BACKGROUND

[0001] Color printing may result from a number of colorants of different colors being superimposed on top of a substrate, such as paper or other media. Examples of colorants include inks, dyes, pigments, paints, toners and powders. The application of colorants to textile substrates such as clothing materials may require additional considerations.

BRIEF DESCRIPTION OF THE DRAWINGS

[0002] Various features of the present disclosure will be apparent from the detailed description which follows, taken in conjunction with the accompanying drawings, which together illustrate features of the present disclosure, and wherein:

[0003] Figure 1 illustrates pattering of an underlayer according to an example;

[0004] Figure 2 is a schematic illustration of a printing apparatus according to an example;

[0005] Figure 3 is a flow chart illustrating a method of patterning an underlayer according to an example;

[0006] Figure 4 illustrates a multi-pixel pattern for an underlayer according to an example;

[0007] Figure 5 illustrates a multi-pixel pattern having multiple layers for an underlayer according to an example; and

[0008] Figures 6 to 8 illustrate multi-pixel patterns for an underlayer according to other examples.

DETAILED DESCRIPTION

[0009] Images, color designs and other visual representations may be provided on textiles such as clothing material by applying printing materials such as different colored inks which may be accompanied by other printing fluids such as, e.g., fixers, overcoats and/or optimizers. Textiles comprise woven fibers which can result in a porous surface onto which the printing material must be adhered. As this can result in non-uniform retention and rendering of printing materials, an underlayer is often first applied to the textile to provide a more uniform surface onto which a subsequent overlayer comprising printing materials representing wanted images and designs may be applied. As an example, the underlayer may be a white ink which may help the wanted image or design to stand out against darker fabrics.

[0010] However, the application of many layers of printing material may affect perceptual characteristics of the textile. Collectively, these perceptual characteristics are referred to as “hand” in the textile industry. Hand may include the stiffness of the textile, its softness to touch, the feel of gathering or bundling the textile, as well as visual properties such as shininess or dullness.

[0011] Color can be represented within imaging devices such as print and display devices in a variety of ways. For example, in one case, a color as observed visually by an observer is defined with reference to a power or intensity spectrum of electromagnetic radiation across a range of visible wavelengths. In other cases, a color model is used to represent a color at a lower dimensionality. For example, certain color models make use of the fact that color may be seen as a subjective phenomenon, i.e. dependent on the make-up of the human eye and brain. In this case, a “color” may be defined as a category that is used to denote similar visual perceptions; two colors are said to be similar if they produce a similar effect on a group of one or more people. These categories can then be modelled using a lower number of variables.

[0012] Within this context, a color model may define a color space. A color space in this sense may be defined as a multi-dimensional space, with a point in the multi-dimensional space representing a color value and dimensions of the space representing variables within the color model. For example, in a Red, Green, Blue (RGB) color space, an additive color model defines three variables representing different quantities of red, green and blue light. In a digital model, values for these quantities may be defined with reference to a quantized set of values. For example, a color defined using an 8-bit RGB model may have three values stored in a memory, wherein each variable may be assigned a value between 0 and 255. Other color spaces include: a Cyan, Magenta, Yellow and Black (CMYK) color space, in which four variables are used in a subtractive color model to represent different quantities of colorant or printing fluid, e.g. for a printing system; the International Commission on Illumination (CIE) 1931 XYZ color space, in which three variables (X, Y and Z or tristimulus values) are used to model a color; the CIE 1976 (L*, a*, b* - CIELAB or ‘LAB’) color space, in which three variables represent lightness (L*) and opposing color dimensions (a* and b*); the YUV color space, in which three variables represent the luminance (Y) and two chrominance dimensions (U and V); and the IPT color space, in which the three variables represent a lightness or Intensity dimension (I), a “Protanopia” red-green chroma dimension (P), and a “Tritanopia” yellow-blue chroma dimension (T).

[0013] A simple color space may be defined using ink vectors which represent the proportion of individual inks within a combination such as an NPac vector. For example, an ink vector may be a vector with a predefined set of elements, where each element represents an ink (e.g. a colorant or color output available to the rendering device) and the value of the element represents a quantity of ink (e.g. [C, M, Y, K]). An ink vector may be used to instruct deposit of inks in a printing device, e.g. where the value represents a particular number of drops or quantity of ink to deposit. An ink vector may be used to instruct the printing of a color in an addressable area of a substrate (e.g. a print “pixel”).

