PrePress Magic - Colour Print

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Understanding commercial colour print

Full colour printing is an illusion built on an illusion. Tom Arah looks at the tricks involved.

In the last Publishing article I talked about the differences between offset and desktop printing and covered the stages involved in getting your work to the service provider ready for imagesetting. That was complicated enough involving understanding the role that Postscript, fonts, images and so on all have to play. However these complexities pale into nothing compared to those involved in ensuring that the printing plates produce the desired results. In particular in the last article I deliberately kept the job simple, by limiting it to black and white.

Sadly you're unlikely to have much success as a designer if you work solely in black and white, so in this article I'm going to expand outward to take our project into full colour. This immediately opens up the world of pre-press, a daunting experience as you'll see if you open any DTP package's advanced print options. This is a Pandora's box full of intimidating options such as screen frequency, bleeds, marks and trapping - all demanding attention. There's far too much to take onboard at one go, so instead we'll build up to full colour and introduce the necessary concepts as we go along.


The first step in adding interest to our project is the simple one of adding some colour even if it's only gray. The problem is that the imagesetter is essentially a binary digital device that can only produce black pixels or leave white spaces. In fact this doesn't prove much of a problem at all thanks to the limitation of the human eye and the resolution of the imagesetter. Because each imageset pixel is smaller than the eye can resolve, if an area is built up with one dot on, one dot off the result will be perceived as a 50% gray. If only every tenth pixel is on this will be perceived as 10% gray.

This optical illusion allows us to add tints to our project to give it variety. More importantly, the principle behind it can be extended to simulate the continuous tones of gray found in black and white photographs. The solution is to break the image into a very fine grid with each square of the grid given an appropriate gray level. To then simulate this gray level the imagesetter creates its own grid and fills in the appropriate number of pixels. On a 16 by 16 grid, for example, it is possible to mimic 256 different levels of gray with the effect of a 50% gray, for example, being created with a dot composed of 128 pixels. This notional grid that breaks the image into cells is called a "screen" and the resulting image that gives the impression of continuous grayscales is called a "halftone".

Here Photoshop has been used to turn the continuous toned eye into an onscreen halftone just as it is when printed. The use of the grid to mimic grayscales becomes visible when the halftone is enlarged.

It's clear that the more pixels there are in each cell the more gray levels can be simulated so the smoother the effect. However if the cells become too big they become visible and interfere with the illusion of continuous tones. In practice this means that there has to be a trade-off between the number of possible grayscales and the size of the grid. This is determined by a combination of the output device's resolution measured in dots per inch (dpi) and the screen frequency measured in lines per inch (lpi). On a 600dpi laser printer, for example, a screen of 100lpi would mean that each grid square was 6 x 6 pixels which would only offer 36 possible grayscales. Decreasing the frequency to 75lpi would allow an 8 x 8 grid that would allow a smoother 64 levels.

The huge advantage of imagesetters is that their pinpoint resolution allows both higher - and so less noticeable - screen frequencies together with more grayscales - and so smoother tones. On a 2400dpi imagesetter for example a fine 133lpi screen still allows an 18 x 18 grid and a theoretical 324 levels. The highest quality imagesetters can even output a 200lpi halftone at 4000+dpi - which is just about indistinguishable from the original photo. For coffee-table work like this though you would have to make sure that your paper stock can hold the definition. If you are printing on absorbent newspaper, for example, the maximum screen is only around 80lpi.

As well as the paper output you must also think about your file source. In fact this isn't too much of a problem as the TIF format is a universal standard supported by all software and all platforms. The only thing you need to do is to gear your images toward their final outputting device. The huge advantage of halftone images is that this is based on the screen frequency (lpi) rather than the resolution (dpi) so that if you are outputting to an imagesetter you should be scanning your photographic images at around 300dpi - not 2400dpi!

Spot colour

Now that we've got halftoning under our belt we can attempt to add some simple true colour, for example, to pick out a masthead or a logo. This is called spot colour and is based on the simple principle of putting the page through the press more than once with a different ink each time. The only thing we have to ensure is that the colours are kept absolutely separate so that the masthead, for example, will only be printed with the coloured ink and the text only printed with the black. The necessary colour-separated plates could theoretically be produced by tippexing out all the elements to be printed in black ink on one sheet and then reversing the process on the other. Nowadays thankfully, any self-respecting DTP program - even Microsoft Publisher - offers the capability of automatically separating spot colours.

