Color separation data generation

Abstract

According to an example, a method (500) comprises receiving (510) image data including instructions for printing an image and generating (520) color separation data based on the image data. The color separation data maps the image data to a nonuniform printing fluid thickness on a photoconductive element (110). The method further comprises printing (530) the image in accordance with the color separation data.

Description

COLOR SEPARATION DATA GENERATIONBACKGROUND

[0001] Liquid electro-photography (LEP) printing systems form an image on a substrate by transferring printing fluid profiles formed on a surface of a photoconductive element to the substrate. The printing fluid profile can be generated by selectively charging or discharging the surface of the photoconductive element and selectively transferring printing fluids to the surface of the photoconductive element. To selectively transfer printing fluid to the photoconductive element, developing units may be used.BRIEF DESCRIPTION OF DRAWINGS

[0002] Features of the present disclosure are illustrated by way of example and are not limited in the following figure(s), in which like numerals indicate like elements, in which:

[0003] FIG. 1 shows a liquid electro-photography (LEP) printing system including a controller to control a set of developing units to transfer printing fluid with a non-uniform thickness, according to an example of the present disclosure;

[0004] FIG. 2 shows an LEP printing system comprising a developing unit including a developer roller, an electrode, and a squeegee roller, according to an example of the present disclosure;

[0005] FIG. 3 shows a line chart representing relative voltage differences of a first component and a second component with respect to a reference voltage, according to an example of the present disclosure;

[0006] FIG. 4 shows an LEP printing system including a photoconductive element and an intermediate transfer member, according to an example of the present disclosure;

[0007] FIG. 5 shows a method for printing an image in accordance with color separation data, according to an example of the present disclosure;

[0008] FIG. 6 shows a method for developing printing fluid with a non-uniform printing fluid thickness, according to an example of the present disclosure; and

[0009] FIG. 7 shows a system comprising a processor and a computer- readable medium comprising instructions to be executed by the processor, according to an example of the present disclosure.DETAILED DESCRIPTION

[0010] For simplicity and illustrative purposes, the present disclosure is described by referring mainly to examples. In the following description, numerous specific details are set forth in order to provide a thorough understanding of the present disclosure. It will be readily apparent, however, that the present disclosure may be practiced without limitation to these specific details. In other instances, some methods and structures have not been described in detail so as not to unnecessarily obscure the present disclosure.

[0011] Throughout the present disclosure, the terms "a" and "an" are intended to denote at least one of a particular element. As used herein, the term "includes" means includes but not limited to, the term "including" means including but not limited to. The term "based on" means based at least in part on.

[0012] Liquid electro-photography (LEP) printing systems can generate images on substrates by transferring printing fluid profiles to the substrate. Prior to transferring the printing fluid to the substrate, a printing fluid profile is formed on a surface of a photoconductive element by transferring printing fluid using a developing unit (e.g., binary ink developers). In an example, at least one developing unit is selectively engaged with the surface of the photoconductive element to selectively transfer printing fluid to the photoconductive element such that a printing fluid profile is formed.

[0013] LEP printing systems may comprise charging elements (e.g., charging rollers to electrically charge a surface of the photoconductive element) and discharging elements (e.g., a writing head to selectively discharge specific regions of the surface of the photoconductive element). In an example, a charging roller may uniformly charge a surface of a photoconductive element of an LEP printing system at a reference voltage. Then, the discharging element may selectively discharge regions of the surface of the photoconductive element whereby a latent image is generated on the surface of the photoconductive element.

[0014] LEP printing systems may transfer printing fluid to the photoconductive element using developing units. To transfer the printing fluid, a developing unit develops printing fluid and subsequently transfers the developed printing fluid to the photoconductive element. As used herein, the expression “to develop the printing fluid” refers to modifying a density of a printing fluid and an electrical charge of the printing fluid by using an electrical source. Once the printing fluid is developed, the developed printing fluid is selectively transferred to the surface of the photoconductive element. In particular, the transfer from the developing unit to the photoconductive element takes place based on a voltage difference between the developed printing fluid and a voltage of a region of the latent image previously generated on the photoconductive element. If an electrical charge of a region of the surface exceeds a charge value, the printing fluid is repelled from such a charged region. On the other hand, if the electrical charge of a region is below a charge value, the developed printing fluid is attracted to such a charged region thereby causing a transfer from the developing unit to the photoconductive element. By subsequently engaging and disengaging developing units of the LEP printing system, printing fluid profiles associated with the image are formed on the surface of the photoconductive element.

[0015] The generation of an image in an LEP printing system may involve forming multiple printing fluid profiles on the surface of the photoconductive element. In some examples, these printing fluid profiles may be formedsequentially (e.g., by engaging a single developing unit with the photoconductive element at the same time). However, in other examples, multiple printing fluid profiles may be formed in parallel (e.g., by engaging multiple developing units with the photoconductive elements at the same time). Once these printing fluid profiles are on the photoconductive element, the printing fluid profiles may be transferred directly to the substrate to form the image. However, in other examples, the printing fluid profile(s) may be transferred to additional elements of the LEP printing system such as intermediate transfer elements.

