The RGB Working SpaceSo what are these strange things, the RGB working spaces, that you choose from the Color Settings RGB menu? They're arbitrary, device-independent RGB spaces. Some real techno-geeks will quibble with applying the term "device-independent" to an RGB space, preferring to reserve the term for purely synthetic perceptually based color spaces like CIE Lab. To those folks, we suggest that while a useful distinction can be made between perceptually based spaces and RGB spaces, that distinction does not revolve around device independence. Photoshop's RGB working spaces don't depend on the vagaries of any given piece of hardware, so we feel it's truthful to call them "device-independent RGB." Why Use RGB Working Spaces?The RGB working spaces built into Photoshop are designed to provide a good environment for editing images. As such, they have two important properties that aren't shared by the vast majority of device spaces.Gray balance. The working spaces are gray-balanced, meaning simply that equal amounts of R, G, and B always produce a neutral gray. This is hardly ever the case with device (scanner, camera, display, printer) spaces. Since one of the easiest ways to bring color into line is to find something that should be neutral, and make it so, gray balance is an extremely useful property.Perceptual uniformity. The working spaces are approximately perceptually uniform, meaning that changing the numeric values in the image by the same increment results in about the same degree of visual change, no matter whether it's in the highlights, the midtones, the shadows, the pastels, or the saturated colors. Again, device spaces generally don't work that way. All color space conversions entail some data loss, but the conversion from capture (camera or scanner) to working space is, in our experience, invariably worthwhile, and when it's done in 16-bit/channel mode, the loss is so trivial it's just about undetectable. Even in 8-bit/channel mode, you're likely to produce much better results editing in a working space rather than a device space. Why not just use Lab?After all, Lab is by design a device-independent, perceptually uniform color space. But Lab has at least two properties that make it less than ideal as a standard editing space.First, Lab is pretty nonintuitive when it comes to making color correctionssmall adjustments to a* and b* values often produce large changes in unexpected directions. A bigger problem, however, is that Lab, by definition, contains all the colors you can see, and as a corollary, it also contains many "colors" you can't see.When we use eight bits per channel to represent this whole range of color, the distance from one value to the next becomes largeuncomfortably large, in fact. And since any real image from a scanner or digital camera contains a much smaller range of color than Lab represents, you wind up wasting bits on colors you can't capture, display, print, or even see. If you work with 16-bit-per-channel images, the gamut problem is much less of an issue, but editing in Lab is still not particularly friendly, and conversions from capture space to Lab generally involve more data loss due to quantization error than the conversion to RGB working spaces. Choosing an RGB Working SpaceThe main difference between RGB working spaces is the gamut sizethe range of color that they can represent. You may think you should just choose the largest gamut available so that you'll be sure of encompassing the gamuts of all your output processes, but (as is almost always the case in digital imaging) there's a trade-off involvedat least if you're using 8-bit-per-channel images.As we explained in Chapter 3, Image Essentials, RGB images are made up of three grayscale channels, in which each pixel has a value from 0 to 255. This holds true for every 24-bit RGB image, irrespective of the working space it lives in. If you choose a very large-gamut space, the 256 possible data values in each channel are stretched to cover the entire gamutthe larger the gamut, the further apart each value is from its neighbors.The practical implication is that you have less editing headroom in a large-gamut space than you do in a small one: When you edit images, you invariably open up gaps in the tonal range as levels that were formerly adjacent get stretched apart. In a small-gamut space, the jump from level 126 to level 129 may be visually insignificant, whereas in a larger space, you'll get obvious banding rather than a smooth transition.The simplest option is to settle on a single RGB editing space for all your work, but you may wish to use a larger space for 48-bit images than you do for 24-bit ones, or a larger space for digital captures than for film scans. In a service bureau environment, you'll have to support all sorts of RGB spaceswhich is easy in Photoshop CS2in which case the default working space should simply be the one you use most.That said, we've probably been guilty in the past of overstating the dangers of large-gamut working spaces. If you're working with legacy images that have already been edited in a small-gamut space, or JPEG digital captures, there's no good reason to convert them to a larger space, and if you do you may encounter