[0014] Certain examples described herein address a challenge with improving the hand of printed textile materials, for example reducing the stiffness of printed textiles or improving their gather feel. This may be achieved by applying a multi-pixel pattern to the underlayer which effectively reduces the amount of printing material applied to the underlayer whilst still providing a sufficiently uniform layer onto which print materials for an overlayer may adhere. In some examples, lines of reduced amounts of printing material may further reduce printed textile stiffness in some directions. [0015] In some examples, printing material such as ink may be applied in pixels which each represent a color having a certain grey level and may be rendered using one or a combination of inks or other colorants. These colorants may be applied according to a halftoning pattern in which drops of ink are distributed about the pixel to render the wanted color to a viewer whilst minimizing unwanted artefacts such as moire. In addition, multipixel patterns are used to adjust the amounts of printing material, such as white ink, applied to the underlayer. The multipixel patterns are generally larger than the halftoning patterns and may incorporate geometric shapes such as circles, squares and lines which encompass more than one pixel and can be thought of as macrohalftoning.

[0016] Fig. 1 illustrates an underlayer representation and an overlayer representation according to an example. Underlayer 110 and overlayer 120 print data may be provided to a printing apparatus for rendering on a textile substrate such as the front of a T-shirt. The underlayer 1 10 may be associated with a uniform white surface 115 which may be rendered using white ink. The overlayer 120 may be associated with an image which can be rendered using colored inks, for example, two different colored inks 125A, 125B.

[0017] In order to improve the hand of the printed textile substrate, print instructions to render the underlayer may be generated in which a multi-pixel pattern is applied to the underlayer when this is rendered by a printing apparatus under control of the print instructions. A modified representation of the underlayer 130 is illustrated in which light areas 135A correspond with a lower density of ink and dark areas 135B correspond with a higher density of ink. The light and dark areas are representative of different densities of the underlayer, e.g., white ink or another light-colored colorant as in the above-mentioned case. These different densities of underlayer are applied according to a multipixel pattern. In this example, the amounts of printing material to be applied to render the overlayer 120 is unchanged, however it is alternatively possible to apply multilayer patterns to the overlayer as well.

[0018] A detail 140 of part of the modified underlayer representation 130 is shown which illustrates a number of pixels 150. Each pixel represents drops of print material, e.g., a color such as a particular shade of white or green and may be rendered by a printing apparatus using one or a combination of colorants such as ink drops applied to the substrate. In the example, the original underlayer 110 is associated with a uniform white which corresponds to each pixel having the same color. However, the modified underlayer 130 has a multipixel pattern applied in which properties of the print material to be rendered for the underlayer are adjusted. For example, the lighter areas 135A may correspond to pixels 150 in which the density of the underlayer is unchanged whereas the darker areas 135B may correspond to areas in which the density of the underlayer has been changed, for example by increasing or reducing the density of ink droplets and/or changing the grey level of pixels impacted by the dark areas 135B. Both the lighter 135A and darker 135B pixels may have their ink densities and/or grey levels adjusted up or down in order to form the multipixel pattern. The lighter areas 135A and the darker areas 135B extend over two or more adjacent or contiguous pixels and so these respective groups of pixels form a multipixel pattern which may extend across a part of all of the underlayer.

[0019] The modified underlayer representation 130 and the overlayer representation may be incorporated into print instructions for controlling a printing apparatus to render the modified underlayer and subsequently the overlayer onto the textile substrate.

[0020] Fig. 2 illustrates a printing apparatus according to an example. The printing apparatus 200 comprises a controller 205 and a printhead 210. The controller 205 comprises an input/output interface 212, a processor 215 and memory 220 and may implement a color printing pipeline for example. Such a pipeline includes color management which receives a source file 250 comprising print data in a source color space, for example CIE 1976. The color management stage generates a device color output corresponding to colors that the printer apparatus can actually render onto a substrate - for example the image data may be mapped from a CIE 1976 color space to a CMYK color space. The device color output can be processed by a color separation stage that maps input from the device color output to amounts of colorants to be applied to the substrate for each pixel of the image. In an example this can be represented by ink-vectors which may be mapped from the device colors using an ink-vector lookup table (LUT). A halftoning process then generates instructions for the print head 210 to render the images 255. Simple PDF images may be used.

[0021] The source file or print data 250 may comprise a data relating to an underlayer to be applied to a textile substrate initially using colorants available to the printing apparatus. Such a printing apparatus may utilize white ink although other lighter colors may be rendered using combinations of colorants available for the CMYK color space. The print data 250 may also comprise data relating to an overlayer of colorants to be applied over the underlayer. The print data may be in the form of raster images having an array of color pixels corresponding to each of the underlayer and overlayer. In one example the print data may comprise one or more PDF files comprising an image or page for the underlayer and an image or page for the overlayer to be applied over the underlayer.

