Rescreens

The problem with printed photos is that they've already been halftoned. That is, the grays or colors of the image are simulated with little dots, and while our eyes are easily fooled, scanners are not. If you scan these images in Grayscale or Color mode and print them, the PostScript printer rescreens them. The conflict between the original halftone screen and the output screen results in a real mess (see Figure 12-9).

Figure 12-9. The problem with rescreens

The other problem is that in the original screening, a lot of the image detail is lost (the coarser the screen, the less detail remains). So when you scan the screened image, there's not a lot of detail there for the scanner to grab. The goal is to capture (and maintain) as much of that detail as possible, while avoiding the problems of overlapping screens. Don't expect miracles, though. Remember: garbage in, garbage out.

There are two basic approaches to working with screened images:

• Line art. Reproduce the image as black-and-white line art. This only works well with low-frequency images, under about 85 lpi. Note that you can't resize the image in a page-layout program later without making the image fall apart or creating moiré patterns. If your halftone was printed at 100 lpi or higher, it's probably better to convert it into a grayscale image using the techniques from the previous section.

• Grayscale. Scan in grayscale or color, then use filters to remove the halftone pattern while maintaining detail. The essential concept is "blur, then sharpen."

Tip: Make Rescreens Smaller

Since there's not much detail in screened images, you should generally plan on reproducing them at a smaller size than the original. You can use the techniques described here to break up the halftone pattern, and not suffer the loss of detail as much; because the image is smaller, less detail is needed.

Tip: Get Permissions First

This should be obvious, but all too often it is not: if the printed image isn't yours, you should always get permission to use it before you scan it in. Ethics aside, there are certainly copyright issues involved here. Most copyright violations in digital imaging occur when people scan pictures from magazines or books without thinking.

Frequency Considerations

One of the first things to consider when working with rescreens is the screen frequency of the printed images. Our techniques vary, depending on whether we're working with low-, mid-, or high-frequency halftones.

Low-frequency halftones

Low-frequency images are both hard and easy. They can be easy because you can reproduce them as line art, as mentioned above. But they're frustrating because there's so little detail; scanning as grayscale is almost always futile. If the line-art techniques aren't working for you, however, you can try using the methods for medium-frequency images described in the next section.

Medium-frequency halftones

Capturing medium-frequency halftones80 lpi to 120 lpiis perhaps the hardest of all. These halftone spots are too small to re-create in line art, but they're too large and coarse to blend together as a grayscale without blurring the image unacceptably (see "High-frequency halftones," later in this chapter). You know a halftone falls into this category if you can see the halftone dots when the paper is six inches away from your face, but you can't see them (at least, not clearly) when the paper is two feet away.

There are six techniques that we commonly use when scanning mid-frequency halftones (there are other techniques, but we usually find these ones effective). All five attempt to capture grayscale information and remove the moiré patterns that typically occur (see Figure 12-10).

Figure 12-10. Rescreening mid-frequency halftones often causes moirés.

• Median, Despeckle, and Dust and Scratches. The Median and Dust and Scratches filters are probably the most effective methods for removing dot patterning, but they work at a cost. Both filters average several pixels together to get a median value for the group. That means your image gets blurry. Often you can retrieve some of the edges with Unsharp Mask, but sometimes you have to apply the filters so much that the image is damaged. Nonetheless, even a one-pixel Dust and Scratches filter can smooth out many of the problem areas in an image.

  If the resolution of the printed image is above 100 lpi, the Despeckle filter might work better than Median. We often try Despeckle first, and if it doesn't work well enough (or it damages the image in ways we don't like), we undo it and revert to Median.

• Downsampling. Downsampling using bicubic sharper interpolation (see Chapter 3, Image Essentials) is one of the best ways we know to get rid of patterning, because Photoshop groups together a number of pixels and takes their average gray value. The problem, of course, is that you can also lose detail. Your goal is to downsample just enough to average out the halftone dot pattern, but not so much that you lose details in the image.

• Upsampling. After you downsample, you might need to upsample again to regain image resolution. You never get lost details back, of course, but sometimes sharpening the higher-resolution image can make it appear as though you did.

• Rotating. When you rotate an image in Photoshop, the program has to do some heavy-duty calculation work, and those calculations typically soften the image somewhat, breaking up the halftone pattern. If you have a very slight patterning effect after scanning a pre-halftoned image, you might try rotating the entire image 10 or 20 degrees, and then rotate the same amount back again. This double rotation can average out some patterning.

Once you've managed to break up the halftone pattern, you'll need to go after the image with the Unsharp Mask filter to give the impression of sharpness for the detail that remains. Since the image will probably be fairly blurry, you'll have to make the more extreme sharpening moves that we suggest in Chapter 9, Sharpness, Detail, and Noise Reduction, while being careful to avoid bringing the halftone pattern back out.

High-frequency halftones

Scanning pre-halftoned images with high screen frequenciesover 133 lpiis often easier, because the dot patterns blur into gray levels while maintaining detail. You often need to use the techniques listed above, but you don't have to work as hard at salvaging the image. In fact, we often find that just scanning at the full optical resolution of the scanner and downsampling to the resolution you need is enough to get rid of patterning. Or, try placing the artwork at an angle on the scanner, then downsampling and rotating in Photoshop (the Cropping tool lets you do both at once).

Tip: Pay Attention to Actual Pixels

Remember that the most important magnification view in Photoshop is Actual Pixels (100-percent view). If you scan an image and you see horrible moiré patterns at 33-percent view, don't panic. Zoom in to 100-percent view and see what's really going on. The damage is often much less than you first thought. Even if you don't see patterning in the Actual Pixels view, you still may opt to do a little smoothing work (especially if you see patterning when zoomed in to 200-or 400-percent), but it's not essential.