Out of Gamut: Photoshop, Previsualization, and Print Prediction
In my last column, I addressed the question, "How can I get Photoshop to make my prints match what I see on my monitor?" -- and broke the sad news that neither you, I, nor anyone else can actually do that. Monitors can display colors that are either not reproducible in print, or are only reproducible by using spot inks on a press. Color management can't change the physical limits of your printing process. What it can do, though, is to get your monitor to show you how your print will appear, and let you take any corrective action necessary to optimize the appearance of the image on the print.
Photoshop's Proof Setup feature is capable of providing very accurate previews of your prints. But (there's always a "but") to do so it needs profiles that describe the behavior of your monitor and your printer accurately (more on this later). I'll assume that you've already obeyed the exhortations in my aforementioned last column to calibrate and profile your monitor, and to set your color management policies correctly in Color Settings, so that you know you have images whose on-screen appearance is satisfactory. This column looks at getting the images from the screen into hard copy, whether the hard copy is being printed on a desktop printer or on a press.
Print Prediction through Proof Setup
Photoshop's Proof Setup feature offers a very powerful and flexible way to preview prints, whether you're printing to a grayscale, RGB, or CMYK printing process. (Desktop color printers that don't have a Postscript RIP have to be treated as RGB printers even though they use CMY, possibly K and possibly light cyan and light magenta inks.) Proof Setup lets you control the way the image color is converted to the simulated printing space, and also lets you control the way that simulation is rendered back to your monitor. By default, it's set to simulate your CMYK working space. If you're printing to something other than your default CMYK, you need to create a custom setup, and even if you're printing to your default CMYK, you may want to create a custom setup to use some of the more advanced controls. Here's how:
Choose Proof Setup>Custom from Photoshop's View menu. The Proof Setup dialog box appears (see figure 1). Let's look at the various controls it offers.
Figure 1: Photoshop's Proof Setup dialog box gives you a powerful way to preview prints.
The Setup menu, which shows "Custom" when you first open the dialog box, simply lists saved Proof Setups for easy recall (they also appear at the foot of the Proof Setup submenu).
The Profile menu is where you choose the profile for the printer or print process you want to simulate on screen.
The Intent menu lets you choose the rendering intent you want to use to go from the image's color space to the printer's color space. Even better, since Proof Setup offers a live preview, you can see the effect of each rendering intent on the image (see figure 2).
Figure 2: Proof Setup lets you preview the effect of different rendering intents, such as Perceptual rendering (top) and Relative Colorimetric rendering (bottom).
Lurking in between the Profile and Intent menus is a checkbox labeled "Preserve Color Numbers." Normally, you want to leave this unchecked, if it's available. When checked, it shows you what the image would look like if you just sent it to the device whose profile you chose without doing any conversions. (Checking it also dims the Intent menu, because since you're not doing a conversion, rendering intents don't apply.) It's only enabled when you choose a profile from the Profile menu that uses the same color mode as your image -- if your image is RGB, the checkbox is only available when you choose an RGB printer profile, and with a CMYK image it's only available when you choose a CMYK profile.
In the case of RGB printers, "Preserve Color Numbers" typically just shows you why you need to convert the image to the printer profile's space. With CMYK images, it can be useful in helping you determine whether or not an image that was prepared for one printing process may or may not be usable on another.
The Profile and Intent menus let you control how your image gets from the space it's in to the print space you're simulating. The remaining two controls, in the Simulate section of the dialog box, let you control how that simulation is rendered back to your monitor.
When both Paper White and Ink Black are unchecked, Photoshop uses Relative Colorimetric rendering with Black Point Compensation to go from the simulation to your monitor. What that means in English is that the paper white is mapped to your monitor's white, and the printer's black is mapped to your monitor's black. If you're printing to a bright glossy stock, this view is probably the most accurate. But if you're printing to a matte paper it may give you an overly optimistic view (see figure 3).
Figure 3: Leaving Paper White and Ink Black unchecked gives you the best simulation for printing on bright, glossy stock.
The Ink Black checkbox turns off black point compensation in the rendering from the simulation to your monitor, and attempts to show you the actual black that will appear in print. If you're simulating printing to a glossy paper, you'll probably just see a very slight lightening of the shadows. If you're simulating printing to watercolor paper, newsprint, or an uncoated stock that produces relatively weak blacks, the shadows will likely lighten a lot when you check Ink Black. This setting is useful for fine-tuning shadow detail, particularly on stocks that produce weaker blacks (see figure 4.)
Figure 4: With Ink Black checked, which turns off black point compensation, you can more easily fine-tune shadow detail.
The Paper White checkbox makes Photoshop do an Absolute Colorimetric rendering from the simulation space to your monitor. It attempts to show you the influence of the paper color and also the true black (when you check Paper White, it automatically checks, and dims, Ink Black). But to do that, it has to dim all the colors -- to change the color of white, it has to reduce the values in one or more of the channels that it sends to the monitor, because the only way to change the color from RGB 255,255, 255 (your monitor's white) is to turn something down. As a result, your first reaction when checking paper white may be that your image just died before your eyes. I've become accustomed to looking away from the monitor when I check Paper White so that I don't see the change happen. This simple trick makes it a lot easier to accept the paper color displayed on the monitor as a true white.
