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COLOUR COMPLEXITIESThe Need for Specialist Software in the Digital Workflow for Decorative Print

By Nick Langford, chief executive, AVA CAD/CAM Group

The successful introduction of Hall 4.0 dedicated to digital print, at the Heimtextil Exhibition in Frankfurt, was further evidence that digital printing

for decorative textiles and wall coverings has well and truly arrived. The new floor was much visited and most of the major manufacturers of digital printing machines were represented – several with demonstration printers working on their stands.

Conspicuously absent from many of the ‘digital print solutions’ displayed, however, was any detailed reference to the software required to produce a high quality, colour matched textile or wall covering product using the intricate and complex designs usually seen in the home furnishings arena. Several printer manufacturers produced glossy brochures offering ‘all-in-one’ solutions without a single reference to software.

The explanation for this is not difficult to guess; the vast majority of the digital printers sold to date are printing images for signage and/or posters or packaging. Basic

colour calibration software and any driver compatible with the model of printer used should produce an acceptable result when printing a simple (CMYK) image. Successful digital printing for the more demanding decorative industry, however, demands a more sophisticated approach.

The purpose of the remainder of this article is to highlight issues which, if not properly addressed, can easily leave customers who have invested a great deal of money in a new digital printer frustrated because they cannot obtain the end result which they expect. This frustration is typically not caused by any shortcomings in the hardware – the digital printer manufacturers have done an amazing job of introducing faster and more accurate printers, now with the additional invaluable options of pigment and latex based inks and we are expecting further interesting developments at ITMA in Milan this November – but due entirely to the use of inappropriate or inadequate software.

The AVA Software working in realtime repeat

AVA’s Profile Management - the CMS Controller

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Colour ManagementThe traditional workflow for many decorative textile and wall-covering products involves putting a (usually scanned) design into repeat and then separating it into a relatively small (typically 6-14) number of ‘spot’ colours. Putting a complex design into repeat is itself considerably easier with specialist software, fully integrated to the digital workflow with full real time visualisation of changes as they are made, than with cheaper generic or ‘plug-in’ substitutes, but I would like to focus here on the unique colour management issues raised by the decorative print work flow.

There are two main reasons for the ‘traditional’ colour separation process referred to above. Firstly, and most obviously, you needed to separate your design to the number of colours which corresponds to the number of printing stations on your analogue (rotary screen or gravure) printing press. Secondly, and perhaps just as importantly, colour separation provided the basis for re-colouring the original design and producing the multiple colourways which would make it to production. Digital printing removes the first of these necessities but, if anything, the second justification for a good quality colour separation is stronger than ever in today’s digital world.

With the constraint to number of conventional printing stations removed, many of the most successful digital decorative printers are now separating to twenty or more colours in order to gain a degree of fine control over the appearance of their final product which image manipulation (colour replacement) will never give. Sometimes this technique is used even when there will only be one final ‘colourway’ produced. Creating such

complex colour separations is relatively simple for designers equipped with with the latest specialist software.

Another aspect of colour management - accurate matching of the printed design all the way through from the colouring on a computer monitor to the final digital print – which will presumably become entirely redundant one day but which is still a very relevant concern for many digital producers today – is the desirability of being able to match digital to analogue production. Modelling the behaviour of conventionally overprinted tonal spot layers is beyond the scope of conventional colour calibration and profiling software packages. Such packages can provide reasonably accurate matches of the (measurable) flat spot colour ‘swatches’ upon which the colouring of each layer is based but a good match of the actual printed design depends additionally upon accurate modelling of the overprinted tonal spot coloured layers. Such modelling is achieved in the best specialist decorative print software but not if generic packages which were developed for the purpose of matching photographs or images are relied upon. Having it provides the ability to switch designs at will between (legacy) analogue and (new) digital production lines.

The next ‘step’ in the digital workflow is the one from which (slightly confusingly in the case of decorative print) the majority of the software mentioned by the printer manufacturers generally takes its name. The main purpose of the RIPs (Raster Image Processors) typically sold with the printers is, as the name suggests, to create a rasterised (dot based) file suitable for printing. Since the majority of decorative (as opposed to vector based signage or packaging) files will already be largely, or entirely, raster based, this process is not very difficult.

A 3D illustration of a typical printer colour gamut. Typical factors in gamut sizes are print head type, media, type of inks, number of inks, linearization, software, spectrophotometer used and pre and post treatments

Multiple Colourways being generated via the new “Inspire” tool, within the new Layout Window

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Colour SeparationMore interesting is the next step in the digital workflow. Regardless of whether the print is of a simple image or of a colourway made up of many spot coloured layers (effectively re-combined to form an image for the purpose of printing), a further digital ‘colour separation’ now needs to take place as the design is separated to the individual heads on the model of digital printer chosen. In many ways this process is analogous to the ‘conventional’ spot separation described above – the (usually 4-8) individual heads of the digital printer, each potentially fed by a different coloured ink, dye or pigment, might be compared to the printing stations on a conventional press. Crucially, however, in the purely digital decorative print workflow, this (second) separation need bear no resemblance whatsoever to the (first) separation which was used for re-colouring. Although usually invisible to the user (unlike the first separation which can be viewed as individual re-coloured spot layers) this second separation is also critical to obtaining the best possible decorative print. At present most digital print manufacturers rely on generic separations based on simple separation methods which assume that the inks on the printer are (only) CMYK. Research suggests that the use of proprietary separation tables for this (second type of printer-head) separation improves the gamut achievable on some digital textile printers. Linking the first (re-colouring) and the second (digital print head) separations may also have advantages in certain circumstances.

The final step is to send the information resulting from the second ‘head based’ separation to the heads themselves and fire the ink jets (including information on variable drop size and other options which may be offered on the model of printer chosen). This is the process which could most helpfully be described as the actual ‘driver’ of the printer (a term which seems to be used fairly

interchangeably with ‘RIP’ in common parlance). This process is, in software terms, generally fairly simple and can easily be integrated with the preceding steps on the basis of information supplied to the software develop by the hardware (printer) manufacturer.

The above is, inevitably given the space available here, a simplification of the digital workflow for decorative print. The reality is that every real life combination of market, product, substrate and model of digital printer chosen is unique and will give rise to different problems and challenges. Ideally, however, even the most complex of these challenges needs to be overcome by software within a totally integrated workflow which allows the designer/colourist to achieve the desired result on a computer monitor and then simply ‘hit the print button’ to see digitally printed product. Such a workflow is now possible but only, I would argue, with investment in software developed specifically for decorative printing and supported by ‘experts’ in this very specialised field. Printing a fine household or fashion textile or wall covering is a very different business from that of printing posters or signage.

AVA’s new Advanced Colour Separation tool - extremely valuable for both screen and digital printing. Colour matched separations can be seen and controlled in realtime.

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