BACKGROUND

[0001] In some printing devices a pretreatment may be applied to a print medium prior to printing on the medium with colored inks. In some devices the pretreatment may be applied by a print head.

BRIEF DESCRIPTION OF THE DRAWINGS

[0002] Examples of the invention are further described hereinafter with reference to the accompanying drawings, in which:

Figure 1 shows an example of a printing device.

Figure 2 shows an example of a proportional print mask.

Figure 3 shows examples of rules for generating a pretreatment mask.

Figure 4 shows an example of a non-proportional pretreatment mask.

Figure 5 shows a method of generating non-proportional pretreatment masks.

Figure 6 shows a method according to an example.

Figure 7 shows an example of applying a non-proportional pretreatment mask when pretreatment is to be applied to a part of a swath.

DETAILED DESCRIPTION

[0003] Figure 1 illustrates an example of a printing device 100 having a printhead 1 10 that is moveable perpendicular to a feed direction 130 of a print medium 120. Herein, the feed direction is a direction in which the medium 120 is fed, and will be referred to as the y-direction. The direction substantially in the plane of the medium and perpendicular to the y-direction will be referred to as the x-direction. Herein, references such as "along the x-direction," include both positive and negative x-directions. That is, the sign of the x-direction is not significant.

[0004] A printer control section 140 controls the printhead 1 10, and may also control other functions, such as feeding of medium 120.

[0005] The printhead 1 10 may be an ink jet printhead. The printhead 1 10 may have a plurality of nozzles for depositing pretreatment and/or color ink onto the medium 120. The nozzles may be arranged in a matrix.

[0006] The printhead 1 10 is arranged to apply a pretreatment to the medium, and following the pretreatment may apply one or more colored inks to the medium. The pretreatment may improve the image quality, for example by modifying an interaction between the medium and the color ink. The effect of the pretreatment may depend on various parameters, such as the medium and ink, as well as an amount of pretreatment applied, a time between application of the pretreatment and application of the color ink, and ambient conditions (temperature, humidity, etc.) Where the pretreatment parameters are poorly chosen the quality of the printed image may be reduced, for example due to bleed and/or coalescence. In some cases, a period between applying a pretreatment and applying a color ink over the pretreatment can affect wetting of the media by the pretreatment and/or the drying of the pretreatment, which can affect image quality.

[0007] The printhead 1 10 prints on the medium in swaths. Herein a swath refers to a portion of the medium 120 that can be printed on by the printhead 1 10 without moving the medium relative to the printhead 1 10 along the y-direction 130. As the printhead 1 10 is moveable across the medium 120 in the x-direction, the swath defines a strip of the medium 120 that extends across the medium 120 and has a width in the y-direction corresponding to the length of the print footprint of the printhead 1 10 in the y-direction. Here, the print footprint describes the area of the medium 120 that is printable by the printhead 1 10 without relative movement between the printhead and the medium 120. A swath is illustrated in Figure 1 as shaded area 125.

[0008] In operation, the control section 140 may control the printhead 1 10 to perform a plurality of passes over each swath of the medium to place pretreatment or ink onto the medium 120. In each pass the printhead 1 10 is moved relative to the medium 120 in the x- direction. Each pass may extend substantially across the width of the medium 120 in the x- direction.

[0009] When the required number of passes has been completed for a swath, the medium 120 may be fed along the y-direction 130 to expose the next swath to the printhead 1 10.

[0010] For each swath, the printhead 1 10 may perform a plurality of pretreatment passes, and may also perform one or more color passes. Pretreatment is applied in pretreatment passes, and color ink is applied in color passes. In some examples pretreatment may be applied to the whole swath, while in other examples the pretreatment may be applied to only part of the swath. In some examples the pretreatment may be applied uniformly, while in other examples the pretreatment is applied with different concentrations in different parts of the swath. The control section 140 may receive or generate pretreatment data that indicates the portions of the medium 120 that are to receive pretreatment. The pretreatment data may also identify the concentration of pretreatment that is to be applied.

