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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

I saved each selection into an appropriately labeled alpha channel with the proper colors selected in the Channel Options menu so I could view how the selections would look against the shirt background and each other when I made the channels visible. In the end, I had a quick set of separations that I could use as a reference when using the Curves menu to create my final set of separations.

Creating the final set of curves

The tricky part of separating this image by using the Curves menu was finding just the right curve that would isolate the selection of color from the file. I left the reference file open for the light-gray separation and duplicated the original file. Next, I opened the Curves menu with the duplicate file open (Figure 4), slid it off to the side so I could view the color-range selection underneath, and slowly created a curve that isolated a selection similar to my color-range selection. I used some general rules for forcing the Curves menu to do this kind of extraction:

• Make a smooth curve whenever possible. This seems to create the best separation file.

• Moving the black point all the way to the top (or bottom if your Curves menu is set up the other way) to knock out the black.

• Find the point on the curve where the gray level is close to the value of the selection for extraction. If you want to extract a 50% gray, then take the center of the curve (the 50% point) and move it entirely away from the knocked out black point. This creates a new 100% black point, meaning that my 50% gray is now black and my black has now become white by changing the position of these appropriate points on the Curves menu. This is similar to inverting the file, which you can do by shifting the white point all the way up and the black point all of the way down, except that this process inverts the file at a gray point rather than a white.

When the final curved files looked good, I copied them and pasted them into duplicate channels right next to the original test files that I pulled using the Color Range command. I could then view the channels in proper print order and check them against the test files that I used for reference.

Additional curves were necessary, and I still needed to create the highlight white. I applied additional curves to the pasted channels in the midtone regions to adjust for dot gain. I just bumped the midtone point of the curve up 10% to compensate for gain and saved. I quickly created the highlight white by duplicating the original, pasting it as a channel, inverting it, and then sliding the white point over and curving it down to knock out most of the midtones while still leaving a smooth transition to the other colors (Figure 5).

I'm always surprised by the amount of extra subtlety that I can create in the files by using the Curves tool instead of Color Range selector. I believe it's easier in the long run to do the extra work on detailed designs and separate using the Curves menu because other methods, such as color range, calculating channels, or converting the image to quadtone, cause the separation set to lose information. You can't retrieve the information once it's lost. For this reason, I suggest you investigate the use of the Curves menu when separating complicated images that require the controlled reproduction of fine detail.


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