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Pre-Separate Complex Graphics with Index Tables

(February 2010) posted on Mon Jan 25, 2010

Trimingham explains how to use index dots to simplify the process of separating highly detailed garment designs.

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By Thomas Trimingham

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.


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