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Identify and adjust out-of-gamut colors before you convert to CMYK

Added on Monday 6th of September 2010 04:36 am EST
 

Application:

Adobe Photoshop CS2/CS3/CS4/CS5

Operating Systems:

Macintosh, Microsoft Windows

 

 

 

Converting RGB images to CMYK may produce some unexpected—and undesirable—results if colors are out of gamut. Avoid any unpleasant color shifts by identifying and adjusting out-of-gamut colors before you change color modes.

 

To fix out-of-gamut colors, we’ll:

     Explain what gamut is.

     Demonstrate multiple ways you can identify out-of-gamut-colors in Photoshop.

     Show you an easy method to bring your colors back into gamut.

                

When preparing images for your print projects your workflow will probably have you converting RGB images to CMYK. Unfortunately, these two color modes don’t have the same gamut range which means colors that look brilliant in RGB will look dreadfully dull once you convert them to CMYK. Understanding how to identify out-of-gamut colors and bring them back into range is your first line of defense to preserving your image’s color range.

 

The meaning of gamut

The range of colors we see is known as the color spectrum. The color spectrum is usually shown as a band of pure colors that begins with violet on one end and progresses to red on the other end. But, the colors that surround us usually aren’t pure—they vary in intensity. To represent this, the Commission International de l’Eclairage developed the CIE color space graph, as shown in Figure A. The colors are represented as blended areas positioned around a central white point. As the colors move out from the central white point, they become progressively less white and more saturated.

 

Note: For more information about the CIE and its function, go to www.cie.co.at.

 

 

 

A

 

Though the range of color perceived from one person to another does vary, the range of color perceived from a person to a color device, such as a color monitor, varies even more. The range of color perceived by a color device is known as its gamut. The gamut varies from device to device, as shown in Figure B. Colors in a subject that the human eye can perceive but a color device can’t are known as out-of-gamut colors. 

 

B

 

Gamut problems when converting RGB to CMYK

The majority of photos used in print publications start off as RGB images and end up as CMYK images. This means that before you can commercially print an RGB image, you must first convert it to CMYK mode. Since the color range, i.e., gamut, of a digital camera or scanner is somewhat larger then that of an offset press, some of the colors in an RGB image may be out of gamut when you convert them to CMYK.

When you use Photoshop’s Image Mode command to convert RGB to CMYK, it automatically converts all out-of-gamut color. However, it’s best to do this manually. By allowing Photoshop to convert your images automatically, you could end up with unexpected results in your print job. To avoid this problem, it’s important first to recognize when and where a color is out of gamut.

 

Identifying out-of-gamut colors

The best way to identify out-of-gamut colors is to open a photo and examine it. To follow along with our example, download the file gamut.zip from the URL given at the beginning of this article, extract the file gamut.jpg, launch Photoshop and open the file. We’ll show you four ways to identify out-of-gamut colors: using the Color Picker, the Color panel, the Info panel, and the Gamut Warning.

 

To use the Color Picker:

1.       Choose the Eyedropper tool from the Tools panel, and then select a color in your photo that you think is out-of-gamut, as shown in Figure C.

2.      Click on the Set Foreground Color swatch in the Tools panel, and the Color Picker dialog box opens. If a small triangle with an exclamation point appears above your selected color, it’s out of gamut, as shown in Figure D.

3.      Click Cancel in the Color Picker dialog box to close it.

                

C

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