ONE OF MY FAVORITE ways to separate a very detailed illustration is to use the Curves menu in Photoshop. This is a high-quality way of capturing the subtlety in a design that is to be separated for screen printing. This is because you can 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 value ranges. The most popular methods of separating color selections in Photoshop are using the Color Range command and the Magic Wand tool. These methods have significant drawbacks with images that are highly blended, use overlapping transparencies, or contain colors with similar values close to neutral grays.

Even with these drawbacks, these specific tools have their place and use in color separation. The Color Range command is a very useful method of extracting complex selections from images. I commonly will use it first to see what I will get even if it is not the final method that I use to create a selection for a color channel. When you have an image with a lot of gradients of colors blending into each other or into the background (shadowy effects), then the Color Range tool often will create very choppy selections of colors from these gradients.

The Magic Wand tool really is best applied when there are large areas of flat colors that easily can be selected and isolated. Any blending or variation in color tone will tend to throw this tool out of whack so that it will select either not enough information, or everything around the color you need. The reason that the Curves menu works better than these other options with very detailed designs is because it always starts with all of the information available in a specific area that has the curve applied to it. This is instead of just a selection of information inside a selection. In essence this is always better than a second-generation copy of the information, which is going to be slightly worse than the original.

Start Points

To use the Curves menu properly in the separation of colors from a design, it’s best to follow several steps: Isolate areas for preparation, pull practice selections for testing, and then create a final set of curves to extract the necessary colors.

Figure 1

Isolating Areas of a Design for Preparation

The more colors in the design and the more complicated it is, the more areas in it will have to be isolated for the preparation stage of separating. Whenever possible, a complicated design should be saved with every major area in it as a separate layer. In this way, a design can be easier to isolate and separate using the curves method. A complicated design that is not saved in this manner can take many hours to correctly select specific areas for color extraction.

For this reason, the Curves method sometimes is a less efficient way to separate artwork that is provided without an easy way to break it down into manageable pieces. The most common issue at the beginning stages of isolating a design for separation is deciding on what basis should you isolate what areas. Each design will be different, but a good way to approach this issue first is to 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 gray scale. In an average screen-printing company, there is a constant 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. If we look at a basic grayscale generated on the computer next to a grayscale that has been damaged by tonal compression, you will see the loss in the midtone areas and the highlights first (Figure 1). It becomes difficult to tell where the 50-60% line is because they tend to merge together. Avoid this issue by using a pure grayscale depiction of a specific color value that can then be isolated to produce a superior final separation set.

Figure 2

The theory behind the Curves separation method is to isolate a selection of color first and then create an ideal grayscale from it that can then be depicted in the separation set. 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 did for a local martial arts club fundraiser was a good example of a design that can be separated using the Curves menu to recreate the subtle tonal range in a grayscale image (Figure 2).

In the case of an image that already is a grayscale image, it isn’t necessary to do a lot of isolating of parts of the image, particularly if the image is to be printed on a black background. For this example, I didn’t need to isolate any colors so I could move forward to the next phase of the separation process.

Figure 3

Pulling Practice Selections for Testing

The purpose of doing extra work for reference selections paid big dividends when using the Curves menu to separate my design. In many previous designs I found it was very difficult (if not impossible) to create a separation from the Curves menu without some kind of ballpark reference selection to guide in getting the right start and finish points.

The extra work I did with pulling a practice separation was easily accomplished by using the Color Range tool. In the example design, I was going to use a common solution for detailed grayscale images on black shirts: I would 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 on using. An additional Alpha channel was created in front of all of the other channels and labeled “shirt” with the Channel Color option selected as black.

The next step was to open the Color Range menu for each gray channel and use an appropriate gray value to select that section of color from the original design (Figure 3). A good hint for this point in the process was to really watch the fuzziness selector as I slid it up and down on the Color Range tool so when I was getting the gray selections I was creating, they would overlap each other slightly. If the selections didn’t flow smoothly into one another, I knew that the final design would be clunky. It was best to be picky with a Reference channel so I didn’t recreate a mistake with the final curved version.

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 turned on the “eye” switch in the Channel Dialog box. In this manner, I had a quick set of separations that I could use as a reference to create my final set of separations with the curves menu.

Figure 4

Creating a Final Set of Curves for Extracting the Colors

The tricky part of separating this image with 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. (Figure 4)

Next, I opened the Curves menu with the duplicate file open (and slid it off to the side so I could view the color range selection underneath) and slowly created a curve that isolated a similar selection to my color range selection. I used some general rules for forcing the Curves menu to do this kind of extraction:

1. Make a smooth-looking curve whenever possible.
2. For some reason, this seems to create the best separation file.
3. Start by 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.
4. Find the point on the curve where the gray level is close to the value of the selection that you want to extract. If I wanted to extract a 50% gray color, then I take the center of the curve (the 50% point) and move it all the way opposite from the black point that I just knocked out and start from there. This creates a new 100% black point, meaning that the 50% gray now is black and my black has now become white by changing the position of these appropriate points on the Curves menu. In a sense, it is similar to inverting the file (which I could do by shifting the white point all the way up and the black point all the way down) except I am inverting the file at a gray point rather than a white.

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

Figure 5

There was some additional curving to the files that needed to be done at this point, and I still needed to create the highlight white. The additional curves were applied 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 them. I created the highlight white quickly 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’ve always found it surprising the amount of extra subtlety in the files that I created with the Curves command vs. the files that were quickly created using Color Range. I discovered that it was easier in the long run to do the extra work on detailed designs and separate using the Curves method because using other methods (such as Color Range, Calculating Channels, or converting the image to quadtone, etc) would cause the separation set to not have a full complement of information in each separation. There was always some detail that was lost along the way and after it was lost, there was no way to get it back.

For this reason, I would end up using the Curves menu on several of the separations. Eventually, I started using the Curves menu to do the separations for any complicated image that needed the detail reproduced with the best control possible.