Avoid Photoshop's Color Calamities

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Desktop color-management technologies give designers the possibilities of superior color control–but also some serious headaches. A prime example is one of the more controversial color-management features of Adobe's Photoshop 5: the adoption of RGB color-space definitions that are not based only on your monitor. This was a great leap forward for color on the desktop, but created problems with most scanner-driver software on the market. As a result, the image you see in your scanner software often looks different when it arrives in Photoshop. So how can you reconcile the two, end up with an image that has the colors you want, and make sure they stay that way?

Most scanner software does not yet take advantage of Apple's ColorSync color-management technology, and simply sends your image's RGB values directly to your monitor. (If you're fairly new to color management or you aren't sure how ColorSync works, check out Apple's ColorSync Web site at http://www.colorsync.com.) Photoshop 5, however, interprets these RGB values according to the settings you've made in the RGB Setup dialog box and uses your ColorSync System Profile to transform the values that get sent to your monitor (see the screen shot "Your Monitor's Profile"). This disagreement between Photoshop and your scanner software creates color havoc–how can you tell what your image will really look like?

Your Monitor's Profile   To use a custom monitor profile on your Mac, you must load it in the ColorSync control panel as the System Profile. (Currently, only AppleScript uses the RGB Default and CMYK Default settings, so if you aren't scripting ColorSync conversions, you can ignore them.

You have a number of options for avoiding the color-fidelity problems that arise when you're scanning into Photoshop 5. For one, you can just hobble Photoshop 5 by using your monitor RGB as Photoshop's RGB workspace, but this isn't a great idea (see "Workaround 1: Downgrade Photoshop 5," below)–plus it defeats part of the purpose of upgrading to Photoshop 5 in the first place.

Other options let you reap the benefits of Photoshop 5's advances but involve a little more effort with your scanner software. For example, sometimes your best bet may be to do the bulk of your editing in the scanner software (see "Workaround 2: Correct the Prescan Image," below). Most scanners can capture more than 8 bits per channel but are limited to 8 bits on output. You can obtain significantly better quality by making as many of your large edits as possible in the scanner software, where you'll have more data and more detail to work with, rather than editing the reduced data in Photoshop.

When you edit the high-bit data in your scanner software, you still end up with a full 256 levels per channel in the final 24-bit image. But when you edit this image in Photoshop, you immediately start to lose some of the possible levels in each channel (see the screen shot "Image Quality Control"). For example, when you make tone or color corrections on a 24-bit image, some levels that were formerly adjacent get stretched apart. If they get stretched too far apart, you start to see obvious jumps of color instead of smooth gradations–a phenomenon called posterization . Similarly, some levels that were formerly apart get squashed together, resulting in loss of detail. So there's a significant advantage to editing the high-bit data in the scanner software instead. But then again, if the scanner software displays the image differently than Photoshop, you may wind up with an image that looks beautiful in the prescan display but awful in Photoshop.

Image Quality Control   The 16-bit-per-channel image on the right and the 8-bit-per-channel image on the left may look similar, but their histograms show a big difference. The higher-bit image contains a full 256 levels in each channel, but the 8-bit image has lots of missing levels that will cause the image quality to deteriorate under subsequent editing.

If your scanner has a color profile, you can use that to convert the color values to those in Photoshop's RGB profile (see "Workaround 3: Convert from Your Scanner Profile," below). But some scanner drivers insist on making automatic corrections and don't let you turn this feature off–in this case it's impossible to create a ColorSync profile for the scanner. To profile a scanner (or any other device), you must be able to keep it in a consistent state; automatic corrections mean that the scanner scans each image differently.

Even better is when your scanner driver allows you to scan to an output profile (see "Workaround 4: Choose Your Output Profile," below). Heidelberg CPS's LinoColor, Imacon's ColorFlex 1.8, LaserSoft's SilverFast, Nikon's scanner software, and Second Glance's ScanTastic all let you do this, and other scanner-software makers are likely to follow suit in future.

The best option of all may be to bring a high-bit image into Photoshop if your scanner software can save high-bit scans (a growing number of scanner drivers can do this). When you save a high-bit scan, you're grabbing all the data the scanner can capture (see "Workaround 5: Import Masses of Raw Data," below). This will create a huge file, but all the image's data will be there for you to edit as needed in Photoshop.

Depending on the capabilities of your particular scanner software, you can use one or more of the workarounds discussed below (presented from least to most desirable) to get around the mismatch between Photoshop's RGB and your scanner's RGB. At least one of these solutions should work for you.