[0016] As used herein, “printing fluid” refers generally to any substance that can be applied upon a substrate by a printer during a printing operation, including but not limited to inks, electro-inks, overcoat materials, water, and solvents other than water. As used herein, the term “electro-ink” refers to a combination of liquids and solids. The liquid may correspond, for instance, with an oil such as dielectric oil. The solid may be a pigment or another type of solid, such as pigmented resins in addition to other compounds. In an example, the solids of the electro-ink contain particles electrically charged or to be electrically charged using an external electrical source.

[0017] LEP printing system may include a plurality of developing units. In some examples, each developing unit is associated with a different type of printing fluid. Accordingly, a controller of the LEP printing system may process the image data associated with the image to be printed based on the types of printing fluid available at the LEP printing system. In an example, different types of printing fluid may have different colors (e.g., cyan, magenta, black, yellow, white, orange, green or violet). However, in other examples, alternative types of printing fluid (such as special inks) may be possible. In some examples, an LEP printing system comprises four developing units associated with cyan, magenta, yellow, and black.

[0018] To effectively create a printing fluid profile on a surface of a photoconductive element, components of the LEP printing system are electrically charged at reference voltages. For instance, the chargingelement of the printing system may electrically charge a surface of a photoconductive element at a reference voltage of within a range from - 1400V to -700V (e.g., -1200V to -900V). Similarly, the writing head may electrically discharge the surface of the photoconductive element at voltages within a range from -400V to 0V (e.g., -200V to 0V). To develop printing fluid, a controller of the system may control a charging element of the developing unit to maintain a developer roller at a reference voltage (e.g., a voltage between -950V and -150V, a voltage between -750V and -350V, a voltage of -850V, or a voltage of -500V). In some other examples, other elements of the developing unit may be maintained at a reference voltage (e.g., a squeegee roller or an electrode such as a main electrode).

[0019] The image quality of the image obtained in an LEP printing system may depend on how color separation data is generated. As used herein, the term “color separation data” will be used to refer to a mapping of the image data to at least one color map which is indicative of a type of printing fluid to be transferred at a particular location of a surface of the photoconductive element. Color separation data may include electrical charge data associated with a latent image to be formed on the photoconductive element. In some examples, a color present in the image to be printed may be separated into multiple color separations based on the types of printing fluid available in the printing system. In some examples, the color maps may be generated using halftoning techniques. As used herein, the word “halftoning” refers to the generation of electrical charge data that maps a color (or colors) using discrete dots of printing fluid varying in size and / or spacing such that a continuous-tone image is simulated.

[0020] However, the image quality obtained when generating color maps using halftoning techniques may be constrained by the size and / or spacing of the printing fluid dots associated with the color maps. In particular, a visual inspection of the printed image using a high level of magnification may show a poor color shift due to the presence of empty spaces between printing fluid dots and / or non-continuous colors. As a result, the imagequality resulting from a printing operation in the printing system may be negatively affected by the generation of color separation data.

[0021] As used herein, the term “color shift” refers to a level of how spatially uniform and how continuous a color transition from a first color to a second color under high magnification is. For instance, when transitioning from a color A to a color B, a smooth color shift includes a more uniform and continuous color transition (e.g., color gradients in a color transition) compared to a poor color shift (e.g., use of different colors next to each other in a color transition).

[0022] Disclosed herein are examples of printing systems, methods, and computer-readable mediums comprising instructions that may be used to generate color separation data to cause an LEP printing system to print an image with an improved color shift thereby improving the image quality. In particular, the examples described herein improve the image quality by generating the color separation data with an improved color shift across regions of the image.

[0023] According to an example, a printing system comprises a photoconductive element to be electrically charged in accordance with an image to be printed on a substrate, a set of developing units, and a controller. The controller is to receive image data associated with the image, generate color separation data based on the image data, control the set of developing units to develop printing fluid in accordance with the color separation data, and control the set of developing units to transfer the developed printing fluid. In particular, the controller is to control at least one developing unit of the set of developing units to develop printing fluid with a non-uniform printing fluid thickness and to control the set of developing units to transfer the developed printing fluid with the non-uniform thickness to the photoconductive element.

[0024] As used herein, the term “controller” will be used to refer to any combination of hardware and programming to implement the functionalities described herein. In some examples, such combinations of hardware andprogramming may be implemented in a number of different ways. For example, the programming of modules may be processor-executable instructions stored on at least one non-transitory machine-readable storage medium and the hardware for modules may include at least one processor to execute those instructions. In some other examples, multiple modules may be collectively implemented by a combination of hardware and programming, as described above. In some other examples, the functionalities of the controller may be, at least partially, implemented in the form of electronic circuitry.

[0025] Throughout the description, the term “printing fluid thickness” will be used to refer to the height of the printing fluid transferred from the developing unit to the photoconductive element. Similarly, the term “non- uniform printing fluid thickness” will be used to refer to a printing fluid thickness that is variable across at least one of a longitudinal and a transversal direction of the printed substrate. As used herein, a printing fluid thickness is considered as non-uniform upon a printing fluid thickness at two different regions of a printing fluid profile differ by more than 20% (e.g., 30%, 40%, 50%, or 90%).