some of the aforementioned issues. But if you work with scans from modern scanners, or raw captures from today's digital cameras, large-gamut spaces are not only safe, but may be needed to do full justice to the image. Bruce has been using ProPhoto RGB, which has a very large gamut indeed, for almost all his work for the past seven years, and has found that images are no more likely to fall apart in ProPhoto RGB than in smaller spaces. Gamut Size RevisitedThe naive presumption is that since RGB spaces are bigger than CMYK, they can hold all the CMYK colors we can print. That's not really the case. RGB color spaces all have a characteristic three-dimensional shape, where maximum saturation happens at fairly high luminance levels. Print spaces have a different characteristic gamut shape, where maximum saturation is reached at lower luminances. If you bear in mind that you increase RGB saturation by adding light, and you increase print saturation by adding ink, this makes perfect sense.Two-dimensional gamut plots disguise this fact, which is why they can be seriously misleading. Three-dimensional gamut plots make the relationships between gamuts much clearer. Figure 5-5 shows three views of Adobe RGB and U.S. Sheetfed Coated v2 CMYK plotted in three dimensions. The differences in size are obvious, but note the difference in shape. Figure 5-5. 3D gamut plotsFigure 5-6. Adobe RGB and Epson UltrachromeFigure 5-7. Working RGBs and U.S. Sheetfed Coated v2[View full size image] Monitor RGBSoft-Proofing Controls," later in this chapter). Other RGB Working SpacesAssign Profile" later in this chapter) to assign a profile other than the working space to an imagebut if you're going to be working with a bunch of images in the same space, it makes life slightly easier to load that space as the working space. The following is by no means an exhaustive list, but we've found each of these spaces useful. ProPhoto RGBFormerly known as rgbMaster, and before that as ROMM (Reference Output Metric Method) RGB, Kodak's ProPhoto RGB is an extremely wide-gamut RGB space, so wide that its primaries are imaginarythere is no light source that could produce these colors, and we couldn't see them if there was. It needs these extreme primaries to be able to accommodate the dark saturated colors we can readily achieve in print and that get clipped by smaller spaces. It's wide enough that we recommend doing major edits only on high-bit filessmall tweaks to 8-bit-per-channel files, however, are safe.Bruce uses it for most of his work, especially for digital captures converted through Camera Raw. Before he switched from film to digital capture, Bruce was hesitant to make ProPhoto RGB his unequivocally recommended working space for two reasons:It's slightly less convenient than any of the four recommended spaces, in that you have to press the More Options button in Color Settings before you can choose it from the RGB working space menu.If you deliver ProPhoto RGB to someone who doesn't use color management correctly, the results will be much worse than if you deliver the color in a smaller space (though the results may still be pretty bad). With the advent of digital capture, though, a relatively new problem crops up. Digital does a much better job of capturing dark saturated colors than film ever did, and converting digital raw to Adobe RGB or a smaller space often turns the gradations in things like dark green foliage or dark brown wood into solid blobs.Figure 5-8 shows a relatively innocuous image converted into ProPhoto RGB via Camera Raw, with no saturation boost and only very mild tonal adjustments. Many of the foliage and flower colors lie well outside the gamut of Adobe RGB, and when the image is converted to Adobe RGB or a smaller space, all these out-of-gamut colors get clipped to the gamut boundary, wiping out subtle detail. Figure 5-8. Color spaces and input clipping[View full size image] EktaSpaceDeveloped by photographer Joseph Holmes, EktaSpace is a large-gamut space that's a little more conservative, and hence a little more manageable, than ProPhoto RGB. There's been some debate over whether EktaSpace really covers the entire E6 gamut. Our experiments suggest that, while it may be possible to capture colors in-camera (without resorting to games like exposing the film with a monochromatic laser) that will be clipped by EktaSpace, it's not likely. EktaSpace will hold any colors you're likely to encounter on E6 film that's shot and processed under normal conditions. It's possible to use 8-bit-per-channel images in EktaSpace, but you'll get much more editing headroom with 16-bit files.Bruce uses EktaSpace for transparency scans when it's important to preserve the characteristics of the individual film stock. He finds it's easier to do this in EktaSpace than in any other space he's tried. You can download it from [ www.josephholmes.com ]. BruceRGBUnlike both ProPhoto RGB and EktaSpace, BruceRGB is a small-gamut space. Bruce designed it to offer the maximum editing headroom on 8-bit-per-channel images destined for CMYK printing. It's basically a compromise between Adobe RGB (1998), which is a little too big, and ColorMatch RGB, which is too small. It covers most of the gamut of CMYK