[0011] For each pass in which pretreatment is applied, referred to herein as a pretreatment pass, a pretreatment mask is applied that defines the portions of the swath that may receive pretreatment in that pass. The number of pretreatment masks is equal to the number of pretreatment passes to be applied to the swath.

[0012] The medium may be considered as a plurality of pixels that may each receive pretreatment and/or ink. The pixels may be arranged in a rectangular grid, for example. Within a single pass the pretreatment is applied only to those pixels identified as printable by the pretreatment mask. In some examples, each of the pixels of the swath is identified as printable in at least one pass. In some examples, each of the pixels of the swath is identified as printable in exactly one pass. In some examples, each of the pixels of the swath is identified as printable in more than one pass, the number of passes in which each pixel is printable may be the same for all pixels.

[0013] Figure 2 shows an example of pretreatment masks for a swath having 4 passes.

Figure 2a illustrates a matrix of pixels that are printable by the print head. Each cell corresponds to one pixel, and the number in each cell corresponds to the pass on which that pixel is printable. "0" corresponds to the first pass, "1 " corresponds to the second pass, "2" corresponds to the third pass, and "3" corresponds to the fourth pass. Figure 2b shows each of the pretreatment masks: the grid represents an array of pixels, and a shaded square indicates a pixel that can be printed in the corresponding pass.

[0014] Figure 2 shows a proportional mask, in which each pass has substantially the same number of printable pixels. There are 32x32=1024 pixels in total, so each pretreatment mask has 1024/4=256 printable pixels. The non-printable pixels in each pretreatment mask are shown in unshaded in Figure 2b. The proportion of printable pixels to the total number of pixels in a pretreatment mask may be referred to herein as the weight of the mask. For example, each of the masks in Figure 2 has a weight of ¼ or 25%.

[0015] Figure 2 illustrates a random mask, in which the pixels are assigned to the four pretreatment masks at random, subject to the constraint that there are equal numbers of printable pixels in each mask.

[0016] Figure 3 illustrates rules that may be applied in generating a pretreatment mask.

Figure 3a illustrates a rule that a pretreatment mask may not include any pair of neighboring pixels: The shaded square represents a printable pixel of the current mask, and "x" represents a pixel that may not be printable in the same mask. Figure 3b illustrates a rule that no horizontal or vertical neighbors (edge-sharing neighbors) may be printable in a pretreatment mask immediately following the current mask. The shaded square represents a printable pixel of the current mask, and "x" represents a pixel that may not be printable in the immediately following pass. The rules of Figure 3a and 3b may be applied alone or in combination, or may not be applied at all. Other rules could also be applied, by applying constraints based on nozzles or groups of nozzles corresponding to the pixel or pixels, and/or rules based on layers (e.g. half-tone value dependent, etc), for example. In some examples the rules may include weighters indicating a probability of printing a pixel in a particular pass; the weighters may depend on nozzles or groups of nozzles corresponding to the pixel or pixels, for example. In some examples the distribution is based on, or similar to, a distribution that is known to produce satisfactory image quality, such as a distribution based on blue noise or white noise.

[0017] Figure 4 illustrates a non-proportional mask for a swath having four passes. Figures 4a and 4b are similar to figures 2a and 2b, respectively, except that the pretreatment masks of Figure 4 have different numbers of printable pixels in each of the passes (i.e. have different weights). By allowing the passes to have different numbers of printable pixels flexibility is improved.

[0018] In the example of Figure 4, each of the second to fourth pretreatment masks has a lower weight that the preceding mask. This is clear from a comparison of the number of printable pixels (shown as back squares) in Figure 4b. Thus, the weight of the masks decreases with increasing sequence number (i.e. numbered in sequence according to order of application).

