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Using Curves to Separate Grayscale Images

(November 2006) posted on Wed Nov 08, 2006

Explore how you can use Photoshop's Curves menu to produce high-quality separations that retain tonal information in your garment prints.

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By Tom Trimingham

The Curves menu in Adobe Photoshop is a favorite of mine when it comes to separating a very detailed illustration for screen printing, because it allows me to extract the information in a systematic and structured way while minimizing the loss of tonal range. Many of the other methods of pulling colors from an image in Photoshop tend to be choppy, inconsistent, and even ineffective depending on an image's color composition and tonal values.

The Color Range command and the Magic Wand tool are among the most popular methods of separating color selections in Photoshop. But both of these approaches fall short with images that are highly blended, use overlapping transparencies, or contain colors with similar values that are close to neutral grays. However, these tools have their place in color separation. The Color Range command is a very useful method of extracting complex selections from images, and I commonly use it first to see what I'll get, even when it's not the technique I ultimately decide to use when creating a selection for a color channel.

The Color Range tool will often create very choppy selections of colors when you have an image with a lot of gradients blending into each other or into the background (shadowy effects). The Magic Wand tool is really best applied when the image has large areas of flat colors that you can easily select and isolate. Any blending or variation in color tone tends to throw this tool out of whack so that it either selects not enough information, or everything around the color that you need.

The Curves menu works better with very detailed designs than these other options because the Curves menu always starts with all of the information available in a specific area that has the curve applied to it—not just a portion of the information inside a selection. In essence, this is always preferable to a second-generation copy of the information, which is slightly worse than the original. To use the Curves menu properly in the separation of colors from a design, isolate areas for preparation, pull practice selections for testing, and then create a final set of curves to extract the necessary colors.

Isolating areas for preparation


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