[0026] Referring now to FIG. 1 , a printing system 100 is shown. The printing system 100 comprises a photoconductive element 110, a set of developing units 120, and a controller 130. The set of developing units 120 comprises five developing units. However, in other examples the number of developing units may be different (e.g., two developing units, three developing units, four developing units, or more than five developing units). In some examples, the number of developing units may be selected based on the number of different types of printing fluid available at the printing system 100 (e.g., a printing system may include four developing units corresponding to CMYK). Each developing unit of the set developing units 120 is movable between an engaged position at which the developing unit is in contact with the photoconductive element 110 and a disengaged position at which the developing unit is away from the photoconductive element 110. In thesystem 100, one developing unit is at the engaged position and four developing units are at the disengaged position.

[0027] As explained above, the photoconductive element 110 may be electrically charged in accordance with an image to be printed on a substrate via, for instance, a charging element. In some examples, a discharging element may be used to create a latent image on the photoconductive element 110. In an example, the photoconductive element 110 may correspond to a photo imaging plate (PIP).

[0028] In the system 100, the controller 130 controls the photoconductive element 110 to rotate in a counterclockwise direction. As the photoconductive element 110 rotates, the controller 130 may control at least one developing unit of the set of developing units 120 to selectively engage with the photoconductive element 110. For instance, in the system 100, the controller 130 controls the third developing unit to move to the engaged position. While the developing unit is at the engaged position, a printing fluid transfer from the developing unit to the surface of the photoconductive element 110 may take place based on a voltage difference between the latent image generated on the photoconductive element 110 and the developing unit. At the engaged position of each developing unit of the set of developing units 120, respective transfer regions are defined at specific locations of the photoconductive element 110.

[0029] As used herein, the term “transfer region” refers to a region of a photoconductive element where a developing unit at an engaged position and the photoconductive element are close to contact (or contact) with each other. Since the photoconductive element may engage with multiple developing units, multiple transfer regions may be defined for the same photoconductive element. During a printing fluid transfer operation, the printing fluid developed by the developing unit is transferred to the photoconductive element via the transfer region. For instance, in the system 100, a transfer region is defined for the developing unit positioned at the engaged position and the photoconductive element 110.

[0030] The controller 130 of the system 100 is to receive image data associated with an image to be printed on a substrate. The image data may be part of a print job submitted by a user of the system 100. The controller 130 is to generate color separation data based on the image data. The color separation may be generated by mapping a particular color at a particular region of the image to be printed as a combination of colors and respective printing fluid thicknesses at a particular region of the available space. In an example, the color separation data may include electrical charge data indicative of a region of the photoconductive element to receive printing fluid and thickness data indicative of a printing fluid thickness to be developed by the developing unit. As a result, different printing fluid thicknesses may be obtained across the image transferred to the substrate. By generating the color separation data as a combination of the thickness data and the electrical charge data, the printing fluid profiles formed on the photoconductive elements are obtained with an improved color shift compared to alternative color separations in which the printing fluid thickness used for the printing fluid transfer operation is a uniform printing fluid thickness. The improved color shift results in an improved image quality.

[0031] In some examples, the generation of color separation data by controller 130 comprises generating electrical charge data representing locations on the photoconductive element 110 and thickness data representing printing fluid thicknesses to be transferred to the latent image formed on the photoconductive element. In an example, the electrical charge data may be used to control a discharging element (such as a writing head) to create a latent image on the photoconductive element 110 and thickness data may be used to control a developing unit to develop printing fluid with a preferred printing fluid thickness.

[0032] To print the image on the substrate with an improved color shift (e.g., smoother color shift), the controller 130 of the system 100 may control the set of developing units 120 to develop printing fluid in accordance with thecolor separation data. In particular, the controller 130 is to control at least one developing unit of the set of developing units 120 to develop printing fluid with a non-uniform printing fluid thickness. The subsequent formation of a printing fluid profile using printing fluid developed with a non-uniform thickness, as previously explained, reduces the discontinuities across the substrate and allows for obtaining the improved color shift. Then, the controller 130 is to control the set of developing units to transfer the developed printing fluid with the non-uniform printing thickness to the photoconductive element 110. The transferred printing fluid forms a printing fluid profile on the photoconductive element 110. Then, the controller 130 may control the printing system to form the image on the substrate by transferring the printing fluid profiles generated on the photoconductive element 110 to the substrate (not shown in FIG. 1 ).

[0033] The development of printing fluid with a range of printing fluid thicknesses may be carried out in different ways. For instance, the controller 130 may control the developing unit to modify at least one internal voltage such that a developer roller of the developing unit receives a layer of printing fluid having a preferred thickness. In an example, the controller 130 may control at least one of an electrode (such as a main electrode) and a squeegee roller of the developing unit to modify their voltage value with respect to the developer roller of the developing unit over time. In some examples, to develop printing fluid with a preferred printing fluid thickness, a reference voltage of the developer roller of the developing unit may be maintained during the printing fluid development operation, and a voltage value of at least one of an electrode (such as a main electrode) and a squeegee roller of the developing unit may be adjusted (e.g., varied over time). To obtain an enhanced image quality, non-uniform printing fluid thicknesses are used for forming printing fluid profiles on the photoconductive element 110.

