Two Colour Print - Spot On

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Make The Most of Two Colour Print

Tom Arah shows you how to avoid the traps and make the most of your spot colour print.

 Quark's trapping controls

Regular readers of the RW Publishing/Graphics section will have noticed a recurring theme, namely the huge difference between producing in-house and commercial print. For work destined for your local inkjet all colours are output in a single pass of the sheet of paper through the device. For commercial full-colour print all colours are recreated by passing the paper through the printers' press four times, once for each of the four "process" colour components - cyan, magenta, yellow and black. The extra work involved in commercial printing and especially the initial setup in creating the CMYK colour-separated plates means that full-colour work generally proves too expensive for short runs. Trying to output serious quantities from an average inkjet though is even more expensive per page and just not a practical option.

In other words there is a huge gap between as-you-need-it in-house print and full-blown full-colour offset print. Unfortunately this gap covers many of the needs of small businesses who often require print runs of a couple of hundred reports, brochures, posters or letterheads. The obvious solution is to cut down on the number of colour separations and so print passes needed. By printing two so-called "spot" colours, rather than the four necessary for process print, the work involved and so cost of the print run can theoretically be cut in half. Of course this also immediately cuts out the possibility of producing full colour photographs and choosing any colour and combination of colours that you want, but even so for many jobs it's a very sensible decision. Even the smallest splash of colour does a huge amount to make an otherwise black and white design stand out and, by using inks outside the usual CMYK range, or tints, or three colours you can produce spot colour print with just as much impact as full colour.

Moreover it's much easier to produce the necessary separations as each spot colour is output on its own plate rather than being broken down into its CMYK percentages. In fact in theory the separations can even be produced manually and many readers will have had the experience of printing out two copies of a publication and tippexing out all the areas of one colour on one sheet and vice versa on the other. A slight refinement on this is to create two copies of a file and to obscure the relevant areas of colour with white boxes in the page layout view of your WP package. It certainly can and has been done and, if you've spent months producing your publication in a package like Word only to discover at the last minute that it doesn't offer in-built colour separation, it's often your only option!

Spot colour seps

Spot colour work is output as single colour separations for each ink.

Obviously though such home-made solutions aren't recommended. The main problem is "registration". To produce successful colour-separated print the colour plates must line up exactly and with typical laser output there just isn't the necessary accuracy. If you laser print two copies of a box, for example, and lay the two pages over each other they are very unlikely to match up exactly. On top of this the laser printer's control over the size and placement of its toner-based dots just isn't precise and consistent enough to enable the creation of tints without obvious banding. For such in-house originals then it's important to keep to solid colours well separated from each other. In other words highlight clear regions of the page, such as headings and company logos, and live up to the "spot" in spot colour.

Of course rather than producing workarounds like this what you should really be doing is using a dedicated DTP package rather than a WP. One of the defining features of any DTP application is that it is designed to help you produce photoset spot and process separations (well it is now that Publisher 2000 has finally recognised the requirement). When you define a colour for use in your publication you will usually find an option to define it as a spot colour which means that it will automatically be output to its own plate. In fact, rather than mixing your own spot colour, you will normally select it from a provided colour library. Different packages offer different spot colour libraries, such as the Japanese TOYO and DIC systems, but by far the most common is the Pantone Matching System (PMS) of just over a thousand predefined colours.


The Pantone Matching System is much the most popular library of predefined spot colours.

Once you've chosen your spot colour you can apply it to any text or internal drawn object, such as a line or frame, and you can also apply it to imported bitmaps to ensure that they are output to the right colour plate. In most packages you can also create tints of the spot colour which greatly increases the available palette and creative possibilities. In XPress 4 you can go even further and define a Multi-Ink composed of set percentages of your existing spot colours though you need to be careful. To begin with you have to ensure that the mixing spot colours won't be output at the same screen angle as one colour will then obscure the other (the usual choices are 45 and 105). Secondly you shouldn't trust the onscreen simulation as it's very difficult to predict exactly what colour you will end up with as, unlike the process colours, the Pantone colours were not created with mixing in mind.

In fact you shouldn't trust the onscreen representation even of solid Pantone colours. The reason is simple. The gamut that the Pantone colours are capable of producing is determined by the combination of Pantone's proprietary inks. This gamut is completely different both to traditional CMYK and also to the RGB gamut that your monitor can produce. The positive side of this is that using Pantones you can print many colours - bright oranges, violets and blues to name a few - which are simply unprintable with process print. The downside is that the onscreen Pantone representations are just closest approximations and often woefully inaccurate.

