The Critically Important Color and Gamma Calibration Article
This article will explain how to understand and work with Photoshop's Color Management System (CMS), including how to create a custom profile, how to create an ICC profile for a monitor, how to set Photoshop's CMS defaults, how to use the Color Settings dialog box, and much more. It is excerpted from Inside Photoshop CS, by Gary D. Bouton. (Sams, 2004, ISBN: 0672326442).
The Critically Important Color and Gamma Calibration Article - Conversion Options: Which Engine to Use (Page 11 of 16 )
Now we'll look at the Conversion Options section of the Color Settings dialog box (item 4 in Figure 2.7). The default color management engine used in new Adobe products is ACE (Adobe Color Engine). Windows 2000 and XP users can choose to use the Microsoft ICM engine instead of Adobe's engine, and Macintosh users can use the ColorSync engine instead, if they prefer. All three engines are similar because all three are based on Linotype AG's LinoColor Color Management System.
At first glance, the Adobe engine provided in all new Adobe products seems like a great choice because having the same engine available on both Macintosh and Windows makes trading files between the two operating systems entirely compatible. But because the Adobe ACE engine can be accessed only by Adobe products, you might not want to use Adobe's color management engine. Color management engines really should belong to the operating system so they are available to all programs that use color management.
In the ideal world, we'd all be 125 pounds, blonde, rich, never flame a jerk in a newsgroup...and only one color management engine would be used, and it would work exactly the same on any operating system. But we haven't reached—and are not likely ever to find ourselves in—such a world. Unless you count Hollywood as a "world."
So what engine should you use? Use the one that is used by the most people who will handle the file. If you, your colleagues, clients, service bureaus, and printers use only Windows ME or higher, choose the Microsoft ICM system; if everyone in the chain uses Macintosh OS systems, choose the ColorSync engine. If your files move across platforms now, or may in the future, your best bet is probably to choose the Adobe ACE engine because Adobe graphics products are the leading products on both platforms and the ACE engine works identically on both platforms. What you want to strive for is consistency. For the purposes of this book, we will assume that you are using the Adobe ACE engine.
Intent, in the context of color management, is not exactly what it sounds like. It does not mean what your plans are. Instead, it asks, "What overall rules do you want to use when you're moving an image from one color gamut to another?" Whenever you change the profile an image uses, the color management engine must somehow decide how the numbers that define the colors are changed, or how the interpretation of those numbers changes to fit within the confines of the new profile. Exactly how this conversion takes place is governed by the source and destination profiles. When the source and destination profiles are created, they usually are assigned a default rendering intent. This default intent can be overridden by applications capable of designating rendering intent, like Photoshop.
Four intents have been defined by the ICC: Perceptual (sometimes called Image), Saturation (sometimes called Graphic), Absolute Colorimetric, and Relative Colorimetric. These four intents are used by all color management engines. Only one of the four intents can be applied during a conversion, but any one of the four could be specified. Which rendering intent you choose as the default intent depends on which qualities of your original image you want to preserve during a color transformation from one gamut to another. As mentioned earlier, it is important to understand what these intents do because you are asked to choose an intent whenever you convert an image's color space, when you choose a custom soft-proofing profile, and when you assign a print profile. A brief description of each of the four intents follows.
Perceptual intent is usually the best choice for working with photographic images. When Perceptual is chosen, the white points of the source and the destination color spaces are matched to each other. Then all the colors in the source space are shifted to new color values that maintain the original relative difference between colors. This means that the actual color values (the numbers) are changed in a way that preserves the overall look of the image rather than preserving the actual colors.
Because photographic and photorealistic images most often are moved from a large RGB editing working space to a smaller RGB or CMYK printing space, either source colors have to be clipped, or the gamut of colors needs to be compressed. The Perceptual intent avoids having to clip colors, which would result in loss of image detail, by desaturating the colors in common between both spaces. Desaturating the common colors produces the room needed to assign color slots to colors that would otherwise be clipped. Consequently, using Perceptual rendering sacrifices absolute color fidelity to preserve detail and the overall look of the image. Perceptual's strategy of using desaturation works particularly well for photographic images that are making the large-to-small color space transition because the human eye doesn't notice the desaturation of colors as much as it notices color clipping or posterization.
When images are being converted the other way around, from a small color gamut to a destination with a larger color gamut, the Perceptual intent would not be the best choice for a photograph or photorealistic image. Because almost all the colors will fit within the new, larger space, desaturation of common colors is no longer necessary to avoid excessive loss of detail due to clipping. Consequently, conversions of photographic and continuous tone images from smaller to larger color spaces usually turn out better if the Relative Colorimetric is chosen for the conversion intent.
It is also important to note, since Perceptual intent maintains the relationships between colors by remapping most, if not all, colors in an image by compressing them to fit into the new gamut, Perceptual would not be the correct choice when the destination gamut is very small—a flexographic newsprint press, for example. In that instance, it would be better to take the clipping hit and try to remap manually the colors that have turned to mud.
Saturation is a good intent to choose for images in which the actual color (hue) is not as important as the purity or distinctiveness of the color. The Saturation intent is most often used for business graphics, such as bar graphs, pie charts, and presentation graphics. These kinds of graphics typically don't require precise color matching; rather, they need non-subtle, easily distinguishable color that makes reading data easy or that doesn't wash out when projected. The rules inherent in the Saturation intent essentially tell the conversion process to focus on producing distinctive colors rather than maintaining an exact color specification. The Saturation intent is also good for re-creating psychedelic posters of the 1960s and for producing cartoons.
Absolute Colorimetric Intent
Absolute Colorimetric is the conversion intent most often used when the most important goal is to ensure that as many colors as possible in the source image are matched exactly in the destination image. Colors that cannot be matched exactly in the destination space will be clipped. White points in the source and the destination color gamuts are not matched.
Color clipping (total saturation of an area in an attempt to render a specific color) will occur during a move from a profile with a large color gamut to a profile with a smaller color gamut, but the colors that do fall within the output gamut are faithfully preserved. When a color is clipped, it is generally moved to the edge of the new gamut, which generally translates as "muddy." Clipping can also take several dissimilar colors and assign them to the same color in the new gamut. With the power of preview soft-proofing, the artist has the opportunity to manually remap those colors that will be clipped before the change occurs. This intent is the best one to use when you are working with corporate logos, spot colors, or other specific colors that must be used in an image.
Relative Colorimetric Intent
Relative Colorimetric intent is our pick for best overall choice, and we're sorry if we made you muddle through the other options—but to be a Photoshop guru, ya gotta know this stuff. Relative Colorimetric intent maps the white point (the hottest point in an image—absolute white) of the source profile to that of the destination profile and then shifts all the colors so that they maintain the same relative position to the white point. Source colors that fall out of gamut in the destination profile's color space are clipped (changed to the nearest color in gamut). Resorting to clipping colors (reducing the number of unique colors) instead of preserving the absolute number of different colors by desaturating some of them is what makes Relative Colorimetric intent different from Perceptual intent.
Relative Colorimetric intent is a good choice if the destination space is capable of producing almost all the colors or if you have done a lot of tweaking to bring colors into the destination's gamut. Examples of this would be if you turned on gamut warning and then used color correction techniques to bring the color used in the image into the CMYK gamut, or if you used only Web-safe colors when creating the image. You should base your default rendering intent on the nature of the images you work with and the kind of output to which these images typically are sent.
The Conversion Options section of the Color Settings dialog box contains two other options, which we'll look at next.
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.