How to Create a Digital Underbase for a Direct-to-Garment Printer

Follow these procedures to lay the foundation for great inkjet prints.
April 1, 2008

Digitally printing the bright red portions of this fire truck — especially on a dark garment — requires a fairly heavy white underbase and, possibly, some highlight white on top.

By Dane Clement

The secret to producing the best possible quality dark shirt print with a digital direct-to-garment printer relies on your ability to create a good underbase. This ensures the print has a soft hand and the colors are vibrant. Creating an underbase is not difficult, but most people don't know how to do it. They rely on the RIP software that came with their printer. RIP is an acronym that stands for raster image processor, and not all RIPs automatically create a good underbase for digital direct-to-garment printing. This article will show you how to control your RIP to produce the best prints from your inkjet garment printer.

Once you know which tools to use, it's pretty simple.

For example, if you want to print bright, fire engine red on a dark shirt, you will need a lot of white on your base channel under the red — at a density somewhere between 80 to 100%. However, if you intend to print a darker shade of red, your underbase may need to be only 40% to 50% as dense. You learn from experience where to boost the underbase and how to handle certain colors. I create my underbases exactly the same way 90% of the time; however, with certain colors like reds, you may have to go in and make some adjustments.

One of the most important things to do when trying to produce an underbase for digital printing or screen printing is to have your art on a transparent layer. Once your art is on a transparent layer you can switch the background layer from black to white. You must have your art on a black-and-white background for this to work properly. This is the case whether you use the RIP software to create the underbase or create that channel manually as I'll show you in this article.

There are two basic types of underbases: solid and tonal (see box, p.80). Which you choose depends on the type of artwork you plan to print — vector artwork or raster (a.k.a., bitmap) art. I mostly print raster images, which require tonal underbases. I create mine using the luminosity of an image, which I can manipulate in Adobe Photoshop. To access the luminosity of an image, you have to use the Lightness channel in the Lab color mode in Photoshop.

To get started creating a digital underbase, first be sure your artwork is on a black background then duplicate your image and change the mode to Lab color. (From the top menu, pull down Image > Mode > Lab Color). Once that's done, go to your Channels palette and select the Lightness channel. Now, using your mouse, click-drag the Lightness channel from your duplicate window and drop it on your working file. With this channel selected, invert the channel by selecting Image > Adjustments > Invert from the top menu, or use the keyboard shortcut Cmd-I/Ctrl-I (for MacOS and Windows, respectively).

Next, go to Image > Adjustments > Curves to open the Curves dialog box. Be sure your grid is in 10% increments and not 25%. All you have to do is put your cursor on the grid and Opt-click (Mac) or Alt-click (PC) inside the grid to change the size of the squares. Drag the nodes at each corner to look like the image below. That's it! This is the adjustment I use 90% of the time. Occasionally, I make some further adjustments if necessary.

Look at the artwork and determine in what areas you may need a denser white underbase. The underbase should be darker under the lighter colors of the image. Remember: if you are printing a bright red on a dark shirt, you'll need a lot of white under that red to make the final print bright.

You can learn a lot about separations and color from the Info palette in Photoshop. If you're not familiar with this palette, make it a point to start using it. The Navigator/Info/Histogram palette should be open by default. If not, just press the F8 key or, from the top menu, select Window > Info.

Turn the preview "eye" icon on and off of your channels and experiment. With the "whitebase" channel visible, move your cursor around the image and watch the Info palette. It will show you exactly how much white you have in any area based on the density shown in the Info panel. Be sure to mouse over red areas of your image. If the Info palette shows 40% to 50% coverage, you may want to increase that to 80% to 100%. That is a hard-and-fast rule. If the final print should be a brilliant red, you should boost the underbase to ensure it prints nice and bright. Otherwise, the final print will look like brick or maroon red if printed without enough underbase white.