The obvious question this raises is how do you know what your spot colour will look like? The only sure way is through a printed reference which is why every professional designer in the country will have a copy of the Pantone Formula guide. Armed with this it's possible to know exactly how your colour will print. Or rather it would be if this didn't also depend on the paper stock on which it is printed which is why Pantone always provides coated and uncoated versions of its libraries. Tints are another matter again so you'll also need Pantone's Colour Tint Selector and if you want to mix colours you better get the Colour + Black Selector and the Colour + Colour Selector. And to be on the safe side you better get the Solid to Process guide to see the nearest CMYK equivalent to your spot colours in case you might want to repeat them in four-colour jobs. Oh and finally, whatever you do, don't forget to keep them up to date as the colours fade every couple of years! Then again I suppose you could always just accept that the onscreen colour is an approximation and borrow your printers' copies when required.

Apart from the provision of dedicated spot colour libraries as you produce your design, DTP packages also score over WP apps with their in-built colour separation when you output your work. Somewhere in the print dialog all the spot colours used in the publication will be listed and you can select them for output to their own plates. Effectively the DTP software automatically undertakes the manual task of hiding everything but one colour in turn and printing that as a separate page. Because we are now outputting to a photosetter the accuracy and consistency is good enough to allow for the production of unbanded tints and also to ensure that two copies of a page laid over each other will match exactly.

The separations might match each other exactly on the film but these now have to be turned into printing plates that line up exactly on the press if we're going to be able to produce registered print. To enable this all DTP applications also offer the ability to output crop marks - small tick marks at the corner of each page which can be used for lining up separations and then for cutting once the job has been printed. That's great for most work but for some jobs, such as the creation of separate business cards, you want to have multiple sets of crop marks on each page. To enable this high-end packages offer the use of a predefined "registration" colour which looks like ordinary black but which prints on all separated plates. This enables the interactive creation of crop marks, but it's slightly dangerous as it's easy to apply the "registration" colour by mistake and end up overprinting and overinking. As such, if you can, it's a good idea to redefine your registration colour as an unmistakable pink or orange.

Crop and registration

Automatic crop marks and use of the registration colour help produce tight registration.

Thanks to crop marks and tight registration we can now produce work where different colours abut each other, but what about where they overlap? If we print a blue headline half straddling a yellow square the text will seem to change colour over the background. In fact the only way that we can be sure that we will get the colour we specified is if it prints directly on the white paper. The only way that this can happen is if all colours "knockout" any colour below them so that any area of colour on one plate will be white on all other plates. Knocking out solves the colour change problem, but it creates a new one. Ultimately offset printing is a mechanical process and as such while our registration is tight it can never be perfect. If our colour separations are out by even the slightest amount, little slivers of knocked-out white will appear and ruin the design.

So what can be done? In fact in most two-colour jobs it's not necessarily a problem as one of the colours will usually be black. Black is the one colour that is unaffected by any underlying colours so the colour change problem disappears. In all professional DTP packages therefore you'll find an option in which solid black can be made to "overprint" rather than knock out. In fact you'll usually be given rather more control so that you can specify whether black lines and fills overprint and also be able to set a point size below which all text will do the same. Such global overprint control is normally limited to solid black but in some packages you'll also find an option to set a percentage threshold above which a black tint will be treated as solid.

Overprinting black

By setting black to overprint you can avoid many potential problems.

Overprinting black solves the misregistration problem for many print jobs, but it doesn't help where two different colours are involved. The only solution here is to create a slight overprinting overlap called a "trap" between the two colours. The trap will be slightly darker than the darker of the two objects which can cause a slight outlining effect, but that's much less distracting to the eye than a white sliver would be. This outlining effect is kept to a minimum by ensuring that it is always the lighter object that overlaps the darker. This means that there are two kinds of trap. A lighter object on a darker background expands slightly to produce a "spread" whereas a lighter background tightens slightly around a darker foreground object to produce a "choke."

For most jobs, the DTP software is able to determine whether to spread or choke depending on the respective density/luminosity of the two colours. With such automatic trapping all that the user has to do is to set the desired trapping width. Bizarrely XPress defaults to 0.144 points which is so small as to be almost useless. The ideal setting depends on factors such as the press and paper - so always check with your printer - but generally for a screen ruling of 100 lpi should fall somewhere between 0.4 and 1.4 points and for 200 lpi should be half that at between 0.2 and 0.7 points. If you are proofing on a Postscript printer you can increase the level to make the traps more evident but remember to set it back before outputting to film!

