Real World Adobe® Photoshop® CS2 [Electronic resources] : Industrial-Strength Production Techniques نسخه متنی

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).

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).