Ask Designorati: Converting CMYK To Spots + Transparency

Converting a full color (CMYK) EPS file into a one-color or two-color image can be tricky in Photoshop, but there are a couple ways to do it right. We’ll also show how to lose a white background in order to create transparency.

Jared Young, a marketing writer in Bend, Oregon, writes,
I have a full-color company logo saved as an eps file. I would like to use the logo on merchandise that requires I use one or two colors. How can I turn my full color logo into a mono or duotone image (can’t use gradients)? Also, is it possible to turn the white background into a transparent background on this image?
First of all, you should make absolutely sure this is a bitmap graphic and not a vector graphic. EPS files are often vector graphics created in Illustrator, especially logos like this one. Open your image in Illustrator and zoom in closely (see Figure 1). If the edges are sharp and clear like the detail on the left, then this is a vector image. If it’s pixellated, then it’s a bitmap image. A more accurate test is shown in Figure 2: open the file in Illustrator and choose Select –> All (Cmd/Ctrl-A). A vector image will show points along its edge (left detail), marking the vector path, while a bitmap image has no vector paths so selecting it in Illustrator will only show a box around the image (right detail).

fig1Figure 1: The resolution test in Illustrator. Vector on the left, bitmap on the right.

fig2Figure 2: The selection test in Illustrator. Vector on the left, bitmap on the right.

Quick tip: To select every shape with a specific fill or stroke, select one and then use Select –> Same –> … to select them all.

If your EPS file is made of vectors, then it’s a snap to bring it into Illustrator, select each colored area and make it a spot color. Quick tip: To select every shape with a specific fill or stroke, select one and then use Select –> Same –> … to select them all. Placing an EPS with spot colors in your page layout application will output spot color plates just fine. And unless there is a white shape below the graphic, there will be no background when the graphic is placed and printed so transparency is no concern. Jared, I’ll bet that if you tried Illustrator you will find your task a lot easier than if you used Photoshop.

BUT MAYBE NOT

What if your EPS file is a bitmap, not a vector? It can happen; there’s lots of designers who unknowingly rasterize EPS files because they don’t understand the difference, and not only does it convert spot colors to full color (CMYK, RGB or another color mode) but it also adds a white background that eliminates transparency. If that’s the case, you will need Photoshop or another bitmap graphics editor. Unfortunately Photoshop isn’t well-suited for spot color work; it’s designed to work with full color modes such as CMYK and RGB, which are additive and subtractive color models. Spot color, on the other hand, presents the color as it is and doesn’t mix colors to create its tonal range. There are three options that come to mind to convert process to spots in this situation:

Add spot channels to a grayscale image. This is the option that I recommend. Convert your image to grayscale, then select all the elements of a particular value range and move them to a new spot channel (created in the Channels palette). You can output this file as an EPS or PDF file and output the plates fine.

Convert to duotone. This option involves converting your image to grayscale with two or three distinct value ranges (black, dark gray and light gray, for example) then converting to duotone and using the Duotone Options dialog box to map spot colors to those three values. For more information on this technique, check out my recent tutorial on creating letterpress effects with duotones.

Add spot channels to a multichannel image. This technique is similar to adding spot channels to a grayscale image, except your color mode will be Multichannel (Image –> Mode –> Multichannel) and you can output this file in only five possible formats, the most notable one being DCS, a legacy (read: obsolete) format used in years past to bring spot color channels into QuarkXPress and other desktop publishing apps. Unless your workflow requires it, I’d recommend staying away from this one.

Because it’s often a pain to select precisely all the pixels you want to colorize (Color Range helps, but it’s still not perfect) I really recommend getting the file into Illustrator first and hoping it’s a vector graphic.

TRANSPARENCY

Transparency in bitmaps has come a long way in the past few years. InDesign has been working with transparent bitmap graphics for several years and QuarkXPress has finally decided to play catch-up. Layered Photoshop files (which can be natively imported into InDesign) are easily given a transparent background by removing its background layer. Transparency will be marked by a checkerboard pattern, which can be altered in color and size in Preferences –> Transparency & Gamut…. I prefer small and light cyan squares so they’re more subtle.