PageMaker's print options are looking long in the tooth, but the program's popularity with service providers make it a safe choice for commercial outputting.

Spot colour is really pretty straightforward, but there are two difficulties. The first is colour accuracy. You can choose any colour you want on the computer, but it will have absolutely no effect on the colour-separated imageset output which will always be black and white (or halftoned). The actual colour the plate is finally printed in depends purely on the ink that the printer mixes up. Of course colour is a very subjective matter which is why an independent and objective standard is necessary. The most common of these is the Pantone library, a printed palette of some 1,000 colours each based on a precise combination of inks. All you need to do to ensure accurate matching is to select a colour from the palette, the printer can then reproduce it with the precise mix of inks defined in its Pantone formula.

The second problem only develops when we get more adventurous and start adding a couple of overlapping colours. If I want a green heading over a yellow tint, for example, if one ink is printed over the other the result will be a blue. In a way this "overprinting" can be very useful in that it allows the creation of new colours, but what if I really want the green heading? The only way it can be achieved is if a corresponding white version of the heading is created - "knocked out" - on the yellow plate. This really is dangerous territory - if you don't set the right settings for knocking out and overprinting, your end results will be completely different to your screen-based intentions and almost certainly a complete mess. Fortunately with black - the most common colour of all - the problem is limited as it is unaffected by underlying colours and so can simply be set to overprint by default.

Unfortunately while knockout solves the problem of unwanted colour blending it creates another. If the two colour-separated printing plates are even slightly misaligned it will immediately become apparent through the presence of a white fringe around the knocked-out object. The main solution is to set your output to include registration marks. As these appear in exactly the same position on each plate they can be used for precise alignment. Because they mark the exact corners of the page they are also used for exact cutting to size - particularly important if you have a tint or picture bleeding off the edge of your design. For these crop marks to be visible you'll have to set a paper size large enough to cope. That's not a problem with the roll-fed imagesetter, but you might have to temporarily reduce-to-fit when creating proofs on a desktop printer.

Tight registration is crucial, but with factors like friction and paper slippage it can never be completely perfect. The professional designer has to take this into account and build in some leeway. This latitude is created by imperceptibly enlarging one of the elements to create a slight overlap wherever two colours meet, a process called trapping. To make the overlap as unnoticeable as possible the extra width should be kept to the absolute minimum and preferably limited to the lighter colour. If the trapped colour is on top - ie knocking out - it is called a "spread" while if it is underneath - ie being knocked out - it is called a "choke". Often global settings can be made to control automatic trapping for a publication as a whole, but the most advanced programs such as vector-based draw programs enable precise object-level trapping. To enable a DTP program to import and output such pre-trapped graphics you really need to use the EPS graphic format. (This and the many other advantages of EPS were discussed in issue 37.)

Process Colour

Now our project really is getting somewhere. From tints and photographic halftones we've moved into the world of colour and colour separation. To add a new colour we can simply use the same principles to add a new ink and put the page through the press again. Yes this is true, but we soon hit a major problem. Obviously each plate and each time through the press adds to the cost. More importantly how can you possibly print a full colour photograph? Putting the same sheet of paper through the press a million times is not really a starter!

So how are all the continuous photographs and solid colours in all the magazines and brochures and packaging that surround us actually created? The answer is a combination of two of the effects we've already seen. The apparent effect of continuous tones is created through the use of halftoning while each of the apparent colours is produced through blending. Rather than creating each colour by blending inks, however, it is again left to the limitation of the human eye. By placing coloured dots too small for the eye to distinguish next to each other, the eye's automatic averaging makes them seem like one combined colour.

Amazingly, to produce the full range of colour that we see in printed material, it only takes a combination of three inks: cyan, magenta and yellow so that to produce black takes 100% of all three. To create each effect the dots are not placed directly on top of each other but are slightly offset. This is achieved by setting the inks' halftone screens at an angle to each other. This is the reason for the strong rosette pattern visible if you look at a printed colour under a magnifying glass.