[0019] In some examples color passes may be performed on the swath following the pretreatment passes. In such cases, the pretreatment applied in the first pretreatment pass has more time to wet the medium or to dry than the pretreatment in subsequent pretreatment passes. Accordingly, in the example of figure 4 the pretreatment from the first pretreatment pass has longer to dry than the pretreatment applied in the second pretreatment pass, which in turn has longer to dry than the pretreatment in the third pretreatment pass, etc. This arrangement may take advantage of the improved drying time that results from applying the pretreatment in multiple passes, while increasing the average time between applying pretreatment to a pixel and applying color ink to a pixel.

[0020] By using a non-proportional pretreatment mask, the curing time and/or time for an initial drying process between applying a pretreatment and applying a color ink on top of the pretreatment can be flexibly controlled. In some examples this may reduce or eliminate a need for a delay or pause in printing between pretreatment passes and color passes. In some examples this may reduce or remove the need for additional components, such as a heater or dryer to control the curing of the pretreatment. Some examples allow proper (or desired) rheological behavior of a pretreatment to be obtained with little or no increase in print time specifically to allow for drying, and/or without requiring forced drying/curing (e.g. by a heating or drying element). Thus is may be possible to rely on natural drying of the pretreatment.

[0021] Figure 5 shows a method 500 of producing a non-proportional pretreatment mask, such as the mask shown in Figure 4. Where there are to be P pretreatment passes for each swath, P pretreatment masks are required. The method begins at step 505, and at step 510, N proportional masks are generated, with N>P. The proportional masks have equal weighting of 1/N, such that the number of printable pixels in each mask is K/N, where K is the total number of pixels in the mask. The proportional masks may be generated randomly and/or according to rules, such as those described in relation to Figure 3.

[0022] At 520 P non-proportional masks are generated by combining one or more of the N proportional masks to produce each of the P non-proportional masks. Each of the N

proportional masks are assigned to, or associated with, exactly one non-proportional mask. The printable pixels in each non-proportional mask correspond to all of the printable pixels in the proportional masks from which it is generated. For example, if the set of printable pixels in the ith proportional mask is N,, the set of printable pixels in the non-proportional mask generated from the first and second proportional masks is N-| N ₂ .

[0023] The ith non-proportional mask has a weight of SjK/N, where s, is the number of proportional masks assigned to the ith non-proportional mask. To produce the non-proportional masks, not all of the weights of the non-proportional masks are equal. Thus, at least one pair of non-proportional masks are generated from different numbers of proportional masks.

[0024] As an example, where there are to be 4 passes (P=4), it is possible to generate 10 proportional masks (N=10). The proportional masks may be combined as in the following table to generate the 4 non-proportional masks.

[0025] The method 500 terminates at 530.

[0026] Figure 6 shows a method 600 according to an example. The method begins at 605. At 610 a pretreatment mask is allocated for at least first and second passes over a swath. At 620 the printhead 1 10 pretreats the medium 120 according to the allocations of 610. Each pretreatment mask allocated at 610 indicates a corresponding set of pixels to which the pretreatment may be applied in the corresponding pass. The allocations of 610 are such that the pretreatment masks of the first and second passes are such that a weight of the first pretreatment mask is different from a weight of the second pretreatment mask. It is noted that there may be additional passes, possibly including passes before the first pass, and/or between the first and second pass.

[0027] Figure 7 shows an example in which the pretreatment is not to be applied to a whole swath. In the example of Figure 7, Figure 7a shows a group of pixels that are to receive a pretreatment within a part of a swath. In this example the shaded pixels in on the left are to be pretreated, but no pretreatment is to be applied to the unshaded pixels on the right.

[0028] Figure 7b shows an example of a non-proportional print mask, where two pretreatment passes are to be performed on the swath. The pixels labeled "0" are to receive pretreatment in the first pass, and the pixels labeled "1 " are to receive pretreatment in the second pass.

[0029] Figure 7c shows the pixels that receive pretreatment in the first pass as solid circles. Pixels that are printable in the first pass (based on the corresponding pretreatment mask), but are not to be printed in this swath are shown as open circles. Figure 7d shows the pixels that receive pretreatment in the second pass as solid circles. Pixels that are printable in the second pass, but are not to be printed in this swath are shown as open circles. Within a particular pass, a pixel has pretreatment applied only if it is to receive pretreatment in that swath, and if it is a printable pixel according to the mask applied to the current pass.

