Where is screening performed in the workflow?

In my last post I gave an introduction to halftone screening. Here, I explain where screening is performed in the workflow:

 

Halftone screening must always be performed after the page description language (such as PDF or PostScript) has been rendered into a raster by a RIP … at least conceptually.

In many cases it’s appropriate for the screening to be performed by that RIP, which may mean that in highly optimized systems it’s done in parallel with the final rendering of the pages, avoiding the overhead of generating an unscreened contone raster and then screening it. This usually delivers the highest throughput.

Global Graphics Software’s Harlequin RIP® is a world-leading RIP that’s used to drive some of the highest quality and highest speed digital presses today. The Harlequin RIP can apply a variety of different halftone types while rendering jobs, including Advanced Inkjet Screens™.

But an inkjet press vendor may also build their system to apply screening after the RIP, taking in an unscreened raster such as a TIFF file. This may be because:

  • An inkjet press vendor may already be using a RIP that doesn’t provide screening that’s high enough quality, or process fast enough, to drive their devices. In that situation it may be appropriate to use a stand-alone screening engine after that existing RIP.
  • To apply closed loop calibration to adjust for small variations in the tonality of the prints over time, and to do so while printing multiple copies of the same output, in other words, without the need for re-ripping that output.
  • When a variable data optimization technology such as Harlequin VariData™ is being used that requires multiple rasters to be recomposited after the RIP. It’s better to apply screening after that recomposition to avoid visible artifacts around some graphics caused by different halftone alignment.
  • To access sophisticated features that are only available in a stand-alone screening engine such as Global Graphics’ PrintFlat™ technology, which is applied in ScreenPro™.

Global Graphics Software has developed the ScreenPro stand-alone screening engine for these situations. It’s used in production to screen raster output produced using RIPs such as those from Esko, Caldera and ColorGate, as well as after Harlequin RIPs in order to access PrintFlat.

Achieve excellent quality at high speeds on your digital inkjet press: The ScreenPro engine from Global Graphics Software is available as a cross platform development component to integrate seamlessly into your workflow solution.
Achieve excellent quality at high speeds on your digital inkjet press: The ScreenPro engine from Global Graphics Software is available as a cross platform development component to integrate seamlessly into your workflow solution.

The above is an excerpt from our latest white paper: How to mitigate artifacts in high-speed inkjet printing. Download the white paper here.

What is halftone screening?

Halftone screening, also sometimes called halftoning, screening or dithering, is a technique to reliably produce optical illusions that fool the eye into seeing tones and colors that are not actually present on the printed matter.

Most printing technologies are not capable of printing a significant number of different levels for any single color. Offset and flexo presses and some inkjet presses can only place ink or no ink. Halftone screening is a method to make it look as if many more levels of gray are visible in the print by laying down ink in some areas and not in others, and using such a small pattern of dots that the individual dots cannot be seen at normal viewing distance.

Conventional screening, for offset and flexo presses, breaks a continuous tone black and white image into a series of dots of varying sizes and places these dots in a rigid grid pattern. Smaller dots give lighter tones and the dot sizes within the grid are increased in size to give progressively darker shades until the dots grow so large that they tile with adjacent dots to form a solid of maximum density (100%). But this approach is mainly because those presses cannot print single pixels or very small groups, and it introduces other challenges, such as moiré between colorants and reduces the amount of detail that can be reproduced.

Most inkjet presses can print even single dots on their own and produce a fairly uniform tone from them. They can therefore use dispersed screens, sometimes called FM or stochastic halftones.

A simple halftone screen
A simple halftone screen.

 

A dispersed screen uses dots that are all (more or less) the same size, but the distance between them is varied to give lighter or darker tones. There is no regular grid placement, in fact the placement is more or less randomized (which is what the word ‘stochastic’ means), but truly random placement leads to a very ‘noisy’ result with uneven tonality, so the placement algorithms are carefully set to avoid this.

Inkjet is being used more and more in labels, packaging, photo finishing and industrial print, all of which often use more than four inks, so the fact that a dispersed screen avoids moiré problems is also very helpful.

Dispersed screening can retain more detail and tonal subtlety than conventional screening can at the same resolution. This makes such screens particularly relevant to single-pass inkjet presses, which tend to have lower resolutions than the imaging methods used on, say, offset lithography. An AM screen at 600 dots per inch (dpi) would be very visible from a reading distance of less than a meter or so, while an FM screen can use dots that are sufficiently small that they produce the optical illusion that there are no dots at all, just smooth tones. Many inkjet presses are now stepping up to 1200dpi, but that’s still lower resolution than a lot of offset and flexo printing.

This blog post has concentrated on binary screening for simplicity. Many inkjet presses can place different amounts of ink at a single location (often described as using different drop sizes or more than one bit per pixel), and therefore require multi-level screening. And inkjet presses often also benefit from halftone patterns that are more structured than FM screens, but that don’t cluster into discrete dots in the same way as AM screens.

 

The above is an excerpt from our latest white paper: How to mitigate artifacts in high-speed inkjet printing. Download the white paper here.

