Figure 4B shows an example of an identified region of interest in halftoned image data. In this example, there are three non-zero power levels, a low, indicated by a 1 , a medium indicated with 2 and a peak, which would be indicated with a P if present. This region is identified by scanning the search mask over image data until there is a match for each pixel of the search mask (wherein the pixels marked x may match with pixels of any power level).

[0065] Once a region of the halftoned data has been identified as matching the search mask, a power level criterion may be applied thereto. In this example, the total power level associated with the central three columns is considered, i.e. in a 3 by 5 pixel array, and compared to a threshold which provides the power level criterion, although another pixel region may be defined in another example. In this example, the value may be a simple sum of the numbers associated with the pixels, but it may be noted that a medium power level pixel may not be associated with double the energy of the low power pixel and thus in other examples, other weighted sums may be determined. Moreover, in some examples, other criteria, such comparing a count of peak power pixels to a threshold, may be used.

[0066] In this example, the power level criterion of the region of the halftoned data identified using the search mask is not met, and therefore the power level associated with the central pixel is increased to be equal to the peak power level as shown in Figure 4C.

[0067] Figure 5 is an example of a print apparatus 500, in this example an electrophotographic print apparatus such as a liquid electrophotographic print apparatus. The apparatus 500 comprises processing circuitry 502, which may act as a controller thereof and further comprises a light source 504 (e.g. a laser) and a photoconductive surface 506 (in this example, in the form of a drum having a PIP wrapped around the surface thereof).

[0068] In use of the apparatus 500, the light source 504 selectively discharges electrostatic charges on the photoconductive surface 506 to provide a latent pattern corresponding to an intended print agent pattern, and the photoconductive surface 506 receives a print agent pattern according to the latent pattern. In other examples, the light source may interact with a photosensitive surface in some other way. [0069] In use of the apparatus 500, the processing circuitry 502 determines if a region (e.g, at least one, or any region) of halftoned image data does not meet a predetermined power level criterion and if so, the processing circuitry 502 modifies the halftoned image data to increase a light source power level associated with at least one pixel of the region. In some examples, the modification may be conditional on there being at least one pixel which is eligible for an increase in power level, as set out above.

[0070] As described above, the halftoned image data comprises an array of pixels, wherein each pixel is associated with a power level of the light source. For example, the power levels may comprise one of a zero power level and a peak power level, and in some examples at least one intermediate power level. For example, determining if a region of halftoned image data does not meet a predetermined power level criterion comprises identifying a region of pixels (e.g. an n x m array, where n and m may be any integer, or a continuous group of non-zero power level pixels, or a region identified by matching some pattern, for example as set out in a search mask), wherein (i) the number of pixels which are associated with a non zero power level is less than a threshold number of pixels, and/or (ii) the number of pixels which are associated with a peak power level is less than a threshold number of pixels, and/or (iii) in which a cumulative power level is less than a threshold power level, or the like. In some examples, candidate regions may be identified using a search mask, as set out above, or by inspecting a region local to each pixel in some other way.

[0071] Moreover, the processing circuitry 502 may, in use of the apparatus 500, control the light source to provide power according to the modified halftoned image data.

[0072] In some examples, the processing circuitry 502 may carry out any of the methods described in relation to Figure 1 , at least blocks 202 to 222 of Figure 2 and Figure 3 as set out above. In some examples, the processing circuitry 502 may identify a region of the halftoned image data using a search mask, for example as discussed in relation to Figure 4. In some examples, the processing circuitry 502 may be provided separately from the print apparatus 500.

[0073] In some examples, the apparatus 500 may comprise a memory to store image data, halftoned image data and/or machine-readable instructions for use by the processing circuitry 502. In some examples, the memory may store a plurality of threshold screens, for example threshold screens as described above in relation to Figure 2. However, in other examples, such data may be stored elsewhere. [0074] Although not shown herein, the apparatus 500 may also comprise additional print apparatus components, for example print agent application unit(s), a charging unit(s) for charging the photoconductive surface, an Intermediate Transfer Member (ITM) which may receive an image from the photoconductive surface before transferring this image to a substrate, substrate handling apparatus, colorant curing or drying apparatus, and the like. The processing circuitry 502 may for example control the apparatus 500 to carry out the method of blocks 224, 226 and/or 228 of Figure 2 set out above.

