Adobe Photoshop CS5 for Photographers - Martin Evening [285]
Conversion options and rendering intents
You have a choice of three Color Management Modules (CMMs): Adobe Color Engine (ACE), Apple ColorSync or Apple CMM. The Adobe color engine is reckoned to be superior for all RGB to CMYK conversions because the Adobe engine uses 20-bit per channel bit-depth calculations to calculate its color space conversions.
The rendering intent influences the way the data is translated from the source to the destination space. The rendering intent is like a rule that describes the way the conversion is calculated and we will be looking at rendering intents later on pages 670–673.
Black Point Compensation
This maps the darkest neutral color of the source RGB color space to the darkest neutrals of the destination color space. Black Point Compensation plays a vital role in translating the blacks in your images so that they reproduce as black when printed. As was explained in Chapter 3, there is no need to get hung up on setting the shadow point to anything other than zero RGB values. It is not necessary to apply any shadow compensation at the image editing stage, because the color management will automatically take care of this for you and apply a black point compensation obtained from the output profile used in the mode or profile conversion. If you disable Black Point Compensation you may obtain deep blacks, but you will get truer (compensated) blacks if you leave it switched on.
You will want to use Black Point Compensation when separating an RGB image to a press CMYK color space. However, in the case of a conversion from a CMYK proofing space to an inkjet profile space, we must preserve the (grayish) black of the press and not scale the image (because this would improve the blacks). For this reason Black Point Compensation is disabled in the Print dialog when making a proof print to simulate black ink.
Use Dither (8-bit per channel images)
Banding may occasionally occur when you separate to CMYK, particularly where there is a gentle tonal gradation in bright saturated areas. Any banding which appears on the display won't necessarily always show in print and much will depend on the coarseness of the screen that's eventually used in the printing process. However, the Dither option can help reduce the risk of banding when converting between color spaces.
Blend RGB colors using gamma
This option allows you to override the default color blending behavior. There used to be an option in Photoshop 2.5 for applying blend color gamma compensation. This allowed you to blend colors with a gamma of 1.0, which some experts argued was a purer way of doing things, because at higher gamma values than this you might see edge darkening occur between contrasting colors. Some users found the phenomenon of these edge artifacts to have a desirable trapping effect. However, many Photoshop users complained that they noticed light halos appearing around objects when blending colors at a gamma of 1.0. Consequently, gamma-compensated blending was removed at the time of the version 2.5.1 update. If you understand the implications of adjusting this particular gamma setting, you can switch it back on if you wish. Figure 12.32 illustrates the difference between blending colors at a gamma of 2.2 and 1.0.
Figure 12.32 In this example we have a pure RGB green soft-edged brush stroke that is on a layer above a pure RGB red Background layer. The version on the left shows the combined layers using the normal default blending where ‘Blend RGB Colors using Gamma’ is deselected, and the version on the right shows what happens if you check this item and apply a gamma of 1.0. As you can see, the darkening around the edges where the contrasting colors meet will disappear.
Desaturate monitor colors
The ‘Desaturate Monitor Colors By’ option helps you to visualize and make comparisons between color gamut