New to inkjet? Come and see us at Hunkeler Innovationdays

Martin Bailey, CTO, Global Graphics Software
Martin Bailey, CTO, Global Graphics Software

If you are new to inkjet and are building your first press no doubt you’ll have many questions about the workflow and the Digital Front End.

In fact, you’re probably wondering how to scope out the functionality you need to create a DFE that is customised to exactly what your customers require. Among your concerns will be how you’re going to achieve the throughput you need to keep the press running at rated speed, especially when handling variable data. Or it might be handling special colours or achieving acceptable image quality that is keeping you awake at night.  And how to achieve this without increasing the bill of materials for your press?

At Hunkeler Innovationdays we’ll have a range of resources available to address just such questions with some real case study examples of how our OEM customers have solved the problems that were causing them a headache using our technology and the skills of our Technical Services team.

For instance, how, on a personalised run, when every label or page might be different, can you stop the press from falling idle whilst the RIP catches up?  Our ScreenPro™ technology helps Mark Andy cut processing time by 50% on the Mark Andy Digital Series HD, enabling fully variable (every label is different) continuous printing at high-speed and at high-quality.

How can you avoid streaking on the image if your substrate is racing under your printheads at speeds of up to 300m/min for aqueous and maybe 90m/min for UV.  Or mottling? The Mirror and Pearl Advanced Inkjet Screens™ available with ScreenPro have been developed specifically to address these problems.

During the lifetime of the press, how can you avoid variations in quality that look like banding because your printheads have worn or been replaced?  Take a look at what Ellerhold AG has achieved by deploying PrintFlat™.

The ScreenPro screening engine is one of the building blocks you’ll need for your inkjet press. Our Fundamentals components provide other functions that are essential to the workflow such as job management, soft proofing, and colour management.

Using a variety of white papers, print samples, video footage and case studies , we’ll be sharing our experience.  So, come along and meet the team:  that’s me, Jeremy Spencer, Justin Bailey and our colleague Jonathan Wilson from Meteor Inkjet if you want to chat about their printhead driver electronics that are endorsed by the world’s leading industrial inkjet printhead manufacturers.

 

Join us at Hunkeler Innovationdays 2019

 

Screening for the next-generation high-definition devices

In days gone by, almost every job was more or less 600 dpi in both directions. Now there is a drive to higher definition, with higher resolutions and smaller drop sizes.

So we’ve introduced a new feature in ScreenPro™ that allows the resolution of a job to be “upscaled” meaning that a RIP can still render at 600 dpi through an existing workflow and then ScreenPro can upscale the job to the printer resolution. The benefit is that you don’t need to change your existing workflow, can cut down on RIP time by RIPping at 600 dpi, but print on a 1200 dpi machine for increased definition.

There are various ways of achieving higher resolutions: use the new generation of print heads running at 1200 dpi, use multiple print bars, or use scanning head printers for multiple passes. Sometimes it really is increased resolution that is required and other times it is higher addressability and, for example with textile printing, sometimes you just need to be able to put down more ink in any given location.

Once manufacturers have achieved 1200 x 1200 dpi there are other problems to solve. There is four times as much data generated that needs to be passed through the workflow pipeline to the press compared to a 600 dpi data path. There are some applications where the higher addressability isn’t needed, and 600 dpi is ok, in this case you could run the press twice as fast and get twice the production if you ran it at 1200 x 600 dpi, or three times as fast at 1200 x 400 dpi.

To solve the problem of too much data slowing down processing times we have implemented resolution upscaling in the latest release of ScreenPro. The typical example is that we have an existing press and workflow to go with it at 600dpi. The RIP delivers data at this resolution. We then have a choice – to send it to the 600 dpi printer, in which case we screen as normal, or we send to the 1200 dpi machine.

In this simple case we use ScreenPro to double the number of dots it produces in both directions. For non-square resolutions we multiply in one direction only. Also for non-square resolutions we have to change the shape of the screens, a circular screen will be distorted by the non-square printer resolution so we have to correct for that up front.

What this means is that you can continue to RIP at 600 dpi and keep the same workflow right up to the last process of screening. You keep the same PC processor requirements, same network data transfer speeds. Only at the last stage use ScreenPro to upscale to your desired resolution.

This will be a really useful feature for many customers developing the next generation of high definition digital printers.

Hunkeler Innovationdays 2019
Join us at Hunkeler Innovationdays 2019 to learn more about the new features in ScreenPro.

ScreenPro™ with PrintFlat™ removes banding on large format posters

Watch our latest video showcasing our award-winning technology, ScreenPro with PrintFlat.

Global Graphics Software’s Technical Services team worked with Ellerhold AG, the leading poster printing house in Germany, to enhance the printing quality of its large-format posters. The result was 100% customer satisfaction and an increased market share of outdoor advertising products in digital printing.

Questions from the Inkjet Conference, Düsseldorf

The Inkjet Conference Düsseldorf has been and gone for another year and we’re already looking ahead to the 2019 events that will be organised by ESMA.

This year delegates in the audience were able to submit questions via an app for the first time. I’m grateful to the IJC for sending me the questions that we either didn’t have time to cover after my presentation, or that occurred subsequently. So here they are with my responses:

Is it possible to increase the paper diversity with software by e.g.  eliminating paper related mottling?