offset printing and is a reasonable match to most RGB inkjet printers. It clips some cyans and oranges, but not much more so than Adobe RGB.Nowadays, Bruce uses it only as an "emergency" space, mostly for legacy images that were scanned on flatbed scanners in the early 1990s and then edited to within an inch of their lives. He notes that today's scanners and cameras are so much better than those from 1998, when he developed the space, that he has much less need for BruceRGB. But it still comes in handy for those rare images that simply fall apart in larger spaces. You can define it yourself using the Custom RGB setting (see the next section). Custom RGB SpacesIf you're a hard-core imaging geek who likes to live dangerously, you can define your own RGB working space. It's not that difficult, because an RGB working space is defined by just three primary xy values for red, green, and blue; a white point; and a gamma value. For example, if you use a high-quality scanning-back digital camera that lets you set the gray balance for each image, you may want to define a working space whose primaries are the same as the camera's, thereby (in theory) ensuring that your working space matches your input device.To define a custom RGB working space, you first need to click the More Options button in Color Settings. This lets you choose Custom RGB from the RGB menu, which in turn opens the Custom RGB dialog box (see Choosing a CMYK Working Space." Figure 5-9. Custom RGB[View full size image] GammaThe Gamma field lets you enter a value for the gamma of your working space. This is completely independent of your monitor gammathere's no reason to match your working space gamma to your monitor gamma, and there may be plenty of good reasons not to do so. To oversimplify (the long explanation would be very long), the gamma of the editing space controls the distribution of the bits over the tone curve. Our eyes don't respond in a linear fashion to changes in brightness: A gamma of 2.2 is generally reckoned to be more or less perceptually uniform, so we recommend using that value for your working space gamma. It has the added benefit of devoting more bits to the shadows, which is where we find we usually need them during editing. White PointThis setting defines the white point of the RGB working space. You can choose one of the ten built-in white points, or choose Custom to define a custom white point by entering xy chromaticities (the xy components of a color defined in CIE xyY). As with the gamma setting, the white point of the working space is quite independent of the monitor white point. It's also independent of the output white point. For a variety of reasons, we suggest using D65 as the white point for most RGB spaces. (For a more detailed discussion of white points, see the sidebar "How White Are Your Whites?" earlier in this chapter.) If you use one of the built-in spaces, this setting will be made for you automatically. PrimariesThe Primaries setting lets you choose from the Primaries menu, which contains six sets of phosphor-based primaries for common monitors and three sets of abstract primaries. Or you can enter custom xy values for R, G, and B to set the boundaries of the color gamut.Tip: Finding the xy Values for PrimariesTo find the xy chromaticities of the primaries for a chosen built-in space, first load that space from the RGB menu, and then choose Custom from the Primaries menu. The Primaries dialog box appears, showing the xy chromaticities for the red, green, and blue primaries. If you wish, you can even plot them on a chromaticity chart like the one in Figure 5-10. Figure 5-10. xy chromaticities[View full size image] Defining custom RGB spaces isn't for the faint of heart, and there are now so many RGB spaces available that relatively few people should need to build their own. But it's always nice to know that you can.Tip: Custom RGB Settings and ICC ProfilesWhen you save custom RGB settings, you're actually creating an ICC profile. On Mac OS X, save them in the Library/ColorSync/Profiles folder. On Windows 2000/XP, save profiles in the WinNT/System/Spool/Drivers/Color directory. That way, the profile that describes your RGB working space will be readily available to other ICC-aware applications.Figure 5-10 shows a chromaticity plot of some of Photoshop's built-in spaces, compared with the chromaticities of SWOP inks. A word of caution: color gamuts are complex three-dimensional objects, and a chromaticity plot is very much an abstraction. We include this figure primarily as a visualization tool to help you get your head around the implications of different RGB primaries, not as an exact comparison of their color gamuts. RGB OutputIt's possible to load an RGB output profile as your working space. This may even seem like a good idea if you're one of the many Photoshop users whose final output is a desktop inkjet printer. But even if you only ever print to an RGB printer, using your RGB output profile as your RGB working space is a bad idea. RGB output spaces have two properties that make them very difficult to use as working spaces: They're rarely gray-balanced, and they're far from perceptually uniform.
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