[0022] The print apparatus 200 may be operated by a print service provider having many different customers that require different types of images to be printed onto textile substrates, for example printing onto T-shirts, wall and furnishing coverings, car matts and other items. By applying an underlayer first, some of the colorant from the underlayer fills porous holes and saturates fibers which might otherwise consume colorant from the overlayer resulting in unwanted visual artefacts. Darker colorants of the overlayer may also appear more pronounced or visible when rendered over a lighter underlayer.

[0023] The memory or storage medium 220 comprises an instruction set 225 arranged to apply patterning to the underlayer. This may be achieved by first determining print data corresponding to an underlayer of print material to be applied to a textile 210. This may be implemented by extracting a PDF file from the print data 250 which is for the underlayer. In an example the PDF illustrates a white page.

[0024] A pattern may then be applied to this, the pattern having a series of lines and/or geometric shapes which have a different color value in the PFD image, for example a less or ore white color value. Color ay be represented in many ways in PDF image files, including in many different color spaces such as CYMK. The PDF image may correspond to a plurality of image pixels each associated with one or more colors each associated with a respective intensity or grey level. A multipixel pattern may be applied by adjusting the colors and/or grey levels associated with some pixels in which groups of two or more adjacent pixels have the same color and grey level in the PDF file.

[0025] The instruction set then generates print instructions to control the application of print material to the substrate, the print instructions incorporating a multipixel pattern 240. This may be implemented by processing the modified underlayer print data, such as a PDF file with pattern applied, through the color processing pipeline of the printing apparatus to generate print instructions 230 for controlling the printhead 210. In an alternative arrangement, the multipixel pattern may be applied within the color processing pipeline, for example by adjusting the amount of colorants to be applied according to a multipixel pattern.

[0026] The printhead 210 is controlled to apply printing material such as inks to the textile substrate 260. An underlayer 270 is applied initially, the underlayer incorporating the multi-pixel pattern in which the density of the applied printing material varies according to the multipixel pattern. The printing material for the underlayer may comprise drops of white ink which are applied more or less densely.

[0027] One or more overlayers 280, 285 are then subsequently applied onto the underlayer 270. The overlayers 280, 285 may represent component parts of an image, such as a background color 280 and a foreground design 285. The underlayer 270 may be allowed to dry before rendering the overlayers 280, 285.

[0028] Fig. 3 is a flow chart illustrating a method of patterning an underlayer. The method 300 may be implemented by the printing apparatus 200 of Fig. 2 or any other suitable apparatus. Some parts of the method may be implemented in a computing apparatus separate from a printer, for example to apply a multipixel pattern to print data for an underlayer before forwarding this to the printer.

[0029] At 310 the method receives print data for an underlayer of print material to be applied to a textile substrate and an overlayer of print material to be applied over the underlayer. This may be in the form of a PDF file comprising image data for the underlayer and an image for the overlayer. The image data may be represented by a plurality of image pixels each associated with a color in a color space. The color for each pixel may be associated with a grey level.

[0030] At 320, the method applies a multipixel pattern to the image data for the underlayer. This may be implemented by adjusting the color and/or grey level of some of the pixels of the underlayer image data according to a predetermined multipixel pattern. This may result in adjusted numerical values in the PDF file associated with respective image pixel colors.

[0031] Different multipixel patterns may be used depending on the wanted hand characteristics of the printed textile. For example, a dense pattern with many adjusted pixel values may be employed to reduce the stiffness of the printed textile. Alternatively, an open pattern with far fewer changes in pixel values ay be employed to ensure that the overlayer design is visually bold even at the expense of some increased stiffness of the textile material. Various multipixel patterns are described in more detail below.

[0032] At 330, color processing is applied in which the print data is converted from a source color space into the color space of the printing apparatus where combinations of available colorants such as inks will be used to render the underlayer and overlayer of the print data. This may be implemented using any suitable color processing pipeline.

[0033] At 340, the method applies a halftoning process. This uses patterns to the position printing material within each pixel in order to provide a high-quality rendering of the underlayer and overlayer. Any suitable halftoning process may be employed. The halftoning process results in print instructions for controlling a printhead to apply available printing materials to the substrate

[0034] It is noted that whilst halftoning applies a pattern within each pixel, the multipixel pattern applied to the underlayer comprises pattern elements which extend across multiple pixels. For example, geometric shapes and lines of the multipixel pattern each comprise groups of two or more adjacent or contiguous pixels.

[0035] It is also noted that whilst in this example the multipixel pattern is applied to the underlayer represented in the print data, it may alternatively be applied within the color processing pipeline or halftoning process. For example, the colors to be rendered on the substrate may be represented by ink vectors following color processing. Some of these ink vectors may then have their values adjusted according to the multipixel pattern. The halftoning process ay then be applied to generate printing instructions.

