Publishing tip #7: Grayscale conversion and desaturation
This article is published as part of the exclusive AG blog series Avoiding 10 Illustrator Publishing Pitfalls 
A common requirement for designers and publishers is to convert artwork into pure grayscale, or “greyscale” if you’re on our side of the pond. This process may at first glance appear to be simple and fool-proof using Illustrator’s native command introduced in Illustrator CS3. (Sorry Illustrator CS2 users; the is not as advanced, disallowing certain objects including gradients to be converted.) But this simple approach does not always produce the best results. Other native Illustrator methods are available, which will be discussed.
The other major aspect of this article, as indicated by its title, is the production of “warm” desaturated results as catered for with the Desaturate tool found in the Phantasm CS range. Technically, true “grayscale” artwork only exists in the black separation, using only black ink (or any other single ink at output stage). However, it’s sometimes more suitable to produce artwork which appears to be monochrome like grayscale, but contains a mix of inks to add some perceived warmth.
Illustrator’s own grayscale conversion methods
Ryan Putnam wrote an excellent article in his Vectips blog entitled Four Ways to Convert to Grayscale. This is a recommended read on the different ways to convert vector artwork to grayscale directly within Illustrator. Unfortunately, for this article, the two methods using the Recolor Artwork tool are discounted as it does not allow conversion of embedded images, which is often of critical importance.
The two methods native to Illustrator that converts all embedded object types including vector, text and images are and . The latter produces the following control window:
In order to produce grayscale artwork, you need to select from the pull-down menu, opt for and preferably tick so that you can see the changes before committing.
The advantage this tool has over the simpler Convert to Grayscale method is that it’s possible to lighten the result by sliding the control left (negative percentage) or darken it by sliding it right (positive percentage). If maintained at a value of 0%, the results from both methods are identical.
That’s it then?
Unfortunately, you may find the results slightly different to what you expect from the equivalent in Photoshop. To illustrate this discrepancy, we’ll be using the following example artwork throughout the article:
The artwork consists of the main background image, which was converted to CMYK in Photoshop and contains no spot inks or transparency masks (ie. as simple as possible). The left and right sides feature expanded blends between colors picked from within the image using Illustrator’s Eyedropper tool. Therefore the image contains both vector and raster objects for a more comprehensive test. Also, all tests in this article have been conducted from within Illustrator CS5, although no differences were detected in operation compared to Illustrator CS3 or CS4.
By selecting all objects and opting for , we get the following result:
The results immediately appear to be very contrasty, especially the background image. To compare to Photoshop’s conversion based upon an identical, albeit purely raster copy of the same artwork using , see the result below:
The method Illustrator uses to arrive to its result is not immediately clear. Other experiments involved using the tool and choosing (plus the live Effect equivalent), as well as using the CMYK image as the basis of an Opacity Mask for a pure black rectangle. Each method within Illustrator resulted in the same contrasty image. Checks were also made using different images – some converted from RGB to CMYK within Illustrator, others using Photoshop – and changing the color profile assigned to the document (). Color management issues were ruled out by checking the gray levels of the results rather than just visually comparing on-screen which can be affected by the toggling of Overprint Preview and the settings defined in .
A glimmer of hope came when the grayscale conversion process took place in an RGB Illustrator document. This resulted in much better tonal results and closer to those found in Photoshop’s results. However, when converting back to CMYK by changing the document’s color mode, the result lightens up due to the interpretation of rich RGB black to pure CMYK black – a topic that has been touched-upon throughout this series.
Alternative conversion methods
If working with images within Illustrator, and a conversion to pure grayscale is necessary, the best advice is to convert using Photoshop. There is obviously the caveat that the image has to still be externally stored (linked images can be converted directly using Photoshop) or you have Phantasm CS Studio or Publisher to in-line edit embedded images (covered in a previous article).
This, however, doesn’t solve conversion to grayscale of vector artwork or maintaining a workflow within Illustrator.
An alternative approach to convert to grayscale comes with the Phantasm CS range. Desaturate may appear to be the simplest tool within the plugin’s color toolbox, but it does allow for the creation of both pure grayscale artwork or a warmer desaturated result. Critically, the Filter and live Effect variants (the latter allowing you to remove the conversion at any later stage) operate with all embedded object types including vector, text and images.
By following either in Illustrator CS2 and CS3, or with Illustrator CS4 or above, or the live Effect equivalent found at , the window shown above may be found.
The option Gray Tone is only available in CMYK documents. For the purpose of this initial test, the Filter version of the Desaturate tool will be used and ticked. The original example produces the following result:
It is important to note that Desaturation is not the same process Photoshop uses to derive a grayscale result and therefore the result will not be identical. However, it appears tonally improved compared to Illustrator’s contrasty method of conversion. (Phantasm CS does provide Brightness/Contrast, Curves and Levels tools allowing further adjustment to suit, if required.)
Whereas the Gray Tone option produces purely grayscale objects – ie. defined as grayscale within Illustrator and therefore only using the Black ink channel, if used within a CMYK document, it can sometimes be visually nicer to produce a warmer, desaturated result. This is achieved by un-ticking to gain the following:
It must be noted that the result now produces separations across all process channels and therefore requires a full process print run to reproduce:
Other desaturation methods
Both forms of desaturation found in the Desaturate tool are simple short-cuts to what’s possible using Phantasm CS’ Hue/Saturation tool. For purely grayscale artwork (the equivalent of the Gray Tone option), open the Hue/Saturation Filter or live Effect window and choose the method, then set the level to 0 as shown below:
Equally, if you wanted to achieve the warmer tone possible in the Desaturate tool, re-visit the Hue/Saturation tool, untick and set the level to -100:
Experimenting further with the Hue/Saturation tool allows for alternative desaturated colors rather than the default warm earthy hue produced above. The best method is via the Colorize mode where the Saturation level is not quite zero – for example, a level of 10. The Hue then controls the result’s color mood. In the example below, a Hue level of 210 produces a steely cold desaturated result which is closer in appearance to pure grayscale but is visually richer…
This richness usually translates well to print where a combination of all 4 inks produces a less harsh result, often representing tonal change more subtly.
Conclusion
It’s hoped that this guide is of some help in picking the right approach to producing grayscale or desaturated objects in your artwork. Apart from a necessity to produce purely grayscale results due to print requirements, there’s no absolute right or wrong method. Experiment and see what looks best for the artwork you’re working on.
Example image courtesy of Fotolia, supplied freely with Computer Arts issue 130.