Welcome halftone

Welcome to Astute Graphics' Vector Halftone Week! Each day this week, we will be publishing a significant new vector halftone-based blog article allowing you to both explore the possibilities of using vector halftones in your designs as well as providing free material. Today, this article will introduce what both a Halftone and Vector Halftone is, as well as preview what's coming…

What is a Halftone?

The need for Halftones arose with printing. How was it possible to reproduce an image or photo using a mechanised print machine where tints of any color should be represented on the page? Think of the problems the first people to experiment must have faced; how to reproduce just about any color in the real world using as few inks as possible.

Through years of experimentation and fine tuning, printers developed several techniques, but the classic dot-type halftone became the common technology. By varying the size of the dots, and making them extremely small, it was possible to produce convincing contones in print, where the eye did not notice the dot structure.

Key to the art of reproducing artwork using dots is the ability to display a range of colors. As any color ink can be produced, there is no limit to the range of colors that may be placed on a printed page. But a combination of Cyan, Magenta, Yellow and Black (know as "Key" – hence "CMYK") was found to allow a useful wide color gamut to be reproduced, and therefore re-used as a standard print setup for most print jobs. By overlaying regular grids of dots of varying sizes in each of these 4 inks (also known as "separations", or "plates"), the semi-translucent inks would combine to produce the desired color.

Original image
To illustrate this, the image shown right is an original photo that we want to reproduce as a halftone. This image was chosen as it contains a full range of colors.

When a halftone is applied, for example when printing commercially, below is an example of the outcome and a zoomed-in detail of a small area showing how the image is now constructed from small dots of 4 ink colors:

Halftone zoomed-in detail

Separations stack diagram
The diagram, left, shows how the 4 separation plates are stacked on top of each other to produce the final composite image. You can already notice from this diagram that each ink/separation's halftone "grid" is at a different angle; this is very important in reproducing a smooth full image.

The halftone grid angle of each ink/separation is precisely calculated to ensure that the visible patterns of interacting colors (remember that each ink is semi-transparent) are minimal. Using the the common default angles, also found buried in Illustrator's Print > Output window, is advised to get a subtle "rosette" effect (if you squint, you can make out repeating rosette patterns in the mid-tone areas of the example halftone above). These angles are shown below:

Halftone angles

It's possible to change these angles on PostScript printers – or the Phantasm CS Halftone tool – to produce different patterns. But in doing so, it would be very easy to produce subtle or very strong moiré patterns which can interfere with the eye. Further details about this can be found on Gordon Pritchard's in-depth print blog "Quality in Print".

It would not be practical to detail all technicalities that go into halftones, so if you want to read up further, a reference section may be found at the end of this post.

So, what is a "Vector Halftone"?

Normally, halftones are generated as raster artwork in print RIPs and sent on as plates to a large commercial print device. The resultant raster halftone images (one for each ink used) would be very large in terms of the number of pixels, but wouldn't be too problematic to handle electronically as these images only needed to be "black and white" (ie. a dot or no dot).

By default, Photoshop and Illustrator contains a native and very basic raster-based halftone tool. Try loading a full color image into Photoshop, converting it to CMYK (Image > Mode > CMYK Color) and then opt for Filter > Pixelate > Color Halftone… (OK the default setting). By viewing each channel individually via the Channels panel (Window > Channels), you can see each halftone separation grid. But if you zoom into the image, the raster-based nature of it will mean that it becomes increasingly pixelated. Therefore if you want to reproduce this halftone effect, not only are you very limited to the options offered, but it's unsuitable to scaling up.

This is where Illustrator's vector nature is ideal. In its most basic form, a simple halftone is a grid of circles each of varying size. What Phantasm CS Designer, Studio and Publisher's Halftone tool does is to produce these classic halftones, and more, as individual vector objects. This means that you can zoom in or out endlessly without any loss of quality!

Image to be vector halftoned
To illustrate this, the image shown left is an original photo placed into Illustrator. When using Phantasm CS, it doesn't matter what the underlying artwork is constructed from – vector shapes, images, text, etc. – but in this case it's a bitmap image.

Next, the Halftone Filter of live Effect produces the result shown right. The settings used are producing the classic CMYK halftone "rosette" and is shown here as a screenshot. However, the vector result can be seen if you click on this PDF…. Within your PDF viewing software, zoom again and again into the halftone effect and you'll see the dots remain perfectly sharp at all levels:

It's now possible to print a poster as high as a sky-scaper and if you stand back far enough, you would see the original image once more!

Expanding on the possibilities

The standard halftone effect is all very nice, but there's a lot more potential to vector halftones. What if the dots were replaced by other shapes such as squares, lines, ovals, text characters, stars… or anything? This is where the power of Phantasm CS' Halftone tool and Illustrator combine to provide endless creative possibilities.

By being in control of every aspect, including the dot shape, size, aspect ratio, dot gain (the scale of each dot relative to the underlying artwork's tint), color type and much more, the following vector results of the previous example image are within easy reach:

Example of possibilities

(PDF versions of these examples, and more, may be found on this page…)

The most important aspect, however, remains the vector nature of all these results.

What are the limitations?

By reproducing artwork as a series of dots or shapes, you can – and often do – end up with a massive array of objects in a single Illustrator document. This isn't a problem as far as object control is concerned; if using the tool as a live Effect, you don't have to even see the individual objects unless subsequently expanded. But internally Illustrator has to control lots and lots of shapes, often set to be transparent. Therefore if you use a high resolution, or halftone a dimensionally-big bit of artwork, Illustrator can slow down or even (politely) refuse to render the result.

Vector halftones were primarily intended for creative tasks. Having made that point, some customers do use it for specialist screening jobs, including security printing as the tool is designed to professional grade.

In terms of technically reproducing artwork types, there are very few limits. The Phantasm CS Halftone tool will correctly allow precise control of screen angles, dot gain, dot shape (you can even use any custom Illustrator Symbol as a dot shape), spot colors, etc.

What's to come this week…

What is coming

We've laid on a series of great vector halftone posts:

We hope you'll join us throughout the week…

References

If you want to find out more about halftones – and there is much to learn! – the following references should be of use:

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I didn't know Illustrator could do that!

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