By blending 256 levels of each of the three coloured inks as halftone dots, the theoretical maximum of 16.8 million colours (256 x 256 x 256) is achieved. On one hand it's a beautifully simple and elegant solution that has worked well on countless occasions, but on the other it's a horrible kludge. In particular it's important to recognise that full colour print is one optical illusion - blending - built on another - halftoning. When you think of full colour printing as essentially a complex magic trick, it's not really surprising that it's difficult to get to grips with - and that sometimes it fails.

Corel Draw's colour-separated print preview and tabbed printing dialog give you all the pre-press power you need, but the sheer number of options can be intimidating.


One immediate complication to the CMY system is caused by the colour black.  While theoretically 100% of each ink creates black, in practice it's more like a very dark brown. Moreover, because dots are set at angles to each other, there would be no way of producing pin-sharp text. The answer is simple - add in another plate for black. The result is the four-colour "CMYK" system that is the universal standard for commercial print. (The "K" is used to stand for blacK to avoid confusion with the Blue in RGB). The system is also commonly referred to as "process" colour - a useful reminder that each of the apparent colours the eye perceives does not actually exist on the page but rather is built-up as an illusion.

The extra black plate serves another useful function. Different types of paper will each absorb different amounts of ink. If a particular colour is produced with 100% cyan, 50% magenta and 100% yellow the result - 250% coverage - could lead to over-inking and smearing. Under-colour removal (UCR) could ease the problem by moving the shared 50% component to the black plate. With the extra additional 50% of yellow and black this would produce a total of 150% coverage - and so in this case cut the total amount of ink by nearly half. The problem is that the converted colour is not completely identical to the original so, unless it's strictly necessary, UCR should be avoided. As knowing when to use UCR depends on both the colours and paper to be used, this is really a question to discuss with your printer.

The same factors of paper, press and ink have an even more important role to play. As we've seen ultimately the whole illusion of colour comes down to the placement and size of the halftone dots. The massive advantage of imagesetting is the control offered over these dots, but it's not complete control. In the move to plate or paper it's easy for the all-important size of the dots to change, resulting in the dreaded "dot gain". Even the minutest changes can affect the end results. When your printer comes up with apparently ridiculous excuses for disappointing results it's worth listening. The same ink on different paper really does produce completely different colours. More than this, obscure factors like air humidity really can have an effect so that running the presses in the morning can indeed produce slightly different results than running them later in the day!

The photomechanical uncertainties of ink, paper and press conspire to mean that exact colour matching can never be guaranteed. In fact the situation is far worse. It's not just the end colour that that you can't be sure of - you can't even be confident of the original intended colour! The problem is that the computer screens we design on are built on an entirely different colour model to the printing model. The monitor's additive RGB system means that 100% Red Green and Blue light produce white. The CMYK model on the other hand is subtractive so that increasing inks lead towards black. Crucially there's no one-to-one correspondence between hues in the two systems to make accurate conversion possible. Even worse, many RGB colours simply do not exist in the CMYK range - they are literally unprintable.

With both beginning and end of the CMYK colour printing model mired in intrinsic uncertainty it's hardly an ideal situation. However there's no reason to panic and there are a number of practical steps to be taken. To begin with you should calibrate your monitor to make it as accurate as possible. Then when you choose a colour in your DTP program you should always base it on the CMYK model if your work is going to be colour-separated. Even then you shouldn't trust the screen representation, but refer to one of the readily available printed CMYK palettes.

The most practical step of all though is to build up your understanding of the printing process and its inherent problems. Once you know, for instance, that not all screen colours can be printed you'll understand why you would want to use Photoshop to massage your RGB scans into the CMYK colour space. Likewise if you know that vivid purples are outside the CMYK gamut you'll know not to pick them for your corporate colours - or at least appreciate that you'll need to print a fifth Pantone plate to reproduce them accurately if you do. Such matters are complex but once you know how the illusion of colour is created, they're all completely logical.

Ultimately successful colour-separated print is a trick, but the steps involved in pulling it off are anything but magical.

Tom Arah


Tom Arah



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