Preparing Line/Stipple Art for Print Publication
Preparing line art is a relatively simple procedure in itself. One does not need a thorough understanding of the printing process. The purpose of this presentation is to present the all information needed to produce your files. If your goal is to learn more about what happens to your files after you drop them off to the printer, I recommend the Print Publishing Guide by Adobe Press.
Resolution: I refer to resolution as dpi (dots per inch). Please note that this is actually incorrect since a scanner scans in pixels, not dots. But most folks use the term dpi for resolution so I will follow suit here.
A line art digital file for publication is composed of black and white pixels (no grays). In Photoshop, this is the Bitmap mode (Image>Mode>Bitmap). Unless you have a very good scanner, your image will start out in the Grayscale mode (Image> Mode> Grayscale) and need to go through the steps that follow. A raw scan is rarely of the quality to send out for printing. This procedure cleans and sharpens the scan.
1. Begin by scanning the image. The resolution should be set at 1200 dpi. Scan in the gray mode. On many scanners, this is called the B&W photograph mode. DO NOT scan under the line art setting. Please note that an image that is 8.5 inches x 11 inches will produce a very large file at this point. This file size will be dramatically reduced by the end of the procedure.
The high resolution is needed so your final art will not have the jaggies. Theortically, you should scan at the resolution of the image setter (part of the printing plate-making process) at the printer. This is usually 2400dpi or higher, however 1200 dpi is quite sufficient. It also keeps the file size manageable. It does not matter whether your scanner has a true optical 1200 dpi resolution in this case. Interpolation will not change the results.
2. Import or open the image in Photoshop.
3. Double-click the zoom tool or magnifying glass. Double clicking the zoom tool shows the actual pixels. I then usually press the f key to make the image full-screen. The more that you can see, the better. After you have done these steps you will only see a small portion of your image.
4. Use the hand tool to move to a part of the image that represents the rest of the image. In other words, you should see lots of lines and/or stipples. This image has no stipples.
5. Adjust the brightness/contrast so that most of the pixels are black and white. There are several ways to make this adjustment. I simply use Image>Adjustments>Brightness/Contrast. There is really no need to use the more sophisticated Levels or Curves here. They come into play later when working in color. I slide the Brightness up to get rid of most gray noise introduced in the scan in the white areas. It is okay if some of the noise is left behind. It will go away in subsequent steps. Then I move the Contrast up to make the lines mostly black again. The edges will still be fuzzy and grey.
6. Sharpen the image to clean up the edges of each line and/or stipple. Use Filter>Sharpen>Unsharp Mask. Radius is 6.0, Threshold is zero. Adjust the amount until the line/stipple edges look clean. At this point your image should be mostly black and white pixels.
7. Use threshold adjustment to make all pixels either black or white. Go to Image> Adjustments>Threshold. This converts all pixels to either black or white. By moving the slider up or down, you can increase or decrease the thickness of the lines/stipples. You need to approximate the thickness of the line with the slightly gray edges. Use the Preview button to toggle back and forth to gauge where to move the slider. I typically use something close to the default which is 128. Note that you can see the jaggies here. Keep in mind that you are viewing the image much larger than it will be printed. Those small steps in the lines will not be apparent in the published piece.
8. Finally, convert your document to Bitmap mode. Go to Image>Mode>Bitmap. In the dialogue box, make sure the Output is 1200. Then select 50% Threshold and click ok. The document will be drastically reduced in size (disk space). Please note that the document will likely look terrible on the monitor! Don't worry it is fine. (You can soft proof it by printing it. Keep in mind that the printer may limit the resolution. For example, if you have a 600dpi laser printer, the printer will only print at 600dpi. So at that resolution, you may see some jaggies introduced by the printer. I get pretty good results printing to an inkjet with a 1200 dpi resolution. These prints are very close to the final published image.) Save the document in TIFF format for the printer/publisher.
9. Below is a scan of the original image above the published image. For comparison, I have not retouched either image. Note that the image was published on a cream paper and was reduced slightly. By comparing line for line, you can see that the published version compares nicely with the original. this image is a sketch of a blueberry meadow in Maine. It was published in the Understory, a newsletter for the Irvine Nature Center, in December 1999. It was printed on uncoated stock on a sheet fed press.
Wm. Keith Harrison is an artist
specializing in botanical art and scientific illustration.