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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 more colors in the design, and the more complicated it is, the more areas you'll have to isolate for the preparation stage of separating. You should save a complicated design with every major area in it as a separate layer whenever possible. This method can simplify the use of the Curves menu in isolating and separating. Neglecting to save a complicated design can lead to many hours of work to correctly select specific areas for color extraction. For this reason, the curves method is sometimes a less efficient way to separate artwork. The most common issue at the beginning stages of isolating a design for separation is selecting image areas. Each design is different, but a good way to approach this issue is to first address the theory behind the curves method and the supporting concept of tonal range.

The Curves menu depicts tonal range with the absolute black point on one corner and the brightest white point on the opposite. The function of an effective tonal range is to demonstrate a noticeable division between the values on a typical grayscale. Many screen shops fight to save the tonal range from dot gain, moiré, screen-washout issues, and printing inconsistencies. What commonly occurs from tonal-range problems is tonal compression or the loss of the ability to differentiate between values in the highlights, midtones, or shadows.

When you look at a basic gray-scale generated on the computer next to a grayscale that is damaged by tonal compression, you will note the loss in the midtones and highlights first (Figure 1). You'll have trouble discerning where the 50-60% line is because the midtones and highlights tend to merge together. To avoid this issue, use a pure grayscale depiction of a specific color value that you can then isolate to produce a superior final separation set. The idea is to first isolate a selection of color and then create an ideal grayscale from it that can then be depicted in the separation set.


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