The trapping width is much the most important setting, but professional packages offer further refinements. The overlap effect is less noticeable with black, for example, so some packages offer a separate Black Width which is usually set to around double the value of the default. Others offer a separate "indeterminate" value which it uses when trapping over multiple colours. Both XPress and PageMaker can also finesse their trapping depending on the difference in luminosity between the two colours. XPress offers a "proportional" option which creates bigger traps for bigger differences in colour, but the benefit of this is debatable and the effect is difficult to predict so it's probably best avoided. Rather more useful is PageMaker's ability to set a Centreline Threshold which means that rather than choking or spreading similar colours, the program instead creates a trap that straddles the line where they touch.

Such global controls are adequate in many circumstances, but if you really want to master the trapping process you need to be able to specify the trapping of each of your spot colour inks. PageMaker enables you to set up overprinting versions of a given ink by creating a 100% tint and setting it to overprint in its Edit Colour dialog. This pales into insignificance next to the control XPress offers through its Trap Specifications dialog. This takes the currently selected colour and allows you to set whether it overprints, knocks out, chokes or spreads, and to what extent, in relation to each of the other spot colours in your publication. By default each colour-to-colour trapping relationship is reciprocal, so that a foreground spread becomes an equal background choke, but you can even override this is you want.

Such ink-based trapping is useful, but for absolute control you need to be able to set trapping on an object basis. PageMaker allows you to force any object's line or fill to overprint rather than knockout using the Element>Fill and Line command. To trap imported bitmaps it offers its Keyline utility which automatically creates an underlying white rectangle marginally smaller than the overprinting photo and set to knockout. Again XPress leaves PageMaker for dead with its Trap Information palette. Like the Trap Specifications dialog this allows any object or selected text to be set to overprint, knock out, choke or spread and for the level of trapping to be set. More than this, it enables full trapping control over the inside and outside of frame borders even down to the individual settings for stripes and dots and dashes!

Quark's trapping controls

XPress offers extensive object and ink-based trapping control.

By this time the full potential complexity of successful trapping is probably becoming clearer and I haven't even begun on trapping imported vector drawings (essentially manage the trapping manually as overprinting non-scalable outlines and then save to EPS format). Even when you have dealt with all the variables that are under your control you'll find that there are some situations that you can do nothing about. In XPress, for example, only the endcaps of rounded dashed lines are dealt with correctly and an overlying object can't simultaneously choke to one colour and spread to another. In PageMaker on the other hand it's impossible to trap type on type or background type to foreground objects.

It's as well to be aware of these limitations, but don't panic as there are two major consolations. The first is that in most cases using the correct automatic defaults and a bit of common sense about likely trouble spots will usually ensure that you produce the results you want. The second is that in the near future the entire task of trapping looks likely to become redundant. Or rather the trapping will still happen but it will occur after the print command has been given. The current leading package in this field is Sequent Trapwise, a post-processing program that analyzes how inks intersect on a page and traps them wherever necessary. This edge-based detection system not only offers the highest quality, for example intelligently trapping against gradient backgrounds, but also - mercifully - requires that designers don't set any trapping themselves!

Certain bureaux already offer such painless trapping at a price, but in the future it will become standard as Adobe has already integrated similar in-RIP technology into Postscript 3 and manufacturers are now beginning to produce devices to take advantage of it. Potentially even more significant is the advent of a PDF trapper for Postscript 3-based Extreme printers due later this year. These printers offer "acceptable" quality direct digital separations from multiple processors simultaneously. In other words they are targeted exactly at that gap between in-house and offset print. Even better, because they will work with any Acrobat PDF file, it means that in the future even if you've created your publication in Word you'll still be able to output it as fully-trapped commercial print.

Best of all of course is the fact that the use of Extreme digital printers will offer affordable full-colour print with an unlimited choice of design colours and integrated colour photographs. Eventually this could even mean the end, or at least a serious erosion, of two- and three-colour work. For the moment, however, spot colour still reigns supreme as the best compromise between impact and cost for medium length print runs. In fact, whatever happens in the future, well managed spot colour will always give full colour print a serious run for its money.

Tom Arah

Sept 1999

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