A second trick is to view the image in full-screen view with the menu bar and the palettes hidden. That way, you ensure that the paper white is the brightest thing on the screen, and your eyes have a chance to adapt to it as the true white: If you have white user interface elements appearing on screen, the paper white will seem visibly dim and tinted (see figure 5.)
Figure 5: The simulation with Paper White checked appears dimmer than with it unchecked. In this case, it also shows the slightly bluer white of the paper.
An obvious question is, which of these three simulations is correct? The truth is, all of them are, and none of them are. No proofing system has ever provided a perfect match to the final product: We learn to interpret proofs, and therefore, like any other proofs, you have to learn to interpret the soft proofs offered by Photoshop. Each one tells you something slightly different about the way the image will appear in print. A handy rule of thumb, though, is that for glossy stocks, the default view with Paper White and Ink Black unchecked will be the closest, while for uncoated stocks, the view with Paper White checked will generally be the most accurate.
The Proof Setup simulation is "live" so you can work inside it. One technique I find effective is to use adjustment layers to optimize the image for a particular print process while working with the soft proof simulation turned on. I then save the adjustment layers in a layer set that's named descriptively for that print process. This allows me to make different optimizations for different print conditions, and turn them on and off as needed.
Printing from Photoshop
Photoshop also offers the ability to make conversions on the data it sends to the printer. If your printer is a printing press, these features probably aren't useful to you, but if you're printing to a desktop printer, they can be very useful indeed. Photoshop adds to each printer driver radio buttons that let you choose the Source Space, and menus that let you choose the Print Space and Rendering Intent (see figure 6). These same controls are replicated in the Print Options dialog box -- any changes you make in one are carried over to the other.
Figure 6: Photoshop's color management controls as they appear in a QuickDraw driver (top) and a Postscript driver (bottom).
But placing these controls in the printer driver has created a fair bit of confusion. The key thing to realize is that they don't control the printer: Instead, they control the data that Photoshop sends to the printer. Since many printer drivers also offer color management features, the number of possible ways to print from Photoshop can be very large. Unfortunately, it also means that many of these ways will be incorrect. You need to decide whether you want Photoshop to do the color management, or the printer driver to do the color management, because if they both try to do the color management, you'll get a double correction that usually produces pretty hideous results.
So, with the clear understanding that the buttons and menus control Photoshop's behavior, not that of the printer driver, here's what they do:
The Source Space radio buttons let you choose between Document and Proof Setup. If you choose Document as the source, Photoshop uses the unaltered values in the image, along with the profile that describes their color meaning, as the source for any conversion you define in the Print Space Profile menu. Let's set the Proof Setup option aside for now.
There are really only three rational choices you can make in the Print Space Profile menu:
- The profile for the printer, paper and ink to which you're actually printing.
- Same As Source.
- Printer Color Management (PostScript Color Management when you print to a PostScript printer.
When you choose a specific profile in Print Space, Photoshop converts the image from the Source Space to the profile you've selected, using the rendering intent you select from the Print Space Intent menu. This is usually the most effective method of printing when you have a paper-specific printer profile available, but as previously noted, you must make sure that any color conversions are turned off in the printer driver.
When you choose Same as Source, Photoshop simply sends the source image data unchanged to the printer driver. I only use this option when I'm printing profiling targets, where the numbers in the file are all that matters, or when I've already converted the image to the printer's space.
Printer/PostScript Color Management also sends the data unchanged to the printer driver, but also includes the profile that describes the source space so that the printer driver's color management can interpret it correctly. This is the only option that works reliably when printing from Windows systems to printers that have a single profile that covers all media, such as most Epson printers (see sidebar "Canned Profiles, Custom Profiles, and Pretend Profiles"). If you use this option, you must, of course, turn on the ICM color management features in the printer driver. If you have custom profiles, this limitation doesn't apply. (The same approach also works with ColorSync print matching in the driver on the Mac, but since most Mac printer drivers include real profiles for each supported paper stock, I prefer to let Photoshop do the color matching.)
The Proof is in the Printing
If you choose Proof Setup as the Source Space, Photoshop executes the conversion you've simulated in Proof Setup before passing the data to the Print Space conversion. The main use of Proof Setup as source is to let you produce proofs of another printing process without actually converting your source data. For example, if you're working on an RGB image that will eventually be printed on a SWOP press, you can set Proof Setup to provide an on-screen simulation of the SWOP press print. If you then want to proof that SWOP press print on a desktop printer, you can choose Proof Setup as the source space, and Photoshop will then convert the print data to the SWOP press space, using the rendering intent you defined in Proof Setup, before it makes the conversion to your desktop printer's space.
If you've set Proof Setup to simulate the desktop printer you're actually printing to, choosing Proof Setup as the source space might seem to invite the danger of a double correction. In fact, it doesn't matter, but only because Photoshop is smart enough to recognize that when a source and a target profile are the same, no conversion is necessary. It's probably a good habit to avoid specifying Proof Setup as the source space unless you really want your desktop printer to emulate some other printing process, because other software may not be as smart as Photoshop, and double conversions are a frequent source of color management trouble.
Obviously, there are many other possible combinations of settings in both Proof Setup and Print. Unless you fully understand both what you're trying to do, and exactly how these settings will help you do it, my advice is to stick to the options I've outlined here. If you do so, the only limitation on the accuracy of your output will be the accuracy of your profiles.
Read more by Bruce Fraser.
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