[0034] Referring now to FIG. 2, an LEP printing system 200 including a developing unit 220 and a controller 230 is shown. In an example, thedeveloping unit 220 may correspond to a binary ink developer (BID). As previously explained, the developing unit 220 may serve for several functions comprising developing printing fluid, applying printing fluid to a surface of the photoconductive element (e.g., photoconductive element 110) of the LEP printing system 200, and removing residual printing fluid from the surface of the photoconductive element. The controller 230 may control the developing unit 220 and the sub-components of the developing unit 220 to perform actions.

[0035] The developing unit 220 comprises a developer roller 221 , an electrode 222, and a squeegee roller 223. The electrode 222 may correspond to a main electrode of the developing unit 220. To perform a printing fluid development operation in which the developing unit 220 is to develop printing fluid, the controller 230 sets the developer roller 221 at a predetermined electric potential while being rotated (clockwise in FIG. 2). As previously explained, the transfer of printing fluid to a surface of a photoconductive element (not shown in FIG. 2) takes place based on a voltage difference between the developer roller 221 and a voltage at a surface of a photoconductive element. To develop printing fluid to the developer roller 221 , the controller 230 adjusts a voltage of the electrode222 and / or a voltage of the squeegee roller 223. During this process, the printing fluid is developed with a non-uniform thickness to obtain an improved color shift thereby resulting in an improved image quality. In an example, during a printing fluid development operation, the controller 130 may vary a voltage of at least one of the electrode 222 and the squeegee roller 223 with respect to the voltage of the developer roller 221 such that a printing fluid thickness of a layer of printing fluid to be transferred to the developer roller is non-uniform.

[0036] In some examples, when generating printing fluid with a non-uniform thickness, the electric potential of the electrode 222 and the squeegee roller223 is adjusted relative to the developer roller 221 over time. In an example, a non-uniform printing fluid thickness may be obtained by varying an electricpotential of the electrode 222 relative to the developer roller 221 , the electric potential being within a range from -2200V to 0V (e.g., a range from -2000V to 0V). In another example, a non-uniform printing fluid thickness may be obtained by varying the electric potential of the squeegee roller 223 relative to the developer roller 221 , the electric potential being within a range from - 700V to 0V (e.g., a range from -500V to 0V). In some other examples, both electric potentials may be varied within their respective ranges. As a result of the different voltage differences between elements (e.g., developer roller 221 , electrode 222, and squeegee roller 223) of the developing unit 220, a range of different printing fluid thicknesses may be provided using the developing unit 220.

[0037] In an example, to generate printing fluid with a preferred printing fluid thickness, a voltage of at least one of the electrode 222 and the squeegee roller 223 may be adjusted in accordance with voltage-to-thickness data. In some examples, the voltage-to-thickness data may associate a preferred printing fluid thickness developed by the developing unit 220 to a voltage difference between the voltage values of the electrode 222 and / or the squeegee roller 223 and a voltage value of the developer roller 221 . In some examples, the printing fluid development operation may be based on the voltage-to-thickness data and the thickness data defined by the color separation data.

[0038] Referring now to FIG. 3, a line chart 300 representing a relative voltage difference of a first component and a second component with respect to a reference voltage over time is shown. In an example, the reference voltage corresponds to a voltage at which a developer roller of a developing unit (e.g., set of developing units 120 in FIG. 1 or developing unit 220 in FIG. 2) is maintained during a printing fluid transfer operation in which the printing fluid developed in the developing unit is transferred to a surface of a photoconductive element (e.g., photoconductive element 110 in FIG. 1 ). The components may correspond, for instance, to an electrode (e.g., electrode 222) or a squeegee roller (e.g., squeegee roller 223).

[0039] In line chart 300, the X-axis 301 represents time and the Y-axis 302 represents the relative voltage of components used in a printing fluid development operation with respect to the reference voltage. Line chart 300 includes a reference line 305 that represents a maximum relative voltage difference between the voltage of the component and the reference voltage. The maximum relative voltage difference may result in different absolute voltage for each of the first and second components, as the components may operate at different voltage values. In an example, the maximum voltage difference between the components and the reference voltage results in a maximum printing fluid thickness.

[0040] Line chart 300 represents a first voltage data 310 and a second voltage data 320 over time. The first voltage data 310 is associated with a first component and the second voltage data 320 associated with a second component. In FIG. 3, the first and the second component are part of different developing units. To simplify the explanation, each of the first voltage data 310 and the second voltage data 320 are periodic and linear. However, in other examples, the voltage data may be non-periodic and / or non-linear. In FIG. 3, the first voltage data 310 repeats over time with a first period T1 and the second voltage data 320 repeats over time with a second period T2, the second period T2 being greater than the first period T1 .

[0041] As previously described, internal voltage modulation in a developing unit (e.g., developing unit 220 in FIG. 2) results in a change in a printing fluid thickness of the printing fluid developed by the developing unit. Accordingly, each of the first voltage data 310 and the second voltage data 320 results in the formation of printing fluid profiles with a non-uniform printing thickness on a surface of a photoconductive element (e.g., a photoconductive element 110). The formation of these printing fluid profiles results in improved image quality, as the printing fluid profiles include a smoother color shift compared to the printing fluid profiles generated by simulation of a continuous image tone (e.g., by modifying the size and / or spacing of the printing fluid dots).

