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Pre-Separate Complex Graphics with Index Tables



Properly separating a complex graphic for T-shirts will make or break the final product. It is the difference between a soft and subtle printed shirt that makes customers take notice rather than just shrugging and asking what else you have. The key is to be able to split the design into multiple colors that work together to reproduce the original design as accurately as possible in the least amount of time. An additional concern is always how often the separations do not work on the press and need to be adjusted, which can be very costly to correct.

Properly separating a complex graphic for T-shirts will make or break the final product. It is the difference between a soft and subtle printed shirt that makes customers take notice rather than just shrugging and asking what else you have. The key is to be able to split the design into multiple colors that work together to reproduce the original design as accurately as possible in the least amount of time. An additional concern is always how often the separations do not work on the press and need to be adjusted, which can be very costly to correct.

One way to jump-start the process on difficult designs is to quickly pull a set of index separations to use as a guide for a final set of separations. This may seem like duplication of effort, but in many cases it takes very little time and the resultant color splits that are defined in the index separations can make all the difference in the final file.

Another advantage to using this process is that it enables the separator to use pieces of these test files in the final if, along the way, the traditional methods don’t appear as accurate. Practice with a combination method can often yield high-quality results, depending on the artwork and preparation methods. There are always a select few cases where the index separations work out so well on the quick test that there’s no reason to continue to use the traditional method—and the time savings can be several hours.

The time savings in using index tables are appealing, but there are several other advantages to consider. The index dot is a square dot that’s the actual size of a pixel, which means that the dot size is equal in the whole separation set. The old saying, “If you can hold one dot you can hold them all,” applies to the area or file that you are separating using this method. Close inspection of this process clearly reveals the difference between dot styles (Figure 1).


An ongoing argument exists among printers who use traditional dots or index dots. Many believe that one is better than the other. The truth is that each method has its own advantage and that combining them, using both styles together in one image, can sometimes create the best possible file. Regardless, knowing how to use both styles of separations allows for simpler separations overall.

Index dots can be tested quickly and can often give a result in minutes, rather than an hour or two. That’s why they can work great as a separation pre-test to get a quick snapshot of a working set of positives.

Testing a graphic using a quick index-dot set
The best images for this process are ones that tend to cause trouble with a standard process. Dark shirt designs with blended colors, memory colors (colors that viewers recognize if they appear incorrect, such as wood, metal, or flesh), and lots of detail are good candidates for a pre-test using an index method. You can use one base design of a tiger to show two different, challenging printing methods: a more realistic version of the animal with a gothic background and a more modern approach using very bright, saturated colors (Figure 2).

The first design, which has realistic animal colors, is a good candidate for an index-separation set on the tiger and then a traditional halftone on the outside edge of the design that has the beveled elements. Soft, gradient transitions such as those outside of the design tend to look better using traditional halftones due to the reduction of the dot size as the halftone fades out on the print. An index dot in those areas would have a more abrupt look that might appear grainy if the dot size were all the same in the fading areas. An exception to this is when you can print and maintain a very fine dot size on screen, which typically requires high mesh count and tension, as well as refined ink viscosity and opacity control. In other words, it’s not something the average printer should attempt casually. To test the realistic design using a quick index separation set, I used the following steps:

