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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 Thomas Trimingham

A simple way to start to understand the curve separation process is to use a grayscale image and pull several colors from it. An illustration that I made for a local martial arts club fundraiser is a good example of a design that can be separated by using the Curves menu to recreate the subtle tonal range in a grayscale image (Figure 2). You won't have to isolate a lot of an image that is already in grayscale, particularly when the image will be printed on a black background. I didn't need to isolate any colors in this example.

Pulling practice selections for testing

Doing extra work for reference selections paid big dividends when using the Curves menu to separate my design. Creating a separation from the Curves menu can be very difficult—if not im-possible—without some kind of ballpark reference selection to guide me in getting the right start and finish points.

In the example presented here, I decided to use a common solution for detailed grayscale images on black shirts: pull three gray colors (dark gray, medium gray, and light gray) and a highlight white. I used the Color Range tool to quickly extract selections that I saved as alpha channels for each of the gray colors that I intended to use. I created an additional alpha channel, moved it in front of all of the other channels, and labeled it "shirt" with the Channel Color option selected as black. Next, I opened the Color Range menu for each gray channel and used an appropriate gray value to select that section of color from the original design (Figure 3). I really had to watch the Fuzziness selector as I slid it up and down on the Color Range tool so the gray selections I created would overlap each other slightly. If the selections hadn't flowed smoothly into one another, the final design would have been clunky. Being picky with a reference channel can prevent the recreation of mistakes in the final curved version.


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