The Critically Important Color and Gamma Calibration Article - Black Point Compensation
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The tonal range of an image is determined by the number of intermediate grays the image contains between pure white (the white point) and pure black (the black point). ICC profiles have rules that govern how and when white points are matched to each other when conversions take place, but surprisingly, they don't have rules about how black points should be matched. When black points are not considered during an image's color space conversion, the translation between color spaces does not always look as good as it should or would if the black points had been evaluated. Adobe has developed a fudge factor, called black point compensation, that evaluates source and destination black points and then makes corrections to help ensure that the blacks in the converted file aren't blocked in or washed out. But like most workarounds, it doesn't suit all situations. The rule of thumb commonly used is that the Use Black Point Compensation option should be enabled when you're converting an image from RGB to CMYK or from one CMYK profile to another, and that it should not be enabled when you're converting from one RGB space to another.
Note - Controlling contrast
The human eye is more sensitive to tonal changes in the low end of the spectrum than it is to changes approaching the white point. This is another reason it is important to control the contrast of darker tones.
Black point compensation should also be avoided when the conversion from RGB has a destination gamut in which the paper and inks used have a low black density, such as CMYK newsprint, which has a washed-out black.
Use Dither (8-Bit per Channel Images)
Dither refers to a process (dithering) that uses different colored dots, shapes placed close to each other, or patterns made up of different colors, to fool the viewer's eye into seeing a color that is not actually there. When small specks of colors are intermingled, the brain blends the viewed colors together and interprets them as the color that would be produced if the colors were actually mixed together. This phenomenon of human vision is what makes both the painting style of Pointillism and CMYK halftone printing work. Activating the Use Dither (8-bit per channel) option enables profile-conversion processes to use dithering to reduce the perceived amount of banding that is caused by color clipping. This is a good option to use, but it will increase file size and make file compression techniques less effective.
Advanced Controls
The last set of options in the Color Settings dialog box is Advanced Controls, consisting of Desaturate Monitor Colors By and Blend RGB Colors Using Gamma (item 5 in Figure 2.7). Adobe recommends that only advanced users use the first option, but we're not sure that it is useful even for advanced users. Desaturating the monitor colors by a user- definable percentage could, if you have a really good imagination, give you a general idea of what colors that cannot be displayed on the monitor might look like when output. The second option, Blend RGB Colors Using Gamma, is more useful than desaturating your monitor, but only if a specific image will be created and output from Photoshop. The default gamma setting of 1.0 for this option produces slightly better color choices on the edges of sharp color transition in RGB images. You probably shouldn't bother to enable either of these options.
Wow! We've looked at all the options in this dialog box. You might want to recalibrate your eyeballs now! The only other thing to do is to click the Save button to save a color settings file if you've made changes that aren't the preset color settings files. And that, Fellow Photoshoppist, brings us back to where we were before we started looking at the settings in this dialog box.
Choosing Between Assigning and Converting to Profile
Although you have specified the default color settings and set the Color Management Policies in the Color Settings dialog box, it is not the only place where you can assign profiles to images. The Image, Mode menu is another. It has two very important entries, Assign Profile and Convert to Profile, that perform very different functions. And it is quite important that you understand the difference.
When we look at an image we've created and saved, it is irresistible not to imagine that an actual image of some sort exists inside the file image—a cyber version of a photographic print. In actuality, the image is just a bunch of numbers that represent the individual flecks (pixels) of color. Photoshop reads these numbers, figures out what to do with them in terms of color, and then puts them onscreen. Our eyes and our brain then take in all the bits of color and decide what they represent, what they look like. Is it a representation of a loved one or just a splash of color?
Some of the rules Photoshop uses to figure out what to do with the numbers it finds in image files are found within Photoshop's own program code. This part of the process of making an image out of numbers is out of Photoshop users' hands. It reflects the logic, decisions, and preferences of the programmers who wrote the Photoshop application. If a CMS is used, Photoshop's actions are guided and modified by an additional set of rules: the rules laid out in ICC profiles that are used in concert with the image. You decide which profile rules are associated with an image file.
 This chapter is from Inside Photoshop CS, by Gary D. Bouton. (Sams, 2004, ISBN: 0672326442). Check it out at your favorite bookstore today. Buy this book now.
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