Adjusting the red areas of the image to get more white underbase is pretty simple. Here's how: With your Color palette selected, from the top menu pulldown Select > Color Range... When the dialog box comes up, mouse over the image and click on a bright red area of the design. Adjust the "Fuzziness:" slider to capture most of the color you want to boost. Be careful not to get too much. While the area is selected, highlight the whitebase channel. Go to Image > Adjustments > Curves to open the Curves dialog box. Grab the middle of the diagonal line and pull down to make the image darker. I boosted mine to about 80% under my red.

Blues and greens normally don't require extra underbase, though they sometimes do require a little tweaking. Red almost always needs extra underbase. By combining heavy and light areas throughout the shirt design, you give the final print the softest hand possible while also applying enough white underbase to make the reds really pop.

In Photoshop, there are 10 ways to do everything. For this column, I'm going to show just one way. Learn this method first, then you can experiment and do things on your own.

Only direct-to-garment printers with the capability to print white ink will need an underbase channel in the image files it prints. These are the machines that are marketed as printing on darks. Inkjet garment printers that don't have the extra white inkset are meant for printing only white or light-colored shirts.

If your printer does have the extra white ink for printing darks, it also must have RIP software so it will know what to do with those extra channels.

Some RIPS do a better job of underbasing than others. Most will auto- generate underbases, but those generally don't meet my standards. Keep in mind that if you are still shopping for your first direct-to-garment printer, you will probably get a RIP bundled with it. When comparing various printers, I think it's important to ask, "Can I create my own underbase or am I locked into the one that the RIP generates?" Most bundled RIPs will let you bypass the automatically generated channels, though some don't allow that flexibility.

The logic behind auto-underbase is that most people don't know how to create an underbase channel themselves and don't want to go the extra step. However, I believe you can do a better job than the RIP, and it usually only takes a minute or two. Personally, I don't want the RIP creating channels for me; I want to bypass the RIP or use it only after I have created my own channels.

And not every shirt color needs an underbase. Obviously, whites and lights do not need an underbase. Examples of light colors are typically pastels such as yellow, khaki, light blue or light gray. If it's a lighter color, you can use just a highlight white to finish it off.

Creating one underbase that will print a piece of art well on every color of dark shirt is the most efficient way to go. Some people who want to conserve ink might say, "Well, if I'm going to print a red shirt, I want the red of the shirt to be in the red areas of the design." That requires one custom underbase. The inefficiency comes when you want to print that same job on five other colors; you're going to drive yourself nuts creating individual underbases for each one.

One of the most common mistakes when creating underbases is not enough coverage. It makes the images look dull and dead. You can fix those things, but you have to test print.

PRETREATMENT
All inkjets that print white ink on dark shirts require some sort of garment pretreatment. Usually the pretreatment is a spray, so you spray the shirt and hit it with a heat press. This lays down all the fibers, giving you a nice, smooth garment to print. The pretreatment also helps ink to adhere to the garment, giving you a more vibrant final print and little or no fibrillation (small fibers sticking up out of the ink).

Once you have created your underbase, print a test shirt. Then, go back in and tweak the colors a little bit if necessary. That's the learning curve. Once you get a feel for it, it's not that tough and you'll find you can create an effective underbase channel for any artwork in just a minute or two.

HIGHLIGHT WHITE
In order to get a really bright white in the finished inkjet print, you will need to print white as a highlight in just the white areas of the design. This is when a Highlight White channel comes in. It prints only over the white areas of the image and basically gives the white area a second hit of ink. An easy way to create the highlight white is to follow these steps:

With the artwork layer selected, go to Select > Color Range... In the Color Range dialog box, click on an area of the image that is bright white. Adjust the "fuzzy" slider — I usually stop around 30 on the slider — and click OK. While the white area is selected, create a new channel in your channels palette. Fill the selected area with Black. This data will print as white on the printer.

Dane Clement, well-known for his expertise in computer graphics and color separations, works as an application specialist at SPSI in Minneapolis. Dane is also the founder of Great Dane Graphics, a supplier of high-quality vector artwork. For more information or to comment on this article, contact Dane via greatdanegraphics.com.

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