[0075] Figure 6 is an example of a tangible, non-transitory, machine readable medium 602 in association with a processor 604. The tangible machine readable medium 602 comprises, or stores, instructions 606 which, when executed by the processor 604, cause the processor 604 to inspect a pixel array representing halftoned image data, wherein each pixel is associated with a power level, to identify at least one pixel associated with a low likelihood of print agent transfer; and modify the halftoned image data to increase a laser power associated with at least one of the identified pixel and a neighbor pixel of the identified pixel. In some examples, the neighbour pixel is a direct neighbour, whereas in other examples, the neighbour pixel may be a pixel in a continuous region of non-zero power level pixels. In some examples, a neighbour pixel may be any pixel identified as being in a local region, for example within an n x m pixel window, where n and m are any integer.

[0076] For example, identifying a pixel with a low likelihood of print agent transfer may comprise inspecting a pixel associated with a non-zero power level to identify adjoining pixels associated with a non-zero power level to define a pixel group or cluster. For example, a group/cluster may comprise a column or row as described in greater detail above, or a two-dimensional continuous area. In some examples, inspecting the pixel may comprise applying a search mask centered on the pixel (or registered therewith in some other way), for example as discussed in relation to Figure 4. At least one size of the pixel group may be determined in some examples. For example, the size may comprise a pixel count. This size may be compared to a threshold size and, if the size is less than a threshold size, the instructions 602 may cause the processor 604 to determine if the power levels associated with the pixels of the group meet a predetermined criterion. If not, the instructions 602 may cause the processor 604 to increase a power level associated with at least one pixel. Other methods for identifying whether a region of the halftoned image data meets a power level criterion, and thus whether the pixel has a satisfactory likelihood of print agent transfer, such as determining a cumulative power and/or determining a count of peak power pixels, have been discussed above.

[0077] In some examples, the instructions 606 may cause the processor 604 to carry out any or any combination of the blocks of Figure 1 , and/or any or any combination of at least blocks 202 to 222 of Figure 2, and/or the instructions 606 may cause the processor 604 to operate in line with the methods described in relation to Figure 3 or 4. In some examples, the instructions 606 may cause the processor 604 to control print apparatus to carry out any or any combination of blocks 224 to 228 of Figure 2 and/or may cause the processor 604 to act as the processing circuitry 502 of the apparatus 500 of Figure 5.

[0078] Aspects of some examples in the present disclosure can be provided as methods, systems or machine-readable instructions, such as any combination of software, hardware, firmware or the like. Such machine-readable instructions may be included on a computer readable storage medium (including but not limited to disc storage, CD-ROM, optical storage, etc.) having computer readable program codes therein or thereon.

[0079] The present disclosure is described with reference to flow charts and block diagrams of the method, devices and systems according to examples of the present disclosure. Although the flow diagrams described above show a specific order of execution, the order of execution may differ from that which is depicted. Blocks described in relation to one flow chart may be combined with those of another flow chart. It shall be understood that at least one block in the flow charts, as well as combinations of the blocks in the flow charts can be realized by machine readable instructions.

[0080] The machine-readable instructions may, for example, be executed by a general-purpose computer, a special purpose computer, an embedded processor or processors of other programmable data processing devices to realize the functions described in the description and diagrams, and which may for example comprise at least part of the processing circuitry 502, or the processor 604. In particular, a processor or processing apparatus may execute the machine-readable instructions. Thus functional modules of the apparatus and devices may be implemented by a processor executing machine readable instructions stored in a memory (for example, the machine readable medium 602), or a processor operating in accordance with instructions embedded in logic circuitry. The term ‘processor’ is to be interpreted broadly to include a CPU, processing unit, ASIC, logic unit, or programmable gate array etc. The methods and functional modules may all be performed by a single processor or divided amongst several processors.

[0081] Such machine-readable instructions may also be stored in a computer readable storage that can guide the computer or other programmable data processing devices to operate in a specific mode.

[0082] Such machine-readable instructions may also be loaded onto a computer or other programmable data processing devices, so that the computer or other programmable data processing devices perform a series of operations to produce computer-implemented processing, thus the instructions executed on the computer or other programmable devices realize functions specified by block(s) in the flow charts and/or in the block diagrams.

[0083] Further, the teachings herein may be implemented in the form of a computer software product, the computer software product being stored in a storage medium and comprising a plurality of instructions for making a computer device implement the methods recited in the examples of the present disclosure.

