Adobe Photoshop CS5 for Photographers - Martin Evening [289]
Undercolor Removal (UCR)
The UCR (Undercolor Removal) separation method replaces the cyan, magenta and yellow ink with black ink in just the neutral areas. The UCR setting is also favored as a means of keeping the total ink percentage down on high-speed presses, although it is not necessarily suited for every type of print job.
Choosing a suitable RGB workspace
The RGB space you choose to edit with can certainly influence the outcome of your CMYK conversions, which is why you should choose your RGB workspace carefully. The default sRGB color space is widely regarded as an unsuitable space for photographic work because the color gamut of sRGB is actually smaller than the color gamut of CMYK (and that of most inkjet printers). If you choose a color space like Adobe RGB or ProPhoto RGB, you'll be working with a color space that can adequately convert from RGB to CMYK without significantly clipping the CMYK colors. Adobe RGB has long been a favoured space for professional photographers and you should really notice the difference here if you are able to view the photos you are editing on a decent display like the high-end Eizo or NEC displays I mentioned earlier in Chapter 2. These are capable of displaying something like 98% of the Adobe RGB gamut and therefore just about all the gamut of a typical CMYK space. It can make a big difference if you can use such a display to accurately preview the colors you are editing and soft proof them for print (see Chapter 13).
Which rendering intent is best?
If you are converting photographic images from one color space to another, then you should mostly use the Relative Colorimetric or Perceptual rendering intents. Relative Colorimetric has always been the default Photoshop rendering intent and is still the best choice for most image conversions. However, if you are converting an image where it is important to preserve the shadow colors, then Perceptual will often work better. For these reasons, I recommend that you use the Soft proofing method described in the following chapter to preview the outcome of any profile conversion and check to see whether a Relative Colorimetric or Perceptual rendering will produce the best results.
Rendering intents
Whenever you make a profile conversion, such as when converting from RGB to CMYK, not all of the colors in the original source space will have a direct equivalent in the destination space. RGB spaces are mostly bigger than CMYK and therefore those RGB colors that are regarded as being ‘out of gamut’ will have to be translated to their nearest equivalent color in the destination CMYK space. The way this translation is calculated is determined by the rendering intent. In the Color Settings dialog you can choose which rendering intent you would like to use as the default method for all color mode conversions (Figure 12.38), but you can also override this setting and choose a different rendering intent whenever you use the Edit Convert to Profile command (Figure 12.39), or soft proof an image using View Proof Setup Custom (Figure 12.40). I have provided here a guide to the four main rendering intents you'll find in Photoshop and their suggested uses.
Figure 12.38 The default rendering intent is set by choosing ‘More Options’ in the Color Settings dialog and mousing down on the Intent menu in the Conversion Options.
Figure 12.39 The default rendering intent setting can be overridden when using the Convert to Profile command.
Figure 12.40 You can also change the rendering intent in the Custom Proof Condition dialog. This allows you to preview a simulated conversion without actually converting the RGB data.
Perceptual
Perceptual (Images) rendering is an all-round rendering