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Tuesday, June 19, 2007

Mysteries Of (Color) Space

We've covered it before and we'll cover it again because the ins and outs of color management begin with a firm grasp of how devices handle color space

Typical Color Management

Implementation of color management begins at the capture of an image. Starting with the input profile from the scanner or digital camera, we go from the device space, through a PCS, into a working space such as Adobe RGB. Because we're displaying via a monitor with its own device space, the monitor must also be profiled to display the image properly.

To go from the working space to the correct output space, the output device also is profiled. The image will be taken from the working space and converted to the destination space for output (which could be offset press, inkjet printer or even sRGB for Internet use).

The real work begins when opening the image for processing. More often than not, the software used is Adobe Photoshop, and Photoshop needs to be set up properly—you might actually want to get some dialog messages to pop up when opening a file or when cutting and pasting a file from one image into another. The messages that need to be mentioned are “Missing Profile” and “Embedded Profile Mismatch.”

In order to receive these messages, you must enable them in the Color Settings preferences. Photoshop 7 and CS have Color Settings under the Photoshop pull-down menu on the Mac OSX platform. In CS2, it has been moved under Edit. Either way, the shortcut remains the same: Shift > Command K.

For the most part, I find U.S. Prepress Defaults to be a quick and standard setting because it uses Adobe RGB as the Working Space, sets all of the Color Management Policies to Preserve Embedded Profiles, and enables the Profile Mismatches and Missing Profiles warning messages. With my editing space set to Adobe RGB, however I acquire my files, Photoshop is on the lookout for files in which a different profile is embedded.

This lets me know that the image being opened isn't in my preferred working space. Under the “What would you like to do?” choices, I'd choose to “Use the embedded profile (instead of the working space).” It's typically best to keep and work with embedded profiles. Even though Adobe RGB has a larger gamut than sRGB, it will be of no benefit to bring an image into that larger working space. I won't be gaining color, as you can't make up color with a conversion. If the color isn't there to begin with, then you won't have it. A general rule of thumb is to do as few transformations as possible; only do them if necessary. With every transformation, you're affecting the color to some degree, either massively or minutely. You may find that some files don't have an embedded profile.

Because the source of the image is unknown in this situation, there's nothing wrong with experimenting with different profiles. You might consider simply using sRGB, or your usually preferred working space. You're now ready for editing.

Preparing For Output

When the image is ready for output, the workflow continues by taking the image from its working space and preparing it for output—in this case, a print from an Epson printer. Select File > Print With Preview in Photoshop. Here's where the output space is defined by using a printer profile.

After the proper printer profile is entered, click on Print. Make sure that No Color Adjustment is checked. Failure to do so will put a double whammy on your color. Even if Color Controls is selected and everything is zeroed out, you're still affecting the color. Set the proper quality settings, paper stock and resolution in Print Settings as well (which can be found in the same pull-down menu as Color Management).

The last important consideration is dealing with rendering intents, as there are decisions to be made in the Color Settings preference and the Print With Preview dialog. Rendering intents basically allow for choices to be made on the types of transition from one color space to another. There are four basic options of rendering intents: perceptual, relative colorimetric, absolute colorimetric and saturation.

Perceptual Rendering

Notice there are four points of color (represented by black dots). Two are “out of gamut” to the smaller gamut, and two reside in both gamuts. When converting from the larger gamut to the smaller one, tonal compression must occur. With perceptual rendering, the software attempts to maintain proportion in the overall color relationships. Notice in the illustration that all color points have moved, even those that were well within the smaller gamut, in order to maintain those relationships.


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