[0030] In some examples, each mask may be defined for the whole swath. In other examples, each mask may be defined for a part of the swath and repeated, mirrored , or alternated with one or more other partial masks to generate the mask for the whole swath. In some examples, the mask is defined for the pixels within the print footprint of the printhead, and repeated across the swath.

[0031] In some examples, after the pretreatment passes, color ink is applied to the swath over the pretreatment in one or more color passes. Where multiple color passes are performed, color masks may be applied to the color passes, in an analogous manager to the pretreatment masks described above.

[0032] According to some examples, the color masks may be proportional masks. According to some examples, the color masks may be non-proportional masks. This can further increase flexibility, and allow further tuning of the pretreatment parameters.

[0033] According to some examples, the non-proportional color masks have an increasing weight with sequence number. Accordingly, an average time interval between applying a pretreatment to a pixel and applying a color ink to the pixel may be further increased.

[0034] In some cases it is desirable for the pretreatment masks to have decreasing weight with sequence number. However, there may also be cases in which improved results can be achieved with increasing weights or non-monotonic weights with pretreatment mask sequence number.

[0035] In some examples it may be possible to apply different amounts of pretreatment and/or color ink to each pixel. For example, it may be possible to apply ink drops of different sizes. This does not change the operation of the pretreatment masks and color masks described above.

[0036] According to the examples described above, all passes are completed over one swath and then the medium is fed such that the next swath is below the printhead 1 10, and the next swath is printed by a plurality of passes. However, in some examples the medium is fed only a fraction of the swath width (in the y-direction). For example, the medium may be fed by 1/2 or 1/3 the width of the swath. In this case, the masks may be modified to take into account the overlap of the swaths.

[0037] In some examples, the printing process may be an inkjet printing process, such as a thermal or piezoelectric printing process. Some examples the printing process may be a print- on-demand process. Some examples may make use of a latex ink system.

[0038] In some examples the pretreatment may be a water based vehicle with a cationic polymer that increases its viscosity when in contact with the different color pigments. In some examples the pretreatment may include other components, such as surfactants, dispersants, etc.

[0039] In some examples the color inks are include water as a solvent. Other solvents could be used. In some examples the color inks include latex polymer particles and pigment particles.

[0040] In some examples the above masking arrangement may be applied to a post-treatment instead of, or as well as, a pretreatment. In some examples the weight of post-treatment masks may increase with sequence number, which may increase the average time period between applying a color ink to a pixel and applying a post-treatment to the pixel. Post- treatments may enhance image print quality, and may include a varnish and/or a fixer, for example. Herein, the term "treatment" is used to mean pretreatment and/or post-treatment.

[0041] The control section 140 may be implemented using any combination of hardware and/or software, and may include one or more of a processor, volatile memory, non-volatile memory, etc.

[0042] Throughout the description and claims of this specification, the words "comprise" and "contain" and variations of them mean "including but not limited to", and they are not intended to (and do not) exclude other moieties, additives, components, integers or steps. Throughout the description and claims of this specification, the singular encompasses the plural unless the context otherwise requires. In particular, where the indefinite article is used, the specification is to be understood as contemplating plurality as well as singularity, unless the context requires otherwise.

[0043] Features, integers, characteristics or compounds described in conjunction with a particular aspect or example are to be understood to be applicable to any other aspect or example described herein unless incompatible therewith. All of the features disclosed in this specification (including any accompanying claims, abstract and drawings), and/or all of the steps of any method or process so disclosed, may be combined in any combination, except combinations where at least some of such features and/or steps are mutually exclusive. The invention is not restricted to the details of any foregoing examples. The invention extends to any novel one, or any novel combination, of the features disclosed in this specification

(including any accompanying claims, abstract and drawings), or to any novel one, or any novel combination, of the steps of any method or process so disclosed.