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By Root 1969 0

that can extend the printing color gamut beyond the conventional limitations of CMYK. This advanced printing process is currently available only through specialist print shops and is suitable for high quality design print jobs. Millions have been invested in the four-color presses currently used to print magazines and brochures, so expect four-color printing to still be around for a long time to come, but Hexachrome will open the way for improved color reproduction from RGB originals. Photoshop supports six-color channel output conversions from RGB, but you will need to buy a separate plug-in utility like HexWrench. Multimedia publishing is able to take advantage of the full depth of the sRGB color range (which is probably about the limit of most LCD displays). If you are working in a screen-based environment for CD, DVD and web publishing, RGB is ideal. And with today's web browsers, color management can sometimes be turned on to take full advantage of the enhanced color control these programs can now offer.

Output-centric color management

Printers who work in the repro industry naturally tend to have an ‘output-centric’ view of color management. Their main concern is getting the CMYK color to look right on a CMYK press and printers can color correct a CMYK image ‘by the numbers’ if they wish. Take a look at the photograph of the young model in Figure 12.3. Her Caucasian flesh tones should contain equal amounts of magenta and yellow ink, with maybe a slightly greater amount of yellow, while the cyan ink should be a quarter to a third of the magenta. This rule will hold true for most CMYK press conditions and the accompanying table compares the CMYK and RGB space measurements of a flesh tone color. However, you will notice there are no similar formulae that can be used to describe the RGB pixel values of a flesh tone. If you were to write down the flesh tone numbers for every RGB device color space, you could in theory build an RGB color space reference table. From this you could feasibly construct a system that would assign meaning to these RGB numbers for any given RGB space. This is basically what an ICC profile does except an ICC profile may contain several hundred color reference points. These can then be read and interpreted automatically by the Photoshop software and give meaning to the color numbers.

Figure 12.3 The tables shown here record the color readings in the CMYK and RGB color spaces of a typical Caucasian flesh tone. As is explained in the text, while the CMYK readings are all fairly consistent, this won't be the case if you try to compare the RGB values.

Profiled color management

The objective of profiled color management is to use the measured characteristics of everything involved in the image editing workflow, from capture through to print, to reliably translate the color at each stage of the process. In a normal Photoshop workflow, the color management begins with reading the profiled RGB color data from the incoming file and, if necessary, converting it to the current Photoshop RGB workspace. While an RGB image is being edited in Photoshop the workspace image data is converted on-the-fly to the profiled display space and sent to the computer display, so that the colors are viewed correctly. When the image is finally output as a print, the RGB workspace data is converted to the profile space of the printer. Or, you might carry out an RGB to CMYK conversion to the CMYK profile of a known proof printer.

A workspace profile is therefore a useful piece of information that can be embedded in an image file. When a profile is read by Photoshop and color management is switched on, Photoshop is automatically able to find out everything it needs to know in order to manage the color correctly from there on. Note that this will also be dependent on you calibrating your display, but essentially all you have to do apart from that is to open the Photoshop Color Settings from the Edit menu and select a suitable preset such as the US Prepress Default setting. Do this and you are all set to start working