[0042] In some examples, each of the first voltage data 310 and the second voltage data 320 may be defined to achieve a specific color variation down the substrate to which the printing fluid profile is transferred. Accordingly, alternative patterns for the first voltage data 310 and the second voltage data 320 may be possible. In an example, a particular color variation for a particular printing fluid type may be associated with a particular pattern for the relative voltage defined between the electric potential of the component and the reference voltage. Accordingly, the patterns for the first voltage data 310 and the second voltage data 320 may be defined based on thickness data indicative of a printing fluid thickness to be transferred to a photoconductive element.

[0043] In some examples, the voltage of a component of the developing unit may be adjusted such that a printing fluid with a specific printing fluid thickness is developed on the developing unit. In an example, the specific printing fluid thickness may be obtained from thickness data defined by the color separation data. The specific printing fluid thickness is subsequently transferred to a specific region of the surface of the photoconductive element such that a printing fluid profile with the specific printing fluid thickness is generated on the surface. Accordingly, different combinations of voltages in each developing unit allow for obtaining printing fluid profiles with a wide range of printing fluid thicknesses for each color. For instance, in line chart 300, each of the first and second developing units may generate a respective printing fluid profile on the surface of the photoconductive element. When transferring these two different printing fluid profiles on a substrate, the resulting printing fluid profile may have a non-uniform printing fluid thickness. As a result, the use of voltage modulation when developing printing fluid allows for obtaining a wide range of printing fluid thicknesses without compromising the image quality resulting from the printing operation.

[0044] Line chart 300 further includes a first vertical line 306, a second vertical line 307, and a third vertical line 308. These lines represent a first, a second, and a third time, respectively. At each of the first, second and thirdtimes, a different color results from the combination of a first printing fluid developed using the first developing unit and a second printing fluid developed in the second developing unit. In particular, the differences in printing fluid thickness for the first and second printing fluid and the different ratios between the first and second printing fluid result in a different colors and saturations.

[0045] Although the explanation of FIG. 3 has been made referring to a first component and a second component that belong to different developing units, it should be noted that, in other examples, the first and the second component may belong to the same developing unit. As previously explained, the first component may correspond to an electrode (e.g., electrode 222) and the second component may correspond to a squeegee roller (e.g., squeegee roller 223) belonging to the same developing unit (e.g., developing unit 220). By providing the electrode and the squeegee roller with a voltage modulation, the printing fluid profiles formed on a surface of a photoconductive element may be obtained with an improved color shift (e.g., smoother color shift). However, it should be noted that, when controlling the voltage of two components belonging to the same developing unit, the first voltage data 310 and the second voltage data 320 may be controlled with different patterns than the ones represented in FIG. 3 (e.g., the first voltage data 310 and the second voltage data 320 may increase and / or decrease over the same time frames). As previously described, a maximum printing fluid thickness developed using a developing unit including the first and the second component is obtained when setting a maximum voltage difference in absolute terms between the reference voltage associated with a developer roller and each of the first and second components of the developing unit. Similarly, a minimum printing fluid thickness may be obtained when setting a minimum voltage difference in absolute terms between the reference voltage associated with the developer roller and each of the first and second components of the developing unit.

[0046] Referring now to FIG. 4, a printing system 400 is shown. The printing system 400 comprises a photoconductive element 110, a set of developing units 120, a controller 430, an intermediate transfer member 440, and a drum 450. The photoconductive element 110 is to be electrically charged in accordance with an image to be printed on a substrate 451 removably attached to the drum 450. In some examples, a charging element (not shown in FIG. 4) and a discharging element (not shown in FIG. 4) may be used to create a latent image associated with the image to be printed on a surface of the photoconductive element 410, as previously described.

[0047] The set of developing units 120 may comprise at least one developing unit such as the developing unit 220 previously explained in reference to FIG. 2. As previously explained, each developing unit of the set of developing units 120 is movable between an engaged position and a disengaged position. In the system 400, the set of developing units 120 comprises four developing units in the disengaged position and one developing unit in the engaged position. However, in other examples, more than one developing unit may be selectively engaged with the photoconductive element 110 at the same time.

[0048] The controller 430 of the printing system 400 is to control components of the set of developing units 120 such that at least one printing fluid profile with a non-uniform thickness is formed on the surface of the photoconductive element 110. As previously explained in FIGs. 1 to 3, printing fluid with a non-uniform thickness can be developed in a developing unit by adjusting an internal voltage of the developing unit over time. In some examples, the controller 430 may generate color separation data that maps a respective printing fluid thickness to be transferred to a particular location on the surface of the photoconductive element 110. As previously explained in reference to FIG. 1 , the generation of calibration data may comprise defining electrical charge data and thickness data.

[0049] The printing system 400 further comprises the intermediate transfer member 440 and drum 450 including the substrate 451 attached thereon. Asexplained above, the set of developing units 120 forms a printing fluid profile on the photoconductive element 110. In FIG. 4, the intermediate transfer member 440 receives the printing fluid profile and transfers the printing fluid profile to the substrate 451 . In an example, when the set of developing units 120 is to form more than one printing fluid profile, the controller 430 may control the intermediate transfer member 440 to transfer the formed printing fluid profiles to the substrate 451 in a single pass.