Yes, we have yet to come across a media/ink combination ScreenPro™ will not work well with. The major artefact we correct for is mottle. This may mean you can print satisfactory results with ScreenPro on papers where the mottle was unacceptable previously, so increasing the diversity of papers that can be used.

It sounds like ScreenPro is very good at tuning a single machine. How do you also then match that output quality among several machines?

There are two technologies in ScreenPro, the screening core itself with the Advanced Inkjet Screens (AIS), and PrintFlat™ to correct for cross web banding. ScreenPro generally improves print quality and Mirror and Pearl screens (AIS) work in the majority of screening situations. PrintFlat, however, needs to be tuned to every press and if the press changes significantly over time, if a head is changed for example, it will have to be recalibrated. This calibration actually makes subsequent ink linearization and colour profiling more consistent between machines as you have removed the cross-web density fluctuations (which are machine specific) from the test charts used to generate these profiles.

“We haven’t found ink or substrate that we couldn’t print with.” Does this include functional materials, such as metals, wood, rubber? or is it limited to cmyk-like processes?

No – with ScreenPro we have only worked with CMYK-like process colours, i.e. print that is designed to be viewed with colour matching etc. ScreenPro is designed to improve image quality and appearance. I see no reason why ScreenPro would not work with functional materials but I would like to understand what problems it is trying to solve.

What is the main innovation of the screening software in terms of how it works as opposed to what it can do?

“How it works” encompasses placing the drops differently on the substrate in order to work around common inkjet artefacts. The innovation is therefore in the algorithms used to generate the screens.

Advanced Inkjet Screens are standard in the ScreenPro screening engine

 

ScreenPro™ makes a real impact on those hard-to-solve killer jobs

By Tom Mooney, product manager for Global Graphics Software

I’ve just returned from a road trip in the US to inkjet press manufacturers who are all interested in using ScreenPro.

The meetings have gone in a very similar manner with the opening line: “We have a print shop that wants to print this job, but take a look at this area.” They point to an area of the image, usually in the shadows, and it is either a muddy brown mess or crusty and flaky, the typical ‘orange peel’ effect. We all agree the print is unacceptable and cannot be sold, so we discuss what can be done.

Firstly, we look at the ink limitation, but this kills the color saturation in the rest of the print. We look at color management and under color removal, but this only moves the problem to a different area on the image.

Then we see what ScreenPro can do.

We try our Advanced Inkjet Screens™ and use Pearl screen on the muddy mess and Mirror on the orange peel. This does the trick and makes the prints acceptable, so the print shop can sell that print job.

As long as this quality threshold is met the customer is happy. This quality is achieved by a combination of hardware, media and ink and software. Color management is only part of the story with the software – ScreenPro makes a real impact on those hard-to-solve killer jobs.

ScreenPro’s Advanced Inkjet Screens solve two common problems: Mirror avoids orange peel mottle on non-absorbent, poorly wettable substrates and Pearl counteracts streaking on reasonably absorbent and/or wettable substrates
ScreenPro’s Advanced Inkjet Screens solve two common problems: Mirror avoids orange peel mottle on non-absorbent, poorly wettable substrates and Pearl counteracts streaking on reasonably absorbent and/or wettable substrates

 

 

 

A problem solved

They say a problem shared is a problem halved. Well, two weeks on from our launch of our Advanced Inkjet Screens it’s been gratifying to see how much the discussion of inkjet output quality has resonated among the press vendor community.

Advanced Inkjet Screens are standard in the ScreenPro screening engine

Just in case you missed it, we’ve introduced a set of screens that mitigate the most common artifacts that occur in inkjet printing, particularly in single-pass inkjet but also in scanning heads. Those of you who’ve attended Martin Bailey’s presentations at the InkJet Conference ( The IJC) will know that we’ve been building up to making these screens available for some time. And we’ve worked with a range of industry partners who’ve approached us for help because they’ve struggled to resolve problems with streaking and orange peel effect on their own.

Coalescence on inkjet is directional and leads to visible streaks.

Well, now Advanced Inkjet Screens are available as standard screens that are applied by our ScreenPro screening engine. They can be used in any workflow with any RIP that allows access to unscreened raster data, so that’s any Adobe PDF RIP including Esko. Vendors can replace their existing screening engine with ScreenPro to immediately benefit from improved quality, not to mention the high data rates achievable. We’ve seen huge improvements in labels and packaging workflows. Advanced Inkjet Screens are effective with all the major inkjet printheads and combinations of electronics. They work at any device resolution with any ink technology.

Why does a halftone in software work so well? Halftones create an optical illusion depending on how you place the dots. Streaking or graining on both wettable and non-absorbent substrates can be corrected. Why does this work in software so well? Halftoning controls precisely where you place the dots. It just goes to show that the assumption that everything needs to be fixed in hardware is false. We’ve published a white paper if you’re interested in finding out more.

The Mirror screen mitigates the orange peel effect common when printing onto tin cans, plastics, or flexible packaging