Note: All the workarounds assume that your scanner driver operates as a Photoshop plug-in. If you scan to disk with a stand-alone application and you have Photoshop set up to warn you of profile mismatches (see the screen shot "Automatic Mismatch Warning"), you can use the Profile Mismatch Handling feature instead of Profile To Profile to make conversions, using the same settings mentioned in each workaround below.

Automatic Mismatch Warning   You can make Photoshop's Profile Mismatch Handling feature let you convert an image as you open it. To do this, choose Profile Setup from the File menu's Color Settings submenu. In the Profile Setup dialog box (above), set all the Assumed Profiles settings and Profile Mismatch Handling settings to Ask When Opening.

When Photoshop's Profile Setup is configured like this, you'll see the Missing Profile dialog box (below) when you open a file that has no embedded profile. You can then convert the image into your RGB color space as it opens, instead of using Profile To Profile after the fact.

The simplest solution is to make Photoshop 5 behave like older versions of the program. I recommend using this approach only if your scanner software doesn't let you turn off automatic corrections, doesn't support high-bit export, and doesn't support output profiles– and you can't be bothered to learn to work with the new behavior of Photoshop 5.

What to Do In Photoshop 5, choose RGB Setup from the File menu's Color Settings submenu. In the resulting dialog box, choose Monitor RGB from the RGB pop-up menu, and turn Display Using Monitor Compensation off (see the screen shot "Downgrading Photoshop").

Downgrading Photoshop   To make Photoshop 5 behave essentially like Photoshop 4, choose Monitor RGB from the RGB pop-up menu in the RGB Setup dialog box, and then turn Display Using Monitor Compensation off. Photoshop will then simply send raw RGB values to your monitor.

Pros The problem disappears, because Photoshop now simply sends the RGB values in the file straight to the screen.

Cons The fairly substantial disadvantage is that you lose all the benefits of the RGB color-space approach introduced in Photoshop 5. Your color gamut is limited to what your monitor can display, and your editing space won't be perceptually uniform (see "Why Use RGB Color Spaces?" for a definition). Plus, if you send the files to someone else, they'll have to run the files through a conversion process to get them to display properly. And even if they are also using Monitor RGB as their color workspace, it's extremely unlikely that their monitor's color quirks will match yours.

Even if your scanner software doesn't let you turn off automatic corrections, doesn't support high-bit export, and doesn't support output profiles, there's a better solution than downgrading Photoshop 5. If you correct an image based on the prescan, you can make your monitor, not your scanner, the source profile for the image.

Prescan Editing   If your scanner software sends raw RGB values to the monitor, you can use that software to edit the prescan and use the monitor profile to bring the image into Photoshop's RGB workspace.

What to Do Scan as usual, making the image look good in the prescan (see the screen shot "Prescan Editing"). Once the image is in Photoshop, choose Profile To Profile from the Image menu's Mode submenu. In the Profile To Profile dialog box (see the screen shot "Converting Your Monitor Profile"), choose your monitor profile in the From pop-up menu. In the To pop-up menu, choose RGB Color. For the Engine pop-up menu, I generally recommend choosing Built-In, but in most cases you'll get virtually identical results no matter which engine you use. Then choose Perceptual (Images) from the Intent pop-up menu, turn Black Point Compensation off, and click on OK to perform the correction. Once it's done, the image should look the same in Photoshop as it did in your scanner driver.

Converting Your Monitor Profile   You can convert your image from the Monitor RGB color space into Photoshop's RGB color space using Photoshop's Profile To Profile command. Just choose your monitor profile in the From pop-up menu, and choose RGB Color (which selects whatever color space you've chosen in the RGB Setup dialog box) in the To pop-up menu.

Pros This approach is fairly simple, lets you scan the way you're used to scanning, and allows you to take advantage of most of Photoshop's new color-handling features.

Cons The colors in the scan are limited to those your monitor can display. And since the conversion is performed on 24-bit data, you may lose some image quality.

If you have a profile for the scanner–and the scanner driver allows you to disable all automatic corrections–converting the profile is a more favorable approach.

What to Do Scan the image with no corrections. In Photoshop, choose Profile To Profile from the Image menu's Mode submenu. In the Profile To Profile dialog box (see the screen shot "Converting Your Scanner Profile"), choose your scanner profile in the From pop-up menu, and RGB Color in the To pop-up menu. Then choose Built-In from the Engine pop-up menu, and Perceptual (Images) from the Intent menu. Turn Black Point Compensation off and click on OK. Once the conversion is done, the resulting image should look very much like the original.

Converting Your Scanner Profile   To convert an image from your scanner's RGB color space into Photoshop's RGB color space, use Photoshop's Profile To Profile command. Choose your scanner profile in the From pop-up menu, and RGB Color in the To pop-up menu.