[0050] As used herein, the term “pass” refers to a subsequent printing transfer operation in which printing fluid profiles are transferred from the intermediate transfer member 440 to the substrate 451 attached to the drum 450. The expression “single pass” refers to transferring all the printing fluid profiles from the intermediate transfer member 440 to the substrate 451 in a single subsequent printing transfer operation. Alternatives to a single pass may be possible, such as transferring each printing fluid profile from the intermediate transfer member 440 to the substrate 451 in multiple passes (i.e. , involving more than one subsequent printing transfer operations). Transferring printing fluid profiles in a single pass allows for improving the alignment of the printing fluid profiles with respect to the substrate 451 compared to a transfer in multiple passes. The expression “single pass” may be alternatively referred to as one-shot mode and the expression “multiple passes” may be alternatively referred to as multi-shot mode. In addition, it should be noted that the expressions “single pass” and “multiple passes” may be used to refer to a printing fluid transfer from a photoconductive element directly to a substrate (for instance, a printing system not including an intermediate transfer member).

[0051] Although in the examples explained in reference to FIG. 4 the substrate 451 is attached to the drum 450, it should be noted that alternative examples may be possible. In an example, the substrate 451 may correspond to a web substrate, and the web substrate may be partially supported by the drum 450 such that the web substrate receives printing fluid from the intermediate transfer member 440.

[0052] According to an example, a method for printing an image comprises receiving image data comprising instructions for printing the image, generating color separation data based on the image data, the color separation data to map the image data to a non-uniform printing fluid thickness on a photoconductive element, and printing the image in accordance with the color separation data.

[0053] Referring now to FIG. 5, a method 500 for generating color separation data is shown. Method 500 may be carried out to improve image quality resulting from a printing operation performed using an LEP printing system (e.g., printing systems 100, 200, and 400).

[0054] At step 510, method 500 comprises receiving image data comprising instructions for printing an image. In an example, the image data may be part of a print job submitted by a user of a printing system (e.g., printing systems 100, 200, and 400).

[0055] At step 520, method 500 comprises generating color separation data based on the image data, the color separation data to map the image data to a non-uniform printing fluid thickness on a photoconductive element (e.g., photoconductive element 110 in FIGs. 1 and 4). In an example, the color separation data may map the image data to multiple color separations, each color separation being associated with a respective developing unit. In some examples, step 520 comprises generating color separation data including at least two color separations indicative of at least two printing fluid profiles overlapping on the photoconductive element. As previously described, the printing fluid profiles are the result of a printing fluid transfer operation from the developing unit to the photoconductive element.

[0056] At step 530, method 500 comprises printing the image in accordance with the color separation data. As previously explained in reference to FIGs.1 to 4, printing fluid may be developed with a non-uniform thickness by modifying an internal voltage value in at least one of the developing units during a printing fluid development operation. In an example, printing the image at step 530 may comprise developing, during a printing fluid transferoperation, printing fluid with the non-uniform thickness on a developer roller of a developing unit (e.g., developing unit 220) and transferring the developed printing fluid to the photoconductive element to form a printing fluid profile with a non-uniform thickness. In some examples, developing printing fluid with the non-uniform thickness may comprise dynamically adjusting an internal voltage value of the developing unit (e.g., at least one of a squeegee roller voltage and an electrode voltage) during the printing operation.

[0057] In an example, generating color separation data based on the image data at step 520 comprises generating electrical charge data representing locations on the photoconductive element and thickness data representing printing fluid thicknesses to be transferred to the latent image formed on the photoconductive element. In an example, the electrical charge data may be used to control a discharging element (such as a writing head) to create a latent image on the photoconductive element and the thickness data may be used to control a developing unit to develop printing fluid with a preferred printing fluid thickness.

[0058] In other examples, printing the image at step 530 comprises forming a printing fluid profile on the photoconductive element and transferring the printing fluid profile to the substrate, the printing fluid profile having a non- uniform printing fluid thickness in at least one of a longitudinal direction and transversal direction of the substrate. In some examples, transferring the printing fluid profile to the substrate may comprise transferring the printing fluid profile to an intermediate transfer member (e.g., intermediate transfer member 440) and transferring the printing fluid profile from the intermediate transfer member to the substrate. As previously explained in reference to FIG. 4, a transfer from the intermediate transfer member to the substrate may take place in a single pass or in multiple passes.

[0059] In some other examples, generating color separation data at step 520 comprises generating color separation data including at least two color separations, each color separation being associated with a respectivedeveloping unit. At step 530, printing the image comprises transferring printing fluid to the photoconductive element by engaging more than one developing unit at once (e.g., two developing units). Referring back to FIG.1 , 2, and 4, engaging more than one developing unit at once should be understood as moving more than one developing unit to its engaged position such that more than one printing fluid profile is generated at the same time (e.g., over a single revolution of the photoconductive element 110 in FIGs. 1 and 4 a first and a second developing unit of the set of developing units 120 are engaged with the photoconductive element 110). The engagement of more than one developing unit simultaneously allows for forming printing fluid profiles with different types of printing fluid on the photoconductive element. In addition, the engagement of two developing units at the same time results in a more productive printing process compared to examples where a single developing unit is engaged at once.