1. I isolated the area of the design that I wanted to separate using the index method, which was easy for this design because I built it using every element on a separate layer. If you receive artwork that is flattened, ask for a layered version; otherwise, you’ll have to separate the different pieces manually. I turned off the visibility on the layers that I didn’t want to use, and then duplicated the file for safety.
2. I examined this design to determine the number of colors needed, starting with the most saturated primary (blue, yellow, red) and secondary colors. I then decided on other colors to use and wrote them down on paper. I preferred to do it this way so I could see them as they were separated.
3. I went through my color list and made sure that the colors selected made sense and that I really needed all of the colors in question. I then determined whether the press could support the number of colors needed. This step exposes the designs that require editing in the art stage before costly seps and screens are made.
4. I then made sure my image was actual printed size. I wanted an index dot that could print easily, so I changed the image resolution to 190 dpi to force the size of the index dot to be the same as one pixel size.
5. Next, I changed the image into an index color (>Image>Mode>Index Color). I then flattened the image. I made sure the preview was off when the index dialog box came up so I could select colors from the original image. I then selected a custom palette from the index dialog box, which brought up the existing palette. I moved the cursor to the bottom right, clicked, and dragged it to the upper left, which automatically opened the dialog to select the first color. I made it white. The dialog to select the last color then opened, and I also made it white, which cleared out my palette (Figure 3).
6. I then clicked on squares to activate the color picker, and picked the colors from the original chosen image. I also selected some variations of those colors to create using light underbasing—or no underbase—and a blended color or two (these are placed on separate rows). Finally, I specified a white and a black (Figure 4) and then accepted the color-mode change.
7. The resulting image wasn’t all that great. Many shadows looked brownish, and the tiger needed more colors in the face transition areas. I simply used the undo command and then added extra colors on another custom index table. Photoshop is nice enough to save your custom color table when you undo, and you can always save it as well if you need to close the file. The resulting file looked a lot better with the adjustments (Figure 5).
8. This process took less than 10 minutes and produced a working index file to use. I saved this file separately from the original and then converted this file back to RGB. I then opened the original file and copied and pasted the index version into my original file as a new layer. I used it as a reference for pulling the final separation set. In this case, the index version was good enough to use as a final for the tiger. I quickly pulled a gray plate for the gothic pattern in the background. Creating separations from the index layer involved using the Color Range tool on a low fuzziness (to prevent the appearance of ghost colors) and pasting the selections into alpha channels. This worked really well with my index copy because all of the colors were already isolated into pixels.
9. I used a duplicate of the original file to create the background gray plate.
I filled the tiger layer with black to knock out the tiger design and then inverted it and adjusted the levels. I combined this selection with the gray selection from the tiger index. The index was solid black dots, so I knew that I could print that channel with grayscale fading combined and it would still hold the index dots and only convert the gray values to halftones (Figure 6).
10. Generating the underbase file was a simple matter of selecting the index colors that needed an underbase using the Color Range tool, avoiding those that didn’t, and then filling those selections onto an alpha channel for an underbase file. I added a couple of underbase areas under the gray color as well. The whole design was color-separated and ready to go to films in less than 15 minutes.

I knew from experience that an index dot would probably look too grainy on the second, more colorful, tiger for what the press could handle at a 190-dpi dot. Going with a smaller dot would likely cause the ink layer to be too thin and make the colors less bright. The answer for this file was a traditional-dot separation set. The index-separation method could save time on these separations, and I could then use the file as a basis for creating my traditional positives. I used the following steps to create an index guide for the colorful tiger:


1. I started with the same process I used for the other file, but I didn’t need to isolate the background because it was designed with most of the tiger’s colors and looks.
2. I checked the file’s size and resolution to make sure it was original size and at a resolution of 300 dpi to prevent concerns about the dots being too small.
3. I duplicated the file (never forget to do this!) and then created an index table using the bright colors in the image. I made sure to use a lot of blends between the colors, which I did by clicking on one square in the table, dragging across a couple of squares, and then putting in a first and last color for the blend. This allowed me to replicate the file quickly and still have an accurate reference.
4. Once I had a good index version (this required an undo step or two), I then copied and pasted the file as a layer again. This time, however, I used the colors that I created as defined selections with the Color Range tool. Modifying the fuzziness enabled me to quickly select and save a complicated set of overlapping separations for recreating this multicolor tiger in less than 30 minutes (Figure 7). The gray values in the color range selections were curved out or converted to traditional halftones in the final positives.

This method worked well with the boldly colored tiger because the colors were defined. Had the colors been very similar, with a lot of gray or brown tones, such an approach wouldn’t be appropriate. The image’s great detail and defined colors worked with the process, and I saved a lot of time in separating. As you can see, index-table pre-separating can simplify even the most complex garment designs, regardless of which type of dot you intend to print on press.



Let’s Talk About It

Creating a More Diverse and Inclusive Screen Printing Industry

LET’S TALK About It: Part 3 discusses how four screen printers have employed people with disabilities, why you should consider doing the same, the resources that are available, and more. Watch the live webinar, held August 16, moderated by Adrienne Palmer, editor-in-chief, Screen Printing magazine, with panelists Ali Banholzer, Amber Massey, Ryan Moor, and Jed Seifert. The multi-part series is hosted exclusively by ROQ.US and U.N.I.T.E Together. Let’s Talk About It: Part 1 focused on Black, female screen printers and can be watched here; Part 2 focused on the LGBTQ+ community and can be watched here.

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