If your background layer has bitmap data you need to retain, or if your image is flattened (in Jared’s case this is almost certainly the case) you’ll need to perform some more selections to grab the white background and delete it, revealing the transparency underneath. If your image is flattened (look for the italicized Background layer name and the lock on the layer) double-click the background and convert it to a layer (the lock will disappear and your layer will be renamed “Layer 0″ and unitalicized). Again, it can be tough to grab all the white areas with your selection tools; in the case of Jared’s logo, it’s probably hard-edged so that makes it easier. If you have some white fringe pixels along the edge that you can’t select, consider using some matting options (under the Layer menu) to defringe or remove the white mat.

Jared, I hope this helps you out with your situation. If any readers have other problems that are stumping them, or just questions about some of Photoshop’s features, drop me a line below or e-mail me directly at jeremy@jeremyschultz.com!

16 thoughts on “Ask Designorati: Converting CMYK To Spots + Transparency”

  1. Hi.
    This is Sung Lee, a graphic designer at ABC LABELS.
    I would like to convert CMYK to Pantone Color in illustrator.
    There is an option to convert Pantone to CMYK, however, I can’t convert CMYK to Pantone (Spot Color)
    Please give some advise regarding the matter.
    Thank you so much for your time.
    Sung Lee

  2. i want to know how i convert an image with CMYK color to spot color in photoshop CS. with maximum output to print by screen printing.

  3. Photoshop works with spot colors differently than Illustrator—spot colors require a spot color channel (create one in the Channels palette) and AFAIK there’s no way to “convert” a CMYK color with a click of a button. As outlined in the story, create a new channel that specifies the spot color and then select, copy and paste particular elements to that channel.

  4. Is there any way in which I can take the alpha channel created for a spot color (magenta) and replace it for the “real” channel in a CMYK image? Was I clear? I’ll try again: I’m printing in two channels (M and K) but I haven’t yet figured out how to convert a grayscale image into CMYK and NOT use the C nor Y channels. I understood the use of spot colors but I’d like to give the printman a “normal” document in CMYK… thank you thank you thank you mr. designorati.

  5. Mr. Designorati…I’ve never been called that before. I think I kind of like it!

    If I understand correctly, you want to create a CMYK image that uses just the magenta and black channel. What you’re starting out with is an image with an alpha channel and black. I would use simple cut-and-paste, because with the Channels palette you can paste data into single channels. Copy your alpha channel image and paste it into a new file’s magenta channel. Do the same thing with your black.

    That should give you a CMYK file with the C and Y channels empty. A printer should be able to see this and output just the two plates.

  6. That was quick! And very helpful indeed! Thanks! I feel dumb — I tried that before, without too much conviction, but since you said it worked, I tried harder… it took me awhile because I had transparency as backgroud — and it has to be white, or some opaque pixels… true?

    And if I may, one more question: Is there any way to gracefully transform a duotone image into CMYK that only uses two channels… Am I interpreting duotone wrong? Shouldn’t this be possible if the two tones are, for example, pure M and pure K? (…probably not)

    Thanks a lot for your time, Mr Designorati. (I’ll come up with more questions for sure!!)

  7. I’m not sure you need an opaque background layer—if you copy image data and paste it in a channel, I don’t think it matters what kind of layer you have. One thing to remember though is, if you’re having trouble and you’re working with an unflattened image or even a layered image, it gets complicated to make sure you are copying from the right layers with the right channel selected.

    RE: duotone to CMYK, even if you are using black and process magenta as your two spot colors, it’s almost certain that your color settings will convert those two colors into mixes of the four CMYK colors. There are two things you can do:

    (1) Create two monotone images, one with black and one with magenta, and with the same duotone curves you would have used. You can copy these images into individual channels of a CMYK document.

    (2) Read my upcoming article this week on how to use a secret of Photoshop’s Color Settings to separate limited-palette images like these into CMYK images without the hassle posed by rich black!

  8. Hi there:

    I have a question,
    How do I convert a CMYK image to spot color in Photoshop to print in black tshirts?

    Thanks!!!

  9. Sandra, that’s a complex question—maybe I will write an article about it. The key thing to remember is shirt inks don’t mix the way inks do on paper, and a white ink will probably be needed.

  10. Your second technique (duotone) works best for complex illustrations that were created with many layers stacked beforehand in different blending modes. Simple, fast and effective.
    Thank you Jeremy!

    Albert

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