[0060] Referring now to FIG. 6, a method 600 for developing printing fluid with a non-uniform thickness in a developing unit is shown. Method 600 may be performed during a printing fluid development operation. In some examples, when having a printing system with multiple developing units, multiple printing fluid development operations may be performed in parallel to develop printing fluid with a non-uniform thickness via each of the multiple developing units.

[0061] At step 610, method 600 comprises maintaining a developer roller at a reference voltage. The reference voltage may correspond to a voltage at which a printing fluid transfer from the developer roller to the surface of a photoconductive element of the printing system takes place. In an example, the reference voltage may be a voltage within a range defined from -1400V to -700V (e.g., a range defined from -1200V to -900V.

[0062] At step 620, method 600 comprises modifying at least one of a squeegee roller voltage and an electrode voltage to obtain a voltage difference with respect to the reference voltage of the developer roller. As previously explained in reference to FIG. 3, the voltage difference may beassociated with a printing fluid thickness to be transferred to the photoconductive element. In an example, at least one of the squeegee roller voltage and the electrode voltage may be adjusted in accordance with voltage-to-thickness data which associates voltage values of the main electrode and / or the squeegee roller with respect to the reference voltage of the developer roller. In some examples, step 620 may comprise modifying the squeegee roller voltage and the electrode voltage in accordance with voltage-to-thickness data.

[0063] At step 630, method 600 comprises transferring printing fluid with the non-uniform printing fluid thickness from the developer roller to the photoconductive element.

[0064] In some examples, printing the image at step 530 of method 500 may comprise developing printing fluid in accordance with method 600 such that a printing fluid profile is formed by transferring printing fluid with a non- uniform printing fluid thickness to a surface of the photoconductive element.

[0065] According to an example, the methods described in reference to FIGs.5 and 6 and the instructions described in reference to FIGs. 1 to 4 may be implemented by way of non-transitory computer program code that is storable on a non-transitory storage medium. Examples of non-transitory computer-readable storage media include, for example, electronic, magnetic, optical, electromagnetic, or semiconductor media. Other examples of suitable computer-readable storage media include a hard drive, a randomaccess memory (RAM), a read-only memory (ROM), memory cards and sticks, and other portable storage devices.

[0066] According to an example, a computer-readable storage medium may comprise instructions that, when executed by a processor, cause the processor to perform a plurality of actions. In an example, the processor processes image data corresponding to an image to be printed to generate color separation data associated with printing fluid thickness distributions to be transferred to a substrate, applies internal voltages to developing basedon the printing fluid thickness distributions, and controls the developing units to the printing fluid thickness distributions to a photoconductive element.

[0067] Referring now to FIG. 7, a system 700 comprising a processor 710 and a computer-readable medium 700 comprising instructions 721 , 722 and 723 is shown. The instructions 721 , 722, and 723, when executed by the processor 710 of the system 700, causes the processor 710 perform a series of operations. In an example, the processor 710 may control elements of the printing systems previously explained in reference to FIGs. 1 , 2, and 4 (e.g., printing systems 100, 200, and 400). Computer-readable media include, for example, electronic, magnetic, optical, electromagnetic, or semiconductor media. More specific examples of suitable computer- readable media include a hard drive, a random-access memory (RAM), a read-only memory (ROM), memory cards and sticks, and other portable storage devices.

[0068] In FIG. 7, the computer-readable medium 700 comprises a first instruction 721 , a second instruction 722, and a third instruction 723. The first instruction 721 , when executed by the processor 710, causes the processor 710 to process image data corresponding to an image to be printed on a substrate to generate color separation data that maps printing fluid thickness distributions to be transferred to the substrate. To obtain the improved image quality on the image to be printed, the color separation data is generated such that at least one printing fluid thickness distribution has a non-uniform printing fluid thickness. The second instruction 722, when executed by the processor 710, causes the processor 710 to, for each printing fluid distribution having the non-uniform printing fluid thickness, set a respective developing unit at a respective internal voltage based on the non- uniform printing fluid thickness. In an example, the respective internal voltages may be set using voltage-to-thickness data. The third instruction 723, when executed by the processor 710, causes the processor 710 to control the developing units to transfer the printing fluid thickness distributions to a photoconductive element (e.g., photoconductive element110 in FIGs. 1 and 4). The transferred printing fluid thickness distributions form printing fluid profiles on the photoconductive element.

[0069] In an example, the computer-readable medium 700 may comprise further instructions to cause the processor 710 to transfer the printing fluid profiles formed on the photoconductive element to the substrate. In an example, the transfer of printing fluid profiles is executed in a single pass such that an improved registration of the printing fluid profiles with respect to the substrate is obtained.

[0070] In other examples, the first instruction 721 causes the processor 710 to process image data to generate color separation data including electrical charge data mapping regions of the photoconductive element to be charged and thickness data mapping printing fluid thickness distributions to be transferred to the photoconductive element. In an example, the processor 710 may control a charging element (such as a writing head) to create a latent image on the photoconductive element based on the electrical charge data and a developing unit to develop printing fluid with a preferred printing fluid thickness based on the thickness data.

[0071] In some examples, the second instruction 722 causes the processor 710 to set a voltage of at least one of a squeegee roller (e.g., squeegee roller 223) and an electrode (e.g., electrode 222) at a respective voltage difference with respect to a reference voltage of a developer roller (e.g., developer roller 721 ) during a printing fluid transfer operation, the voltage difference being associated with a printing fluid thickness to be developed during the printing fluid transfer operation.