[0084] While the method, apparatus and related aspects have been described with reference to certain examples, various modifications, changes, omissions, and substitutions can be made without departing from the spirit of the present disclosure. It is intended, therefore, that the method, apparatus and related aspects be limited by the scope of the following claims and their equivalents. It should be noted that the above- mentioned examples illustrate rather than limit what is described herein, and that those skilled in the art will be able to design many alternative implementations without departing from the scope of the appended claims. Features described in relation to one example may be combined with features of another example.

[0085] The word “comprising” does not exclude the presence of elements other than those listed in a claim, “a” or “an” does not exclude a plurality, and a single processor or other unit may fulfil the functions of several units recited in the claims.

[0086] The features of any dependent claim may be combined with the features of any of the independent claims and/or other dependent claim(s).

Claims

1. A method comprising, with processing circuitry: receiving halftoned image data comprising a pixel array, wherein each pixel of the pixel array is associated with a power level; identifying a region of the pixel array which does not meet a predetermined power level criterion; and when a region is identified, increasing a power level associated with at least one pixel of the region.

2. A method according to claim 1 wherein identifying the region of the pixel array which does not meet a predetermined power level criterion comprises identifying a pixel which comprises part of a continuous cluster of pixels associated with a non-zero power level of less than a threshold number of pixels.

3. A method according to claim 1 wherein identifying the region of the pixel array which does not meet a predetermined power level criterion comprises identifying a pixel which comprises part of a continuous cluster of pixels comprising less than a threshold total power level.

4. A method according to claim 1 wherein identifying the region of the pixel array which does not meet a predetermined power level criterion comprises using a search mask to identify a region of the halftoned image data matching a pattern defined by the search mask.

5. A method according to claim 1 wherein each pixel of the pixel array may be associated with one of: a zero power level, at least one intermediate power level and a peak power level.

6. A method according to claim 5 wherein increasing the power level associated with at least one pixel of the region comprises increasing the power level associated with at least one pixel of the region which is associated with an intermediate power level.

7. A method according to claim 1 wherein increasing the power level associated with at least one pixel of the region is conditional on a pixel meeting a criterion to have the power thereof increased.

8. A method according to claim 7 wherein the criterion for increasing the power level associated with a pixel comprises at least one of: the pixel is associated with a non-zero power level; the pixel is associated with a non-peak power level; and the pixel is associated with a zero power level and the corresponding pixel in image data is associated with a non-zero greyscale level.

9. A method according to claim 7 comprising, when a pixel meeting the criterion is not identified, reducing the power level associated with at least one pixel to a zero power level.

10. A method according to claim 1 comprising halftoning image data by applying at least one threshold matrix thereto to generate the halftoned image data, wherein at least one pixel of the halftoned data is associated with an indication of a greyscale level associated with that pixel in the image data.

11. A print apparatus comprising: a photosensitive surface, which is to receive a print agent pattern; a light source to provide a latent pattern in the photosensitive surface corresponding to an intended print agent pattern; and processing circuitry to: determine if a region of halftoned image data comprising an array of pixels, wherein each pixel is associated with a power level of the light source, does not meet a predetermined power level criterion and if so, modify the halftoned image data to increase a light source power level associated with at least one pixel of the region; and control the light source to provide power according to the modified halftoned image data.

12. Apparatus according to claim 11 wherein determining if a region of halftoned image data does not meet a predetermined power level criterion comprises determining if a pixel of the halftoned image data is associated with an identified region of pixels which are associated with at least one of: a number of pixels of a non zero power level which is less than a threshold number of pixels; a number of pixels of a predetermined power level which is less than a threshold number of pixels; and a cumulative power level which is less than a threshold power level.

13. A tangible machine-readable medium comprising instructions which, when executed by a processor, cause the processor to: inspect a pixel array representing halftoned image data, wherein each pixel is associated with a power level, to identify a pixel associated with a low likelihood of print agent transfer; and modify the halftoned image data to increase a laser power associated with at least one of the identified pixel and a neighbor pixel of the identified pixel.

14. The tangible machine-readable medium of claim 13 wherein identifying a pixel with a low likelihood of print agent transfer comprises: inspecting a pixel associated with a non-zero power level to identify adjoining pixels associated with a non-zero power level to define a pixel group; determining at least one size of the pixel group; comparing the size to a threshold size and, if the size is less than a threshold size, determining if the power levels associated with the pixels of the group meet a predetermined criterion.

15. The tangible machine-readable medium of claim 13 wherein determining if the power levels associated with the pixels of the group meet a predetermined criterion comprises determining if the group comprises a threshold number of pixels associated with a maximum power level.