[0036] At 350, the method controls a printhead according to the print instructions to apply printing material to the substrate to render the underlayer 270 onto the textile substrate 260. The printing instructions 230 incorporate the multilayer pattern for the underlayer, whether this was implemented by adjusting the print data or internal data used by the printing apparatus such as ink vectors. [0037] In some examples, the underlayer may be applied in more than one pass of the printhead. Using multiple passes builds the underlayer up in multiple layers and allows for different thicknesses of printing material for the underlayer. This enables the multipixel pattern to be applied by adjusting the thickness of the underlayer according to the pattern.

[0038] At 360, the method applies the overlayer 280, 285 over the underlayer 270. The print material of the underlayer may be allowed to dry sufficiently before applying the print material for the overlayer.

[0039] Fig. 4 illustrates a multipixel pattern for an underlayer according to an example. The multipixel pattern 430 comprises circles 435B of one color and/or grey level within a background 435A of another color and/or grey level. The different parts of the pattern 435A, 435B may correspond to different densities of print material to be applied of the same color. In an example, this may correspond to more white ink drops per pixel in one part of the pattern 435B then in the other part of the pattern 435A. This may be achieved using different grey levels for the white ink to render different intensities of white color and/or additional amounts of white ink may be achieved by applying white ink to additional layers of the underlayer in additional passes of the printhead. In this example, the darker parts of the pattern 435B may correspond to additional white ink which provides a strong visual background for the colors of the overlayer to be subsequently applied. The lighter parts of the pattern 435A correspond to lower amounts of white ink which allows for additional flexibility of the printed textile along these lines. This reduces the perceived stiffness of printed textile.

[0040] In one example, the printing apparatus may have a resolution of 600ppi or 600 pixels per inch and the ellipse 435B may be 40 pixels in the y direction and 80 pixels in the x direction.

[0041] The multipixel pattern 430 may comprise a predetermined ratio between the grey level, ink amount or ink density of the two parts 435A, 435B of the pattern. For example, a background 435A white grey level may have a grey level of 150 in an 8-bit color whilst the ellipses 235B may have a grey level of 240. [0042] In another example, the multipixel pattern may comprise more than two tones, colors, such as three or four different grey levels. Further, whilst white ink has been used as an example for the underlayer, rendering a white underlayer may use a combination of other colorants such as CMY inks or dyes. Furthermore, other light colored printing materials may alternatively be used such as light yellow or cream colors.

[0043] Fig. 5 illustrates a 3D perspective view of the pattern illustrated in Fig. 4 of a printed textile according to an example. The textile substrate 560 has a white underlayer 570 rendered onto it. The underlayer comprises areas 535 of raised white ink so that the underlayer is thicker at these positions. The apparent heights of these areas are exaggerated for purposes of illustration and indicate higher concentrations of white pigments (higher opacity) relative to surrounding areas. The thinner underlayer between these areas 535 has a lower concentration of white pigment which makes the printed material more pliable and flexible and contributes to reducing the stiffness associated with application of the underlayer 570. The underlayer 570 helps to hide or minimize visibility of dark substrate 560 in the interstitial space between areas 535. An overlayer may then be applied over this underlayer including areas 535. Additional ink in the areas 535 increases lighter colored opacity relative to a flatter/less ink topology.

[0044] Fig. 6 - 9 illustrate other examples of multipixel patterns for the underlayer. In these patterns, the white and black represent one or more of the following: different grey levels of the same color; different colors; different densities of ink droplets per pixel; different thicknesses of printing material for the underlayer. The values of these parameters that black and white represent may be reversed for different multipixel patterns. Smaller patterns may be better for image quality and larger patterns for reducing stiffness.

[0045] In Fig. 6, the multipixel pattern comprises overlapping geometric shapes - diamonds and squares.

[0046] In Fig. 7, a hexagonal or honeycomb multipixel pattern is presented. [0047] In Fig. 8, a multipixel pattern having adjacent squares and diamonds.

[0048] The same or different multipixel patterns can also be applied to the overlayer in some examples. This may further reduce stiffness and improve other hand characteristics but may do so at the expense of image quality. An appropriate trade off of image quality and hand may then be applied depending on the desired outcome.

[0049] Examples may be suitable for any type of ink or other printing material.

[0050] The preceding description has been presented to illustrate and describe examples of the principles described. This description is not intended to be exhaustive or to limit these principles to any precise form disclosed. Many modifications and variations are possible in light of the above teaching. It is to be understood that any feature described in relation to any one example may be used alone, or in combination with other features described, and may also be used in combination with any features of any other of the examples, or any combination of any other of the examples