Pros You can preserve colors that lie outside your monitor's gamut.

Cons Since the conversion is performed on 24-bit data, you may lose some image quality.

If your scanner driver allows you to choose an output profile, use the Photoshop 5 RGB color space. You can create a ColorSync profile for your RGB workspace by clicking the Save button in Photoshop's RGB Setup dialog box (see the screen shot "Saving a ColorSync Profile"). To make this profile available to other applications and to your scanner software, make sure you save it in your ColorSync Profiles folder, which you can find at the root level of your System Folder.

Saving a ColorSync Profile   To create a ColorSync profile of your Photoshop workspace, first load the space you want to use in Photoshop's RGB Setup dialog box, then click Save. Save the resulting profile in the ColorSync Profiles folder so that your scanner software (and other ColorSync-savvy applications) can locate and use it.

What to Do Scan as usual, making global corrections in the scanner software. When the image lands in Photoshop, it will look the same as it did in the scanner software.

Pros You can preserve all the colors your scanner can capture. Plus, you'll get better image quality because the color conversion is performed on the high-bit scanner data rather than the 24-bit final image.

Cons You're making all your edits based on a smallish prescan image, so you can't see all the detail in the final scan. Also, your scanner software's tools are most likely not nearly as comprehensive as Photoshop's.

If your scanner driver allows it, bring the raw high-bit scanner data–with no prescan corrections–into Photoshop as a 48-bit RGB file, and make all your corrections in Photoshop.

What to Do Set the image crop and resolution in your scanner software, and bring in all the data you can. If you have a profile for your scanner, you can use Profile To Profile to convert the image from scanner RGB to Photoshop's RGB color space before you make any corrections. This should get the raw image closer to the appearance of the original so you'll have fewer corrections to make.

Pros You correct the full-resolution image using Photoshop's tools rather than correcting the low-resolution prescan image using the more limited tools scanner software offers.

Cons The file size is twice as large (at least initially) as that of a 24-bit image. Once you've done the conversions, you can downsize the image to 8 bits per channel by choosing 8 Bits/Channel from the Mode submenu on the Image menu. However, you could make a good case for archiving the high-bit file for future conversions to output devices, since you'll always get a better result doing the conversions on 48-bit data than you will on 24-bit data.

The new color architecture in Photoshop 5 may cause a few folks to do some head scratching, but this feature really does offer some major advantages over the old ways. Eventually all scanner software will integrate seamlessly with Photoshop 5, but for now, one of these methods should help you work around the mismatches. No matter which method you choose, you should get predictable color from your scanner.

Applications without color-management features simply send the RGB values in your image directly to your monitor. This creates several problems.

• Since every monitor behaves differently (sometimes a lot differently), when you send the same RGB numbers to different monitors, the images don't look the same. Even when you calibrate monitors to the same specifications, it's rare for them to match exactly.

• Monitor color gamut–the range of colors a monitor can reproduce–is generally larger than the gamut of most output devices, including printing presses. However, it's much smaller than the gamut of film, and it also limits some of the colors you can reproduce on a press, particularly cyans, and adjacent blues and greens. If you work in monitor RGB, you limit the range of colors you can capture in your scans, as well as the range of colors you can print.

• Monitors are inherently nonlinear, and their RGB color spaces are perceptually nonuniform . This means the same increment of change you gain by using editing tools such as Photoshop's Levels, Curves, or Hue/Saturation may produce varying perceptual changes in different parts of an image's tonal range or color gamut. The same move may result in a barely perceptible change or in a large jump, depending on where you apply it.

Photoshop 5 lets you address all three of these issues by uncoupling the RGB color space from the monitor–it's an arbitrary color space that doesn't depend on the quirks of any particular device. For maximum editing flexibility, you can select a perceptually uniform RGB workspace with a large enough gamut to encompass your devices. Plus, you can standardize on a single RGB workspace for your workgroup and then move files around from one machine to another without worrying about color variations.

By default, Photoshop 5 automatically embeds a ColorSync profile in every image when you save it. When you move the image to another machine, Photoshop knows what actual colors the RGB values represent–or, to put it another way, the program knows what the colors are supposed to look like.

Photoshop's Display Using Monitor Compensation feature (see the screen shot "Downgrading Photoshop") uses the information stored in your ColorSync System Profile to perform a customized on-the-fly conversion of the RGB values in the image file as they're sent to your monitor. This makes it possible for the image to appear the same on everyone's monitor, as long as the ColorSync profile describes your monitor accurately.

September 1999 page: 105

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