A command of color is vital to accurate production in textile – more so than other print workflows. LMNS expert Steven Harnie explains the options in this article that was written for Caldera’s Gamut Magazine.

While digital textile printing traces back to around 1990 it remained relatively undeveloped in potential until recent technological innovations opened the doors to the high-volume textile printing market. This market segment’s demanding clients require the rapid reaction of technology providers, who must introduce radical changes in order to help the buyer adaptation to the potential of this emerging market. Accurate reproduction is a critical factor in textile printing and improvements to the color workflow for digital textile printing are therefore in high demand.

The challenge

Probably the most time-consuming process in the digital textile production workflow ls proofing designs for color accuracy. Textile differs from other printed applications in several ways. The hardest colors to reproduce are blacks, greys and dark, saturated tones, and the gamut of an average textile color profile is around two-thirds that of an offset process.

Measuring color on woven textiles is more complex than on flat surfaces, like paper, and the color appearance changes more than average under different viewing conditions. Fabrics are liable to contain a surprising amount of optical brighteners, while color transformation during fixation creates additional difficulties for calibration. Textile buyers also demand color matching to within the kinds of tolerance demanded by the manufacturing industry, which are very tight. In short: not only is it harder to achieve good results when printing textile, but the requirements are high compared with most other printing applications.

A typical textile design is a repeatable pattern filled with up to 12 reference colors: they rarely contain photographic images. During reproduction two factors are very important: the colors printed on the fabric must represent closely the reference colors that were chosen by the designer, while a gradient between any two colors should be perceptually linear, producing smooth graduations without color shifts. While achieving grey neutrality during color calibration is a challenge, no matter the print process, printing a deep black on textile is even more problematic.

These are but a few of the challenges that will keep software developers busy for the next few years. Among the premier providers, Caldera has been noticeable for its commitment to addressing these issues and provides its textile customers with solutions for black generation issues and improved color predictability.

Color workflow for designers

Designers often ask me which color space they should use for textile design. Most work in either RGB or L*a*b* working spaces, rarely in CMYK. All working spaces have their advantages and purposes so, once the media, ink type and therefore print process have been defined, the choice of working space becomes clearer. Caldera provides additional options specifically designed for textile applications that can prove to be particularly handy.

L*a*b* is a valid working space which has several advantages: it is device-independent, removing the need for ICC profiles throughout the color workflow, and because its L component closely matches a human’s interpretation of light, it approximates the eye’s visual spectrum to provide perceptual uniformity. But it also has several downsides: the gamut volume is so large that most colors can’t be printed due to poor predictability and L*a*b* images require 16-bit/channel depth to maintain acceptable step widths, thereby guaranteeing gradient reproduction without banding.

RGB working spaces are designed for image editing and have the advantage of being grey balanced, perceptually uniform and containing a balanced gamut volume. The Adobe RGB (1998) color space was developed to encompass most of the colors achievable on CMYK color printers and has become the de facto standard for digital textile design.

CMYK working spaces were developed for offset printing and should generally not be used for textile design. The additional color transformations needed to bring an image to output will probably lead to inferior color accuracy. Many textile printers can print colors that are outside the color gamut of CMYK working spaces, meaning that it would restrict these printable colors. The one exception is when colors must be matched between offset and digital textile.

Working in CMYK from the outset automatically restricts the designer to the gamut of an offset press. This can be an advantage because the design is almost guaranteed to contain no non-printable colors, providing therefore very good color predictability. This is possible because offset presses are calibrated to pre-defined standards, as seen with the measurement of tolerances concerning offset papers, inks and machinery.

Digital textile can be considered as more an industrial manufacturing process where, at the end of a production line, a variety of products is completed. These could be carpets, clothing, wall decoration, sportswear, car seat upholstery, interior blinds, plus many more. The base materials – in this case, textiles – are chosen for their desired properties, such as the hand (or touch and feel), color intensity, resistance to UV, or wear and tear. Each requires its own compatible ink, and the ink must be adapted to the application itself.

Adding in the print machinery itself, the combination could produce a huge matrix of processes, for which we can’t create the type of standard we see in offset. We would end up with thousands of working color spaces in Photoshop, whereas, today, only a few additional spaces will be added to compensate for the way designers and printers work closely together. These custom spaces need to be defined as RGB or CMYK ICC profiles. However, many textile printers use additional profile colors to increase the gamut and profiles generated with traditional color profilers are CMYK+n (multi-color) profiles which are incompatible with Photoshop.

Profile Expert, the color profiler included in Caldera’s TextilePro solution, includes the option to create RGB profiles for CMYK+n color printers. The generated profiles can be imported in Photoshop, where they can be used as a working space to make adjustments to designs. The working spaces in turn match gamut with the printer, and the designer can make color adjustments using RGB channels that provide better perceptual uniformity and grey balance than the working space of CMYK. This provides excellent output color predictability for the designer and, effectively, reduces production downtime from color proofing – one of the biggest added costs in digital textile printing.

Advanced black generation

On the other hand, a perfect neutral black in colorimetric terms – the theoretical result of proper color profiling – won’t be one that satisfies the customer. This is because humans prefer blacks with a blue or red hue when viewed on textile. Traditional color profilers don’t allow the type of black to be specified, nor are they particularly effective at creating GCR (grey component replacement) curves from chart measurements. Calibration software tends to overcompensate for fluctuations in dark color mixes and produces ICC profiles that contain irregularities.

An additional step in Caldera’s RGB workflow allows the user to choose the desired black before the creation of an ICC profile. The black chart tool generates a black swatch book to help with choosing the proper parameters for ink mixing. The use of four black separation methods provides more control in the generation of the grey component. While ‘black only’ uses black and light black inks only, ‘black mixing’ allows the user to choose their own Ink formulation; ‘GCR’ mode is based on the traditional GCR method with some additional parameters and ‘no black’ only uses colored inks to create black.

The combination of black choice and black mixing options prevent excessive fluctuations in the black mixing and the ICC-profiler generates smoother curves. As a result, the color gradients in the dark region stay more perceptually linear until they reach the deepest black achievable on the print engine,

“Black separation is the central concern in RGB to CMYK conversion in ICC profile generation,” explains Arnaud Fabre, Product Manager at Caldera. “Software can be very smart, but when it comes to color perception, giving more control to end users and allowing them to choose their preferred rich black is the smartest thing to do. The RGB workflow feature in TextilePro is the result of years of research and experimentation in textile printing to make sure we let customers do exactly that.”

While producing technically acceptable color is satisfying to the expert, the real target in digital textile is to produce results that hit the gamut of customer expectation. Without software solutions that assure color output predictability, expensive textile production lines can quickly turn into waste generators. The RGB workflow and black generation tools developed by Caldera provide a method for dealing with several issues encountered in digital textile print production and are likely to lead to further adoption amongst existing producers.

_{This article has been published in Gamut #5 and was written by Steven Harnie, LMNS expert and owner of Printrix.  He is an expert in digital print technologies providing services to print companies, manufacturers and software developers. He specializes in setting up efficient production workflows, advanced color calibrations, color management, training, support and technical consulting services for industrial textile printing companies. Additionally, he works with several manufacturers on development of different products related to the digital printing market.}