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8 Resolution and Scanning > Scanning previously printed images

Scanning Previously Printed Images

If you are sent an image that has been printed using the four-color process and that you are now supposed to scan, you have two problems. The first is copyright. Does the client have the right to use this image? Check up. Otherwise, you may be liable.

The second problem is that all the correct screen angles for the CMYK colors have already been taken (see Chapter 2). If you simply scan the print again, the result will be a moiré pattern. This is because as your scanner moves across the image, it captures whole rows of pixels at a time, each one of which is called a “sample.” The number of samples per inch (SPI) is therefore the same as the number of dots, or pixels, per inch that you have specified. All these samples put together create a grid of pixels at a new screen angle—hence the moiré.


  

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