With all the frenzy on the Photoshop scene that surrounds cool effects like fractalization, motion blurs, and drop shadows, it's easy to lose sight of the basics. And there are few scanned images more basic than line art-those black-and-white images (or "bitmap images," in Photoshop terminology) with no halftoning, dithering, or anything else-are as simple as can be. Each pixel is either on or off, black or white, and you aren't concerned with levels of gray or halftones. Scanning, manipulating, and printing these things should be easy. And it is-at least compared to the vagaries that surround grayscale and color images.
Nonetheless, we've found that most people's line art images don't begin to approach the quality of (even mediocre) photographic reproduction. Edges are jaggy, fine lines break up, and dense patterns clog up. Many of you are going to be surprised when we tell you that it doesn't have to be that way.
With line art, you can actually produce an image that matches the original to an extent that just doesn't happen with grayscale and color images. With just a few techniques under your belt, you can achieve that ethereal, Platonic state of perfect line art reproduction, and with very little effort. The tricks lie in scanning mode, resolution, sharpening, and thresholding.
It's essential that you scan in Grayscale mode to take advantage of the techniques covered in this chapter. If you scan in Bitmap mode (line art or 1df-bit), you can't do much of anything to improve your image. In Grayscale mode, however, you can sharpen, adjust the black/white threshold to control line widths, and increase your effective line art resolution; each of these techniques helps create a beautiful reproduction.
So avoid the temptation to scan line art as line art, and scan it as grayscale instead. Sure, your files are eight times as large, but it's only temporary. You can convert them to Bitmap mode when you're done with your manipulations. And the quality difference with these techniques is like the difference between... well... black and white.
When you're printing to an imagesetter, you need very high image resolution to match the quality of photographically reproduced line art. That means 800 ppi minimum image resolution. You can see the difference between 800 and 1,200-ppi line art (see below), so you may want to opt for the higher resolution if your printing method can hold it.
Of course, you never need image resolution higher than your output resolution. If you're printing your final artwork on a 600-dpi laser printer, for instance, you don't need more than 600-ppi image resolution. The additional data just gets thrown away.
Tip: Fast Line Art Imagesetting. It may seem like 800 and 1,200-ppi images are going to make for big files and slow print times, and that's often the case. But there's a little-known back-door trick that might speed things up for you. Most imagesetters that are based on Adobe PostScript RIPs (and perhaps some others) can process same-resolution bilevel images very quickly.
For example, if you send a 1,200-ppi line art (bitmap) image to a 1,200dpi imagesetter, it can say, "Hey! That's a 1,200-dpi bitmap, and I'm printing 1,200-dpi bitmaps, so I'll just blast it down onto the page, dot for dot." The end result? A 1,200-ppi line art image may print much faster than an 800-ppi image. Try it on your system and see if it works.
(By the way, this doesn't work with desktop laser printers, at least according to the testing we've done.)
Tip: Scan Big for High Resolution. "Great," you're saying. "They say we need 800-ppi images, but all we've got is a 300-ppi scanner. And they said back in the Image Essentials chapter that upsampling is useless. What are we supposed to do with this business card the client gave us?"
You've got two ways to get a higher resolution out of a low-resolution scanner. First, you can scan a large original at your scanner's highest optical resolution and scale it down, increasing resolution. If you reduce the image to 50 percent, for instance, you double the resolution.
You can either scale the image in a page-layout program, or adjust the size in Photoshop's Image Size dialog box while the File Size checkbox is turned on (see Chapter 3, Image Essentials).
If you don't have a larger version of the artwork, you can enlarge your small version on a stat camera or quality photocopier, and scan that. You still get a higher-quality image because the photographic enlargement doesn't cause pixelization (there may be some cleanup work involved after the scan, of course).
The second solution (which you can use in combination with this enlargement/ reduction technique) is covered in the next tip.
Tip: Doubling Your Scanner's Line Art Resolution. The second method for going beyond your scanner's resolution essentially "steals" information from an 8-bit grayscale scan, converting that information into higher line art resolution.
1 . Scan your artwork as a grayscale image at your scanner's maximum optical resolution (let's use 300 ppi for this example).
2. Double the image resolution (thus quadrupling the file size) using the Image Size dialog box (make sure the Resample Image checkbox is turned on with Bicubic as the resampling method). In our example, you'd upsample to 600 ppi. Note that if your scanning software can interpolate up to this same resolution, you can use that as you scan, and save yourself a step.
If you're yelling, "Hey! You said interpolation was useless," you're right-we did. This is the exception (we can't think of any others).
3. Sharpen and threshold the image as outlined later in this chapter.
4. Switch to Bitmap mode at the same resolution (600 ppi in our example), with the 50% Threshold option selected (see below).
Voila! A 600-ppi line art image from a 300-ppi scanner. While it isn't a true 600-ppi scan, it's so close that we dare you to find a difference. You may be able to raise the image's resolution above two times optical resolution, but that's pretty much the point of diminishing returns.
Note that you can use this tip alongside the previous one to res up to 800 ppi or higher.
If the arithmetic of scaling and resolution is giving you trouble, you might want to take a look at the tip "Figuring Scaling and Resolution" in Chapter 13, Capturing Images.
Nothing will do more for the quality of your line art images than sharpening the grayscale scan (see below). 'Nuf said. We recommend running the Unsharp Mask filter twice with the settings 500/1/5. If your scanning software can sharpen, you may be able to save yourself a step (Hewlett-Packard's DeskScan software does it, for instance, though not as well as Photoshop's Unsharp Mask filter).
When you scan line art in Grayscale mode, lines aren't captured as hard lines, but as a collection of pixels with different values (see below). But although you scanned in grayscale, you ultimately want a straight black-and white image. The way you get there is via the Threshold command (from the Adjust submenu under the Image menu). Threshold turns gray pixels above a certain value to black, and pushes all other pixels to white.
By adjusting the break point in the Threshold dialog box where pixels go to black or white, you can control the widths of lines in your scanned-as-grayscale line art image (see beow).
For simple line art images that don't include very detailed and dense shadow areas, just set Threshold to 2 and press Return. With images that do include densely detailed shadows, try values up to about 55. As you move the slider to the left, you can see the fine lines start to break up. As you move right, the shadow areas start to clog. It's a lot like working the trade-off between shadow and highlight detail with the Levels dialog box on a grayscale image.
The last step is changing the image from a grayscale (8-bit) to a bitmap (1-bit) mode. Use Photoshop's Image>Mode>bitmap command. It would be prudent to save a copy of the grayscale before converting.