[0072] In some examples, the third instruction 723 causes the processor 710 to control two developing units to engage with the photoconductive element at once, as previously explained in reference to method 500 of FIG. 5. As mentioned above, engaging two developing units at once allows for providing a more productive printing process while generating an improved color shift (e.g., a smoother color shift).

[0073] What has been described and illustrated herein are examples of the disclosure along with some variations. The terms, descriptions, and figures used herein are set forth by way of illustration only and are not meant as limitations. Many variations are possible within the scope of the disclosure, which is intended to be defined by the following claims (and their equivalents) in which all terms are meant in their broadest reasonable sense unless otherwise indicated.

Claims

CLAIMSWhat is claimed is:1 . A method comprising: receiving image data comprising instructions for printing an image; generating color separation data based on the image data, the color separation data to map the image data to a non-uniform printing fluid thickness on a photoconductive element; and printing the image in accordance with the color separation data.

2. The method of Claim 1 , wherein printing the image comprises: developing, during a printing fluid transfer operation, printing fluid with the non-uniform printing fluid thickness on a developer roller of a developing unit, and transferring the developed printing fluid to the photoconductive element to form a printing fluid profile with a non-uniform thickness.

3. The method of Claim 2, wherein developing printing fluid with the non- uniform printing fluid thickness comprises modifying, during the printing fluid transfer operation, at least one of a squeegee roller voltage and an electrode voltage in the developing unit.

4. The method of Claim 3, wherein developing printing fluid with the non- uniform printing fluid thickness during the printing fluid transfer operation comprises: maintaining the developer roller at a reference voltage, modifying at least one of the squeegee roller voltage and the electrode voltage to obtain a voltage difference with respect to the reference voltage of the developer roller, the voltage difference being associated with the non- uniform printing fluid thickness to be transferred, and transferring printing fluid with the non-uniform printing fluid thickness from the developer roller to the photoconductive element.

5. The method of Claim 1 , wherein the color separation data includes at least two color separations indicative of at least two printing fluid profiles overlapping on the p photoconductive element.

6. The method of Claim 1 , wherein: generating color separation data based on the image data comprises generating color separation data including at least two color separations, each associated with a respective developing unit, and printing the image comprises transferring printing fluid to the photoconductive element by engaging more than one developing unit at once.

7. The method of Claim 1 , wherein printing the image comprises forming a printing fluid profile on the photoconductive element and transferring the printing fluid profile to the substrate, the printing fluid profile having a non-uniform printing fluid thickness in at least one of a longitudinal direction and transversal direction of the substrate.

8. A printing system comprising: a photoconductive element to be electrically charged in accordance with an image to be printed on a substrate; a set of developing units; and a controller to: receive image data associated with the image; generate color separation data based on the image data; control the set of developing units to develop printing fluid in accordance with the color separation data, wherein at least one developing unit of the set of developing units is to develop printing fluid with a non-uniform printing fluid thickness; and control the set of developing units to transfer the developed printing fluid with the non-uniform printing fluid thickness to the photoconductive element.

9. The printing system of Claim 8, wherein the developed printing fluid forms a printing fluid profile on the photoconductive element, the system further comprising an intermediate transfer member to receive the printing fluid profile, the intermediate transfer member to transfer the printing fluid profile to the substrate in a single pass.

10. The printing system of Claim 8, wherein the controller to generate the color separation data based on the image data comprises the controller to separate a color map corresponding to the image data into at least one printing fluid thickness profile to be developed by the set of developing units.11 . The printing system of Claim 8, wherein each developing unit comprises a developer roller, an electrode, and a squeegee roller, wherein the controller is to adjust a voltage of at least one the squeegee roller and the electrode with respect to a voltage of the developer roller to develop printing fluid with the non- uniform printing fluid thickness.

12. The printing system of Claim 11 , wherein the controller to adjust the voltage of at least one of the squeegee roller and the electrode comprises the controller to reduce a voltage difference of at least one the squeegee roller and the electrode with respect to the voltage of developer roller to reduce a printing fluid thickness transferred from the developer roller to the photoconductive element.

13. A computer-readable medium comprising instructions that, when executed by a processor, cause the processor to: process image data corresponding to an image to be printed to generate color separation data that maps printing fluid thickness distributions to be transferred to the substrate, wherein at least one printing fluid thickness distribution has a non-uniform printing fluid thickness;for each printing fluid distribution having the non-uniform printing fluid thickness, set a respective developing unit at a respective internal voltage based on the non-uniform printing fluid thickness; and control the developing units to transfer the printing fluid thickness distributions to a photoconductive element.

14. The computer-readable medium of Claim 13, wherein the processor to set a respective developing unit at a respective internal voltage comprises: set a voltage of at least one of a squeegee roller and an electrode at a respective voltage difference with respect to a reference voltage of a developer roller during a printing fluid transfer operation, the voltage difference being associated with a printing fluid thickness to be developed during the printing fluid transfer operation.

15. The computer-readable medium of Claim 13, wherein the processor to control the developing units to transfer the printing fluid thickness distributions comprises the processor to control two developing units to engage with the photoconductive element at once.

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